f/4 vs f/2.8 & photographic equivalence

In Tuesday’s post, I made a pitch for more high-quality slowish lenses on full frame mirrorless cameras. I also posted most of my little soliloquy on dpr. Over there, a good time seemed to be had by all; more than a hundred responses in the first 20 hours.

One of those respondents took me to task for recommending f/4 lenses on full frame cameras instead of the “equivalent” shorter f/2.8 lenses on APS-C ones. Nicely played, I thought. But I’m unconvinced. Here’s why I don’t buy the argument.

The first is that I was not pushing for only slow lenses. There are plenty of great uses for fast ones, and having a few in your bag makes a lot of sense. It’s just that you can carry more lenses if the bulk of them are slowish, and the slower ones will be easier to handle on the camera. Note that I’m using “slow” in a relative way; in yesterday’s post I implicitly called a 500/4 lens slow; and it’s only slow when compared to a 400/2.8, not in any other way, considering its focal length.

Here’s a little aside not related to the equivalence thing. In some ways, making the lens a stop slower can completely change the way you use it. The Nikon 300/4 PF lens is a case in point. It, and to a lesser extent its non PF predecessor, can be used handheld. I can’t handhold a 300/2.8 lens for more than a few seconds.

The second is equivalence assumes that there isn’t enough light to fully expose to the right (ETTR) at base ISO. That’s certainly not an assumption that will be satisfied in all scenarios. I am able to use ETTR at base ISO a great deal of the time, and almost all the time if you think that the increased conversion gain ISOs of 640 on the a7RII and 2000 on the a7S qualify in some sense as base ISOs (I prefer to reserve the term “base ISO” for the ISO that yields the highest DR, but see the merit of giving ISO 640 on the a7RII and ISO 2000 on the a7S special prominence).

Example: I have an Otus 85/1.4 that I use mostly for landscapes. Most of the time I use it at f/4 or f/5.6 at base ISO. The fact that it’s an f/1.4 lens does me no good in that usage scenario. I’d love a 85/4 that was as good wide open as the Otus is at f/4.

The third is that equivalence assumes the full well capacity of a sensor with a given number of pixels independent of the physical size of the sensor. It’s not clear that that’s actually true in, say, the small medium format (33x44mm) through MFT (13.5x18mm) region.

Then we get to the more practical issues. The densest FF sensors have between 36 MP (Nikon, Sony) and 50 MP (Canon). The a7RII has 42 MP. However, the densest APS-C camera that I know of is 24 MP, and the lowest-pitch MFT one is 20 MP. You just can’t buy smaller sensors that are resolution-equivalent to the larger ones. As another aside, I note that the Sony 33x44mm CMOS sensor has about the same resolution as the FF Canon one, and the data is sparse.

Also, the APS-C lenses from Zeiss, Sony, Nikon, and Canon tend not to be equivalently designed when compared to the cost-no-object FF ones, so the choice is not there. It’s not just native lenses that exhibit this tendency. When looking for the best glass, shopping for adapted FF lenses is shopping at the big end of the store; shopping for adapted APS-C lenses, the selection is relatively thin.

Am I piling on here? Maybe, but I’ll go on.

When it comes to the features serious photographers want, it appears to me that the FF cameras are more likely to have them than the APS-C ones. I haven’t done a survey, and I could be wrong – and, if I am, I’m sure someone out there will point it out – but that’s my opinion.

So, no, I’m not moving to APS-C anytime soon, although that could certainly change in the future. And I’d still like to see some more top quality f/4 FE lenses from Sony.