> Nicolas, I hope the following data "quenches your curiousity"!
>>>>>>>>>>>>>> The only concern for the manufacturer is that > 'stops' between f2 to f2.8 (and so on....)are acurately a 1 stop > difference, so that automatic "stopping down" can work. I used a SLR system where the exposure measurement wide open did not agree with the reading at the next stop. For ex&le, 1.8 would indicate one speed, 2.8 would indicate .5 stop difference (reflected in the shutter speed). It was within a .5 stop tolerance, but it irritated me nonetheless. I had several lenses made by the mfr for the system and they all did that. I now use Nikon and Contax SLR's, Leica M6 and do not find this discrepancy. Don't forget that the acceptance angle of a hand-held meter is often wider than a spot meter built into the camera, thus affecting your readings.
>>>>>>>>>>>>> Lens f-stop numbers are meant to account for light loss in the glass > - think along the lines of: large format photography and handheld meters. Not true. F/stops do not account for losses in many more elements (not as great a factor before zoom lenses in the '50's, and do not account for improvement made by single or multi-coating. With older lenses with fewer elements, the calculated f/stop was close to the actual, differences were not that great. F/stops are calculated at infinity. Therefore, there will be an inaccuracy the closer you focus. Did you know that the older 55 3.5 Micro-Nikkors for Nikon F had a "compensated" f/stop that was mechanically linked with the focus so that it "opened" up slightly as you focused closer? I have such a lens my father bought new in 1967. That feature became unnecessary, indeed counterproductive when TTL was developed. >>>>>>>>>>>> In TTL cameras, suppose an 'f 1.4 lens' is really an f2 lens, the > camera simply tells us to use a slower (1 stop) shutter speed - the > user will not notice the difference. Pop Photo, testing various Contax SLR's claims that exposure is biased toward .5 stop under. This is claimed to intensify colors with slide film, not to make a lens faster.
>>>>>>>>>I have checked pop photo, photodo etc. I can safely say that 'cheap' primes and zooms are not tested for their f-stop accuracy or MTF. Not true. Check Pop Photo again. The Pop Photo test shows the Zeiss 50 1.4 Planar for the N has an actual f/stop of f/1.50. The Zeiss 50 1.4 Planar MM has an actual f/stop of f/1.68! Even zooms with several elements and multi-coating do not exhibit huge f/stop discrepancies. In fact, the Zeiss 35-70 3.4 tests out at f/3.21-3.37! Photodo does not mention actual f/stop. MTF has nothing to do with actual f/stop.
Therefore, since the 50 1.4 Planar-N is actually a f/1.50, your test is not the f/stop of the lens, but a test of your hand held meter compared to the camera meter. When you eliminate the factor that the f/stop on the lens is grossly wrong, look elsewhere for the differences between your cameras+lenses metering and a handheld.
>>>>>>>>>>>>>> The only concern for the manufacturer is that > 'stops' between f2 to f2.8 (and so on....)are acurately a 1 stop > difference, so that automatic "stopping down" can work. I used a SLR system where the exposure measurement wide open did not agree with the reading at the next stop. For ex&le, 1.8 would indicate one speed, 2.8 would indicate .5 stop difference (reflected in the shutter speed). It was within a .5 stop tolerance, but it irritated me nonetheless. I had several lenses made by the mfr for the system and they all did that. I now use Nikon and Contax SLR's, Leica M6 and do not find this discrepancy. Don't forget that the acceptance angle of a hand-held meter is often wider than a spot meter built into the camera, thus affecting your readings.
>>>>>>>>>>>>> Lens f-stop numbers are meant to account for light loss in the glass > - think along the lines of: large format photography and handheld meters. Not true. F/stops do not account for losses in many more elements (not as great a factor before zoom lenses in the '50's, and do not account for improvement made by single or multi-coating. With older lenses with fewer elements, the calculated f/stop was close to the actual, differences were not that great. F/stops are calculated at infinity. Therefore, there will be an inaccuracy the closer you focus. Did you know that the older 55 3.5 Micro-Nikkors for Nikon F had a "compensated" f/stop that was mechanically linked with the focus so that it "opened" up slightly as you focused closer? I have such a lens my father bought new in 1967. That feature became unnecessary, indeed counterproductive when TTL was developed. >>>>>>>>>>>> In TTL cameras, suppose an 'f 1.4 lens' is really an f2 lens, the > camera simply tells us to use a slower (1 stop) shutter speed - the > user will not notice the difference. Pop Photo, testing various Contax SLR's claims that exposure is biased toward .5 stop under. This is claimed to intensify colors with slide film, not to make a lens faster.
>>>>>>>>>I have checked pop photo, photodo etc. I can safely say that 'cheap' primes and zooms are not tested for their f-stop accuracy or MTF. Not true. Check Pop Photo again. The Pop Photo test shows the Zeiss 50 1.4 Planar for the N has an actual f/stop of f/1.50. The Zeiss 50 1.4 Planar MM has an actual f/stop of f/1.68! Even zooms with several elements and multi-coating do not exhibit huge f/stop discrepancies. In fact, the Zeiss 35-70 3.4 tests out at f/3.21-3.37! Photodo does not mention actual f/stop. MTF has nothing to do with actual f/stop.
Therefore, since the 50 1.4 Planar-N is actually a f/1.50, your test is not the f/stop of the lens, but a test of your hand held meter compared to the camera meter. When you eliminate the factor that the f/stop on the lens is grossly wrong, look elsewhere for the differences between your cameras+lenses metering and a handheld.