question about DOF at f/2.8 with 70-200L

Still having a blast getting used to my 70-200L f/2.8 that I got earlier this year. Have a DOF question. In the shot below, why are the ears out of focus? I was shooting my 5D with center focus point only and my subject was only a few feet in front of me. Would I have gotten a better DOF had the subject moved back a few feet and then I later cropped it in LR?

Shot was taken at 200mm, 1/1000, ISO 400.

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Lower your F-stop to F4.0 or more. Your DOF is thin at F2.8. You can also step away from the subject and then crop it like you said. But lowering your F-Stop is a better solution.

How long have you had all this gear for?

There is a good depth of field calculator at http://www.dofmaster.c​om/dofjs.html. You should probably play with it a bit for a good understanding.

I did not run the numbers at f2.8 but ran the numbers based on a 36 inch (your "few feet") focus distance and at f4. The near limit of focus is 35.9 inches and the far limit is 36.1 inches for a total depth of field (DOF) of 0.15 inches. That is slightly more than 1/8 of an inch so you can readily see you problem and at f2.8 it would have been even less! Bring the focal length in to about 85mm and under the same conditions you get about 2 inches total DOF. The other thing you have under your control is lowering your shutter speed thus yielding a smaller aperture and more DOF. As you can see it is a fine balancing act of all the variables.

One point of interest, in portraiture work, the desired focus point is the area of the eyes; it looks like you might have used the forehead. Once you find a workable combination of aperture and focal length to yield about 4 to 6 inches DOF then you will get a better picture. One way to do this might be to shoot a high quality RAW at 200mm and from 6 to 8 feet from the subject. Then crop out the desired area.

You have framed about 8" wide area. With 200mm on FF, that would mean that you were about 75" away. At that distance, at f/2.8 your DOF zone is from 74.8" to 75.2". If you had used f/16, your DOF zone still would have been only 2.6" deep!

Backing up increases DOF, but then if you crop in post processing, you effectively end up with shallow DOF...DOF is based upon the relative size of your subject in the frame on the final print, and it does not matter if you 'crop in camera' or 'crop in post'

Put another way, as long as the aperture and framing are identical, you will have almost the same DOF regardless of what focal length you shoot at. Backing up and reframing with a longer focal length would give you more background blur (not that you need it here) but would not increase your DOF. The only solution to get everything in focus would be to stop down your aperture.

As far as the correct value to use to get his entire head in focus, I'd refer you to the DOF calculator linked to above. However, keep in mind that the total depth of field it gives you will include space both in front of and behind your focus point. Thus, if your subject is, say, one foot deep, but you're focusing on his eyes, you'll need a depth of field of approximately two feet to get one foot behind the focus point. In reality it's not a 50/50 split between in front of and behind the focus point, but as you get further from your subject, it approaches it. Again, the DOF calculator above is your friend if you want to get an exact number. As Wilt mentioned, though, even at an aperture of f/16, your DOF is still only 2.6", so you may find it challenging to keep his entire head in focus with this framing and focal length.

OP should run some numbers to get a feel for how the variables like zoom, focus distance and aperture affect DOF. Some combinations follow below and show how increasing focus distance has a relatively large affect.

200mm 75" distance @ f2.8 DOF = 0.54 inch

200mm 75" distance @ f16 DOF = 3.07 inch

200mm 150" distance @ f16 DOF = 13 inches

So going from f2.8 to f16 only increases DOF from about a 1/2 inch to 3 inches. Maybe just enough with the eyes the focus point to get the ears in focus. But lengthen the focus distance to 150 inches and now DOF = 13 inches, plenty to get the entire head in focus with the eyes as the focus point.

Here is how to calculate DOF:
http://www.dofmaster.c​om/dofjs.html

Yep. You figured it out. At f2.8 the DOF is very shallow and gets shallower as you get closer.

Enjoy
Mike

Here is a link for a DoF calculator. http://www.dofmaster.c​om/dofjs.html
You can also get a version that runs on an iPhone, maybe Android too.

