"Phase angle" "diffraction" and other mis-conceptions in bird photography
Hi all, Recently I have started to hear more and more about two new guys in town - 'phase angle' and 'diffraction'. Apparently they were always here but somehow I haven't met them in the 30 years I am doing my bird photography.... The reason I post this thread is not to start a war - just to let less experienced bird photographers or those who are starting out now not to worry about those guys as there is enough factors to worry about when it comes to getting an excellent bird photo... Here are a few simple rules that work for me: (please note that I refer to bird photography ONLY) 1. Always have the sun behind you – don’t worry about the precise angle as long as the sun is behind you. Birds have the annoying tendency to walk/fly so stretching out your arms in order to determine the “correct” ‘phase angle’ is not very practical. It is especially true in those parts of the world where bird photography has to be done from a hide. (Like in Australia and many other places.) It is also true when doing BIF photography where trying to control the precise angle is not practical at all. 2. Shoot early morning and/or late afternoon when it’s sunny. Depending on how far you are from the equator –the closer you are the less time you have before light turns too harsh for birdphotography. 3. As to shutter speed when shooting BIF. If your panning technique is decent - anywhere between 1/1600-1/3200 should be more than enough to give you a perfectly sharp image. It won’t hurt to go even faster but don’t do it on the expense of going wide open – especially when using converters as sharpness will suffer. 4. If you got enough light –prefer shooting around f6.3-f8 and don’t worry about ‘diffraction’ (the other new guy in town). Most of my images in the last 30 years were taken between f6.3 up to f14 and I had absolutely no problems with ‘diffraction’. Going wide open will not only hurt the sharpness but will also reduce the depth of field. (Both sharpness and depth of field belong to the more important guys in town….)
5. If you are using the Canon 1D Mark IV or the Nikon D3 - don't worry about going higher than iso 400 if you have to as you'll get very clean files at iso 800 and even more. Just my two cents….
Happy shooting!!!
Last edited by James Shadle; 01-07-2012 at 02:07 PM.
Hi all, Recently I have started to hear more and more about two new guys in town - 'phase angle' and 'diffraction'. Apparently they were always here but somehow I haven't met them in the 30 years I am doing my bird photography....
Ofer, these facts are not "new guys in town." They have always been there. And they are not misconceptions.
Diffraction, in particular is becoming more important because smaller pixels in digital cameras show the effects at faster and faster f/ratios. In film days we didn't see diffraction effects much even with fine grain film at f/8 or f/11. But digital cameras, especially in the last few years where pixel sizes are smaller than about 6 microns, suffer from diffraction effects like never before. So in that sense, what formerly could be ignored can no longer be, as it does affect our images.
I'm attaching a graph from one of my articles. Diffraction reduces contrast. It always has, and always will. Basic physics. At f/5.6 and green light, the diffraction spot diameter is 7.2 microns and it grows to over 10 microns at f/8. So what happens when you record an image made at f/8 with 5.7 micron pixels (1DIV) or 4.3 micron pixels (7D)? Contrast of fine details are reduced! In fact, with a 7D and f/8, pixel to pixel contrast is zero! Diffraction was not as much a factor a few years ago at f/8 with a Canon 1DII (8.2 micron pixels) or even today with a Nikon D3 due to the large pixels.
More on diffraction effects and telephoto reach at: http://www.clarkvision.com/articles/telephoto_reach/
The reason I post this thread is not to start a war - just to let less experienced bird photographers or those who are starting out now not to worry about those guys as there is enough factors to worry about when it comes to getting an excellent bird photo...
But it is important to have your facts correct and not mislead.
Here are a few simple rules that work for me: (please note that I refer to bird photography ONLY) 1.Always have the sun behind you – don’t worry about the precise angle as long as the sun is behind you. Birds have the annoying tendency to walk/fly so stretching out your arms in order to determine the “correct” ‘phase angle’ is not very practical. It is especially true in those parts of the world where bird photography has to be done from a hide. (Like in Australia and many other places.) It is also true when doing BIF photography where trying to control the precise angle is not practical at all.
