Poll: in focus or out of focus

[arash_hazeghi]

Have a look at this thread I started:

http://www.birdphotographers.net/for...924#post445924

Looks like the expansions don't show unless you have the C.Fn set to "expand to all 45"

The 7D has only one expansion mode and that is 4-point expansion, in this case only center of the expansion is highlighted regardless of which point was used for acquisition. However the pattern of the expansion is fixed so you can see whether any of the expansion points had a chance of grabbing the subject or not.

I have no idea why Canon don't record the actual point that was used for acquisition in expansion mode.

[Jay Gould]

Roger:

the active focus is not necessarily where the true focus is actually at, nor where it is projected to be at the time the shutter actually fires

This is what I was poorly trying to say.

Arash, you posted an image of the active sensor - you did not post an image of the focal point at the time the shutter actuallly fires.

If you are suggesting that the active sensor was where indicated on the body behind the head, are you also suggesting that the active point of focus when the image was taken was in the grass where the arrow is pointed?

Certainly, using programs like Breeze Browser you can show us where the point of focus was when you shot this image. It sounds like it is possible that while your active sensor was on the bird the point of focus was miles away in the grass!

I will defer to the scientists and the nonscientist experts like Jim and Chas: assuming that the active sensor was as depicted in the image presented at the moment of shutter release, would you expect to see the actual point of focus 4 - 5 feet from the active sensor?

Regarding bumping focus - the system Jim taught me - I use it with the 7D and do not seem to have a problem with the camera locking on the background IF the active sensor at the time of the bump is on the bird.

[Roger Clark]

Another thing to note In Arash's images, is he has a dark bird on a bright background. A bright background makes it more difficult for the AF system, because the bright background will have a high signal in the AF system. AF would work better in better light on the bird with a darker background.

Roger

[arash_hazeghi]

Jay, I have no idea what you are trying to say, if you are still confused unfortunately I don't know what to say anymore, you can read the links I provided or google "Auto Focus" to find more material.

Good luck with your photography

Roger:

This is what I was poorly trying to say.

Arash, you posted an image of the active sensor - you did not post an image of the focal point at the time the shutter actuallly fires.

If you are suggesting that the active sensor was where indicated on the body behind the head, are you also suggesting that the active point of focus when the image was taken was in the grass where the arrow is pointed?

Certainly, using programs like Breeze Browser you can show us where the point of focus was when you shot this image. It sounds like it is possible that while your active sensor was on the bird the point of focus was miles away in the grass!

I will defer to the scientists and the nonscientist experts like Jim and Chas: assuming that the active sensor was as depicted in the image presented at the moment of shutter release, would you expect to see the actual point of focus 4 - 5 feet from the active sensor?

Regarding bumping focus - the system Jim taught me - I use it with the 7D and do not seem to have a problem with the camera locking on the background IF the active sensor at the time of the bump is on the bird.

[arash_hazeghi]

If you keep the AF sensor covered by the bird BG is not sampled so its relative contrast will not affect the AF tracking, if you can't hold the lens tight and wobble around as you track (have seen many people do this) it might pick the BG. Sometimes it is difficult to keep the AF point on a fast moving bird, in these cases I prefocus at approximate distance and wait for the bird to enter the frame and cover the sensor, then fire a few shots.

[Roger Clark]

If you keep the AF sensor covered by the bird BG is not sampled so its relative contrast will not affect the AF tracking, if you can't hold the lens tight and wobble around as you track (have seen many people do this) it might pick the BG. Sometimes it is difficult to keep the AF point on a fast moving bird, in these cases I prefocus at approximate distance and wait for the bird to enter the frame and cover the sensor, then fire a few shots.

Arash,
This is not my experience. For example, in the attached image I investigated the autofocus performance because I was have trouble getting the correct focus. I was following these birds for a while and when i first starting photographing them I had trouble with focus. I tested numerous positions. A focus point on the bird's head focused on the background. An AF point on the bird's body focused on the background. In fact I could not focus on the bird period with AF, no matter where I placed the AF point (single point enabled on a 5DII)! The focus point is actually at the feet of the bird and the focus is on the water (and in this image slightly in front of the bird). That included placing the AF point on the bird's body with a lot of bird around the AF point.

