Alley in Pisa, Italy

by Lars Kotthoff

submit your photo


Hall of Fame
View past winners from this year

Please participate in Meta
and help us grow.

Take the 2-minute tour ×
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It's 100% free, no registration required.

Why does higher aperture, less light, make an image sharper? Or rather make everything in focus? And how does this lead to lenses having a sweet spot in sharpness?

share|improve this question
    
Not a specific duplicate but there is an answer here. –  Peng Tuck Kwok Jan 30 '13 at 6:36
1  
You're actually asking three related questions here: What exactly determines depth of field? What is a diffraction limit? and a third dealing with the characteristics of a lens used closer to wide open, which I've tried to cover in an answer to Why do some lenses cost ten times as much...? –  user2719 Jan 30 '13 at 7:24
    

2 Answers 2

A higher f-number (technically a smaller aperture) contributes to sharpness in two ways. Firstly the depth of field is increased, thus objects which would appear blurry are now rendered sharp. Secondly a smaller aperture reduces aberrations which cause the image to appear soft even at the plane of focus.

In a perfect lens light coming from an object spreads out, passes through the aperture and then is focussed into a dot on the film/sensor plane. However real lenses suffer from aberrations, such as spherical aberration whereby the light passing through the aperture isn't all focussed at the correct distance, light passing through the edges of the aperture might come into focus in front of or behind the sensor, and thus not form a precise dot but a smear. Closing the aperture simply blocks the light from the edges thus it can't have a softening effect on the image.

image by Lookang

If you make the aperture too small then diffraction occurs, whereby light spreads out, again causing a softening of the image. So for every lens there is a crossover point where the increase in sharpness from reducing aberrations is balanced by the decrease in sharpness from diffraction. This is the "sweet spot"

share|improve this answer
    
Please can you elaborate this sentence:"Closing the aperture simply blocks the light from the edges thus it can't have a softening effect on the image."? What do you mean by edges? –  D4Am Jan 30 '13 at 11:10
    
@Mr.M: that's actually nicely answered here: How does aperture work without “cropping” the image hitting the sensor? –  mattdm Jan 30 '13 at 13:58
    
Thank you, now I understand :) –  D4Am Jan 30 '13 at 15:33
    
Is the two-slit diffraction image (ah, takes me back to high school physics!) really the right illustration? –  mattdm Jan 30 '13 at 18:46

This question is exactly a duplicate of a prior one Why small aperture (less light) has large depth of field?, but that was closed as a duplicate of a more technical response.

A common concept in both this question and the direct duplicate is considering aperture as primarily a control over the amount of light in the exposure — it seems like maybe less light would be bad for image quality.

Aperture is a control of the amount of light, but it works by decreasing the hole through which light passes, which (as explained in the technical answers) has the "side effect" effect of increasing the depth of the image which is in sharp focus. This has nothing to do with the amount of light per se.

The unstated assumption in the answers is that shutter time is increased to make the exposure correct — assume that there's enough light for the image in any case. If the exposure weren't made to be correct, the underexposed result wouldn't be less sharp, but if you've underexposed to the point where the scene is unrecognizable, sharpness is kinda irrelevant. Alternately, if you compensate for the reduced light by boosting ISO sensitivity, the result will inevitably have more noise (because less light is less signal), and while that doesn't degrade sharpness in the same way technically, noise interferes with the perception of sharpness (and noise removal techniques in post processing do have a smoothing/blurring effect).

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.