Consider this analogy: on-camera flash. Does anyone like the light from on-camera flash? The reason few, if anyone, likes on camera flash lighting is because it is low phase angle: the angle from the light to the camera as viewed by the subject. People move their flash off camera to increase the phase angle.
Phase angle is important. If the sun is low in the sky, have it coming over one shoulder or even a little further to one side and not directly behind you. That angle imparts subtle shading that gives form to the subject. You want a little angle but not too much because if too much shadow contrast increases. As the sun rises put the sun more behind you: as Artie says, point your shadow at the subject in that case. With the sun higher, the phase angle remains significant again imparting form-making shading.
Avoid very low phase angles. Also avoid high phase angles (e.g. side light) if you want to avoid high contrast situations. So there need not be much precision; simply not too low and not too high. There is a wide range in between that works well. One need not fret about it in the field, just position so not too low and not too high. The Goldilocks Zone.
Phase angle is simple. Next time you are out with your camera, put it on a tripod and point at your subject. Stand in front of the camera and stretch one arm straight parallel to the lens axis. Now point your second arm straight at the sun. The angle between your two arms is the phase angle. You don't want your arms parallel and usually not one arm out to one side. In between, and closer together than half way to the side is great: The Goldilocks Zone. 2.Shoot early morning and/or late afternoon when it’s sunny. Depending on how far you are from the equator –the closer you are the less time you have before light turns too harsh for birdphotography.
Good advice. I agree. But even at higher latitudes, or any latitude, harsh light can occur when the sun is low in the sky and the atmosphere very clear. Harsh light is from one direction and harsh light is mitigated by clouds and other objects, including trees and grass that reflect light into shadows. Understanding light is key to photography.
3.As to shutter speed when shooting BIF. If your panning technique is decent - anywhere between 1/1600-1/3200 should be more than enough to give you a perfectly sharp image. It won’t hurt to go even faster but don’t do it on the expense of going wide open – especially when using converters as sharpness will suffer.
Shutter for the subject and the available light. There is not one small range for all. Large birds fly slower. Small birds fly faster and need faster shutter speed. Ofer didn't seem to like my 1/4000 second exposure on a pygmy falcon, but the falcon was moving at over 1000 pixels per second, even with my panning. Blur must be less than 1/3 pixel (better if less) to be sharp. Small fast birds large in the frame need higher shutter speeds. Try photographing cliff swallows at 1/1600 second exposure when they are large in the frame and most images will likely be blurred.
4.If you got enough light –prefer shooting around f6.3-f8 and don’t worry about ‘diffraction’ (the other new guy in town). Most of my images in the last 30 years were taken between f6.3 up to f14 and I had absolutely no problems with ‘diffraction’. Going wide open will not only hurt the sharpness but will also reduce the depth of field. (Both sharpness and depth of field belong to the more important guys in town….)
See above. Know your lenses. Most people agree that adding TCs, especially 2X TCs, reduces image quality. A significant part of that loss is due to diffraction. So if you know that your lens is sharp at f/4.5 or 5.6 don't stop down to f/8 unless you need the depth of field. Usually one needs the light, especially near sunrise/sunset.
5. If you are using the Canon 1D Mark IV or the Nikon D3 - don't worry about going higher than iso 400 if you have to as you'll get very clean files at iso 800 and even more. What is clean concerning noise is relative to individuals. I personally don't like the noise on any camera at ISO 100, but that is a physics limitation we all have to live with. I also thought velvia 50 had too much grain. One reason I shoot large format.
Roger
Last edited by Roger Clark; 01-07-2012 at 01:27 AM.
I don't doubt both of you are correct. Many of my photos are great also. 1dmk4 Many times I don't have a choice as to where to shoot from. Just take the da*n picture. I like Ofers way best....
However I respect both of you....
