The effect of aperture on image quality

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Tom Axford Veteran Member • Posts: 9,139
The effect of aperture on image quality

Even if we ignore the effects on exposure and on depth of field, the aperture still affects image quality even when everything is perfectly in focus and properly exposed.

Every so often I like to run some tests on a particular camera and lens.  To simplify things, I often consider only the central part of the image (all sorts of nasty things can happen towards the corners) and I like to take black text on a white background as the target. I recently did such a test with a Panasonic Leica 25mm f/1.4 lens on an E-M1.3 and here are the results (all shown as 100% crops from the centre of the image, processed from raw in Lightroom with all sharpening turned off).









The main conclusions I draw from these tests:

1. Lens aberrations soften the image fully open at f/1.4, but very much less so at f/2.

2. The best image quality is around f/4  to f/5.6 and then diffraction softening gradually increases with increasing f-number, being very noticeable by f/16.

3. The lack of an AA filter on this camera is most obvious in the f/4 and f/5.6 images, in which false colour aliasing artefacts appear in several of places.  Diffraction softening acts as an effective AA filter at the smaller apertures, while lens aberrations act similarly when the lens is wide open.

I fairly strongly object to these false colour artefacts.  For this reason, I quite like f/8 as a good compromise: it retains good image quality but has enough diffraction softening to act as a pretty good AA filter (although a little aliasing is still present).

Of course, you may get different results.  This is one particular camera, one lens and a particular target (the choice of target can affect the results considerably).

Comments welcome, particularly from those who have done similar tests with their own kit.

Olympus E-M1
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