Normal Zoom vs. Normal Prime Lens, Part I

Are kit zooms ready for prime time?

50mm prime lenses rock…but how do they compare to those inexpensive normal “kit” zoom lenses? Here’s a case study.

One of the questions that came up in the wake of my article comparing 50mm f/1.8 to 50mm f/1.4 lenses was: How do Zoom lenses at 50mm settings and equivalent apertures compare? Fortunately, the answers can be found on DxOMark’s extensive web site.

Note: All lab test results courtesy, used by permission.

The executive summary

I specifically focused my attention on kit lenses, which are the least expensive zooms. The test results can be easily summed up: You get what you pay for. Low-end zooms, which can be bought for under $200, did relatively poorly at 50mm against the 50mm prime. (Pro-level zooms, which cost well over $1,000, did noticeably better than kit zooms.) But are they major differences? At 50mm, the zoom lens results I checked showed optimal performance and so the differences at the middle apertures were fairly minor. Test results at the smallest and widest apertures showed a more significant difference.

The details

As a case study, I tested the Nikon 50mm lens against a typical kit zoom lens. Just for fun, I also compared it against a pro-level zoom. I compared test results shot with an APS DSLR, the D5000.

APS Sensor lens tests

I compared the Nikon 50mm f/1.8D lens it with the pro-end 24-70mm f/2.8G and a typical Nikon kit lens, the Nikon AF-S DX 18-55mm f/3.5-5.6G ED II VR lens.  All lenses were tested on the Nikon D5000.

In the “no surprise” department, the 18-55mm f/3.5-5.6 had the worst performance overall. Here are the overall test results; interpretations follow:


Note: I compared results for all lenses at f/8, since that was the only middle aperture they all had in common. (The 18-55mm’s widest aperture at 55mm, for instance, was f/5.6 so we couldn’t compare that as a middle aperture. I also compared the 18-55mm lens at 55mm instead of 50mm, since that was the test data provided.)


Center resolution tests results:


Corner resolution test results:

Resolution: 50mm f/1.8D and 24-70mm showed virtually identical characteristics in both center and corner, while the 18-55mm lens had the lowest lp/mm. At the widest apertures, the 24-70mm lens had the best overall resolution, with both the 50mm and 18-55mm lens turning in similar image quality at both the corners and center.


Vignetting: All three lenses had nearly identical vignetting results, with the worst vignetting coming at the widest apertures. At f/8, the 50mm lens had slight vignetting whereas neither zoom lens showed any visually identifiable vignetting at f/8 and 50mm.


Distortion: Optical distortion for both zoom lenses at 50mm was negligible and were a near match to that of the 50mm.


Chromatic Aberration: This shows the degree of red or blue fringing you might see on an image detail where there’s an abrupt contrast change. While the 50mm and 18-55mm showed similar lateral chromatic aberration characteristics and were fairly well controlled, I was surprised that the 24-70mm lens showed the most pronounced results and the most noticeable fringing at f/8, as you can see in the field maps here.

What about Canon, Pentax, and Sony?

You can check specifics for your brand lenses here, but in general, the performance of 50mm prime lenses outshone that of kit lenses at equivalent focal lengths, while pro lenses typically gave primes a run for their money.

Beyond lab test results

Of course, there are many other reasons to buy a zoom over a prime, even an el cheapo one. Besides pixel peeping, other considerations are important. With kit zooms, light weight, low price and the flexibility of a zoom range are all tempting, and if your goal is a decent-looking 11x14-inch print, that might be enough as long as you’re willing to sacrifice wide apertures. Pro zooms give you prime-lens quality at most focal lengths, the benefit of wider apertures, and the flexibility of zooming, but they cost a lot of money and are much heavier and larger than primes. Primes, of course, are small and provide the widest apertures at the lowest prices.

What about comparing 50mm prime against a couple of zooms on a full-frame DSLR? Look for that in a future article!

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