John
Ofer didn't seem to like my 1/4000 second exposure on a pygmy falcon, but the falcon was moving at over 1000 pixels per second, even with my panning. Blur must be less than 1/3 pixel (better if less) to be sharp. Small fast birds large in the frame need higher shutter speeds. Try photographing cliff swallows at 1/1600 second exposure when they are large in the frame and most images will likely be blurred.
Roger, thanks for mentioning bird flight speed measured as pixels per second. It's a factor in BIF photography that I've never heard put that way and it clarifies the situation very well.
Phase angle is important. If the sun is low in the sky, have it coming over one shoulder or even a little further to one side and not directly behind you. That angle imparts subtle shading that gives form to the subject. You want a little angle but not too much because if too much shadow contrast increases. As the sun rises put the sun more behind you: as Artie says, point your shadow at the subject in that case. With the sun higher, the phase angle remains significant again imparting form-making shading.
Avoid very low phase angles. Also avoid high phase angles (e.g. side light) if you want to avoid high contrast situations. So there need not be much precision; simply not too low and not too high. There is a wide range in between that works well. One need not fret about it in the field, just position so not too low and not too high. The Goldilocks Zone.
Roger, I struggled to understand this concept for a fair while and disagreed through most of it, but finally realized that the point (for me) is when the sun is directly behind you, you will be in alignment with feather edges and their shadows and consequently will not see much if any of the shadow. (Of course, if the sun is really directly behind you the bird would be in your shadow ). So, increasing phase angle would expose more shadow, hence contrast (between lit surface and shadow). But as the shadow grows it begins impinging on the lit parts that you want to see, so the phase angle would be best on the small end but greater than 0 degrees (I think you said 10 degrees is good). Much more (say at least beginning with 45 degrees, but probably more like 30 degrees IMO) the shadow begins to dominate the edge.
I think most bird photographers intuitively see the impact of phase angle most in the sun inclination angle. Three hours from exact sunrise (over flat terrain) is 45 degrees (at the equator and at equinox .). Hence, near dawn and sunset are good times to shoot. Shooting upwards (say, bird in tree) both obscures top parts of the bird due to shooting angle but also increases phase angle significantly.
Side to side phase angle (as opposed to sun inclination angle) has a downside. On the sun side of the angle, the shadow is lost under the feather edge and full shadow begins on the side opposite the sun (think side light shadows). So, the benefit of the phase angle contrast is mitigated somewhat by it's loss in the overall subject sides.
So in the end I find myself agreeing with you of the importance of phase angle in respect to sun inclination angle, but not so much in respect to side to side angle. My takeaway is pointing your shadow at the subject is the best (but only if the subject isn't in your shadow ).
Cheers,
-Michael-
Last edited by Michael Gerald-Yamasaki; 01-13-2012 at 02:38 PM.
Reason: clarified, corrected miscalculation
Roger, I struggled to understand this concept for a fair while and disagreed through most of it, but finally realized that the point (for me) is when the sun is directly behind you, you will be in alignment with feather edges and their shadows and consequently will not see much if any of the shadow. (Of course, if the sun is really directly behind you the bird would be in your shadow ). So, increasing phase angle would expose more shadow, hence contrast (between lit surface and shadow). But as the shadow grows it begins impinging on the lit parts that you want to see, so the phase angle would be best on the small end but greater than 0 degrees (I think you said 10 degrees is good). Much more (say at least beginning with 45 degrees, but probably more like 30 degrees IMO) the shadow begins to dominate the edge.
I think most bird photographers intuitively see the impact of phase angle most in the sun inclination angle. Three hours from exact sunrise (over flat terrain) is 45 degrees (at the equator and at equinox .). Hence, near dawn and sunset are good times to shoot. Shooting upwards (say, bird in tree) both obscures top parts of the bird due to shooting angle but also increases phase angle significantly.
Side to side phase angle (as opposed to sun inclination angle) has a downside. On the sun side of the angle, the shadow is lost under the feather edge and full shadow begins on the side opposite the sun (think side light shadows). So, the benefit of the phase angle contrast is mitigated somewhat by it's loss in the overall subject sides.