When you think about it, the AF detector in this situation has a weak signal from the bird and huge signal from the background. The phase signals are dominated by the background. The situation in your images is similar, though not as extreme.

Roger

[Jim Neiger]

Roger:

This is what I was poorly trying to say.

Arash, you posted an image of the active sensor - you did not post an image of the focal point at the time the shutter actuallly fires.

If you are suggesting that the active sensor was where indicated on the body behind the head, are you also suggesting that the active point of focus when the image was taken was in the grass where the arrow is pointed?

Certainly, using programs like Breeze Browser you can show us where the point of focus was when you shot this image. It sounds like it is possible that while your active sensor was on the bird the point of focus was miles away in the grass!

Jay,

Read my previous post about delay from tracking sensitivity. That is one explaination why the active focal point location and plane of focus may be very different. There are other explanations as well. For example the camera may have been in the process of focusing (on it's way to focus acquisition.) when the shutter was released. If you are focused at 10 yards, and thnen point the camera and focus on something at 100 yards, there is a period of time when the focus is traveling from 10 yards to 100 yards. If you trip the shutter during this time your plane of focus will be somewhere between 10 yards and 100 yards even though the active AF point is positioned on the subject at 100 yards. I hope this helps.

[Jay Gould]

Jay,

Read my previous post about delay from tracking sensitivity. That is one explaination why the active focal point location and plane of focus may be very different. There are other explanations as well. For example the camera may have been in the process of focusing (on it's way to focus acquisition.) when the shutter was released. If you are focused at 10 yards, and thnen point the camera and focus on something at 100 yards, there is a period of time when the focus is traveling from 10 yards to 100 yards. If you trip the shutter during this time your plane of focus will be somewhere between 10 yards and 100 yards even though the active AF point is positioned on the subject at 100 yards. I hope this helps.

Another cause for the focal plane differing from the position of the active focus point is the tracking sensitivity delay. For example: If the focus was on the grass in fron of the bird and then you position the focus point on the bird and immediately shoot, the delay may not have expired and the focus may still be on the grass. The delay may be overridden by bumping the focus. So when the camera was focused on the grass bumping the focus back to the bird would have allowed the shot right away with the focus on the bird as intended.

Jim, I do understand and totally agree. And, I havetracking delay set to slow because I want to hold the bird when it locks even if I slightly move the camera between lock and firing.

My question is different and I thought simple: Arash's image shows an active sensor -the red box - on the bird at the moment the image was recorded. While that was the active sensor, if he moved the camera at the moment he intended to fire and the sensitive delay has not yet expired, you are saying that the focus on the bird will still be active and that would be the focal point at the moment the image was recorded.

In this case the sharpest point of focus is on the grass 4-5' from the bird; 4-5' from where the active sensor point (the red box in the posted image) was located at the moment the image was recorded.

What I and a few others are trying to understand is Why?

DOF works from the point of focus outwards. Shouldn't the sharpest point be where the red box is located even if the camera was slightly moved prior to the sensitivity period expiring?

If the sharpest point in an image is not at the location of the captured "active sensor", doesn't that indicate something is not properly calibrated?

Why is the sharpest point 4-5' away from the active sensor? The only answer I can surmise is that the camera/lens combination at that moment in time was not adjusted properly and either front or back focused, or something like that.

For most us, nonscientific explanations would go a long way to furthering our education. Jim, you seem to understand that very well.

Thanks Mate for staying with this.

[Jim Neiger]

Jay,

The location of the red box indicating the active AF sensor is simply superimposed on top of the image regardless of where the focal plane actualy was. Usually these locations match, but not always. I have given two scenarios that may explain why this may happen. I think you are assuming that the red box indictaes where the camera was focused. It actualy indicates where the camera will try to focus next at the time the shutter was released. If the camera had acquired focus befoe the shutter was released then the red box indicates a point on the plane of criotical focus. If the camera is waiting to focus, or on it's way to focus, the focal plane and red box may be in different locations. I hope this ehlps you understand.

