Question about DoF and apeture

[intothedawn]

If I'm at 35mm (APS-C) shooting a subject at 1000m with the closest foreground at 500m and the furthest background at 2000m the scene should fall between the near/far plane of any apeture according to the DoF table.

I know on my lenses that F8 is considered the sharpest and F11 is where diffraction starts to kick in but is there any difference between using F8 and using a lower F stop like F2.8 to shoot the same scene - apart from letting me have a higher shutter speed?

Would there be more of a drop off in sharpness in the foreground/background using a wider apeture? Would there be a drop in sharpness of the subject?

 

[Alphoid]

Short answer: It depends on the lens

Medium answer: On good lenses, you can open them up quite wide and still have very sharp photos (f/1.8, or even f/1.4). On lousy lenses, you need them stopped down a good bit to have sharp images.

Long answer: Resolution is limited by three things:

1. Diffraction (which gets worse with narrow apertures, and set by physics)
2. Lens aberrations (which get better with narrow apertures, and set by how good your lens is)
3. Sensor resolution

Each lens has a point where diffraction and lens aberrations are about equal. That's where photos are sharpest. For better lenses, that's wider open (usually around f/2.8 or so). For worse lenses, that's stopped down a good bit further.

For most decent lenses, there's a pretty wide range where that's sharper than sensor resolution, and shooting anywhere in there, you won't see a huge difference in sharpness. If your aperture is too wide, lens aberrations will reduce resolution. If you stop too far down, diffraction will limit resolution.

Short short answer: Don't worry about it. Open up as wide as you want, without going to extremes. If you're a stop or two down from the widest your lens goes, you're unlikely to run into issues.

Oh, and as for DoF, use the calculator, but wide angle and long distance means it's essentially infinite DoF. DoF becomes a problem when using long lenses and shooting things close to you.

 

[Tom Axford]

If I'm at 35mm (APS-C) shooting a subject at 1000m with the closest foreground at 500m and the furthest background at 2000m the scene should fall between the near/far plane of any apeture according to the DoF table.

At those distances, even at the maximum aperture of your lens, everything will be sharp whether you focus on the foreground or the background.

At f/2.8 with a 35mm lens on APS-C, the hyperfocal distance is around 20m to 25m. Your distances are so much greater than this that they can all be regarded as infinity.

I know on my lenses that F8 is considered the sharpest and F11 is where diffraction starts to kick in but is there any difference between using F8 and using a lower F stop like F2.8 to shoot the same scene - apart from letting me have a higher shutter speed?

If the lens is equally good at those apertures, there would be no difference.

Would there be more of a drop off in sharpness in the foreground/background using a wider apeture? Would there be a drop in sharpness of the subject?

Not if they are as far away as you say.

 

[intothedawn]

Thanks guys, that makes sense. Good to know I can probably open up the lens a fair bit more when handholding in those cases.

 

[Mark S Abeln]

Some lenses are rather soft in the corners when opened up. So it depends on your lens and your needs.

 

[Leonard Migliore]

If I'm at 35mm (APS-C) shooting a subject at 1000m with the closest foreground at 500m and the furthest background at 2000m the scene should fall between the near/far plane of any apeture according to the DoF table.

Yep. Those distances are known as way far away.

I know on my lenses that F8 is considered the sharpest and F11 is where diffraction starts to kick in but is there any difference between using F8 and using a lower F stop like F2.8 to shoot the same scene - apart from letting me have a higher shutter speed?

Depends on the lens. If you have an f/2.8 lens, most of them aren't as sharp wide open as they are a couple of stops down, like f/5.6. But some are perfectly good wide open. You'll have to check for yourself.

Would there be more of a drop off in sharpness in the foreground/background using a wider apeture? Would there be a drop in sharpness of the subject?

There won't be any visible effect on sharpness due to aperture at the distances you mention. What can have an effect is the atmosphere. Most of the time, you will get loss of contrast and distortion from the air at 2km. Right now, where I live, you can't see as far as 2km; you can just about see to the end of the street. But you may not be in California.

Anyway, in many cases the greatest limitation on sharpness in the kilometer range is the air quality. It won't be sharp no matter what you do.

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Leonard Migliore

 

[Gerry Winterbourne]

If I'm at 35mm (APS-C) shooting a subject at 1000m with the closest foreground at 500m

I assume you mean the closest significant object you want to be sharp. If you hold the camera horizontal at eye level (about 1.5m above ground) then the 35mm field of view will capture everything at ground level from 6.5m away from the camera.

and the furthest background at 2000m the scene should fall between the near/far plane of any aperture according to the DoF table.

Hyperfocal distance (HFD) is a useful concept (note I say concept - it isn't a better way of working than any other). It has two properties: if you focus at infinity the nearest end of DOF is at HFD; if you focus at HFD the near end of DOF is at half HFD and the far end is at infinity.

Look at your DOF calculator and you'll see that your camera-lens gives HFD about 21m at f/2.8. So if you focused there you'd still get your 2000m distance in the DOF and your 50mm distance; but the very closest things between the 6.5m I calculated above and 21m would be softened by being outside the DOF.

As I say, I'm not advocating the general use of HFD - in your case it's better to do what you say and focus on the critical features - but it does give an idea of how much or how little flexibility you get at different settings. I looked at this and by pure chance I picked 300mm to get a restricted field of view of that distant landscape and HFD calculates at 1000m - so DOF would run from exactly your 500m out to infinity, capturing 2000m on the way.

I know on my lenses that F8 is considered the sharpest

You can get details of lens resolution from sites like this https://www.opticallimits.com/ https://www.ephotozine.com/

Here's my 35mm lens from OpticalLimits. You'll see its central resolution peaks at f/5.6 while edge sharpness peaks a bit later at f/8. This is typical of all lenses. Any drop from peak resolution exaggerates the out-of-focus blur that causes DOF, which is one of many reasons why DOF calculations are never better than a guide. Some lenses are pretty soft at the edges; with those the near end of DOF can be pushed away from what the calculations say.

For my lens the drop from f/5.6 at centre is more significant than the gain from f/5.6 to f/8 at edge so I always use f/5.6 for landscapes if possible. It's worth looking at your own lenses to do a similar equation.

and F11 is where diffraction starts to kick in

Strictly speaking diffraction is always present. It depends on how small the aperture is. At wide apertures it's very little but aberrations are significant; stopping down reduces aberrations and increase diffraction. The sweet spot is where they balance; after that diffraction doesn't "cut in", it just gets progressively more severe.

but is there any difference between using F8 and using a lower F stop like F2.8 to shoot the same scene - apart from letting me have a higher shutter speed?

See above. For my lens centre resolution at f/2.8 beats edge resolution at f/8 so it won't be disastrous; but the whole picture will be slightly softer even though it is all still within DOF. Knowing my own gear I'd happily go to f/2.8 for night scenes but only when forced.

Would there be more of a drop off in sharpness in the foreground/background using a wider aperture? Would there be a drop in sharpness of the subject?

Yes - see above. DOF is what appears "acceptably" sharp. Among other things this depends on the size of your image (screen or print) and how close you view it; and, importantly, how good your eyes are. At normal size and distance most things would look just the same because your eyes wouldn't be able to detect the difference. But if you have exceptionally sharp vision, or you print large or if you examine your photos in great detail you'd probably see a slight overall softening.

Oh, and at the extremes the difference would be more noticeable on a camera with a higher MP sensor.

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Gerry
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I'm happy for anyone to edit any of my photos and display the results
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First camera 1953, first Pentax 1985, first DSLR 2006

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