So in the end I find myself agreeing with you of the importance of phase angle in respect to sun inclination angle, but not so much in respect to side to side angle. My takeaway is pointing your shadow at the subject is the best (but only if the subject isn't in your shadow ).
Cheers,
-Michael-
Hi Michael,
In my opinion a little bit of side angle helps. I posted this on October 30. Check the images where I measured the phase angle at time of capture. The sun was low in the sky, so the phase angle was dominantly to the side. To me the 10 degree phase angle image looks flat, where 20 and 30 look better.
Of course, everyone is free to choose whatever angle they want. But for me, when I can, I choose 20 to 30 degrees for most wildlife. Occasionally, side lighting can be dramatic (phase angle 90 degrees), and I do like back lighting (near 180 degrees phase angle). But I really try and avoid less than about 15 degrees. I find it easier to be slightly off zero phase angle than to try and point one's shadow directly at the bird. But I do agree with Artie, that as the sun rises, one should point their shadow at the bird, as the phase angle is increasing and can become too large as you also described.
But as I study this problem more, I find that the best phase angle depends on the subject, and for birds, the microstructure of the feathers. Birds with finer feathers make better images at higher phase angles, than birds with large feathers and feather structure that is coarser (e.g. the GBH on the first link).
Then there is another concept: negative versus positive phase angle. I'll probably introduce that next month.
In my opinion a little bit of side angle helps. I posted this on October 30. Check the images where I measured the phase angle at time of capture. The sun was low in the sky, so the phase angle was dominantly to the side. To me the 10 degree phase angle image looks flat, where 20 and 30 look better.
Okay, sure, that's fine. I was just trying to say, that for such side angle, along with the gain of apparent detail there is also loss. Take an image of, say, a waxing gibbous moon (google waxing gibbous moon and pick a high res one) such as:
The angle between the sun, moon and earth approximate a angle which looks in the first image about maybe 30 degrees (doesn't matter for the point I'm trying to make). On the right side near the dark side of the moon plenty of crater detail. On the side away from the dark side, not so much crater detail.
The downside of increasing phase angle is the decrease of shadow detailing on the sunny side of the subject as in the gibbous moon. To change sun inclination angle requires the photographer to move up or down with respect to the subject (which is a bit harder than moving side to side usually). In other words we're stuck with the time of day of the shot . Side to side angle change when available would gain detail on one side (the shadow side of the subject) but loose detail on the sunny side.
This, of course, may be exactly what the photographer wants.
Okay, sure, that's fine. I was just trying to say, that for such side angle, along with the gain of apparent detail there is also loss. Take an image of, say, a waxing gibbous moon (google waxing gibbous moon and pick a high res one) such as:
The angle between the sun, moon and earth approximate a angle which looks in the first image about maybe 30 degrees (doesn't matter for the point I'm trying to make). On the right side near the dark side of the moon plenty of crater detail. On the side away from the dark side, not so much crater detail.
The downside of increasing phase angle is the decrease of shadow detailing on the sunny side of the subject as in the gibbous moon. To change sun inclination angle requires the photographer to move up or down with respect to the subject (which is a bit harder than moving side to side usually). In other words we're stuck with the time of day of the shot . Side to side angle change when available would gain detail on one side (the shadow side of the subject) but loose detail on the sunny side.
This, of course, may be exactly what the photographer wants.
Cheers,
-Michael-
EXCELLENT!!! Great observation! In the images of the moon, the phase angle is virtually constant across the lunar disk (varies by a mere +/- 1/4 degree). So what you are observing is positive versus negative phase angle. We see these same effects on birds as they have rounded bodies analogous to the moon. I'm attaching an image with two finches photographed on the same perch within minutes of each other, both with a phase angle of 20 degrees. I did not move between images. The bird on the left has the majority of the body and head angle at a negative 20 degrees, while the image at right is at a positive 20 degrees. This gets into body angle and head angle with relation to the sun. The negative phase angle shows more microshading in the feathers. If I was at a lower phase angle, the effect would be less and more like the image on the right regardless of which way the bird is facing.
