Study the MTF chart for your lens (Scientific way to calculate your lens’s sharpness) Do some tests (Practical way to test and it will give you more real life data because everything works in lab)

Old school rule of thumb (not entirely true but a starting point): 2 stops down from wide open (eg: on an f/1.4 lens, f/2.8 would be 2 stops down) is often the “sweet spot” of a lens. More recent lenses have gotten a bit better and may only need a stop. If you’re looking for maximum sharpness somewhere between f/5.6 to f/11 is going to be it but the top end doesn’t depend on the lens it depends on the sensor. The old saying is “f/8 and be there” and in most cases that works. But with many newer lenses f/5.6 is just as sharp and gives you a little shallower depth of field if that’s what you’re after (and if you pushing 40MP on APS-C you might be better off at f/5.6 because of the next factor:)

Conversely, where would you go to learn that a given lens model goes soft after f/11?

The lens doesn’t go soft the diffraction grows as you stop down. There is an “airy disk” of blur that grows the smaller the aperture gets… it’s a bit of a balance… most lenses are soft wide open and stopping down a little improves their optics, and if you want more things to be sharp you stop down to get more depth of field and have more things in focus, but as you do that the blur diffraction grows… and it grows purely based on the aperture, no matter what lens you use. The issue is that if the blur is smaller than the size of a pixel on your sensor you’ll never notice it. Once the blur grows to more than a couple pixels of your sensor wide, then you notice it. So on sensors with smaller pixels (small sensors with high MP) you’ll notice diffraction earlier maybe at f/8. I’ve had some where I’ve noticed it at anything smaller than f/6.3. A larger sensor with few megapixels you’ll be able to stop down to f/11.

When you get a new lens, you need to take a bunch of photos to learn the qualities of your lens. You can to it scientifically or you can just go shoot and experiment.

Generally if I get a new lens I go out take some shots wide open, then 2/3 of a stop down then 1 or 2 stops down from wide open, maybe some at f/5.6 and some at f/8 just to see what it’s like. I have a 50mm f/1.4 that is a bit soft at f/1.4 (if it’s really low light it’s not the end of the world, but if I really want something tack-sharp f/1.4 isn’t ideal) after using it a bit I’ve come to feel that if I stop down to f/2.2 it will be much sharper. But I had to do it for myself over time started at f/2.8 and it worked well but over time took some shots where I’d push it a little… f/2.0 one time but it was a little soft, f/2.5 was still decently sharp, eventually I came to realize intuitively that f/2.2 was the widest I wanted to go unless I was really pushing it.

A depth of field calculator is a good start.

But I’ve found more success with Merklinger’s method, which takes into account the blur of objects in real life instead of on the sensor. So it is an “object space” method instead of an “image space” method for determining depth of field.

Basically, for landscapes, wide deep landscapes where infinity needs to be sharp, you just focus on infinity. Easy, right?

Then you identify the smallest object in your scene, in the foreground, that you want just barely resolved. Suppose this is a blade of grass 5 mm wide. As it so happens, the diameter of the entrance pupil of a lens puts a lower limit on the size of things that can be resolved, anywhere in the scene when focused at infinity, and the entrance pupil width is the focal length divided by the f-number, by definition. You divide the focal length by the size of object that you want barely resolved, and that gives you the f-number needed. So if you are using a 50 mm lens, and want to barely resolve a blade 5 mm wide, you’ll calculate 50 mm / 5 mm = 10, or f/10. This is so easy you can do it in your head. This is true no matter how near or far the object is from the camera; you simply don’t have to calculate distances, and it doesn’t matter what camera you have and what size sensor you have.

This also works if you focus closer, except the 5 mm blade of grass will be sharper the closer it is to the focus distance, but you can calculate that as well: 2.5 mm will be resolved at half the camera-subject distance, and 1 mm will be resolved at when you are 4/5s of the way to the subject, and if you want to estimate blur behind the point of focus, the same thing happens in reverse, so 1/5 of the distance beyond the point of focus will resolve 1 mm as well, and twice the focus distance will again resolve 5 mm, and will increase proportionally out to infinity.

