Krdytkyu

I understand the primary benefit of a prime lens is the fast speed that you can get from wider aperture sizes. But when stopped down (to get a deeper depth-of-view), does it matter which lens you use?

For instance, compare a 50/1.8 with the 18-55/3.5-5.6 kit lens when both are set to f/5.6.

My experience (Sigma 17-70mm (which I found at least as good as my kit lens (a 18-55mm IS) vs Canon 50mm f/1.8, on an EOS 450D) is that you can still see the difference. Crop of a test shot from my window, back in 2011:

An f/1.8 lens also allows the camera body to use its more accurate AF sensor (but I don't think it mattered much here).

It obviously depends on the lenses being compared. Looking at this comparison of the EF-S 18-135mm f/3.5-5.6 IS STM and the EF 50mm f/1.8 STM, both at f/5.6 for example, the prime is sharper, even in the center of the image.

In this comparison with the EF-S 35mm f/2.8 Macro IS STM, both at f/8.0, I see more purple fringing, less sharpness and more distortion in the image from the kit lens.

No amount of stopping down will get the weight of the kit lens anywhere near of those of the two primes.

It depends on your requirements, the specific lenses, the camera, and the metrics you use. The only way to know is to test your lenses.

• As you note, light gathering ability pretty much depends on the max F-stop, for which primes have the advantage. But when wide open, lenses tend to be softer and have more aberrations, so you might end up stopping down a lens anyway. A prime that performs unacceptably wide open has no "advantage".




• All lenses tend to be softer wide open than when stopped down slightly. Some lenses "glow" when wide open in bright light. If you are stopping down a prime to the kit lens' max aperture, the prime is expected to have the advantage because the kit is operating at one of its weakest aperture settings (the other being fully stopped down).




• A prime at the same "level" as the kit would normally be expected to hold the advantage at the same focal length. However, some primes are relatively soft at all apertures. For instance, I have a 50/1.4 that is softer than a 50-230/4.5-6.7 when both are set to F8. In normal photographs, the 50/1.4 offers greater creative control over aperture, but when sharpness is of great concern (or I need greater focal lengths), I know to grab the zoom.



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  If your "kit" lens is some 'L' equivalent glass (such as 24-105/4L), there's less room for improvement. The prime has to be that much better.

  
  
  

  The 50/1.8 is a sharp lens to begin with, and its sharpest setting is around F5.6, where it is sharper than the kit lenses (18-55/3.5-5.6, 18-135/3.5-5.6, 24-105/4L, 24-70/2.8L). The 24-70/2.8L is quite close though.

  
  



• If your prime is old, or particularly weak, a modern kit lens would likely have the advantage. Similarly, a newer prime against an old kit lens would be expected to hold the advantage. But there have been cases where a newer revision of a lens performs worse than the older model.




• If you are comparing a "good" copy of one lens with a "bad" copy of another, the "good" copy has the advantage, regardless of whether it is a prime or kit lens.




• Full-frame lenses are at a disadvantage on crop-sensor cameras because the full imaging circle is not being used. Any lens weaknesses are magnified by the crop factor.




• A prime cannot beat a kit lens in terms of focal-length versatility and convenience. (Unless the kit is so bad as to be utterly useless at all focal lengths.)




• The difference in sharpness between a prime and kit lens may be negligible. For instance, unless you spend all your time photographing brick walls, distortion and corner sharpness aren't important beyond a certain point.




• The camera also matters. If the kit lens is able to resolve beyond the sensor's capabilities, there will be no improvement with a prime, no matter how much sharper it may be.