Wilt wrote in post #15163085
Backing up increases DOF, but then if you crop in post processing, you effectively end up with shallow DOF...DOF is based upon the relative size of your subject in the frame on the final print, and it does not matter if you 'crop in camera' or 'crop in post'

This doesn't make sense to me. DOF increases with distance, but it's absolute, not relative, isn't it? If DOF at 10 feet is, say, 1 inch, and at 80 feet it's 8 inches (for the same aperture and lens FL), then an object at the focus plane will be a full 7 inches more in focus than at 10 feet, regardless of the amount of cropping. Where is my logic failing here?

Snafoo wrote in post #15183478
This doesn't make sense to me. DOF increases with distance, but it's absolute, not relative, isn't it? If DOF at 10 feet is, say, 1 inch, and at 80 feet it's 8 inches (for the same aperture and lens FL), then an object at the focus plane will be a full 7 inches more in focus than at 10 feet, regardless of the amount of cropping. Where is my logic failing here?

Your example numbers are way out of reality. Please try doing some calculations with the DOFMaster calculator and try your question again.

In addition - you must realize that DOF calculations are based on using a whole image and printing (or displaying) that whole image at a standard size and viewing that image from a standard distance. If you change any of the variables, the actual depth of field varies from the calculated values. On top of that, there are several assumptions about human vision involved in the calculation formula. If you have a group of folks analyze a test print (viewing it from the standard distance), not everybody will feel that the depth of field in the test print is the same.

Snafoo wrote in post #15183478
This doesn't make sense to me. DOF increases with distance, but it's absolute, not relative, isn't it? If DOF at 10 feet is, say, 1 inch, and at 80 feet it's 8 inches (for the same aperture and lens FL), then an object at the focus plane will be a full 7 inches more in focus than at 10 feet, regardless of the amount of cropping. Where is my logic failing here?

Folks do not have a fundamental understanding the principle behind DOF and CofC...a point which is out of focus becomes a blur circle. When the blur circle is smaller than a certain size the eye/brain is fooled into thinking that 'the blurry circle is a sharp point'. When you crop a photo and enlarge it more (e.g. take 1/2 of an image, and enlarge it to 8x10" size), you magnify the blur circles by 2x more as well, so that now what they eye was fooled about is suddenly that blur circle is 'blurry' because it is large enough to be detected as blurry. Or, to put it differently, if you shoot a photo and enlarge by 8x to 8x12 and then enlarge the same image by 16x to 16x24 size, the DOF value for 8x12 do not apply to 16x24...and if you trimmed the 16x24 print down to 8x12 size the 16x magnification of the blur circles is what allows the brain to see things as 'blurry', whereas at the 8x magnification it thought 'sharp'.

Snafoo wrote in post #15183478
This doesn't make sense to me. DOF increases with distance, but it's absolute, not relative, isn't it? If DOF at 10 feet is, say, 1 inch, and at 80 feet it's 8 inches (for the same aperture and lens FL), then an object at the focus plane will be a full 7 inches more in focus than at 10 feet, regardless of the amount of cropping. Where is my logic failing here?

The true calculation is that DoF increases by the square of the multiple of the distance. So as Skip says, your assumption and your figures are way out.

Using your 10 feet/1" start point, an object at eight times the focus distance would have 64x the DoF. 64" = 8" squared.

But interestingly, if you then enlarge the subject area by 8x, to give the same image as the the original shot taken at 10 feet, DoF reduces by the inverse of the enlargement factor, so you would have DoF of 64/8 = 8". 8x8 divided by 8.

That is of course the same calculation for the reason that a 1.6x crop camera has 1.6x more DoF than its full frame equivalent.

Ok this is a different lens but it does show how narrow the depth of field can be. In this case just a few inches. Also since your picture was shot at a lower f# it will have a narrower DOF

Mike
1" Mushroom

SkipD wrote in post #15183603
Your example numbers are way out of reality. Please try doing some calculations with the DOFMaster calculator and try your question again.

I just threw those values out as hypotheticals; I didn't think they needed to be accurate for the argument and I was lazy in not bothering to look up the actual values. Excuse me. The actual values are:
- 200mm, f/4, 10 ft: DOF = 1 in.
- 200mm, f/4, 80 ft: DOF = 7 ft.
More or less.