A few days ago, I wrote this post, which attempted to make the case that slower, lighter lenses were — by and large — a better match for modern full frame mirrorless cameras than the traditional faster ones.
I got a lot of pushback. I dealt with the equivalence argument here. Another line of attack went something like this:
People don’t buy f/1.4 lenses to use them at f/1.4. Lenses perform the best when stopped down two or three stops from their maximum aperture. Therefore, if you want a sharp image at f/4, you need to buy a f/1.4 or f/2 lens, and if you want a sharp image at f/5.6, you need to buy a f/2 or f/2.8 lens.
That didn’t sit right with me. While I recognize that most lenses are not at their best wide open, I had always thought that that was a conscious decision on the part of the lens designer. I would have thought that, if performance at f/4 were the most important thing, telling the lens designer that the widest aperture of the lens only needed to be f/4 would allow a lighter, smaller, and probably less expensive lens than if the maximum aperture were, say, f/1.4.
As a thought experiment that indicates that designing a f/4 lens to be its best at f/4 at least wouldn’t cost any more, be any bigger, or weigh any more than designing an f/1.4 lens that is its best at f/4, consider that taking the second lens and modifying it in such a way that the iris wouldn’t open any wider than f/4 would meet the design criterion.
In the real world, is there a breakeven in f/4 (and stopped down from there) performance that comes from saying that the lens doesn’t need to go any wider than f/4? is there a win in size and weight that comes from saying that the lens doesn’t need to go any wider than f/4, considering that if can’t lose performance from that of the faster lens?
I dunno. I’m not a lens designer. I don’t even play one on the Internet. But I’m going to ask around on dpr — where there is at least one actual lens designer — and I’ll report back what I find.