Unfortunately, diffraction effects aren’t easily incorporated into depth of field equations, but diffraction blur is the same width on the sensor for all cameras and all lenses at the same f/stop, which ultimately means that the f-number you can use with equal diffraction is proportional to the sensor width. Simply determine what f/stop on one lens and one camera is too blurry for you, and from that one observation you can extrapolate it to all of your gear. If f/16 looks a bit too ugly on your full frame camera, then you’ll know that f/8 will be too ugly on your Micro 4/3rds camera, since the latter camera has half the sensor width. However, diffraction blur responds well to sharpening, so you can usually go beyond the so-called diffraction limit if you are willing to pump up the sharpening.

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http://www.trenholm.org/hmmerk/TIAOOFe.pdf

Composition and lighting over worrying about pixels and sharpness any day of the week.

If I need to use f/2 or f/16 for the shot, I use it for the shot.

99.9% of the people won’t notice any slight difference in sharpness because they will be remarking how they like my composition.

You’re focusing on the wrong part of photography. All modern lenses for the last 30 years are good enough to take great pictures if you have a good composition and the right lighting for it.

You can study all you want, but it ultimately comes down to experience. You just go out and do it. Make mistakes and make good shots and eventually it’ll stick in your head for what works and what doesn’t.

Granted this is more for landscape photography, but what you are describing is kinda like a hyperfocal length chart.

Try the DoF tab in the PhotoPills app. You can set your specific camera model and lay in your distance from subject, aperture, and focal length. This is really designed to get EVERYTHING in the field of view acceptably sharp and isn’t really used for bokeh. However, this may help to fill in some of the technical numbers stuff you are trying to iron out.

Purely scientific? Depth of Field Calculator. There are many in browser available via phone. Will show what is area(by distance from sensor plane) is in focus at every distance for every aperture and for every focal length for every format.

Aesthetically? Lens Tests. Depth of Field Calculator is great, but won’t tell you about focus rolloff, or bokeh size differences.

Testing can be tedious, but depending on the subject/compositions you plan to use often and the focal lengths available you can focus on that. It’s even easier if you can shoot video with your camera - slap on a lens, light a subject with maybe some foreground/background, mark distances to subject from different camera points and roll for 10 seconds, keep rolling and change the aperture every time. Change iso or light intensity to compensate for image viewability.

Lastly, something small to leave you with - lenses are (typically) designed and calibrated to have the best performance between t/2.8-4. Nothing wrong with with shooting at 1.4 or 22, but the closer you are to those stops the less vignetting, aberration and other unpredictable elements you will find.

Christopher Frost

Shoot shoot shoot. One lens. One camera. Shoot the crap out of it. You don’t learn from forums, reddits, YouTube or reading other people’s opinions online. Shoot.

If you’re on Android you can use an app like PhotoPills to calculate DOF for your lens and camera model.
There are other apps as well for DOF like DOF Simulator or DOF calculator on apkpure.com, both free.
In case you’re on an iPhone, PhotoPills should be available too in the appstore. For other DOF apps you will have to do a search.

Just do it enough and you will gain an intuitive understanding on which aperture to choose. It depends on focal length and focusing distance. You can also use the DOF preview to see what sort of results you would get. When you find something to shoot, shoot it at different apertures that should be about right and analyse the results, see what works in what sort of situations (focal length + focusing distance + what you want out of the image).

It helps if you learn to master few prime lenses first before moving onto zoom lenses.

This is very subjective based on your subject and your goal for what you’re taking a picture of.

You can’t just use one aperture for everything. The reason for photography in manual is to change the exposure settings based on the subject and lighting. It will continuously change because your subject and lighting will change, since I assume you don’t plan to take a photo of the same thing in the same lighting forever…