Depth Of Field Question / Help

It will reduce the apparant depth of field for a given print size and view distance, since the closer-up you see a photo, the easier it is to tell which part is in focus.
For instance, if I shoot a portrait at 150mm and f2.8, the tip of the nose and ears might be out of focus, but when viewed as a whole the entire face will seem sharp. The more I would crop into the portrait, the more you would see that only the plane along the eyes are in fact in perfect focus.

I'll give it a go as I'm curious to 're-learn' this...it's been a long time since I did originally.

My understanding based on the way it was taught to me in the 70's...(maybe physics has changed since then) : A lens can only ever focus on one plane. Any additional 'sharpness' that's achieved by stopping down is only *apparent sharpness*, not real focus, as a function of limiting circles of confusion. As the image is enlarged, that apparent sharpness diminishes. In fact, I seem to recall, that if you were to enlarge an image taken with a wide angle lens to the same magnification as that of a longer lens, the DOF would be identical in both images provided that they were taken at the same aperture.

Also worth noting as it relates to the above, DOF calculations are only accurate at a 1:1 scale of the image...iow, a contact sheet type reproduction. As that image is enlarged, the apparent focus becomes what it really is: out of focus!

As I said, it's been a long time, I'd be happy to hear someone who really know this stuff to comment.

I was considering editing my post to add some of the stuff you mentioned, but yours was more comprehensive. Yes, that's exactly how it works, given infinite resolution only a narrow plane would be in focus, regardless of focal length. (But still limited by diffraction)

The circle of confusion is related to the physical pixel size on the sensor. One recorded you can't see more or less focus except by reducing the image size - then it may look a little better. .

"Additional focus" by stopping down is real! There is also a diffraction issue, which reduces image quality as you stop down to small appertures - IMHO this is negligable in most manufacturer's common lenses - certaily at one less than the minimum.

Yup, view distance / enlargement is a factor in depth of field.

I look at the OP's image on my smartphone's tiny screen and everything in the photo seems in focus to me.

Heya,

Without getting into a ton, you either need more depth of field potential for the distance & aperture of all subjects in your composition (ie, ultrawide + stopped down aperture with closest depth of field to farther depth of field in mind), or, you simply "focus stack." This is a digital technique. Two images or even three images may be needed to do it. You simply focus on your foreground with the same exposure values and mid-ground, and then maybe at the horizon. Then simply layer them and stack the focused areas. There are easy tutorials to do this with photoshop. I find this easier to do that way you can use normal field of view or telephoto even and get everything in focus by simply taking more than one image and stacking the in-focus areas.

Very best,

Copidosoma wrote in post #18081210
Cropping a picture after the fact will have no effect on the DOF.

Cropping an image and printing (or displaying it on a screen) at the same size and the complete image will definitely affect depth of field. To understand this, you need to understand that depth of field is not something that is calculated for the in-camera image size. Depth of field is calculated for a specific image size (nominally 8x10 inches is typical) and a specific viewing distance by a person with a specific visual acuity. If you crop an image and print it to fill the standard image size, you have enlarged the fuzzy points in the image and now they can appear to be fuzzy instead of appearing sharp like they might when printed to a smaller size.

SkipD wrote in post #18081286
Cropping an image and printing (or displaying it on a screen) at the same size and the complete image will definitely affect depth of field. To understand this, you need to understand that depth of field is not something that is calculated for the in-camera image size. Depth of field is calculated for a specific image size (nominally 8x10 inches is typical) and a specific viewing distance by a person with a specific visual acuity. If you crop an image and print it to fill the standard image size, you have enlarged the fuzzy points in the image and now they can appear to be fuzzy instead of appearing sharp like they might when printed to a smaller size.

Skip - thank you for clarifying that it is cropping AND enlarging that changes the depth of field. I was initially a bit confused as to how only cropping an image would change dept of field.

By the way, I very much appreciate your sticky on depth of field. Although, I've been reading some photography books and it keeps popping back into my head when I read the author's take on perspective. I should probably just pass over what they wrote, print your text and reread that instead.

Copidosoma wrote in post #18081210
Ummmmm, care to explain how cropping an image changes the DOF?

If you use a wider lens and move closer to the subject to get the same framing the DOF will decrease (because DOF is primarily a lens characteristic, not a sensor one). Cropping a picture after the fact will have no effect on the DOF.

Actually, Big Al was absolutely right in what he said.

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I will confess that I tend to avoid thinking too much!

But still, there are things to consider -- photography can be a "juggling act"!

I would tend to avoid the scenario of trying to get close things as well as further things in good focus...as was suggested, move yourself to get better results!

One time, though, back in the summer of '08, I was at a POTN meetup in the Columbia River Gorge (East of Portland, OR). We were at a spot looking up a stream with some falls in the background, but the stream flowed down to us, around some rocks in the foreground. I wanted to get the "whole shot", although I was in a "fixed" position -- I knew that with a "normal" f/16 aperture; well, I'd be limited...

So, J went the limit, set the dang lens to f/22 and took the dang shot.

Actually, I was happy with the outcome, it was worth sharing, in fact a POTN friend said "Print it!" and I would have, except that zoomed in, it showed a patch of greenery was blurred, I guess from a breeze, Oh Well!

BigAl007 wrote in post #18080784
One warning on using a wider lens, if you do the DoF will indeed be deeper, but the subjects in the image will be smaller. If you then crop the image to make the subjects the same size as originally the DoF depth will also be reduced, negating the increase in DoF.

Alan

I agree with Alan.

Copidosoma wrote in post #18081210
Ummmmm, care to explain how cropping an image changes the DOF?

If you use a wider lens and move closer to the subject to get the same framing the DOF will decrease (because DOF is primarily a lens characteristic, not a sensor one). Cropping a picture after the fact will have no effect on the DOF.

Not true. Alan IS correct.

Tom Reichner wrote in post #18081505
Actually, Big Al was absolutely right in what he said.

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I agree with Tom & Alan.

Perhaps a word of explanation will help to clarify. When an image is cropped, the net effect is CHANGE THE VIEWING DISTANCE, which most (all?) of us would agree, does change the DOF. Print a photo (this fixes size). Look at from 18". Look at it from 18'. The apparent DOF will be hugely different. Cropping changes the viewing distance.