Best,
Jim

[Jay Gould]

Jay,

The location of the red box indicating the active AF sensor is simply superimposed on top of the image regardless of where the focal plane actualy was. Usually these locations match, but not always. I have given two scenarios that may explain why this may happen. I think you are assuming that the red box indictaes where the camera was focused. It actualy indicates where the camera will try to focus next at the time the shutter was released. If the camera had acquired focus befoe the shutter was released then the red box indicates a point on the plane of criotical focus. If the camera is waiting to focus, or on it's way to focus, the focal plane and red box may be in different locations. I hope this ehlps you understand.

Best,
Jim

Bro, we are getting closer! ;)

The camera can be set to show the focus point at the time of capture in the LCD. Are you saying that the little red box that we see in the LCD when reviewing our images in camera is not necessarily the actual point of focus at the moment of capture?

The 7D Manual at p. 163:

About the AF Point Display: When the AF point display menu option is set to Enable, the AF point which achieved focus will be displayed in red. If automatic AF point selection was used, multiple AF points might be displayed in red.

What you are saying seems to contradict what is written.

Yes, I am assuming based upon the Manual that "the red box indicates where the camera was focused." What other interpretation can I give to the words "achieved focus"?

:confused:

[Jim Neiger]

Bro, we are getting closer! ;)

The camera can be set to show the focus point at the time of capture in the LCD. Are you saying that the little red box that we see in the LCD when reviewing our images in camera is not necessarily the actual point of focus at the moment of capture?

The 7D Manual at p. 163:



What you are saying seems to contradict what is written.

Yes, I am assuming based upon the Manual that "the red box indicates where the camera was focused." What other interpretation can I give to the words "achieved focus"?

:confused:

Jay,

I'm saying that the little red box that your software displays (like in Arash's post) simply shows which focus point was active when the shutter was released. This is not always a point on the plane of focus.

[arash_hazeghi]

Arash,
This is not my experience. For example, in the attached image I investigated the autofocus performance because I was have trouble getting the correct focus. I was following these birds for a while and when i first starting photographing them I had trouble with focus. I tested numerous positions. A focus point on the bird's head focused on the background. An AF point on the bird's body focused on the background. In fact I could not focus on the bird period with AF, no matter where I placed the AF point (single point enabled on a 5DII)! The focus point is actually at the feet of the bird and the focus is on the water (and in this image slightly in front of the bird). That included placing the AF point on the bird's body with a lot of bird around the AF point.

When you think about it, the AF detector in this situation has a weak signal from the bird and huge signal from the background. The phase signals are dominated by the background. The situation in your images is similar, though not as extreme.

Roger

I have seen the phenomenon you describe, previously in 5D/5DMKII/XXD cameras the AF frame was smaller than the actual sensor and worse than that sensor was slightly misaligned in some cases, so it's not clear what exactly it sees. This issue has been solved in 7D, the sensor is exactly as large as the red box indicates and it does not sample the area outside of it, so there is no signal from BG.

Plus I do not think that the image you posted resembles my situation, this is strong back-lit with reflective BG, stationary bird, different light, different BG , my photo was not back-lit and there is still plenty of detail and contrast on the bird as seen in the photo. Camera has absolutely no problem focusing on the bird in the same situation using single shot AF as opposed to AI-servo. Done it a thousand times.

[Roger Clark]

I have seen the phenomenon you describe, previously in 5D/5DMKII/XXD cameras the AF frame was smaller than the actual sensor and worse than that sensor was slightly misaligned in some cases, so it's not clear what exactly it sees. This issue has been solved in 7D, the sensor is exactly as large as the red box indicates and it does not sample the area outside of it, so there is no signal from BG.

