Ugly Reality

I do try to avoid deep-ending on technical details, but the Zeiss Otus thing is sufficiently irritating that I'm going to do some math. Well, arithmetic.

We see, altogether too often, blather about lenses capable of delivering the necessary resolution to satisfy the demands of the newest digital cameras. Let's do some arithmetic, and see what these demands actually are. Take a 50 megapixel full frame camera. Bayer demosaicing throws away something like half of the resolution to give you color, so you're looking at 25 megapixels of resolving power. Doing some arithmetic, we come up with about 4100 pixels on the short side, which works out to 164 pixels per millimeter, or 82 line-pairs per millimeter. 82 lp/mm.

To put that in perspective, the reference film used for amateur lens testing, back in the day, was Technical Pan, which is capable of delivering 320 lp/mm of resolution. Yep. 4x the linear resolution. 16x the 2D resoluion. Now that's a challenging environment. Your 50 megapixel camera? It is to laugh. The much vaunted Reference Camera, the Nikon D810, clocks in a little under 70 lp/mm.

Having done some amateur lens testing in the 1990s, I can attest that actually getting 82 lp/mm to the sensor plane is hard. In reality, most people can't or won't accomplish it. The 50 megapixel wundercamera, while not in any meaningful way a challenging lens test case, has all the resolution normal people can actually use.

In the 1990s, any decent lens was capable of pushing 82 lp/mm onto the film plane. It is only in these modern times where, apparently, lenses have gotten so bad that they struggle.

So what the hell are all these people talking about? There are sensible people talking about how good the Otus lens line is.

Well, there IS something there. Consider some test charts. Suppose you have some black and white lines spaced very close together, so as to give you that 82 lp/mm resolution at the sensor. A very good lens will render them as crisp black lines alternating with white lines. A lesser lens will give you dark grey and light grey lines. The worse the lens, the lower the contrast at the sensor plane, until finally the contrast drops away to unusable levels. In the land of 82 lp/mm, both a very good and a quite moderate lens will deliver plenty of detail, but the better lenses will deliver higher contrast fine detail than the lesser one.

This mattered in the days of film, since recovering that contrast was technically demanding. Now, of course, it's the "Sharpen" slider. That's literally what that slider does. It doesn't make anything actually sharper, it puts the contrast back into the fine detail.

The better lens will also exhibit fewer aberrations, lateral blah and chromatic whatsit. Again, these mattered in the days of film, and are largely correctable in post in this digital age.

Bugger. Well, what on earth can this Otus thing actually do that is not doable with some minor digital corrections with a $200 nifty fifty?

Well. I guess there's fine detail that starts out low contrast? This can get lost, regardless of the sensor/film, when a cheap lens loses the thread of the very weak signal it starts with. This is and always has been the hardest thing to manage in the resolution part of the world, and even in the unrarified air of 82 lp/mm it can be a problem. Frankly, though, that's just a theory, and I suspect you'd be hard-pressed to actually find a situation in which the Otus preserved visible detail that a midrange equivalent lens lost.

Worth noting is that this kind of fine detail is by definition extremely hard to see in the first place, and hence arguably not very important visually, and it's quite difficult to preserve all the way to the print. So, you are (theoretically) saving something that nobody can see and which you will likely lose somewhere later in the process. Handy, that.

Secondly, correcting things in post will tend to add noise and artifacts. I dare say that fine detail from the Otus will look a trifle cleaner than fine detail from a midrange lens together with a touch of extra sharpening.

More importantly, it will look subtly different. So, when you're pixel peeping to see what your new $4000 lens is giving you, you'll be able to rationalize it.

There are other things like extreme field flatness, which is terribly helpful when you mainly take pictures of sheets of paper.