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.