I too am wary of this degenerating into yet another long discussion of Equivalence (there have been any number of threads on that topic already). If I see that happening, I'll just siphon off the appropriate posts into a separate thread. But since there *is* an issue relevant to *this* thread here, I would like to make a few points. I think the idea of using a 135 on APS-C digital becomes that much *more* impressive when looked at this way, and it's actually worth thinking about for a moment.
Originally posted by Douglas_of_Sweden Sure, a 135/2.8 does not give the same DOF on APS-C as on FF. I am aware of that. But f2.8 expose like f2.8 regardless of the format. Even James agree on that. So if I have a dark motife where my 200/4 on film would be a challenge, a 135/2.5 or 2.8 on APS-C has a speed advantage.
Yes, but it's important to note you are introducing a new variable here by mixing film and digital. The issue becomes clearer when you first limit discussion to digital, and make FF versus APS-C the only variable, and *then* consider how this applies to film.
Yes, a 135/2.8 on APS-C will allow a 2X faster shutter speed than a 200/4 on FF *when shooting at the same ISO*. But since a FF digital camera will also have approximately one stop better noise performance due to its larger sensor, the point here is that you could actually get the same results by simply raising the ISO one stop on the FF camera. That is, using the FF camera's noise advantage to nullify the APS-C camera's focal length advantage (or vice versa, depending on how you look at it).
Of course, a 135/2.8 is still somewhat smaller and lighter than even a 200/4 (at least, all the 135/2.8's I've seen are smaller than all the 200/4's I've seen). So even when comparing FF to APS-C in the digital domain, we still see that a 135/2.8 on APS-C is a nice thing, if not maybe earth-shatteringly so.
But now you can see why talking about film changes things in a big way: 35mm film has *no* noise advantage, and indeed, really a pretty significant *disadvantage* compared to APS-C digital. For convenience, I'm obviously equating grain with noise, since despite the different physics/chemistry, the basic effect on image quality is similar.
Since 35mm film has no noise advantage it can use to offset the focal length advantage of APS-C digital, a 135/2.8 on APS-C digital actually *blows away* a 200/4 on 35mm film in terms of how fast a shutter shutter speed you can get for a given level of IQ.
For that matter, in terms of exposure, the 135/2.8 on APS-C digital beats a 200/2.8 and even a 200/2 on film - quite handily! I think most would agree that shooting APS-C digital at ISO 1600 will give you IQ comparable to shooting 35mm film at ISO 400 (and that's probably being generous to film). If you can get a given shutter speed at f/2.8 at ISO 1600, you'd need f/1.4 to get that same shutter speed at ISO 400. So really, shooting a 135/2.8 is a fairly similar experience with respect to exposure (if not DOF) to shooting a 200/1.4 on film! That's pretty incredible! Especially when one considers that in the majority of situations where you might want to take advantage of that sort of exposure, you wouldn't *want* the extremely shallow DOF of a 200/1.4. Being able to shoot an *exposure* similar to a 200/1.4 but with the *DOF* similar to a 200/4 - for a concert shooter like me, that's like having one's cake and eating it too.
Looked at it in this way, the M135/3.5 is in some ways more amazing still. Seems slow by film standards, but if a 135/2.8 on APS-C digital is similar to a 200/1.4 on 35mm film in terms of exposure, then a 135/3.5 is similar to a 200/1.7 (with DOF similar to 200/4.5 or a little higher). Still absolutely unprecedented performance - who's going to quibble that half stop now? - but getting this in a package the size of my fist is just unbelievable.
So as far as I'm concerned, looking at things in this way just makes the 135 club that much *more* special!