So for the best images (in my opinion), many things must line up: sun altitude, atmospheric quality, negative phase angle, head angle to the photographer, head angle to the sun, body angle to the photographer, body angle to the sun, foreground quality, background quality, and of course great subject.
I had not talked about this before because it seemed hard enough to get the phase angle concept across here. Some seem quite out off by it. And of course one can't always control all these factors, but if one knows and understands them, one has a better chance of positioning oneself to maximize the probability of everything lining up.
I have followed this thread, and the other more lengthly one as well, the last couple of responses have given me a much clearer understanding of what Roger was trying to explain to those of us who did not quite grasp the concept he was expounding upon. Thank you for making a difficult and sometimes contentious subject understood Roger. My hats off to BPN and the staff.
EXCELLENT!!! Great observation! In the images of the moon, the phase angle is virtually constant across the lunar disk (varies by a mere +/- 1/4 degree). So what you are observing is positive versus negative phase angle. We see these same effects on birds as they have rounded bodies analogous to the moon. I'm attaching an image with two finches photographed on the same perch within minutes of each other, both with a phase angle of 20 degrees. I did not move between images. The bird on the left has the majority of the body and head angle at a negative 20 degrees, while the image at right is at a positive 20 degrees. This gets into body angle and head angle with relation to the sun. The negative phase angle shows more microshading in the feathers. If I was at a lower phase angle, the effect would be less and more like the image on the right regardless of which way the bird is facing.
Roger,
Okay, so I had my Ah, Hah! moment. With the notion of positive & negative phase angle I'm able to construct for myself a mental model of how this works. How I get it is positive & negative phase angle includes the surface angle on which the shadow falls. That is, there are two angles to consider: the sun, subject, viewer angle and the angle between the viewer and the surface on which the shadow falls. Here's a diagram:
The diagram shows on a curved surface three planes: the plane perpendicular to the viewer direction and two planes + or - 10 degrees from perpendicular. The phase angle (light source, subject, viewer) for each of these is the same (as shown about 48 degrees - chosen for attempted clarity of the diagrammed concept). The diagram attempts to show the variance in shadow length depending on surface angle irrespective of phase angle. This is the same variance that is seen in the Gibbous Moon crater shadows (or the difference in your own shadow lengths during the time of day).
As you observed the phase angle of the lunar surface as viewed from earth varies little from one side to the other, which means the surface angle is the source of the variance in the Gibbous Moon crater shadows.
Since the surface of birds for the most part is also a curved surface, I would think the surface angle would be the dominant source for visual impact on local contrast (tiny shadows). So how would one go about practically applying this notion?
One way might be to consider the surface of the bird as a more reflective surface (think shiny). With a point light source there will points where the direct light will be reflected to the view point (think specular highlights or catch lights). That surface angle would be the angle which one would expect the least local contrast. I think without considering all the angles if one looks closely at an egret's head or neck image one would see the variance in local contrast and perhaps be able to pick out the diffuse specular highlighted area (showing the least local contrast).
So the "trick" would seem to be to place the diffuse specular highlighted area in a place where it matters least.
{SO if the sun is over my left shoulder, I want the perched pointy faced bird, say egret, to be facing to the left so the reduced contrast region will fall on the pointy part of it's head and not interrupt the fine feather detail on the right. But if it's a bushtit I'd rather it be facing to the right so the fine feather detail shows up on the face and the less detailed region falls toward the back of the head (this, of course is all reversed if the sun is over your right shoulder )}
Okay, so I had my Ah, Hah! moment. With the notion of positive & negative phase angle I'm able to construct for myself a mental model of how this works. How I get it is positive & negative phase angle includes the surface angle on which the shadow falls. That is, there are two angles to consider: the sun, subject, viewer angle and the angle between the viewer and the surface on which the shadow falls.
