Depth of Field

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Many videos talking about birds in flight photography talk about the importance of focusing on the eyes, because if you focus on a wing the head might be out of focus. They talk about an extremely shallow depth of field, apparently being a foot or less. Online depth of field calculators show a much greater depth of field. For example, Depth of Field Calc, when set for my Nikon D7100 and a distance of 300ft, f/stop 4.5, focal length of 600mm gives a depth of field of 13.43 ft. Can someone help me to understand.
 
300 ft is a long way and increases the DoF. Also, the actual depth of sharp focus is razor thin, but there is a decreasing sharpness as we move forward and backward from that page of focus. Thus we end up with the concept of "acceptable focus". The idea of acceptable focus has changed over the years as sensors have become higher and higher resolution. Some calculators accommodate for this by having an adjustable setting for circle of confusion.
 
In essence the DOF is a function of the size of the object in the sensor. So for a bird that occupies 50% of the frame (say vertically) the DOF would be same using a 50mm, 100mm or 600mm lens.

DOF calculations are very complicated as Ltkg20 mention the dirty little secret about DOF is the circle of confusion (which basically mean how large are you going to enlarge the image and how far are you going be away from it, when you view it). Billboard look fairly sharp until you see them up close.

I use photopills as my primary tool, highly recommended
 
300 feet is way too far for a decent photo, unless you're going for a documentary type shot. With that much distance you're pretty much guaranteed to get a sharp photo due to the DOF being so large, but then the background becomes more cluttered and in focus, and you'll introduce lots of atmospheric distortion with so much air between you and the subject. With a 500mm F/5.6 lens on a crop sensor body I try to photograph raptors from under 100 feet away, and even then I don't really have to worry about DOF too much because it's around 3 feet at that distance. DOF only becomes an issue for me when they're too close.
 
Honestly, I don't worry about it for BIF. My aperture is always wide open on my telephotos for BIF and even birds at rest. More than enough DOF at typical distances of 20 to 30 yards.. The thing you have to be a bit careful about is where your camera locks focus at short distances. Using Grp, D9 or D25, the camera tends to lock on the thing closest to the camera, so if the bird is close, you can see a sharp wing tip and soft eye, but take it into Topaz Sharpen AI and viola, a sharp eye.
 
In essence the DOF is a function of the size of the object in the sensor. So for a bird that occupies 50% of the frame (say vertically) the DOF would be same using a 50mm, 100mm or 600mm lens.

DOF calculations are very complicated as Ltkg20 mention the dirty little secret about DOF is the circle of confusion (which basically mean how large are you going to enlarge the image and how far are you going be away from it, when you view it). Billboard look fairly sharp until you see them up close.

I use photopills as my primary tool, highly recommended
Photopills is an excellent general purpose photo app, but it has limitations for calculating DOF. The circle of confusion (COF) it uses is determined by selecting the camera. For the Nikon D850 znd Z9 the COF is 30 microns. This is a pre-WW2 standard promulgated by Zeiss and is too large for current high resolution sensors. I prefer TrueDoF-Pro by George Douvos that allows you to specify the COF. For details refer to this article.

Bill
 
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Photopills is an excellent general purpose photo app, but it has limitations for calculating DOF. The circle of confusion (COF) it uses is determined by selecting the camera. For the Nikon D850 znd Z9 the COF is 30 microns. This is a pre-WW2 standard promulgated by Zeiss and is too large for current high resolution sensors. I prefer TrueDoF-Pro by George Douvos that allows you to specify the COF. For details refer to this article.

Bill

The advanced dof calculator at photopills offers many more choices for circle of confusion or you can basically dial your own by adjustng the viewing distance and print size.

 
I can observe sharpness difference between the bird eye and legs when using subject detection. Shooting with nikon 500 mm pf the bird about 15 meters away. Not sure about the depth of the field at such condition but there will be impact on sharpness .
 
Many videos talking about birds in flight photography talk about the importance of focusing on the eyes, because if you focus on a wing the head might be out of focus. They talk about an extremely shallow depth of field, apparently being a foot or less. Online depth of field calculators show a much greater depth of field. For example, Depth of Field Calc, when set for my Nikon D7100 and a distance of 300ft, f/stop 4.5, focal length of 600mm gives a depth of field of 13.43 ft. Can someone help me to understand.
The further away the deeper depth of field.
Its all a balance between aperture shutter and ISO, and trying to get enough depth of field with an nice blurry background...🦘
 
Many videos talking about birds in flight photography talk about the importance of focusing on the eyes, because if you focus on a wing the head might be out of focus. They talk about an extremely shallow depth of field, apparently being a foot or less. Online depth of field calculators show a much greater depth of field. For example, Depth of Field Calc, when set for my Nikon D7100 and a distance of 300ft, f/stop 4.5, focal length of 600mm gives a depth of field of 13.43 ft. Can someone help me to understand.
Think about it....300' is basically the length of a football field. How big is the bird going to be in the frame? Tiny! Yes, it will all be sharp if you can focus on it......but you probably won't even be able to see its eye! LOL!
 
The further away the deeper depth of field.
Its all a balance between aperture shutter and ISO, and trying to get enough depth of field with an nice blurry background...🦘
The COF in the DOF calculation is very confusing- The COF needs to include the enlargement factors as well as the viewing distance. Billboards look reasonably sharp from 100 feet away but not from 10 feet
 
The COF in the DOF calculation is very confusing- The COF needs to include the enlargement factors as well as the viewing distance. Billboards look reasonably sharp from 100 feet away but not from 10 feet

I'll agree and add that cropping also reduces dof, since we usually adjust the size and or viewing distance so the coc is impacted.
 
The advanced dof calculator at photopills offers many more choices for circle of confusion or you can basically dial your own by adjustng the viewing distance and print size.

Thanks for the clarification. As explained in the linked Douvos article, there are two approaches to DOF. The first is just good enough where you take viewing distance and print size into account. This is fine when you have a specific output print size and viewing conditions for your image. This is the approach of Photopills. However, many of us do not know in advance the print size and viewing conditions and want to resolve as near as possible to the sensor resolution, so we can use the image for multiple print sizes and viewing conditions.

As an example, we can consider the approach with the Nikon D850 and Z9 sensors. To resolve to sensor resolution, we need a total blur circle of two times the pixel pitch (4.34 µ times two, rounded to 10 microns). The final blur circle consists of two components: defocus blur and diffraction blur. The Photopills calculator does not take diffraction into account. The diffraction blur is approximated by the size of the Airy disc. At f/16 the Airy disc is 20 µ, so we can't really get a blur circle of 10 µ. TrueDoF-Pro by George Douvos does take diffraction into account and the total blur circle is approximately the sum in quadrature of the two components = sqrt (defocus blur^2 + diffraction blur^2). See the linked article for more details. The table below shows the calculations for total blur circle. Apertures up to f/8 are reasonably acceptable.

Bill

Blur Circle.jpg
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What about field curvature?
Field curvature would throw a monkey wrench into the calculations. Digilloyd ( if you subscribe to making sharp images) suggests looking at the MTF curve at various apertures. A wave form appearnce indicates curvature of field which may be reduced by stopping down to f/5.6 or so. For DOF you would likely stop to f/5.6 or f/8 anyway. Beyond f/8 diffraction softening becomes problematic, although some clalim this can be handled by deconvolution sharpenig. Another approach would be focus stacking in small steps, but I havn't done this or have seen it done in this situation. With well corrected lenses I don't think this is a major problem. For microscopy lenses (objectives) refer to plan achromat or plan apochromat. The latter are wonderful although expensive.

Bill
 
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