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I own a Sony NEX-5R, with the Sony 35mm F1.8 prime and the Sigma 19mm F2.8 prime. I'm deciding whether to buy the Sony 16-50 power zoom. To make that decision, I need to decide whether cropping the pixels from the prime lens will result in a better or worse photo than zooming in with the zoom lens.

From DXOMark, the lenses are rated as having 7 and 11 perceptual megapixels of sharpness respectively. This is a factor of 1.57 (11/7). Taking the square root of 1.57 to convert the area of a rectangle (which corresponds to the megapixel count) to the diagonal of the rectangle (which corresponds to focal length) gives us a factor of 1.252 difference in sharpness. Multiplying this by 35mm gives 43mm.

The conclusion seems to be that the prime lens works better than the zoom lens up to a focal length of 43mm. Given that the lens goes only up to 50mm, it doesn't seem worth buying a zoom lens for only 7mm extra zoom range.

Is this analysis and conclusion correct?

Note that I'm new to interchangeable lens cameras, so I wouldn't value the minute differences that professionals or serious hobbyists pay hundreds of dollars for. For example, I don't care about distortion that can be corrected in-camera or in Lightroom. Things like edge softness also seem like factors minor enough to not influence the decision.

Both lenses are optically stabilized. The 35mm has a much wider aperture of F1.8, compared to F3.5 for the zoom.

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    It's really, really hard to crop a 16mm frame out of a 35mm frame, and only a little less difficult to crop it out of a 19mm frame ;-) – user2719 Jan 2 '14 at 4:22
  • Sure, but I don't want to buy a lens just for an additional 3mm focal range at the wide end. – Vaddadi Kartick Jan 2 '14 at 5:04
  • For anyone who's going to DXOMark checking the effective megapixels of sharpness of their favourite lens.. it depends heavily on the camera model selected. It adds little in this specific case (as it was tested only on NEX-7 which is the best of the crop anyway as far as I know) but future visitors be ware. – Marco Mp Jan 2 '14 at 6:34
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    @KartickVaddadi Personally, I would be much more likely to buy a lens for additional 3mm of focal lengths at the wide end than for some small gain of sharpness to be true. The zoom also offers flexibility - in case you need those 19mm and have no time to change lenses on the fly. – Marco Mp Jan 2 '14 at 6:56
  • I'm okay with buying an ultra-wide-angle lens, but I'd want it to go as wide as 12mm or so. I wouldn't spend $350 for 3mm. – Vaddadi Kartick Jan 4 '14 at 7:18
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In terms of effective resolution, it sounds like you are correct. Actually, working even more against the 16-50, most lenses tend to be sharper in the center than on the outside, so there is a good chance that the effective resolution of the center of the prime is significantly higher than the entire field of view of the prime.

There may be other issues, but it wouldn't surprise me if they overall looked better since primes are much easier to produce a quality lens than a telephoto is and 7 effective megapixels is really nothing to write home about. The best bet is still probably going to be to look at samples though. Things like chromatic aberration can only be corrected so much, though generally I'd expect the prime to have fewer issues there too, even given the crop.

  • Re: "11 really isn't either".. browsing that website for various lenses it's difficult to find much sharper lenses when used on a comparable sensor (IIRC the NEX7 is 24MPix). So far the best I found (for a 24MPix sensor) is Nikon 85mm/1.8 at 15 MP effective sharpness. Canon 85mm/1.2 scores 12MP but on a 20MPix sensor (70D) so not directly comparable, but still (I don't know Canon lineup well enough to drive a better comparison). So, to close, I would tend to consider 11 as not a bad score at all. – Marco Mp Jan 2 '14 at 6:50
  • @MarcoMp - Hmm, ok, after doing a little more research in to the P-Mpix that DXoMark uses, I don't particularly like the number as it doesn't seem to match with reality. It may be a decent comparative measure, but I don't buy the actual relationship to megapixels based on looking at some of the results. 11 and even 7 don't seem to be too bad after looking at what some of the other lenses are. – AJ Henderson Jan 2 '14 at 14:28
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    I totally agree, the name of the measure DxO uses is misleading and I was of the same your opinion before "hey, let me check how my lenses score". – Marco Mp Jan 2 '14 at 17:47
  • @MarcoMp - yeah, what did it for me was that they say the 24-70f/2.8 II is only 18 effective megapixels. I have a very hard time believing that, though perhaps if they are averaging across the entire range of aperture values, it might pull it down enough. – AJ Henderson Jan 2 '14 at 17:55
  • Click 'Measurements-->Sharpness-->Profiles' to see the actual measurements from center (left on chart) to edge (right on chart). Change the apertures to see the results at different Av. Change the focal length (on zoom lenses) to see the results at different focal lengths. The actual test data is very useful, but I rarely even look at the overall scores because they weight them a lot differently from what is often important to me. – Michael C Jan 3 '14 at 6:09

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