Hi Michael,
Yes, more good insight, but a few points.
The case of the moon is a bit different than birds because the rocks and craters on the moon are pretty random, and the feathers on a bird a more regular, like the teeth on a comb. Even the moon reflects light differently from waxing to waning due to the amount of craters on the highlands and mare that have different distributions. While the appearance of shadows and the shadow length is also dependent to the angle to the local surface, the main point is that as the phase angle decreases, the shadow length decreases and when zero, and even very low phase angle, one sees no shadow regardless of the incident angle. Zero phase angle not only hides shadows, but another property occurs called coherent backscatter which increases the light returned more than one would predict from shadow hiding alone. And this scattering adds light at all wavelengths so reduced apparent color. So by working a little off the sun angle (I prefer 20 to 30 degrees), one gets shading, shadows, and better color.
Now one other property we can't control: spacing of the feathers. This is shown in my finches example. If the shadow casting object (e.g. feather) is separated from the object below, the shadow is longer. The finch on the left has separated feathers, making a longer shadow path length, enhancing the negative phase angle effect.
Positive versus negative phase angle is hardest to control in the field, unless a subject is facing a particular way. For example, the finches. When I approached the subjects, I chose a phase angle that was about 20 degrees. I could have chosen a spot that gave me less than 10 degrees, but then I would not have gotten little shading and shadows. So that was my choice. I also chose the 20 degrees rather than 30 degrees because the 20 degree spot had a better background. Then I photographed the birds in that spot and depending on which way they faced I got positive and negative phase angle images, but that was not in my control.
If I was in a flight situation, where a regular flight pattern was being done by the birds, I would have chosen a position that gave me 20 to 30 degrees phase angle with a good background. Of course as the birds flew by I would track them over a large range of angles, but my optimum position would be when they were at the 20 to 30 degree position.
While the appearance of shadows and the shadow length is also dependent to the angle to the local surface, the main point is that as the phase angle decreases, the shadow length decreases and when zero, and even very low phase angle, one sees no shadow regardless of the incident angle. Zero phase angle not only hides shadows, but another property occurs called coherent backscatter which increases the light returned more than one would predict from shadow hiding alone. And this scattering adds light at all wavelengths so reduced apparent color. So by working a little off the sun angle (I prefer 20 to 30 degrees), one gets shading, shadows, and better color.
Roger, I appreciate your explanations. I get lost, though, when you talk about actually being able to shoot at zero phase angle (in regards to bird photography). Perhaps, it's because when you say phase angle, you mean, lateral phase angle irrespective of time of day? Where I live, sunrise to sunset is about 10 hours this time of year. Since I usually don't have a direct sightline to the spherical horizon I would guess it's about an hour from sunrise or sunset that I would have direct sunlight on any natural subject. At that time (an hour after sunrise) if I'm laterally aligned between the sun and the subject I have a phase angle of about 18 degrees (in the summer it could be as little as 12 degrees).
One might say phase angle is the technical reason it is better to shoot when the sun is low in the sky. As a practical matter, phase angle is probably sufficiently off of 0 degrees whenever you're not worrying about casting your shadow on the subject .
Lateral phase angle (seems to me) to be useful in capturing additional contrast depending on feather alignment (shelves of feathers which are more vertically aligned), where vertical phase angle would impact the more horizontally aligned shelves of feathers.
As far as coherent backscatter goes... I suspect that ambient light and reflective light (at least from the ground) on all natural subjects would dominate the effect of backscatter.
Roger, I appreciate your explanations. I get lost, though, when you talk about actually being able to shoot at zero phase angle (in regards to bird photography). Perhaps, it's because when you say phase angle, you mean, lateral phase angle irrespective of time of day? Where I live, sunrise to sunset is about 10 hours this time of year. Since I usually don't have a direct sightline to the spherical horizon I would guess it's about an hour from sunrise or sunset that I would have direct sunlight on any natural subject. At that time (an hour after sunrise) if I'm laterally aligned between the sun and the subject I have a phase angle of about 18 degrees (in the summer it could be as little as 12 degrees).
