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From here: http://photo.stackexchange.com/a/20930/5205

f/8 sounds like a small aperture, and it might be good enough to get everything in focus in a landscape, but the closer (more magnified) your subject is, the shallower the DoF will be at a given aperture.
A smaller aperture may have been more appropriate, as may focusing slightly (millimeters) in front of the candles and letting the DoF bring them into relatively sharp focus. (That's very hard to force yourself to do;
nothing will be perfectly sharp at the focusing aperture and the DoF preview is too dark and small to show you what the result will be.

Didn't understand the above quote thoroughly.
I was under an impression that for ultra sharpness (all around) we need to lower the aperture? Is this true with some conditions applied?

What aperture should be preferred while shooting the subject which is close enough, to get maximum overall sharpness? and why?

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Sounds like there is confusion over the term "sharpness". This is generally a property of the specific lens in question. But it's also important to understand depth-of-field ("area of sharp focus"), which is not really the same as sharpness, but is related. –  djangodude Mar 15 '12 at 15:57

2 Answers 2

up vote 4 down vote accepted

The depth of field (defined as the area of acceptable focus) for any given f/stop is relative to the distance to the subject. It is also relative to the focal length of the lens - but thats another story.

In other words, if you were to use a 50mm f/1.4 lens on a crop sensor body, and shoot a subject 45cm away (which if memory serves is the MFD for that lens) at f/8, your depth of field is going to be 2.19cm. If you moved back, so your subject was 100cm away from you, your depth of field would be 11.6cm. If you were shooting a landscape (for arguments sake, we'll say you can see one mile), at f/8 pretty much anything beyond 16.3 metres will be in 'acceptable focus'.

Because the DoF reduces the closer you get to your subject, you must stop down to get the (approximate) same start and end point of acceptable focus. So to take the example above of f/8 at 100cm away; your near limit would be 94.5 cm and far limit would be 106.1cm, giving you the 11.6cm DoF. If you were to move closer, say to 60cm away, the DoF given by f/8 would reduce, and so you would need to increase it again by stopping down some more to retain that 11.6 (or thereabouts). At 60cm, to get the approx same DoF, you would need to stop all the way down to f/22, giving you 11.5cm of acceptable focus.

The problem with going all the way down to f/22 is that above around f/16 (dependent on the quality of the lens), diffraction starts to creep in. This means you will be sacrificing sharpness of the image even though it will still technically be 'in focus'.

In short, there is no magic f-number at which you will get everything in focus and at its best quality when closeup. I'm not really sure what to suggest for the best... You could perhaps get further away from your subject, and go for that middle f/8-f/11-f/14 range, then crop the photo in post to remove the blurry parts (and thus crop to the same 'image' you would have gotten if you were closer)...

I hope that make sense.

References: I am using this online depth of field calculator to give my figures, above.

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The more you stop down (use a smaller aperture) the lesser the effect of lens aberrations on sharpness. However the more you stop down the greater the extent to which diffraction reduces sharpness.

So there is a crossover where reduction in aberrations is cancelled out by increased diffraction. This is usually about f/8, though for really good lenses (with fewer aberrations to start with) it can be f/5.6 or even f/4.

A secondary factor is depth of field. The more you stop down the more of your scene will be in focus, improving the apparent sharpness as less of your objects will be blurred. Again you get a crossover point where the gains from stopping down are cancelled by the increased diffraction. The crossover point depends on your subject (how flat it is) and shooting distance. It may be the best average sharpness is obtained at f/16 or f/22

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What is "aberrations on sharpness"? –  TheIndependentAquarius Mar 15 '12 at 14:07
    
@Anisha lens aberrations are defects/design consequences that degrade the sharpness of the projected image. –  Matt Grum Mar 15 '12 at 15:35
    
"The crossover point depends on your subject (how flat it is) and shooting distance." What do you mean by how flat it is? How much distance should be considered for F8? My lens is Nikon 50mm 1.8G. –  TheIndependentAquarius Mar 16 '12 at 10:29

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