Plus I do not think that the image you posted resembles my situation, this is strong back-lit with reflective BG, stationary bird, different light, different BG , my photo was not back-lit and there is still plenty of detail and contrast on the bird as seen in the photo. Camera has absolutely no problem focusing on the bird in the same situation using single shot AF as opposed to AI-servo. Done it a thousand times.

Arash,
It may not be exactly your situation, but it is still an effect. A good test of the AF sensitive area is select one AF point and try focusing on the moon in a dark sky. Every camera I have tried is able to focus on the moon with the AF sensor well off the moon. It would be interesting to see that test with the 7D. Note that an out of focus subject will cover a larger area than the AF sensor so contributes to signals that the AF sensor sees. So this is true in your bird images too, and the fact that the background is bright means that some of the background (or foreground) will contribute to the signal on the AF sensor.

Roger

[arash_hazeghi]

Arash,
It may not be exactly your situation, but it is still an effect. A good test of the AF sensitive area is select one AF point and try focusing on the moon in a dark sky. Every camera I have tried is able to focus on the moon with the AF sensor well off the moon. It would be interesting to see that test with the 7D. Note that an out of focus subject will cover a larger area than the AF sensor so contributes to signals that the AF sensor sees. So this is true in your bird images too, and the fact that the background is bright means that some of the background (or foreground) will contribute to the signal on the AF sensor.

Roger

Thanks Roger but I am not interested in focusing on the moon, it doesn't resemble my field subjects at all and I don't find the test relevant.



Thanks all for contributing.

[Roger Clark]

Thanks Roger but I am not interested in focusing on the moon, it doesn't resemble my field subjects at all and I don't find the test relevant.
Thanks all for contributing.

That's fine but it is a clean test if you want to understand your system.
Every point in the field of view that enters the camera of a typical scene has many distances. Those that are not at the focus (wherever the lens may be set) are by definition not in focus (duh) and that out of focus area is larger because the spread of the light cone. That means that out of focus light (e.g. the grass in the background) will spill over into the in focus area (e.g. your bird) and cause an additional signal on the AF sensor. That out of focus light will be a phase error on the AF sensor. If the background is brighter than the subject, it can overwhelm the signals from the subject. That leads to errors in the phase detection AF system. Understanding those issues will help you to understand when your AF system will be accurate and when it will not, thus improving your in focus to mis-focused BIF.

Roger

[arash_hazeghi]

That's fine but it is a clean test if you want to understand your system.
Every point in the field of view that enters the camera of a typical scene has many distances. Those that are not at the focus (wherever the lens may be set) are by definition not in focus (duh) and that out of focus area is larger because the spread of the light cone. That means that out of focus light (e.g. the grass in the background) will spill over into the in focus area (e.g. your bird) and cause an additional signal on the AF sensor. That out of focus light will be a phase error on the AF sensor. If the background is brighter than the subject, it can overwhelm the signals from the subject. That leads to errors in the phase detection AF system. Understanding those issues will help you to understand when your AF system will be accurate and when it will not, thus improving your in focus to mis-focused BIF.

Roger

You want me to focus on a bright object at infinity against a black BG so at extreme defocus condition the halo from the moon would bleed and blind the AF line sensor. How on earth this would resemble focusing on a bird in flight at close distance? Sorry I am not going to do that it is illogical and irrelevant IMO.

Also I respectfully don't need your advice in helping me "understand" the AF system or improving my BIF shots. Fortunately I just happen to have enough technical knowledge on these issues, more than what I need or care about in the field, so no "technical" lessons for me please. As for photography I am here to learn from photographers like Al, Fabs, Doug, Artie, James, Chris, Alan and other experts who actually take high-quality flight shots in the field.


Thank you,

Arash

[Roger Clark]

You want me to focus on a bright object at infinity against a black BG so at extreme defocus condition the halo from the moon would bleed and blind the AF line sensor. How on earth this would resemble focusing on a bird in flight at close distance? Sorry I am not going to do that it is illogical and irrelevant IMO.

