Are 50mm f/1.8 lenses less sharp than 50mm f/2.8 lenses of similar design?
Asked 5/2/2011
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I bought a Carl Zeiss Jena Pancolar MC 50mm f/1.8 to use on a Canon 7D with an M42 adapter, mainly for video and low-light shooting. A friend has a similar 50mm f/2.8 version and says the faster f/1.8 lens will be less sharp because of the larger maximum aperture.
Why can a wider-aperture lens be softer wide open, and how noticeable is the difference? If both lenses are used at the same aperture, such as f/2.8, would the f/1.8 still be less sharp?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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Firstly for video work lens sharpness isn't that important anyway as a 1080p image is about 2 megapixels so there isn't the resolution to pick up the extra sharpness that may be available.
A larger physical aperture tends to produce softer images on account of dispersion due to the larger volume of glass the light passes through, and from aberrations in the lens which result from the larger spread of ray angles that are difficult to correct for when designing a lens.
However stopping down a large aperture lens to match a smaller aperture lens mitigates these problems so you can expect similar sharpness at the same f-stop. There is a theory that because lenses get sharper when stopped down a faster lens stopped down will always beat a slower lens wide open, but this doesn't account for the causes of softness and in practice the results are mixed, with some fast lenses worse stopped down that their slower (but simpler) counterparts.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
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A faster lens can be softer wide open because designing for a larger aperture makes aberrations and ray angles harder to control. In practice, many lenses improve when stopped down, often reaching near-best sharpness around 1–2 stops down.
So yes, an f/1.8 lens may be softer at f/1.8 than an f/2.8 lens is at f/2.8. But that does not mean the f/1.8 lens is inherently worse. If you stop the f/1.8 lens down to f/2.8, it will often be similar to, or even sharper than, the f/2.8 lens used wide open.
The real answer depends on the specific lenses, not just the f-number. Sharpness varies from model to model.
For video, the difference may be less noticeable than in stills because 1080p resolution is relatively low compared with modern photo resolution. The f/1.8 lens also gives you an option the f/2.8 lens cannot: more light and shallower depth of field when you need it.
In short: wider maximum aperture can mean softer performance wide open, but at the same shooting aperture the f/1.8 lens is not automatically less sharp.
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