Help a Newbie: DOF vs. Diffraction

[jentenna]

So, I was reading this thread over in the Landscape Critique section, and it raised an issue that I've been idly wondering about recently.

For the long depth of field, I understand that you stop down the aperture. In Bryan Peterson's books, he recommends f/16 or f/22 for landscape photography. However, with a small aperture, you lose sharpness due to diffraction... possibly starting around f/8. So my questions are:

1. For sharp photos with large depth of field, what do you prioritize? Stopping down the aperture? Or respecting the diffraction limits of the sensor?

2. It makes sense that crop sensors start suffering from diffraction sooner (at lower f-numbers) than full frame. But what about depth of field? It seems like it shouldn't change, if the smaller sensor is really just cropping the image coming through the lens. Am I wrong?

I realize that the best way to understand this out is to go out try it myself at different focal lengths, so this might be my experiment for next weekend. But I'm curious what the experienced photographers think about this. Is there a "sweet spot" on f-stop for the sharpest landscape images?

[ravncat]

Originally Posted by jentenna

1. For sharp photos with large depth of field, what do you prioritize? Stopping down the aperture? Or respecting the diffraction limits of the sensor?

2. It makes sense that crop sensors start suffering from diffraction sooner (lower f-numbers) than full frame. But what about depth of field? It seems like it shouldn't change, if the smaller sensor is really just cropping the image coming through the lens. Am I wrong?

Is there a "sweet spot" on f-stop for the sharpest landscape images?

Diffraction is a tricky beast - as it also is a bit dependent upon print size and pixel size. (you`re not likely to notice diffraction in a 4x6 print.) It`s a real and visible effect, but it is small, and tends to hit very fine details. on to your questions...

1. This is a choice for you to make, if you need the depth of field, you need the depth of field - stop down as far as you want. If you want maximum sharpness - you probably shouldn`t be going past f11 on a 12 megapixel dx sensor (think of that as a guideline and not an absolute limit, some people have different ideas of acceptable sharpness). Prioritize whichever fits what you are trying to do

2. It`s really more a question of pixel density than just crop or full frame - see the links at the bottom. But what about depth of field? Depth of field isn`t dependent upon focal length - but on where you focus, the aperture and the distance to your subject. Depth of field is also related to magnification (which can muddle things)*. A lot of people talk about this and make one of two kinds of assumptions -

a you are using the same lens (or focal length) and you are changing your position to keep the same framing (depth of field and perspective change...)

b you are changing lenses to keep the same framing, and not changing your position or you are keeping lenses and not keeping the same framing (depth of field and perspective stay the same)

The sweet spot changes depending on the system (Camera and Lens)- but it`s usually around f/8.

Here are a couple of links about diffraction.
Understanding Lens Diffraction
Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

Bottom line - don`t worry too much about diffraction unless you are printing large or your goal is absolute sharpness. Never be afraid to stop down into "diffraction territory" if you need the extra depth of field.

*an Aside
if your crop sensor and full frame sensor have the same pixel size ( different resolutions, case a and b are accurate above ) If however your crop sensor and full frame sensor have the same number of pixels (say both 12 megapixel) then you start to see changes in the depth of field as well. This is because the depth of field consists of an area of "acceptable focus" - based on those circles of confusion again. More resolution tends to mean that your area of acceptable focus is smaller - which is why if you follow old depth of field markings on older lenses - you`ll find the image isn`t as sharp through the whole depth of field.

There was a standard agreed upon size of the "circle of confusion" for "acceptable sharpness" - and modern technology doesn't fit well with the old standards. Depth of field is inversely related to image magnification - more magnification = less depth of field. As you magnify your image, those circles of confusion get bigger, and what was acceptably sharp before no longer is. More resolution kind-of does this - by having more pixels to resolve the circle of confusion - so more resolution is kind-of like image magnification. (this is what I mean by muddled above) I apologize for any poor wording.

[jentenna]

Ravncat! Thanks so much for the clear, detailed reply. There's a lot to think about, but I think this has helped clarify the issue for me.

And those links are great... and cambridgeincolour has a bunch of detailed tutorials that look interesting. Could be a dangerous timesink for me in the near future.

[inkista]

As ravncat says, it's up to you. Peterson is firmly in the camp that DoF wins out over diffraction. But he might be shooting with a full-frame camera.

Originally Posted by jentenna

2. It makes sense that crop sensors start suffering from diffraction sooner (at lower f-numbers) than full frame. But what about depth of field? It seems like it shouldn't change, if the smaller sensor is really just cropping the image coming through the lens. Am I wrong?

No, you're not wrong. Technically, the DoF does not change assuming that the same subject distance and focal length are used on both formats.

However, in real-world usage, most people will attempt to frame/compose the image identically somehow between the two formats: either by changing their distance from the subject, or by using a different focal length lens.

With a crop body's narrower FoV vs. a full-frame's FOV, you're likely to either a) move backwards to get more of the scene into the frame, or b) use a shorter focal length (wider) lens. With either of these strategies, you will be increasing the depth of field. This is why most people make the airy assertion that you get a thinner DoF with a full-frame camera vs. a crop body. Technically, you don't, but the effect of moving in closer or using longer lenses does decrease the DoF.

Similarly, you get "a thinner DoF" moving from 35mm film to medium format, and from medium format to large format (which is why Ansel Adams's group, which mostly shot large format, was called f/64 ;-).

That's why the diffraction/DoF tradeoff is more critical the smaller the sensor gets. It's most critical with P&S cameras, where the tiny tiny focal lengths (5mm-100mm) give a huge DoF, and the super-high pixel density and microscopic pixel size makes diffraction far more evident.

[sk66]

I've never worried about diffraction, but I also don't go above f/11-16 unless I really need to

[jentenna]

Ah, thanks Inkista! This makes perfect sense. And although I had accepted that narrow DOF was impossible with my old point-and-shoot, I finally understand why.

Lots of great information. Much appreciated.