Eye of the eclipse...

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

So, is there any reason to shoot with wider aperture except to separate foreground and background? (In theory — don't worry about shutter speed and blur with narrower aperture, etc.) The question is about picture quality in perfect focus with different aperture.

Assuming we have ideal post-processing software which can separate each object from background, we can easily programmatically blur background and get same quality image. Why bother with wider aperture?

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Just so you know - "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... " - HUGE assumption. – rfusca Jun 1 '12 at 5:48
Yes and I want to understand where I'm wrong – yura Jun 1 '12 at 5:55
I'm not sure I quite understand your question, but, is it maybe essentially the same as photo.stackexchange.com/questions/20430/… or photo.stackexchange.com/questions/11205/…? – mattdm Jun 1 '12 at 11:51
Note that physical aperture size decreases for the same f stop for the equivalent field of view as sensor size decreases. That means that if you're using a small-sensor camera, you need to be more aware of diffraction limits. Kirk Tuck advocates shooting micro-four-thirds lenses wide open or as close to it as possible for that reason. – mattdm Jun 1 '12 at 13:29

4 Answers 4

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.

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On the "perfect photoshop" idea: photo.stackexchange.com/a/23526/1943 – mattdm Jun 1 '12 at 14:19

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.

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Although to be fair to that lens, it really reaches excellent sharpness stopped down just two stops to f/2.8, gets a little better at f/4, and basically just doesn't get worse again until past f/8 or so. – mattdm Jun 1 '12 at 13:06
My Canon 50mm f/1.4 always seems sharpest to me around f/4... – Mike Jun 1 '12 at 14:30
Every lens has a "sweet spot" -- the f/# where it is sharpest. For many lenses this seems to happen around f/4 to f/5.6 and stay sharp for a few stops. Around f/11 diffraction begins to degrade image quality. – Joe Jun 1 '12 at 19:20

Assuming we have ideal post-processing software which can separate each object from background, we can easily programmatically blur background and get same quality image.

Your assumption is false. Or at least, if there's software that can infer all the depth information from a single 2D image, I haven't seen it. Individual objects have depth. If I take a photo of someone's face at f/2, their eyes will be in focus but the ear or some of the hair will be slightly out of focus, and the scene behind them will be a total blur.

If you merely separate the foreground objects from the background objects and blur everything in the background, you don't get the same effect that you do with a wide aperture. You'd essentially be cutting out the foreground objects from a shot at f/16, say, and pasting them onto a blurry scene. That's very different from what you get shooting at f/2.

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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.

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