Long lens and DOF

[Raul Quinones]

I saw Chris Van Rooyen post, "Cattle Egrets" and I have a rookie question;
how can both birds be sharp at 500mm and f/4?

Looking at one online DOF calculator and assuming the birds were 20 meters away (65 feet) the total DOF is only .24 meters (9.4 inches)... I normally stop down my lens (giving away speed) in an attempt increase the total DOF, now it seems like a big mistake.

I may have been misjudging the distance to my subjects; the farther away the subject is the total DOF increases.

I guess I need to go to a football field and try to improve how to judge the subjects distance, also I should run some test with my lens/camera to make sure that I have an accurate auto focus.

Please let me know if any of the above makes any sense,
Raul

[Maxis Gamez]

Hi Raul,

For the most part I use my 500mm at f/5.6 and my DOF is fine for my taste. Look at this Canada Goose photographed at f/5.6. This is a huge bird. My main concern is to have the head and eye in focus. Some will disagree but that's OK.

[Alfred Forns]

Raul most of the time I don't have the luxury of stopping down and most images are wide open even with converters. They are plenty sharp but you do have to be careful in going for the eye.

For flight it changes a bit since it is not possible to hold a single AF point on the eye, here dof comes into play more. You need cameras with clean ISO to crank it up if needed !!!

[Mike Milicia]

Agree with all the good advice above.

But also keep in mind that it is difficult to judge subject distance in a posted image
without also knowing how much the image has been cropped.

For example, let's say that you assume that an image is full frame, and, based on that, you estimate
(correctly) that given the focal length, sensor size, and subject size in the frame, that the subject was
n feet away, and therefore the depth of field was x inches.

If, in reality, the image is a 50% crop (i.e. contains 50% of the full frame pixels), then the
subject was actually (1.4 * n) feet away which means the depth of field was actually (2 * x) inches.

If it were a 25% crop, the subject distance was actually (2 * n) feet and the depth of field was
actually (4 * x) inches.

Numbers above are approximate (but very close) to keep things a bit simpler.

[Alfred Forns]

Good point Mike That is one of the ways we recognize big crops !!!! :)

[Charles Glatzer]

Raul,

How can both birds be sharp at 500mm and f/4?

It is difficult to ascertain the degree of critical sharpness from images posted on the web, but ..to my eye both birds do not appear sharp, nor is the image overall AS POSTED.

At f/4 it will not happen unless the subjects are in the same plane as the capture medium, to each other, and a good distance away from the lens. Not the norm for sure....and more than likely even a single bird like an egret will not be sharp throughout with a 500mm @ f/4 if it approaches the lens at 45 degrees.

Chas

[Raul Quinones]

Thanks to all,
I am still green... Next time I will make a conscious effort to shoot wide open or close to wide open (4 and 5.6) instead of the (7.1 to 8) and see what I get.

Raul

[Charles Glatzer]

Thanks to all,
I am still green... Next time I will make a conscious effort to shoot wide open or close to wide open (4 and 5.6) instead of the (7.1 to 8) and see what I get.

Raul

F/8 will provide a much sharper resolution image than f/4 providing you can maintain the shutter-speed necessary to freeze the subject.
The whole thing is a compromise...DOF vs shutter-speed vs ISO. Choose the aperture and/or shutter speed that is most important to capture the image you envision at the lowest ISO necessary to render the image as desired.

Chas

[John Chardine]

The Canon super-teles are designed to be used wide or close to wide open. There is a very slight drop-off, almost unnoticeable, in IQ full open compared to 1/3 stopped down but from there down to f8 there is very little if any improvement. Essentially across the board from wide-open to f8, IQ is excellent. At f8 with bodies having high megapixels and small sensor sites (e.g., 50D), diffraction effects start to come into play which will reduce IQ.

[Charles Glatzer]

The Canon super-teles are designed to be used wide or close to wide open. There is a very slight drop-off, almost unnoticeable, in IQ full open compared to 1/3 stopped down but from there down to f8 there is very little if any improvement. Essentially across the board from wide-open to f8, IQ is excellent. At f8 with bodies having high megapixels and small sensor sites (e.g., 50D), diffraction effects start to come into play which will reduce IQ.

John,

With all due respect...I use this stuff in the field 8 plus months a year every year. I urge you and others to test this for yourself.

f/8 will blow away shooting wide open on my 500/600 And, I am shooting full frame 1Ds Mark III bodies.

Best,

Chas

[John Chardine]

Chas- Sorry I didn't want to come across as contradictory, just laying down what I believe to be true from my own experience and reading Roger Clark's and other's stuff.

I use my 500 pretty well every month of the year and in the summer pretty much daily. I always test my gear and tested the 500 as soon as I got it. Here's a montage of the results. Test conditions as follows:

Canon 50D, Canon EF 500 mm f4L IS USM, mounted on Mongoose 3.5a head on Manfrotto 055 tripod, cable release, mirror lockup, 10s self timer, ISO 400, 1/250s, flash as main light, RAW, no sharpening, no noise reduction. Images shown at 100%. Subject was Canadian $20 bill taped flat to glass. Focus point is centre of images shown and was smallest writing just above big "20". BTW this writing is not readable with the unaided eye and cannot be photocopied as an anti-counterfeiting measure. Distance to subject was min. focus ca. 14'. BTW the biggest writing at the top of the images- "E DU PARLEMENT • THE CEN" - is 20 thousandths of an inch or about 1/2 mm high!

