The Problem with Near Equivalence

Denial is a long river in Africa. Apparently on the Internet it's just as long, too.

Lately I've been hearing a "partial equivalence" claim being repeated over and over by people who should know better. A couple of these posters on Internet fora have been repeating the same half-truths for years, maybe even for more than a decade.

What I'm talking about usually comes up when someone tries to write about equivalence. True equivalence would be taking the same photo in the same place creating the exact same results using two different systems, e.g. m4/3 and full frame (which is usually where I see these arguments coming up all the time). See my article on equivalence.

What happens in these partial equivalence claims is that the poster ignores one or more factors that would create a truly equivalent image. In fact, what sometimes happens is that they mistake the casual definition of exposure (aperture and shutter speed) with the actual definition (light filtered by aperture filtered by shutter speed). The fact that apertures are expressed as a ratio and is not defined as a capture circle (area) is just one of the things that often trips them up. 

What you end up with in these near equivalent statements is something like this: "my m4/3 with an f/1.4 lens is every bit as good as a full frame camera with an f/2.8 lens." Uh, not exactly an equivalence argument. The DOF probably changed. The aspect ratio changed. If the field of view wasn't perfectly controlled, perspective may have changed. When you confront the poster with these and other things, they'll just start arguing that "more DOF and the 4:3 format is preferred, anyway." Really? By everyone, all the time? Don't think so.

But the real issue here is that these posters are usually arguing about only one end of the photographic spectrum, and often using more expensive lenses in doing so (the Olympus PRO f/1.2 primes come up a lot, for example). 

However, where I find the equivalence issue starts to truly raise its ugly head is with the more consumer zooms and particularly at the long telephoto end, especially when you're not in Sunny 16 conditions and trying to photograph something that's moving (i.e., need a shorter shutter speed).

I've pointed this out before: the reason why I don't use m4/3 for wildlife and sports has to do with this. 

Let's consider f/4, because that's the comparison going around right now in various fora: Olympus m4/3 300mm f/4 versus Canikon full frame 600mm f/4. (I'll warn you the comparison tends to get worse when you start using zooms that are f/5.6 and f/6.3 at their maximum telephoto end.)

Reminder: the Sunny 16 rule is 1/ISO at f/16 for exposure settings of something lit by mid-day sunlight.

In Sunny 16 conditions, f/4 means that we're four stops away from f/16, which is good. If we're shooting at a base ISO of 200—the typical m4/3 base—that means we can set a shutter speed of 1/3200 (four stops faster than 1/200), which is fast enough to capture pretty much anything that moves. No problems in sunny conditions. We open up the aperture to max and dial down the shutter speed an equivalent amount to freeze the action.

Deep shade can be several stops away from Sunny 16, though. Edge of day conditions can add even more stops of difference. 

Let's say that light has dropped by four stops (4EV), a fairly common condition I encounter when I'm shooting wildlife. Light can go even lower than that in my wildlife shooting, and in sports, it can be as much as 8-12EV lower than Sunny 16 shooting under some conditions. But let's stick with four stops less light for the moment. To keep our shutter speed above 1/800—and 1/800 is already too low for many things that move, but we'll use it as our anchor point anyway—we're going to bump the shutter speed down two stops from 1/3200 to 1/800 and the ISO up two stops (we can't open the aperture because we're already at the maximum of f/4). We're now at ISO 800, which is as far as I really want to go with the Olympus sensors (though I know others who'll tolerate further). 

So I'm personally at my limits with that f/4 lens on an m4/3 body in a common situation (e.g. 1/800, f/4, ISO 800). Plus I really want a higher shutter speed much of the time. I don't want to go to ISO 1600 to get that if I can at all avoid it, so I certainly won't go above those settings I just mentioned unless desperate. My absolute ending point is usually 1/800, f/4, ISO 1600, or five stops below Sunny 16 (one stop in shutter speed, four in aperture if you're trying to follow along).

So what about a full frame camera? I'll take my full frame Nikon Z6 as an example here, whose base ISO is 100 (oh, oh, that's going to affect things, isn't it?). Conditions dropped by four stops of light from Sunny 16 again. I'm now at 1/800, f/4, ISO 400. Where's my typical stopping point now? I'm three stops away (1/800, f/4, ISO 3200). Technically, I find that I often go even a stop beyond that on the Z6 (e.g. 1/800, f/4, ISO 6400) without much worry, but my typical stopping point is 1/800, f/4, ISO 3200, which is six stops below Sunny 16 (two stops in shutter speed, four stops in aperture).

Don't get me wrong. For a lot of folk who are mostly shooting outdoors on sunny or partly cloudy days and who aren't trying to isolate subject from background, m4/3 is a fine choice, and yes, a 300mm f/4 on an m4/3 body is smaller and lighter than a 600mm f/4 on a full frame body. Nothing wrong with saying that that's your choice for conditions where there's enough light and that you'll accept the background being more in focus. 

What's wrong is trying to argue that "it's the same as full frame." It isn't. At one edge of the shooting spectrum with m4/3 you're getting more depth of field at equivalent exposure settings, at the other edge you're getting limited in shutter speed because you're trying to control noise. In between, everything is fine. 

Now that we're getting a number of f/1.2 (and faster) lenses for full frame cameras, we're moving into a realm where focus isolation ability has also increased for the big sensor shooters over the small sensor ones. Meanwhile, the larger sensors having gone to BSI, dual gain, and other technology is also giving those same full frame owners some additional leeway at the other end, where shutter speeds start to plummet as light goes down. 

To me, today's FSI m4/3 works across a modestly broad set of cases, but not well at the edge cases (one of the reasons why I don't think the E-M1X was a good use of development resources, as it's supposed to be a camera for edge case usage, but still has the same mid-case emphasis). 

Most people don't hit the edge cases, though, or can find other ways to mitigate them a bit (typically meaning they have to switch lenses; e.g. take off the 12-100mm f/4 and put on one of the f/1.8 primes). Or: they tolerate the edge case differences (more depth of field or more noise).

Some of us do hit edge cases often, though, and care about them. Portrait, event, sports, and wildlife shooters are much more likely to hit edge cases than landscape, architecture, street, and travel photographers, for example. 

Make sure you know which category you fit into. If you're doing more general and casual shooting and in categories where you might not be hitting the edge cases all the time, then m4/3 and other crop sensor cameras absolutely have a place. The more you're an edge case shooter, though, the more you have to look beyond crop sensors, particularly the smallest ones.

We've got plenty of sensor choice these days in mirrorless: m4/3, APS-C, full frame, and small MF. Each has pluses and minuses. But be wary of those "just as good" and "near equivalence" claims people keep making. 

First of all, it's "just as good" for them, and second, they're wrong at the edge cases. They just haven't encountered the situation where they can't get there from here. 

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