Why are many lenses sharpest a few stops down from wide open?
Asked 6/1/2015
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Why do many lenses resolve finer detail when stopped down a little from their maximum aperture? I understand that lenses often have a "sweet spot," and that stopping down too far eventually reduces sharpness because of diffraction. Besides diffraction, are there other ways a smaller aperture can reduce sharpness or overall image quality?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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When you "stop" down, you're physically blocking (or stopping) light from the edges of the lens from hitting the image. This light often focuses at a slightly different distance to light that passes right through the centre of the lens which causes a loss of sharpness known as spherical aberration. There are other aberrations which affect sharpness, these too diminish when stopping down the lens.
Other than diffraction there's nothing that affects resolution per se (apart from possibly noise) but in can affect image quality in general. Stopping down can affect Bokeh, points of light rendered as circles wide open can become hexagons, octogons, etc. depending on how many aperture blades you have. Dust on the sensor will much more apparent (due to the light passing straight through the centre of the lens casting a more defined shadow on the sensor), and finally you will have less light which can result in increased noise in the image if you don't compensate with a slower shutter speed (which can in turn result in blur from camera/subject motion).
Stopping down can also create a starburst effect around light sources, although this is often seen as an improvement.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
11y ago
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Many lenses get sharper when stopped down a bit because the aperture blocks light from the outer parts of the lens, where optical imperfections are usually strongest. Rays passing through the edges can focus slightly differently from rays through the center, causing aberrations such as spherical aberration and other softness-reducing flaws. Stopping down reduces those edge-ray contributions, so resolution often improves.
This is why many lenses have a "sweet spot" a few stops down from wide open: aberrations are reduced, but diffraction has not yet become dominant.
Past that point, making the aperture smaller increases diffraction, which spreads fine detail and lowers resolution.
Beyond diffraction, stopping down doesn’t usually reduce resolution in the same direct optical way, but it can affect overall image quality in other ways. For example:
- out-of-focus highlights may take on the shape of the aperture blades rather than smooth circles
- sensor dust becomes more visible at small apertures
- if shutter speed gets slower because of less light, motion blur or higher ISO noise can become issues
So the tradeoff is mainly: wide open = more aberrations, very small apertures = more diffraction, and the middle is often sharpest.
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AI11y ago
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