Hi Michael,
Phase angle is the angle between the sun, subject and viewer (camera). What you are describing is azimuth angle. Zero phase angle is the limit, but any phase angle lower than about 10 degrees has much reduced shading and shadows. One can photograph at such low angles at any time of day. For example, say you are photographing birds in the water and you are standing on shore with your camera pointed down. The sun could be quite high in the sky but directly behind you so your shadow comes close to falling on the subject. That is a low phase angle. If the sun is directly behind you, high over your head, and you are imaging in a horizontal direction, then the phase angle can be quite large, e.g. on the equator at 9 am it is about 45 degrees high, so the phase angle is about 45 degrees with the sun directly behind you over your head. Whether overhead or to the side, non zero phase angle creates shading and shadows and whether overhead or to the side only changes the direction of the shading but not the magnitude. Although aesthetically I prefer shading to the side.
Originally Posted by Michael Gerald-Yamasaki
One might say phase angle is the technical reason it is better to shoot when the sun is low in the sky. As a practical matter, phase angle is probably sufficiently off of 0 degrees whenever you're not worrying about casting your shadow on the subject .
There are other reasons too. Low in the sky generally has more scattering so the light is more diffuse, and is warmer in color. As the sun rises, one can keep adjusting phase angle, for example by starting out low, then increasing height, e.g. standing and pointing down to keep phase angle from getting too large, but that obviously has other detrimental effects, including aesthetically poor perspective, light not as warm, and in a clear sky, harsher light.
Originally Posted by Michael Gerald-Yamasaki
Lateral phase angle (seems to me) to be useful in capturing additional contrast depending on feather alignment (shelves of feathers which are more vertically aligned), where vertical phase angle would impact the more horizontally aligned shelves of feathers.
Yes, but ignoring the other qualities of low versus high sun.
Originally Posted by Michael Gerald-Yamasaki
As far as coherent backscatter goes... I suspect that ambient light and reflective light (at least from the ground) on all natural subjects would dominate the effect of backscatter.
yes. One can observe the backscatter as a brightening around the shadow of your head when cast a fair distance, for example onto soils, grasses, bushes, trees and even clouds (e.g. from an airplane).
You didn't hijack the thread, you added a lot of value.
Whether overhead or to the side, non zero phase angle creates shading and shadows and whether overhead or to the side only changes the direction of the shading but not the magnitude. Although aesthetically I prefer shading to the side.
Ah, I think that sums it up for me. I have two takeaways from this discussion:
1. Phase angle is important. (But) If one is shooting more or less at a subject horizontally, the time of day is the primary consideration in phase angle.
2. Surface angle with respect to the phase angle can control the placement of detail contrast. In other words the angular relationship of viewer to subject (parts!) with respect to sun angle has an impact on detail contrast. Specular highlighting shows area of least detail contrast.
As a practical matter, it seems that at the given time of day one is shooting there is little one can do about vertical (azimuth) phase angle unless one is really close to the subject. At 30 feet, going from standing full upright to laying on the ground would only gain at most 10 degrees in phase angle (using chord length for estimate - www.1728.org/circsect.htm for calculator).
Similarly, increasing phase angle laterally (other than the direction of the shading) would require moving outside the phase angle already present vertically. That is, if your vertical phase angle is 20 degrees, you would have to move beyond 20 degrees laterally to have any impact on the phase angle... which would be over 10 feet for a subject at 30 feet.
Shooting options for 2. above seem to have the most latitude (depending on subject's pose change frequency ) and impact on shot selection (e.g. I want to concentrate on shots where the head is turned toward the right rather than the left because the sun is off my right shoulder).
Shooting options for 1. are best taken at set up (shooting here rather than there for this time of day) since changes in the presence of the subject at the required distances to impact phase angle can be problematic.
