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Alternative Imaging Methods
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Old June 20th, 2006, 09:56 AM   #16
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Originally Posted by Bill Porter
No, not all f-stops are equal. Just because two lenses are F1.4 for example, doesn't mean they pass the same amount of light. If you use decent lenses, a 50mm F1.4 probably doesn't pass more light than a 200mm F4.0.

Furthermore, a 20mm lens with a given f-stop, for example, won't pass as much light as a 50mm lens with that same max aperture.
I see your point about different lenses at the same f-stop, and it's a good distinction to make, but I disagree that any SLR lens at f/4 could possibly provide as much light as another SLR lens at f/1.4 or even 2.8. That's been my experience with Nikon's glass -- the 1:1.4 50mm fully open provides a dramatically brighter image than either of the 1:2.8 28-70mm or 1:2.8 80-200mm I have. I'm under the impression that Nikon makes good lenses.

Otherwise, while subtle differences of similar lenses across manufacturers are a factor, my bet is that they will never approach the exponential light loss that full stops exhibit. The leap from f/1.4 to f/4 (3 stops) requires 8 times an increase in lighting to get approximately the same exposure. There's plenty of good shooting to be had at f/2 and f/2.8 -- but many of the wide/tele lenses start at f/3.5 or f/4 and rob you of the chance to work that way.

Dennis Wood did a great write-up about this on DVXUser.
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Last edited by Jim Lafferty; June 20th, 2006 at 11:17 AM.
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Old June 20th, 2006, 11:28 AM   #17
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Originally Posted by Jim Lafferty
I see your point about different lenses at the same f-stop, and it's a good distinction to make, but I disagree that any SLR lens at f/4 could possibly provide as much light as another SLR lens at f/1.4 or even 2.8. That's been my experience with Nikon's glass -- the 1:1.4 50mm fully open provides a dramatically brighter image than either of the 1:2.8 28-70mm or 1:2.8 80-200mm I have. I'm under the impression that Nikon makes good lenses.

Otherwise, while subtle differences of similar lenses across manufacturers are a factor, my bet is that they will never approach the exponential light loss that full stops exhibit. The leap from f/1.4 to f/4 (3 stops) requires 8 times an increase in lighting to get approximately the same exposure. There's plenty of good shooting to be had at f/2 and f/2.8 -- but many of the wide/tele lenses start at f/3.5 or f/4 and rob you of the chance to work that way.

Dennis Wood did a great write-up about this on DVXUser.
I can't say as that's a valid example since wouldn't expect an F2.8 lens even at 70mm to be brighter than an F1.4 50mm.

Furthermore there's a heck of a lot more than "subtle differences" between similar lenses across manufacturers. Nick Bartleet illustrated just how dramatic that difference can be; check out some of his previous posts about his Nikons.
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Old June 20th, 2006, 12:43 PM   #18
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Originally Posted by Bill Porter
I can't say as that's a valid example since wouldn't expect an F2.8 lens even at 70mm to be brighter than an F1.4 50mm.
It's certainly a valid example of the argument I'm making. I can only assume you meant to say "I wouldn't expect an F2.8 lens even at 70mm to be brighter than an F1.4 50mm." In which case, we're in agreement completely.

Otherwise, I was using the term "subtle" in reference to relative exposures at the same f number. Even within the Nikon family of lenses, there are differences in exposure for different models of the same lens -- but the difference is subtle when compared to any lens at f/1.4 or 2.8 versus any other lens at f/4.

To belabor my original point in case it's not clear, many zoom lenses have a starting point of f/4 versus others which start at f/2.8. This means the minimum illumination required for a proper exposure on the GG with these lenses is a factor of 2x (or, one stop). Granted, choosing a lens is a question that involves more factors than simply minimum exposure, but sometimes available light makes it impossible to shoot with anything over 2.8.
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Last edited by Jim Lafferty; June 20th, 2006 at 01:14 PM.
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Old June 20th, 2006, 01:06 PM   #19
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Originally Posted by Jim Lafferty
70% is terrible. Most flip options utilize front-surface mirrors -- something like three pairs of such mirrors, each suffer something like .5% light loss. When calculated, you're losing something like 5-10% light loss from the flip module, IIRC. 30% is way off the mark, especially when added to other factors (light loss of the 35mm lens, which goes up as DOF and FL goes up; and of course light loss of the diffuser in use).
It's the right place to talk about diffusers lightloss in percents which is more realistic than in F-Stops. Did anyone measure lightloss of different GG(optosigma, nikon, wax, zenith,... etc.)? It seems that major avarage is about 1 T-Stop or little less. 1 T-Stop makes 50% lightloss!
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Old June 20th, 2006, 02:53 PM   #20
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F-stops are relative to focal length, so (theoretically) 200mm @ f5.6 should let in as much light as 50mm @f5.6 EVEN IF the aperture size at 200mm is the same as 50mm would be at f1.4. The aperture size is the same at 50mm f1.4 and 200mm f5.6 but the f-stop is NOT.

This is part of the reason why fast telephoto lenses are expensive; the glass must be huge. But fast wide angle lenses are also very expensive. 50mm, for some reason, seems to be the sweetspot.

Yes, there is some disparity between t-stops and f-stops, but it is NOT this significant. Half a stop or so at most for a decent lens I'd expect... Assuming decent lens quality, f1.4 should be roughly equivalent to f1.4 on all lenses. (Although vignetting, etc. may cause some to fall off a stop or two near the edges.)
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