How Far Apart Are Mediocre and Superb Lenses?

There are two ways to look at lens quality - what can you measure and test, and what can you see in prints. They 'ain't' the same thing at all.

For example, I made good use of my Sony A6000 on holiday, and the most used lens was the 55-210 - which according to all the usual test sites (photozone.de, slrgear.com) is mediocre. Yet, I have made some lovely images with this lens, and sharpness to me, in the print was perfectly fine. I even tried it on the A7r before selling that camera, and it wasn't bad, but tried removing the baffle to get even wider coverage on the big sensor camera and definitely the edges are poor, but it was never intended for full frame sensors so can hardly be blamed for that.

It's so small and light that it's a perfect match for the A6000, and one of the things that put me off the A7r in the end was that with my 70-200 Canon (that was noticeably sharper than the new Sony 70-200 even in prints) the camera ends up huge.

Issues of quality apply to all formats. Theonlinephotographer recently published an article on lens quality - how to use a lens to it's best, and how to stress it and it makes good reading.

An example of stressing a lens is stopping down well beyond the start of diffraction. Peruse any photo magazine that features landscape and you will find plenty of examples of f22 on a 35 mm. format wide angle - well within diffraction, and when viewed on screen at 100%, clearly showing that the benefit of increased depth of field did not outweigh the fuzziness introduced by the small aperture - but the magazine print, some of them two page spreads, look fantastic. Likewise 13x19 prints show the same thing - what is seen on screen isn't shown in print.

I started to notice this when I went to 20+ megapixels - what was clearly out of focus on screen looked fine in normal sized prints. It's all very well making test crop sections from really enlarged prints, but if you never actually make the uncropped huge print, what you are testing isn't reality.

This is a common phenomenon - just think of cars that handle beautifully at 150 mph, but fortunately are never driven at that speed, cars that cost a lot so they can do wheelies and burn rubber, but you don't because the tires are expensive, and besides, you grew out of that when you turned 21 (ok, maybe 40 for some).

Consider too, the subject matter. A lens that is sharp in the centre but has weak corners and even edges will be fine in portraits, sports, news, streetshooting, and just about anything other than distant landscapes and architecture.

We also need to consider that a superb lens on a small format will not look as good in a large print as a medium quality lens on a larger format, everything else being equal.

Lenses like the Zeiss Otis are mostly about being extremely sharp, corner to corner, wide open, and at this the lens has no peer. But how many of us need corner to corner sharpness at f1.4, or for that matter, the high contrast this lens has wide open. Often 1.4 is for people pictures and a bit of reduced contrast isn't a bad thing, and how often are the corners in the same millimeter deep area of focus anyway?

Bottom line: modest price primes and top quality zooms do a pretty decent job.