In one of our previous articles we talked about vignetting, chromatic aberrations, and diffraction as well as about how to avoid these three optical errors that are so undesirable but omnipresent almost in all lenses.
These three effects have to do with aperture. And the same way, depending on the lens maximum aperture, it has a sweet spot where it’s at its sharpest, allowing you to minimize vignetting, chromatic aberrations, and diffraction. Continue reading and learn how to find it for your lens.
As we remember, in order to minimize vignetting and chromatic aberrations, you will need to stop the aperture of your lens down about 2 or 3 stops. This way vignetting will go because the aperture hole will be smaller and the lateral sunrays won’t be blocked by the lens barrel.
Chromatic aberrations are avoided by stopping down too because by doing so you make the depth of field wider and put everything in focus, getting rid of annoying purple fringes in high-contrast areas.
However, diffraction is different. If you stop down the aperture too much, the light won’t be able to enter the lens that easily and the rays will be interfering with one another, which causes a loss in sharpness. Just imagine that you squinting your eyes when looking at something that’s far from you. Squinting your eyes just a bit really helps, but if you squint them too much, you won’t be able to see a thing.
That’s why every lens has a diffraction limit in aperture that you should not reach in order to get sharp shots. And the balance in aperture between the three—vignetting, chromatic aberrations, and diffraction—is called the lens’ sweet spot.
There’s a rule of thumb that works almost always when you want to find your lens’ sweet spot. For lenses with the maximum aperture of f/1.8, it will be more or less at f/4. Lenses with the maximum aperture of f/3.5 will have their sweet spot at f/6 to f/8.
But apart from this rule, there are more concrete ways to find the exact aperture sweet spot for your lens.
Firstly, we advise you to check out diffraction limit calculator on this page. Secondly, you can place your camera on a tripod in aperture priority mode, set the smallest ISO possible, tweak the light, and shoot a page in the book with different apertures. Try starting with your lens’ maximum aperture plus 2 stops and then increase the aperture by another 2 or 3 stops for every shot. For an f/3.5 lens this will look like that: f/5.6, f/8, f/11, f/16, f/22.
The diffraction limit of a quality professional lens will be at about f/14, whilst entry-level lenses may be diffraction limited already at about f/8 to f/11. Keep learning, practicing, and good luck!
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