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I recently bought a second-hand Carl Zeiss Jena PANCOLAR MC 1.8/50mm Lens off eBay (still waiting for it to arrive though) and am going to mount it to my Canon 7D via a M42 adaptor. I primarily shoot video and am not a fan of high ISO so I thought the 1.8 would be a good way to go.

My mate has a similar lens, but in the 2.8, and he reckons that the images won't be as sharp because of the larger aperture.

Can anyone explain why this might be? And how noticeable it may be?

Cheers.

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See also photo.stackexchange.com/questions/24/… –  mattdm May 2 '11 at 12:06

4 Answers 4

up vote 4 down vote accepted

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.

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Great answer - thank you. I hadn't thought about the resolution of video, definitely a valid point. –  Chard May 3 '11 at 0:46
1  
cheers. In actual fact sharpness can be a big problem with video DSLRs as they subsample the sensor in video mode which gives rise to aliasing (weird artefacts when viewing lines/grids/manmade textures). In fact some people use anti-aliasing (blurring) filters over the lens to avoid this. –  Matt Grum May 3 '11 at 11:20

Well, sharpness varies from one lens to another. A 50mm f/1.8 might not be as sharp as an f/2.8 lens if sharpness in considered at the widest possible aperture, but, if you step down the f/1.8 lens to f/2.8 you got yourself a contender! For example, Canon 50mm f/1.8 is almost equal in sharpness if compared to 50mm f/1.4, but you have bigger aperture in the 2nd lens and therefore better low light performance. Lenses with slightly smaller aperture always give you better sharpness but sharpness might not always be the thing you'll need, faster lenses generally have shallower DOF and better bokeh which should be considered as well. Remember, you can always step down a f/1.8 lens to f/2.8 if sharpness is the only thing needed for a particular photo but you can not step up a f/2.8 to f/1.8 if need be! Again, like almost every other photography gears, this selection is very subjective and depends on your own choice.

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Fast lenses typically perform best (or near their best) when stopped down about 2 stops from the maximum aperture. For example, see the resolution chart for the Canon 85mm f/1.2. Note that while all lenses are different and I haven't been able to find resolution charts for the Zeiss 50mm f/1.8, but the characteristic of producing near optimal sharpness 2 stops from the maximum aperture holds for most lenses.

Your 50mm f/1.8 is likely to be as sharp or sharper than the f/2.8 lens at the same aperture.

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Thanks for the link and the info, mate. Very interesting. –  Chard May 3 '11 at 0:45

My mate has a similar lens, but in the 2.8, and he reckons that the images won't be as sharp because of the larger aperture.

These factors are independent of the maximum aperture. A lens with a maximum aperture of f/1.2 might provide considerably better sharpness then a lens with a maximum aperture of f/4. The reverse could be true as well. It depends on the lens elements quality, among other build factors.

For the most part, a lens with a wide aperture such as f/2.8 is considered a higher quality more professional lens then something in the f/4-5.6 range, but since you are comparing f/1.8 to f/2.8 it is difficult for a blanket recommendation.

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