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Example:
The prime lens Nikon 1.4F G is said to be very sharp at F3.5.
So, if I compare that with the normal or telephoto zoom lens's F3.5, will the level of sharpness be same?

If no, then will the sharpness of the prime lens be better as compared to its counterpart at the max aperture (without diffraction)?

Assumptions:
- The sensor size, and brand for both lenses are same.
- The normal/telephoto len's aperture starts from F3.5 to F16.
- The prime len's aperture starts from F1.4 to F16.
- The focal length used for shooting is 50 mm for both.

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2  
This seems like it's going to be somewhat specific to the two lenses involved. People generally seem to suggest overall better optical performance for prime lenses, however I wouldn't expect that this would mean for ANY TWO zoom/prime comparison the prime would always come out sharpest... –  forsvarir Jan 18 '12 at 12:05
    
@forsvarir What is "optical performance"? –  TheIndependentAquarius Jan 18 '12 at 12:09
    
you can't really compare sharpness at different focal lengths, Nikon don't make any f/1.4 telephotos! –  Matt Grum Jan 18 '12 at 12:11
1  
@MattGrum Sorry, assuming that the focal length for both is 50 mm. :doh: –  TheIndependentAquarius Jan 18 '12 at 12:20
1  
I think there's a basic, simple fact of optimization at work here: if, for a given budget, one could design a zoom that was "optically better" (in some well-defined sense) at a focal length equal to that of a competing prime, then the best solution for the prime would be to manufacture the zoom and freeze it at that focal length! This demonstrates that when you hold all relevant factors the same and you keep your design objectives fixed, the prime cannot be any worse than the zoom. –  whuber Jan 19 '12 at 22:55
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3 Answers

up vote 12 down vote accepted

There is no answer to your general question.

Prime lenses are usually sharper than zooms at the same focal-length and aperture, mostly at wider apertures when the sensor out-resolves the lens. At one point lenses can out-resolve the sensor and then you will see equal sharpness in your images despite a potential difference in lens sharpness.

If someone were to built a prime and a zoom today with the best technologies, then the prime will be sharper since there are less variables in its design. However, once you compare lenses introduced at different time and even different quality levels, you will have to compare case-by-case.

Now, manufacturers choose to use different qualities of materials, different designs and tolerances today which sets the price-point of lenses. That is why you see standard zoom lenses which are very soft at their widest apertures. Other premium zooms can be extremely sharp from wide-open.

share|improve this answer
    
At one point lenses can out-resolve the sensor What does that mean? –  TheIndependentAquarius Jan 18 '12 at 14:46
    
It means the lens can let in more details than the sensor can capture. This is usually the case with quality lenses which is why you do not throw them away when you upgrade your camera. Although as cameras resolutions go higher, more and more lenses get out-resolved by the sensor. –  Itai Jan 18 '12 at 14:49
    
@AnishaKaul: You should join the PhotoSE chat room sometime SOON. You have LOTS of questions here, and I think we could help you much better with a real-time chat than here on the forum. I'm in the chat room most of the time, although not always watching. If you join and stick around, I'll keep an eye on chat and hopefully we could have a real-time conversation and clear up a lot of the questions you have here. –  jrista Jan 19 '12 at 20:48
    
@AnishaKaul: Go here for chat. –  jrista Jan 19 '12 at 21:25
1  
@AnishaKaul: You can certainly ask individual questions here, and I would recommend you do. It just seemed like the answers on this particular question were just creating a deluge of new questions for you, and I thought I could clear some of them up, so you could better understand the answers here. Chat may not be visible on google, however it is fully archived, and all content there is available for perusing. –  jrista Jan 20 '12 at 1:22
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In general a prime lens is easier to build and need less glass due to the single fixed focal length.

