Wildlife Photography: Understanding the DoF Misconception of Telephoto Lenses

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Great article, Ronnie! And amazing photos, too. Thanks for writing and sharing it.

I'll readily admit I have only a cursory grasp of the mechanics of imaging, but isn't it true that you can effectively omit focal length from the DoF discussion? That is, if your subject fills the frame in the same way, with a given aperture the DoF will be same for any focal length (with camera-to-subject distance obviously changing to achieve the same framing with the different focal lengths).

So, really, it's the way you're filling the frame with your subjects (which makes for the best wildlife photos) that leads to the tiny DoF. The sensor size is only relevant in that it determines what focal length you need to use to achieve a given framing.

Someone please correct me if any of these assertions is wrong.

- K

You are correct, but you can't just omit focal length because it is part of the formula for DoF. This article was not directed for those that fill the frame with their subject since when you do fill the frame the DoF will never be enough to capture all of the subject. Which honestly is only a small percentage of people photographing wildlife. This article was directed at people that shoot images like this.

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300/4 Wide-Open at 125 Feet by Phocal Art, on Flickr

The above image was taken with the 300/4 shot at ƒ4 from a distance of 125 feet with a jaw dropping 6'4" DoF. Despite this I will regularly see photos like this where they have stopped down to ƒ8 or more with the reasoning to get all of the bird in focus. Why do they stop down? Because they have the misconception about DoF being razor thin with telephoto lenses, which it is but only when you are filling the frame with your subject.
 
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This claim appears to be based on

  1. A circle of confusion invariant of sensor pixel pitch.
These assumptions aren't supported in the article, none of them seem especially realistic, and I can't really replicate the 2x depth of field claim since more plausible values give 1–2x depending on the tradeoffs made. Probably most influentially, the constant circle of confusion performs comparison between 20 MP μ43 and a 135 body with an 80 MP sensor, something which isn't functionally relevant as no 80 MP 135 bodies exist.
As far as I knew, the CoC is format dependent, not pixel pitch dependent. Then I have checked wiki, and they agree: https://en.wikipedia.org/wiki/Circle_of_confusion

And as example, since one needs to enlarge 2x a m43 image as compared to the same FF image in order to print a given size (say A4) picture, then the CoC for the m43 sensor would be half the size of that of the FF sensor, hence the 2x DoF for m43 as compared to FF
 
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As far as I knew, the CoC is format dependent, not pixel pitch dependent. Then I have checked wiki, and they agree: https://en.wikipedia.org/wiki/Circle_of_confusion

And as example, since one needs to enlarge 2x a m43 image as compared to the same FF image in order to print a given size (say A4) picture, then the CoC for the m43 sensor would be half the size of that of the FF sensor, hence the 2x DoF for m43 as compared to FF
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pdk42

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You are correct but I think you may have missed the reason why I wrote the article or I wasn't clear enough in my writing. I regularly see photos posted in forums like this one where someone has stopped down, sometimes significantly. I understand that most lenses are sharpest stopped down (with µ4/3 it is typically one stop) and it is why I always ask whenever I see a photograph where they stopped down. Maybe 1 out of 10 times (probably more like 1 in 50) they will say because it is the sharpest point of their lens. The rest of the time the answer is "to get more DoF so I can get the entire bird in focus". When I get that answers I always point out that wide-open would have had enough DoF and that by stopping down they have increased ISO which has directly effected the IQ of their photograph. Those are the people the article was directed at, the ones that will stop down because they believe they need to when they do not.
Yes, I can understand your point entirely. I never stop down tele lenses unless it’s to improve sharpness. But as I’ve got older I’ve realised that spending the money on the best lenses (esp long lenses) pays off in the long run, so even that isn’t really important.
 
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Yes, I can understand your point entirely. I never stop down tele lenses unless it’s to improve sharpness. But as I’ve got older I’ve realised that spending the money on the best lenses (esp long lenses) pays off in the long run, so even that isn’t really important.

I will stop-down on occasion to get more DoF, especially when shooting the 150/2 on gators.
 
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Very interesting OP / thread (and I did read the off-site article on Vocal as well).

I never realised that my ORFs include focus distance data. I looked (in exifgui) at my garden bird pics from the past year since I bought a 75-300mm. For the "across the garden" shots (as far away as is even worth bothering with), the dof is circa 30cm (or twelve of your "inches"!)

For the closest shots I can generally manage of small birds, the dof gets as low as about 5cm (or two inches), which is still approx the size of a small garden bird, which I guess demonstrates your main point. Happy days!
 
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Very interesting OP / thread (and I did read the off-site article on Vocal as well).

I never realised that my ORFs include focus distance data. I looked (in exifgui) at my garden bird pics from the past year since I bought a 75-300mm. For the "across the garden" shots (as far away as is even worth bothering with), the dof is circa 30cm (or twelve of your "inches"!)

For the closest shots I can generally manage of small birds, the dof gets as low as about 5cm (or two inches), which is still approx the size of a small garden bird, which I guess demonstrates your main point. Happy days!

Glad you enjoyed the article.
 

PakkyT

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I have been a Mac user for a really long time. I did switch to a windows PC for a few years (switched back beginning of this year) and really loved having access to EXIF Tool. Unfortunately it is a Windows only program.
Assuming you are talking about Phil Harvey's excellent Exif Tool, it has always been available for Mac but you do have to run it from the command line in Terminal if run stand alone. But some other apps use this tool such as GraphicConverter, my main "photo toolbox" app for all things not related to actual editing (although it does that as well) where all exif info is displayed via Mr. Harvey's Exif Tool in a tab.

In GraphicConverter if I click on a photo and then click on the ExifTool tab, the focus distance can be found under the "---- Olympus ----" section of data (which is the eighth section down on the photo I am looking at right now). The command line running of Exif Tool gives a bit cleaner output though. Assuming Exif Tool is installed, open terminal app, type exiftool followed by a space then drag and drop the photo onto the terminal window to fill in the file path and hit return. You get a nice text based output which can easily be cut and paste elsewhere. Easy peasy.
 
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PakkyT

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I can't really replicate the 2x depth of field claim since more plausible values give 1–2x depending on the tradeoffs made.

His statement was pretty basic in that if you are lying there in the snow and you are 300 feet away from your subject, on the Olympus you mount your 300mm lens and the FF to get the same field of view you mount a 600mm lens. Shooting both at f4, with the Olympus your DoF is 36.59 feet and with the full frame it is 18.18 feet. All the other stuff you went on about seems like odd nitpicking especially the CoC since most calculators factor that in (0.015 for Oly and 0.03 for FF seeming to be the standard numbers they all use).

Since that first part you seemed to get confused about, I assumed the rest of your long reply was suspect as well, so TL: DR. Also it seemed like a classic case of missing the forest for the trees. I didn't need to double check all the numbers to get the main point of what the article was saying.

(online DoF calculator I used)
 
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speedy

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I'm not one for formulas & such in photography, so here's a quick and dirty sample. If you frame the subject the same size, DOF remains the same for aperture despite the focal length. Yes, the background will be a bit more blurry with the longer focal length, but that's not DOF. Example, with a 20X difference in focal length. Focus point was on "Dreamer"
10 test.jpg
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200 test.jpg
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archaeopteryx

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Since that first part you seemed to get confused about
The blog post repeatedly and explicitly relates 300 f/4 to 600 f/8. Not f/4 to f/4 as you seem to be thinking (perhaps TL;DR again?). Have a look at the DoF equation and it should be clear maintaining f/4 to f/8 whilst changing the CoC and making more typical CoC choices whilst using f/4 instead of f/8 have similar effects.
 
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