Anyway, that's what makes most sense to me.
Roger, thanks for the interesting and inspiring discussion...
Roger:
You stated: "So what happens when you record an image made at f/8 with 5.7 micron pixels (1DIV) or 4.3 micron pixels (7D)? Contrast of fine details are reduced! In fact, with a 7D and f/8, pixel to pixel contrast is zero!" I am not sure if this is correct or we are misunderstanding something. The spatial cutoff frequency for an ideal lens at some F# is approximately 1800/F# in LP/mm. Now at f/8 this will yield a cutoff frequency of 225 LP/mm. This would correspond to a period of 4.4 microns. So if two pixels are used to sample this line pair the pixels would be about 2.2 microns. The 7D has pixels of 4.3 microns so the cut off period is 8.6 microns resulting in a cutoff spatial frequency of 116 LP/mm. Thus at f/15.5 there is no pixel-to-pixel contrast. Now perhaps you were thinking of the frequency where the contrast is reduced by 50%. Then I think f/8 is true. Am I misunderstanding you?
Last edited by John Chardine; 02-05-2012 at 10:11 AM.
Reason: typos for clarity
Attached is a demonstration of diffraction with a 7D and 300 f/2.8 lens. Fine detail (e.g. 3px to 5px on the chart) peaks at f/5.6 then drops due to diffraction. Next post I'll show the MTF plot.
Roger
Last edited by Roger Clark; 02-12-2012 at 05:48 PM.
Attached is the MTF results from the previous bar chart. The MTF is for the green channel. From wide open, intermediate fine detail (4 to 6 pixels /cycle) shows improvement by stopping down from f/2.8, while very fine frequencies show less improvement because they are already low contrast (and smeared by the blur filter), while lower frequencies (>6 pixels/cycle) show similar response from f/2.8 to f/8.
Peak contrast on fine detail is at f/5.6 but as one closes the aperture from f/5.6, diffraction is decreasing the contrast steadily with each f/stop. Contrast is very low at 2 pixels/cycle (this is the pixel to pixel contrast) and slower than f/8 the effective zero contrast moves to the right, as diffraction harms more and more fine detail.
Lesson for this lens: avoid above about f/5.6 unless you need depth of field when using a 7D. Cameras with larger pixels can, of course, be used proportionally higher in f/ratio before diffraction becomes a factor.
Thanks Roger for the feedback. My comment was more related the diffraction limit of the 7D assuming no AAF. As you have stated on your web site diffraction can really limit the resolution of a camera in particular the "superzooms" whic loose about 1/4 of their MPs at max zoom. Of course this is never made clear in specs.
Here is a new version of my diffraction plot that shows only f/5.6 and slower. It shows the loss due to diffraction better without the confusion of faster f/ratios that include other aberrations. Note that 3 pixels/cycle = 78 cycles/mm and 2 cycles/pixel = 116 cycles/mm.
Here is another example of the effects of phase angle. The image was make early one morning on the Serenegeti. At 0 phase angle is my shadow and to the left of the shadow you can see the shadow of my 500 f/4. Note the grass near zero phase angle appears quite bright. Now move your view to the left. Beyond 10 degrees phase angle we see shadows in the grass giving form and structure. But also note the color is richer. At low phase angles the shadows are hidden, but also an interesting phenomenon occurs called coherent backscatter that creates constructive interference of the light reflected back to the viewer, and that makes the surface appear even brighter. This also acts to reduce color. These properties occur in all surfaces to some level, including rocks and soils, vegetation (as shown here) and in the fur of animals and the feathers of birds. So working at least 10 to 15 degrees from your shadow landing on your subject enhances color and the shading adds form, structure, and importantly, microstructure, thus enhancing fine detail.
Roger
Last edited by Roger Clark; 02-17-2012 at 11:14 PM.
"Phase angle" "diffraction" and other mis-conceptions in bird photography
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Originally Posted by Roger Clark
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