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At the same focal length, apart from speed and maybe weight, what other advantages does a prime lens like the 50mm have over a zoom lens like the 24 - 70mm?

To put it better, If I set my 24-70mm lens at 50mm and I don’t care about length and speed, what advantage would just using the 50mm offer me over the 24-70mm

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possible duplicate of Would a prime be redundant with a fast zoom? –  mattdm Mar 6 at 16:10
    
@mattdm - the answers to the proposed duplicate are all specifically not what this question is asking about. That question focuses on speed, this focuses on non-speed factors. –  AJ Henderson Mar 6 at 16:13
    
@AJHenderson Fair enough. –  mattdm Mar 6 at 16:14
    
Also see Would a fixed or zoom telephoto lens be better for learning?, which isn't a duplicate at all, but goes into some of the differences in practice. –  mattdm Mar 6 at 16:14

4 Answers 4

up vote 15 down vote accepted

A prime likely still has several advantages over a zoom at a given focal length. (Well, depending upon your needs and habits.)

  • A less complex optical design. All else being equal, a less complex design is likely to have fewer compromises, which means the prime is more likely to have less distortion of any sort -- pincushion, barrel, coma, and chromatic abberations. And on the flip side, a sharper lens.

  • Maximum aperture: an f1.4 prime is two stops faster than a "pro" f2.8 lens. 3 stops faster than a consumer-aimed f4. That lets you use a shallower depth of field and see in dim light much more easily.

  • Focus speed: a less complex design almost definitely means less weight to move to bring the lens in focus and a larger aperture lets in more light to make focus more sure-footed, so the focus speed can be faster.

  • I wasn't sure what you meant by "speed." Focus speed or aperture speed? I covered those. Here's one more: composition speed. With a prime I don't need to twist the zoom ring to get to 50mm -- I'm always there!

  • When comparing to a consumer lens, the prime also typically has a focus scale and a much smoother focus ring (with a longer throw, too).

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4  
I'd argue that composition is slower because rather than turning a dial you have to walk to get the shot you want. In some cases it also results in a better photo, but it isn't faster. Still a very solid answer overall though. –  AJ Henderson Mar 6 at 17:47
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The thing that made me realize composition with a prime was faster was when my son was a baby and I had to hold him. Kid in one arm, camera in the other... and no free hand to easily turn the zoom ring. Prime wins! :) –  Dan Wolfgang Mar 6 at 19:47
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Nah, in that context you just have an overpriced, under-quality prime even if it is a zoom. ;) –  AJ Henderson Mar 6 at 20:07
    
Thanks @AJ-Henderson –  yinka Mar 6 at 20:52
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I love them, they also help you avoid laziness. Instead of taking the easy way out and relying on the zoom, you have to get right in the center of the action. –  Robert Lowdon Mar 6 at 21:39

Prime lenses use simpler optics with fewer glass elements. They don't need the ability to change their focal length, so fewer compromises have to be made with the design. This allows for cheaper and higher quality image reproduction. Less chromatic aberration, more sharpness, better color reproduction, etc are all made much easier because of the fewer design constraints.

That said, a very high end telephoto will still beat a low end prime, but my high end ($2400) 24-70 f/2.8 L II lens from Canon is still beat slightly by the image quality of the midrange ($350) 50mm 1.4. It is close, but the 1.4 has the advantage for most measures.

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Prime lenses almost always give noticeably better image quality. Zoom lenses generally are at their best in the middle of their ranges - only the very best zooms give consistent quality throughout the zoom range.

Prime lenses also generally have larger maximum apertures, even lower end primes. The largest aperture you are likely to find on a zoom lens is f2.8, and then only on expensive examples, whereas even the cheapest primes will go to f1.8.

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Regarding zoom lenses, Sigma actually has a 18-35 mm lens with f/1.8 capability in the entire (although small) zoom range. It's one of a kind though, you won't find any other brands with similar specs. –  Daniel R Mar 6 at 16:09
    
Some of the cheapest primes at around 50mm will go to f/1.8. as you move further away from the normal focal length range in either direction the maximum aperture and the cost of the lens goes up. The Canon EF 14mm f/2.8 L II will run you about $2,360 and is over a stop slower than f/1.8. The EF 20mm f/2.8 is only about $540, but is still over one stop slower than the $125 EF 50mm f/1.8 II. Likewise, the EF 100mm f/2 runs about $500. The 200mm f/2.8L II costs around $820. –  Michael Clark Mar 7 at 0:08

Everyone has covered image clarity, aperture, and F-Stop, but there's three more advantages as well:

Price

A basic prime lens (Canon 50mm 1.8) costs $99 and is a fantastic value. You can buy a cheap prime to experiment with to see if you like prime lenses, before you commit to getting higher-quality prime lenses. There are no zoom lenses cheap enough to throw away, you'll have to rent.

Price Again

Primes can fill out your kit until you commit to bigger budget. I love the Canon 24-70 f/2.8 L II lens, but you shouldn't buy that expensive a lens until you find you need it often. Instead, using a 35mm prime lens and a 50mm prime lens can cover your indoor, low-light shooting.

Size

Physically, prime lenses are short and light. You can add several to your kit without having to buy a bigger bag or get into better shape.

Bokeh

Prime lenses are more efficient with light than zoom lenses, so in the same amount of light, you can use a wider aperture (lower F-stop), and get a blurrier background. Or a faster shutter speed, and get less motion blur. Or a lower ASA, and get fewer image noise artifacts.

I love prime lenses! When you get a few primes, you should practice switching lenses smoothly. I recommend this article: http://strobist.blogspot.com/2006/06/pro-tip-how-to-quick-change-slr-lens.html

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I think the last two are actually covered in the question as already-known (your bokeh point relates to speed, and size and weight are related). But anyway, +1 and welcome to Stack Exchange! –  mattdm Mar 6 at 20:08
    
The EF 50mm f/1.8 II is a great value, but it is not particularly great when compared to other prime lenses strictly on the basis of optical performance. It is the optical better or equal of many zoom lenses priced considerably higher. –  Michael Clark Mar 7 at 0:13

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