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I'm shooting with a Sony A7C and a 35mm f/2.11 ZA Lens. Suppose it's the middle of a Summer day at high altitude. I'm using the Intelligent Auto setting, the camera identifies the scene as landscape, and chooses an aperture of 11 and a shutter speed of 1/250.

I believe this means the camera has decided my best focus / sharpest image / depth-of-field will be at aperture 11: it could have also chosen 13 with a shutter speed of 1/200 or so, still plenty fast to avoid camera shake, but decided 13 would have too much diffraction.

Suppose I'm at this same location and there is less light. My camera might choose aperture 8 and shutter speed 1/60 because it needs a larger opening for a fast-enough shutter speed. I believe I can put the camera on a tripod, set the aperture to 11, and shoot at whatever shutter speed because the tripod will take care of shake.

Ok, here's the question. Suppose I arrive at a new location with my tripod. My camera chooses aperture 8 and shutter speed 1/60. I have not been to this spot before and so I do not know what my camera would choose in a higher-light situation. I'd like to choose the aperture which gives the sharpest image, regardless of shutter speed because of the tripod. How do I determine what that aperture would be?

I realize the phrase "less light" is imprecise: light has direction and quality, not just intensity, but can that be ignored for now?

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The problem with the question is that there is a difference between actual sharpness/"sharpest image" and "acceptable sharpness"/Depth of Field.

Light itself is sharpest when it is not interfered with... i.e. when there is no aperture involved. However, that is not possible; so light itself is sharpest when the aperture is the largest.

But with most lenses there is a tradeoff, because at wide apertures there tends to be greater optical aberrations from the periphery of the lens elements; which are removed by stopping down. (note that the light/traces from the narrow aperture image are also w/in the wide aperture; just not shown for simplicity).

enter image description here

However, if even the smallest dot the lens can project is larger than 2 photosites the actual maximum resolution/sharpness at the focal plane will be limited. If it is only due to diffraction the lens is called "diffraction limited" (having no optical aberrations).

This chart shows the maximum resolution/sharpness possible in MP at various apertures for various formats and wavelengths of light due to only diffraction... I have color coded the FF/35mm columns.

enter image description here

"Acceptable sharpness" for a FF sensor is limited at a .03mm airy disk size based on the standard CoC limit as used by a DoF calculator/scale... that occurs at f/22 for green wavelengths (which are more important).

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Some ideas to consider.

  1. A Landscape will hardly have a depth of field problems. Most of the elements will be far away, so if you focus on them they will be in focus.

  2. To maximize the depth of field, simply use the smallest aperture you can.

  3. But that does not mean that that is the sharpest you will get. You already know this, because you mentioned possible diffraction on f13.

  4. So you probably need to find the sweet spot on your lens (or range of them)

  5. You also need to make some test to find out what relative distance among different planes reacts with that range of sweet spots.

  6. But a blurry natural landscape can also be the result of some wind.

You could get rid of the camera shake but probably some trees will move. So if you find out that you like the sharpness at f5.6, and you do not have many planes, and you can focus all the planes on your framing at f5.6, and there is some wind, you probably what to use that aperture, and not an unnecessary slow f11.

My camera chooses aperture 8 and shutter speed 1/60.

Do not use a program setting, simply use aperture priority. That way you limit the variables to just 1 (besides ISO setting)

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Your title was "How to maximize depth-of-field", but then your question was "I'd like to choose the aperture which gives the sharpest image".

Those are two different subjects.

For maximum depth of field, perhaps needed for a landscape, focus at the hyperfocal distance, and use a wider aperture and a shorter lens. A good depth of field calculator should specify hyperfocal distance for each specified aperture. A wider aperture will shorten hyperfocal to increase range of depth of field (if focused at hyperfocal for that aperture). A tripod makes slower shutter possible to use a wider aperture in the hyperfocal case.

If focused at hyperfocal distance, depth of field will extend from infinity back to half of hyperfocal. This is very helpful to include both infinity and a quite close distance too. A wide aperture on a short lens focused at hyperfocal can extend depth of field back to a few feet (and to infinity too).

Example: full frame camera, 24 mm lens at f/4, hyperfocal is 15.82 feet. If focused at 15.82 feet, depth of field is 7.9 feet to infinity. The sharpest point will be at focus distance. Or if for example, as close as 8 feet is not necessary (if nothing is there), you can focus a bit longer than at hyperfocal, which will increase sharpness at infinity. A DOF calculator will show the details.

If focused at infinity, depth will extend back to hyperfocal (which is wasting the opportunity of extending depth of field to half of hyperfocal), but it will of course improve sharpness at infinity. Sharpness is always best at the focused distance, but sharpness is at marginal limit at the depth of field limits.

For a full frame camera, sharpest image will be at focus distance and will usually be at aperture around f/5.6 or maybe even f/8, but that won't be maximum depth of field.

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