02-12-2012, 01:10 AM
[quote name='mst' timestamp='1326738516' post='14922']
Effective Aperture. Which is a given on any macro lens. But on Canon cameras, the physical aperture is displayed (so a macro lens seems to "keep" its aperture up to its minimum focus distance) while on Nikon cameras macro lenses (all macro lenses!) display the effective aperture.
So, the Tamron 90 will also change the aperture (the effective aperture!) with closer focus distance.
-- Markus
[/quote]
And for those interested:
This is caused by the fact that the AoV on both sides of the lens, subject side and sensor side, stays the same, but the distance between lens and sensor gets bigger when focusing closer, which means that the image circle gets larger. The lens still presents the same amount of light as a quantity determined by the physical aperture to the sensor, but now spread over a larger image cirdle, which gets cropped more and more at shorter focusing distances IOW. At 1:1 the image circle has an area 4 times that of the image circle at infinity, which is why at 1:1 one loses 2 stops of light effectively.
This with a normal macro lens. Once a macro lens gets IF, the picture may change a little, because at shorter distances the focal length gets shorter, and potentially the exit and entry pupil ratios also come into play, just to make things complicated. However, this generally is really only a problem when reversing a lens, amd the general rule stll holds, at 1:1 we lose 2 stops of light compared to infinity.
In the days before we had TTL light measurement, it meant it was very useful to know what the magnification was as a result, so one could look up or calculate at which magnification how much light one would lose and adapt exposure for this loss, but with the advent of TTL 40 or so years ago, the camera does this all for us <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. The only hassle really is that because of this spreading of the light, diffraction gets larger too, and for lens diffraction limits, one really has to take this effective aperture into account rather than the physical one.
This really gets only important once one gets into the macro reach, however. The light loss follows an exponential curve. At 12 X the FL (the light loss depends on distance and FL, magnification IOW), or, e.g., 60 cm for a 50 mm lens, it is approximately 1/3 of an f-stop, at 8.5 X the FL the light loss is approximately 0.5 of an f-stop, at 4.8 X the FL approximately 1 f-stop, and at 4X the FL 2 f-stops. In short, for normal focusing distances, this doesn't pose a problem; most light meters are calibrated to be accurate within 1/4 to 1/3 of an f-stop anyway.
Kind regards, Wim
Effective Aperture. Which is a given on any macro lens. But on Canon cameras, the physical aperture is displayed (so a macro lens seems to "keep" its aperture up to its minimum focus distance) while on Nikon cameras macro lenses (all macro lenses!) display the effective aperture.
So, the Tamron 90 will also change the aperture (the effective aperture!) with closer focus distance.
-- Markus
[/quote]
And for those interested:
This is caused by the fact that the AoV on both sides of the lens, subject side and sensor side, stays the same, but the distance between lens and sensor gets bigger when focusing closer, which means that the image circle gets larger. The lens still presents the same amount of light as a quantity determined by the physical aperture to the sensor, but now spread over a larger image cirdle, which gets cropped more and more at shorter focusing distances IOW. At 1:1 the image circle has an area 4 times that of the image circle at infinity, which is why at 1:1 one loses 2 stops of light effectively.
This with a normal macro lens. Once a macro lens gets IF, the picture may change a little, because at shorter distances the focal length gets shorter, and potentially the exit and entry pupil ratios also come into play, just to make things complicated. However, this generally is really only a problem when reversing a lens, amd the general rule stll holds, at 1:1 we lose 2 stops of light compared to infinity.
In the days before we had TTL light measurement, it meant it was very useful to know what the magnification was as a result, so one could look up or calculate at which magnification how much light one would lose and adapt exposure for this loss, but with the advent of TTL 40 or so years ago, the camera does this all for us <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. The only hassle really is that because of this spreading of the light, diffraction gets larger too, and for lens diffraction limits, one really has to take this effective aperture into account rather than the physical one.This really gets only important once one gets into the macro reach, however. The light loss follows an exponential curve. At 12 X the FL (the light loss depends on distance and FL, magnification IOW), or, e.g., 60 cm for a 50 mm lens, it is approximately 1/3 of an f-stop, at 8.5 X the FL the light loss is approximately 0.5 of an f-stop, at 4.8 X the FL approximately 1 f-stop, and at 4X the FL 2 f-stops. In short, for normal focusing distances, this doesn't pose a problem; most light meters are calibrated to be accurate within 1/4 to 1/3 of an f-stop anyway.
Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....