DOF and picture quality

Question

Is it possible to say that picture quality is better with small f number but on very small DOF, so this perfect quality degrades very quickly?

So basically is there any reason to shoot with big aperture except to separate foreground and background? It is mostly theoretical question, so I do not consider shutter speed and blur with big F etc. The question is about picture quality in perfect focus with different F numbers.

In other words assuming we have ideal photoshop which can separate each object from background, so we can easily programically blur background and get same quality image... - and I want to understand where I'm wrong

Answer 1

Ignoring that fact that a large aperture lets in more light allowing faster shutter speeds etc. there are a number of reasons you'd want to use a larger aperture other than for subject background separation:

• Wide apertures let you shoot through certain obstacles. It's a common technique in amateur motorsport photography to shoot through a chain-link fence using a wide aperture to blur the fence to the point where it becomes almost unnoticeable in the image.

• Most wide aperture lenses are soft wide open, and this can be used for photographic effect, e.g. to give a softer, more flattering look for portraiture.

• You get perfectly circular out-of-focus highlights when the lens is wide open as the aperture blades are fully retracted leaving only the circular hole of the lens barrel itself. When stopped down the aperture can take on a hexagonal/pentagonal shape, depending on the construction of the iris.

Finally it's worth reiterating that the "perfect photoshop" does not and cannot exist with regards to separating subject and background, and producing blur. Thus it will always be better to do it in camera.

Answer 2

Most lenses are actually designed to give the best pure quality further up the f numbers - for example my Nikon 1.4f - has the sharpest pure image quality at around f10.

The main reason for super-low f numbers is to get better low-light shots. yes the tiny DOF that you can get can be good too, but at f1.4 its so tight that nearly nothing is actually in focus if shot at an angle.

Personally I really don't like programically blurring, it looks fake.

Answer 3

It depends mostly on what you want to achieve.

I think that for portraits you want a shallow DOF, so wide aperture, while for landscapes is better to have a wide view.

Sometimes you are just limited by technical reasons, such as low light, and you can't choose your DOF. I'm thinking to low light or strong light conditions.