There's not much to tell between the images but f4 is a tiny bit softer than f4.5, f4.5-8 look very similar to me and f11 and 16 are softer again. I personally doubt if I could see the difference in these settings in a real world situation. Caveat of course is that this is my kit; your mileage may differ.

[Charles Glatzer]

John,

No, worries. We are all trying to get the best out of our gear. They posted images appear as you state above. Were the images AF or manually focused? AF has a tolerence and it is difficult to ascertain crirical focus. Best to manually focus w/ Live preview at 10x. I always try to shoot between 5.6-8, unless the DOF out weighs the diffraction. I have found a big enough difference to not shoot wide open or close to it if I can help it....publishable, yes, the best the lens can deliver, no.

Chas

[Roger Clark]

Hello all,

John's experience mirrors mine. My 500 f/4 is slightly sharper at f/4.5, then is about the same through f/8, then slowly decreases toward f/16 and slower. The main effect of diffraction that you will notice as you close down the aperture is loss of contrast in the fine details. That is seen in John's f/16 image for example. Not only is sharpness down a little, but contrast is lower. You can recover this loss in contrast to some degree with a little unsharp mask, but at the risk of more noise.

For a Canon 50D with 4.7 micron pixel spacing, at f/8, the modulation transfer function (that is the contrast) is 0% for pixel to pixel detail. See Figure 8 at:
http://www.clarkvision.com/imagedeta...ml#Diffraction
The blur filter mitigates this effect to some degree. At f/16, detail spread over 4 pixels gets recorded with 0% contrast, and that is showing in John's f/16 image. Cameras with larger pixels, like the 1D III will not show similar effects until a slower f/ratio.

So in general, my rule is shoot from wide open to about f/8 and then if I really need more depth of field, I go for whatever aperture is required. After all, you can be out of focus from depth of field or slghtly blurry from diffraction at those lower f/stops.

[Charles Glatzer]

Roger,

You know your tech stuff for sure... but I find in the field there is more to a visually sharp image than what the numbers above show relative to a two dimensional plane in controlled conditions. Perhaps, it is a combination of contrast, flare, atmospheric conditions, etc. In addition to res..shooting at f/4.5 severely limits the DOF for any subject(s) not parallel to the capture plane. I will avoid shooting near wide open if at all possible!!! As mentioned ...I always try to shoot between 5.6-8, unless the DOF out weighs the diffraction.

I wish you olny the best in your image making,

Chas

[Cliff Beittel]

. . . In addition to res..shooting at f/4.5 severely limits the DOF for any subject(s) not parallel to the capture plane. . . .

Nothing ruins a bird photo more quickly than OOF feet, which you often get wide open, at least with a full frame camera. Also, shooting wide open gives much more light falloff in the corners--so bad at times that a bird centered in a blue sky will appear spotlighted.

[John Chardine]

Cliff- I can think of several things that kill a bird image before OOF feet, but anyway.... Also remember that one reason fast super-teles are so nice to use is that wide-open the backgrounds are smooth and creamy. Also the speed of these lenses allows use of fast shutter speeds at reasonable ISOs. One reason my 500/4 cost the price of a used car is the speed of the lens and I'm darned if I'm going to throw this away by shooting at f8 all the time. BTW I have never noticed vignetting with the Canon 500/4 wide open on the 5D.

[Charles Glatzer]

John, the reason I opt for fast glass is the brighter viewfinder image and typically faster AF acquisition speed. My f/2.8 teles acquire focus even faster than my f/4 long glass does, but I rarely if ever shoot them at f/2.8. Shoot a blue sky wide open and check the corners for vignette.

Different strokes for different folks, is all. ;)

Best in your image making and good light,

Chas

[Roger Clark]

Roger,
You know your tech stuff for sure... but I find in the field there is more to a visually sharp image than what the numbers above show relative to a two dimensional plane in controlled conditions. Perhaps, it is a combination of contrast, flare, atmospheric conditions, etc. In addition to res..shooting at f/4.5 severely limits the DOF for any subject(s) not parallel to the capture plane. I will avoid shooting near wide open if at all possible!!! As mentioned ...I always try to shoot between 5.6-8, unless the DOF out weighs the diffraction.

Chas,
I agree that sometimes you need the depth of field so I use whatever f/ratio is needed. But sometimes I want a blurred background and a marrow depth of field, so I often image wide open too. And sometimes exposure time limits sharpness. Photography is full of compromises and one should not be afraid to push any of the limits when needed, nor feel limited by something scary like diffraction.

[Charles Glatzer]

Chas,
I agree that sometimes you need the depth of field so I use whatever f/ratio is needed. But sometimes I want a blurred background and a marrow depth of field, so I often image wide open too. And sometimes exposure time limits sharpness. Photography is full of compromises and one should not be afraid to push any of the limits when needed, nor feel limited by something scary like diffraction.

Roger,

I have said before "Photography, is at times the art of compromise" :) Could not agree more!

Best,

Chas