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Diffraction is a fundamental physical property that blurs images. It is caused by the bending of light waves near boundaries. The smaller the aperture (the larger the f-number) or the longer the wavelength, the worse the diffraction blur. Since diffraction is a fundamental physical effect, it is the same for all lenses. For most lenses, performance does not vary significantly at small apertures (large f-numbers), where diffraction is worse than aberrations. The f-number is equal to the lens focal length. f. divided by its aperture diameter, A (f# = f/A). In the classic f-stop (f-number) sequence
f# = { 1 1.4 2 2.8 4 5.6 8 11 16 22 32 45 64 ...}
each stop admits half the light of the previous stop, while the f-stop number is multiplied by the square root of 2 (1.414). When a photographer says, “I increased the exposure by one f-stop,” then the location on the sequence is decreased by one step; e.g., the aperture changes from f/8 to f/5.6.
For most lenses, aberrations tend to be worse at large apertures (small f-numbers), while diffraction gets worse at small apertures (large f-numbers). As a result, lenses tend to have an optimum aperture where they are sharpest, typically around 2 to 3 f-stops below a lens’ maximum aperture (often less for premium lenses). The optimum is fairly broad. |
You can find the optimum aperture by running a batch of images (Imatest SFRplus or eSFR ISO recommended) taken at different apertures, then entering the combined output (a CSV file) into Batchview (Figure 1).
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IAiry and MTF as functions of Q are plotted below.
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Defocus
Defocus (or misfocus or focus error) blurs the image — taking a bite out of MTF. Defocus is a large subject that we can only touch on.
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