|Wesley R. Elsberry
Joined: May 2002
|Quote (midwifetoad @ April 20 2012,08:33)|
|I looked back at my original post and see nothing controversial or argumentative.|
All I said was for folks using autoexposure cameras, the difference between nominal and actual f-stop is inconsequential.
If I select 4.5 (aperture priority), the camera may select a shutter speed somewhere between 1/250 and 1/500. Actually it records the shutter speed in decimal seconds.
If I use shutter priority, the camera uses continuously variable aperture settings.
So It doesn't care what the actual transmission factor might be.
Thom Hogan is an expert on the Nikon system. From a review of the Nikkor 200-400mm lens:
Before I call out individual attributes, there's one thing I should mention: this lens is not f/4. Let me correct that: this lens is not t/4. (A t-stop is the actual transmitted light, an f-stop is the theoretical light transmission.) Actual performance is somewhere around t/5. This is somewhat normal for a zoom lens with complex optics (each air/glass surface is less than perfect in transmitting light), but it's going to be a real issue for many users of this lens. This is one of the reasons why teleconverters aren't a great choice on this lens. Even with a TC-14E the lens is approaching t/8, which is outside Nikon's AF specs.
You may not care about differences between nominal and actual light transmission. A lot of hobbyists with autoexposure equipment may not care. Other people do. That would include people who want to expose according to incident light rather than reflected light measures, or who want to choose a lens with the absolute best performance under difficult low-light conditions (something that isn't changed whether you use TTL metering or not). I'm assuming that it would also include people who want to understand the physics behind the equipment. I think my hackles got raised with the "irrelevant" phrasing in your original comment and the fact that you did at that point include autofocus in your response, and that was plainly incorrect.
Dhogaza, nobody here has at any time disputed the ability of TTL metering systems to adjust for individual light transmission differences. I'm going to have to disagree with a claim of yours. Using a film like Kodak Technical Pan developed for continuous tone (POTA, 15 minutes at 68 degrees F, no push, no pull, no nothing) that has essentially no shoulder in the highlights makes a transmission difference like the one in the lens mentioned above highly relevant and not something that would be swamped by other parts of the process. If you are using TTL autoexposure, that will (mostly) work out, but if you aren't, it will make an actual difference in your results if it isn't accounted for. I feel like I've fallen through the rabbit hole to a place where people who I know to value knowledge in other circumstances are suddenly arguing for the primacy of ignorance in this one.
"You can't teach an old dogma new tricks." - Dorothy Parker