Also I respectfully don't need your advice in helping me "understand" the AF system or improving my BIF shots. Fortunately I just happen to have enough technical knowledge on these issues, more than what I need or care about in the field, so no "technical" lessons for me please. As for photography I am here to learn from photographers like Al, Fabs, Doug, Artie, James, Chris, Alan and other experts who actually take high-quality flight shots in the field.


Thank you,

Arash

Arash,
Your response is totally inappropriate and insulting. You have posted questions to a public forum and everyone has equal opportunity to reply to any question and everyone should be treated with equal respect.

You posted images in which the plane of focus significantly off your subject even though the active focus point is on the subject. You also said your 7D focus point was in perfect alignment and there was no signal from the background:

This issue has been solved in 7D, the sensor is exactly as large as the red box indicates and it does not sample the area outside of it, so there is no signal from BG.

I was showing you that regardless how perfect a focus point is, out of focus detail near the AF point will contribute to the signal in the AF point. That will always be true regardless how perfect the AF rectangle is. Regarding the moon, the shape of the light cone from the lens is hardly changed from infinity to the bird images you posted. It is the light cone of the wide open lens that determines the spread of the light. If you don't like the moon as a test, set up a white paper (or image of a white bird) on a black background at whatever distance you want to demonstrate the effect. It is simple optics, and the effect can be calculated too using simple geometric optics.

It is the effects of out-of-focus bright backgrounds that I bet contribute a lot AF errors than simple tracking and extrapolation errors in many images. And the images you posted have all the earmarks of that problem.

Roger

[Jay Gould]

Jay,

I'm saying that the little red box that your software displays (like in Arash's post) simply shows which focus point was active when the shutter was released. This is not always a point on the plane of focus.

Right! That pertains to the image posted by Arash.

However, and I clearly do not have the technical expertise to answer this: the manual suggests that the little red box that we see in the LCD AFTER the image was created is the point on the plane of focus where "the camera was focused."

That is what I am trying to understand; how do we determine objectively - if we can - where the camera was focused at the moment the image was created?

Roger's description:

You posted images in which the plane of focus significantly off your subject even though the active focus point is on the subject.

Roger, is the location of the little red box shown in the LCD after the image is created the active focus point or "where the camera was focused" at the moment of capture? What does "where the camera was focused" mean?

In Arash's image is the red box showing the active sensor, or is it showing "where the camera was focused" at the moment of capture? If the latter, than Arash has a calibration problem, in my lay opinon. The camera was focused on the body and the sharp point is 5' away!

To state differently, if the former, the active focus point, useless information because all you have to do is bump the shutter button and you know what sensor point you were using; if the latter, the location of the point of focus at the moment of capture - that is valuable information as it tells you how well you are handholding the rig, you can evaluate DOF outward from the red box shown in the LCD, and you can determine if you camera/lens needs calibration.

:confused:

[Roger Clark]

Right! That pertains to the image posted by Arash.

However, and I clearly do not have the technical expertise to answer this: the manual suggests that the little red box that we see in the LCD AFTER the image was created is the point on the plane of focus where "the camera was focused."

That is what I am trying to understand; how do we determine objectively - if we can - where the camera was focused at the moment the image was created?

Roger's description:



Roger, is the location of the little red box shown in the LCD after the image is created the active focus point or "where the camera was focused" at the moment of capture? What does "where the camera was focused" mean?

In Arash's image is the red box showing the active sensor, or is it showing "where the camera was focused" at the moment of capture? If the latter, than Arash has a calibration problem, in my lay opinon. The camera was focused on the body and the sharp point is 5' away!

To state differently, if the former, the active focus point, useless information because all you have to do is bump the shutter button and you know what sensor point you were using; if the latter, the location of the point of focus at the moment of capture - that is valuable information as it tells you how well you are handholding the rig, you can evaluate DOF outward from the red box shown in the LCD, and you can determine if you camera/lens needs calibration.

:confused:

Jay,
The answer is complex. And a camera+lens system can be in perfect calibration and still mis focus because the of the conflicting information the AF sensor receives.