Add to that the fact that most prime lenses have a wider aperture than their zoom equivalents, which requires a better lens design and better glass (including coatings) to allow full use of the benefit of a wider aperture without horrible distortion or colour fringing, stopping a prime lens down can only make the result better as pretty much any lens gets sharper when you reduce the aperture (up to a point, depending on the lens).

As a result, a prime lens will, provided the lenses are from roughly the same area, always give a higher quality image.

(A significantly older prime lens can be worse because of the advances in optics since. A zoom lens also needs to compromise on distortion, especially when it covers for example 24-70mm, wide angle to the beginning of telephoto.)

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This means that at F11, prime len's sharpness will be "better" than a normal/telephoto zoom lens? Is the difference noticeably? –  TheIndependentAquarius Jan 18 '12 at 12:14
    
Perhaps you should touch on the relationship between prime and normal lenses, looks like there's some confusion on that front. –  Imre Jan 18 '12 at 12:17
1  
@DetlevCM It's not necessarily true that the wider the max aperture the sharper the lens will be stopped down, in fact the wider the aperture the more corrective lens elements are required which are at best dead weight when stopped down and at worst extra sources of dispersion. I'd bet the Canon 50 f/1.8 is sharper at f/4.0 than the Canon 50 f/1.0! –  Matt Grum Jan 18 '12 at 12:44
1  
@DetlevCM ahh but if you look at the dpreview test the 50 f/1.8 is sharper than the f/1.4 from . You could spend all day finding examples and counterexamples on various review sites, my point was that the issue is not clear cut, you can't just say x is sharper due to the wider aperture. And if you are correcting for things like spherical aberration then blocking the light causing the aberration with the aperture stop will cause you to lose the benefit. –  Matt Grum Jan 18 '12 at 13:55
2  
@DetlevCM Here you go, although AFAIK no Encyclopaedias are acceptable as references among academics. Here's a UK photog explaining normal lens, never mentioning standard. I'm in Europe, but "standard" would have more likely made me think of a kit lens rather than a medium prime. Now I know better, thanks ;) So perhaps it's a community thing (i.e. who you've been talking to), not a continental difference. –  Imre Jan 19 '12 at 7:49
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If we were to graph out the "sharpness" or various lenses, my guess is that we'd come up with something like this:

enter image description here

Keep in mind that this is hand-drawn based on my beliefs, not an actual graph of results from tests, or anything like that. Since it is hand-drawn, don't try to read too much into things like whether it might not be more accurate to show more overlap between the two, the spot with a negative slope on the left-hand part of the "Primes" curve, etc.

In any case, let me point to a couple of points this is intended to show.

  1. On average primes have higher resolution than zooms, but
  2. There's a lot of overlap between the two
  3. Resolution of zooms tends to vary a lot more widely
  4. Conversely, primes tend to be much more consistent
  5. A lot of zooms have much lower resolution than almost any prime, but...
  6. the best zooms are better than all but a few of the very best primes

As you reduce the aperture (smaller opening/bigger number), differences between lenses tend to get smaller. By f/8, even poor lenses usually produce decent results. By f/11 they're all starting to get worse, and by f/22 they're virtually all getting pretty poor.

share|improve this answer
    
Sorry, your last paragraph left me a little confused (though I love your graph!). Are you saying that the image quality from the lens actually declines past f/11, or are you referring to the image quality due to diffraction on the sensor? Or are you trying to say that there's no difference between cheap and expensive lenses at those apertures? –  drewbenn Jan 19 '12 at 20:12
1  
@drewbenn: The more you stop down the aperture, the less maximum resolution a lens can achieve. Its a simple matter of physics. A "perfect" lens, one which has no optical aberrations, would always produce the highest resolution wide open, and resolution would always decrease the more you stop down. The only reason we sometimes see BETTER resolution when stopping down a lens a little bit is because there are often optical aberrations wide open that impact IQ MORE than diffraction. When optical aberrations are not an issue, diffraction is the only quality detractor. –  jrista Jan 19 '12 at 20:55
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