There are two cases to consider:
The manual you are referring to is probably indicating the focus lock position in single shot mode, so it the red rectangle is indicating where the camera thought it focused. But that isn't necessarily where it actually focused!

In the case of servo, the red rectangle indicates where the servo system was trying to focus, but the actual focus point can be different. The reasons are many.

In single shot mode (there is a canon white paper n this somewhere), the phase is measure and the distance the lens must move is calculated and the lens commanded to move to that point. The shutter won't fire until the lens is at that point. (Not that it is the correct point--more below)

In servo mode, the phase signal is continuously monitored and the lens commanded to the focus point but you can fire the shutter any time. Thus the focus may be way off and on its way, as Jim discussed. This would be called focus lag.

But the problem is not simply lag or AF on the wrong location, but the multiple signals the AF system receives. An example you may have encountered is photographing a bird in a tree with a branch in front. The lens may jump from one focus position to another as it tries to lock on to the bird, a branch in front, or a branch behind. While you may have the focus point perfectly on the bird, the out of focus branches are so out of focus that the light also falls on th AF sensor (even if the bird is in perfect focus). If the bird is dark, for example, and the branch bright, the signal from the branch is larger so the AF sensor locks onto it. While algorithms may try and analyze multiple AF signals, there is always a gray area where the decision is tough. It is obvious to the photographer, but the AF system is not analyzing an image, just a line profile of intensities. I have found that the AF system on the Canon cameras I have used (over a dozen SLRs and DSLRs) that brightness usually wins. So if there is something brighter next to what I'm trying to focus on, the AF system will lock onto that, even though it is outside the AF sensor area. So Arash's image with the focus well in front of the bird, may have locked onto the brighter grass.

In servo mode, this situation can lead to instabilities. Say the AF locked onto the grass (Arash's image), then the lens is commanded to move to the focal point of the grass. As the grass comes into focus, the light from the grass no longer falls on the sensor, which is on the bird. So the AF no longer sees the grass and now sees the bird is out of focus, so starts to move the focus to the bird. But as the bird comes into focus, the stronger signal from the out-of-focus grass is back on the AF sensor, so the AF then sees it and again locks on it. However, in the most recent cameras, software must be analyzing the 1-D plot of phase signals to try and maintain focus on one subject given varying and multiple signals. But again, there is always a gray area where it will be difficult.

Does this help?

Roger

[Jay Gould]

Mate, it does help!

Now it would be nice to here what Chuck Westfall has to say on the issue and the statement in the manual. :D

Thanks,

[arash_hazeghi]

Mr. Clark,

Your comments to me in the past have always been inapproproiate and insulting and I have been offended by it multiple times.

I aks that other moderators step in and solve this issue.


You think you can establish your ideas by repeating them over and over and going in circles, just like in the thread about relative sharpness of 100-400 lens
http://www.birdphotographers.net/for...ad.php?t=55985
you posted two hazy and soft photos of the moon and tried to argue one lens is sharper than the other, at least half a dozen people told you that it was not a good test yet you insisted and insisted and argued with distorted logic and pointless persistence to the point that Fabs and Al were offended and had to change their tone. You know that I am not going to buy your argument here, why insist so much? . What do you want to achieve? why not let go and

I am sorry if I had to drag this, but this is not the first time you are doing this to me and I am really upset, before on multiple occasions we had discussions about noise and image sensors, each of those discussion were extremely pointless and unpleasant for me and I respectfully requested that you kindly refrain from offering advice or comments on my threads, I have kept my word and not posted a single letter in any of your threads since then.

I really ask that Mr. Shaddle to stepo in and solve this conflict.

Arash

[Alfred Forns]

Has been an informative thread but run its course.

Can see both points and understand but best to close down the thread, don't think further comment will be helpful.

From the commenting point of view is best to stick to facts and opinions reporting them as we see it and not getting into personal issues. We all have our own ideas and they should be taken for what they are.