Lens speeds

Hans Moleman

Hans Moleman

Carbonite Member
Lemme get this straight.

Would my 28-200mm 3.5-6.5 Tamron zoom be as 'fast'

as a 28mm 3.5 Nikkor prime? I mean at full wide. Cause if so, Why would I bother getting the nikkor if the tamron isn't losing more light at 28mm?
 
You'd get the Nikon if you cared about more than about lens speed. Nominally f/3.5 on any lens passes the same amount of light - that is, the illumination at the focal plane should be same for the same source. But I'll bet the light coverage from the Tamron is not a patch on the evenness of the Nikkor. And distortion on the Tamron will be way greater than on the Nikkor (though probably nothing as bad as we put up with on our fixed lens camcorders). And, I'll also bet the Nikkor gives lovely crisp contrasty images with minimal coma and astigmatism at full aperture, compared to a fuzzy image from the Tamron at full aperture. If you care about image quality, Hans, you'd shell for the Nikkor. One final bet - which I welcome being corrected on - that particular Nikkor will be up there with the Cookes and Zeiss cine primes.
 
this is not true - f stop is not a measure of light transmission - it is a mathamatical measurement of depth of field - although it is related to an amount of light the actual transmission of a lens is different - this would be the t-stop which is the f stop with the transmission loss factored in.

For example lens A at f3.5 with clean glass might be t3.5 - but lens B at f3.5 with worse glass might be t4.0
 
Hans Moleman said:
Lemme get this straight.

Would my 28-200mm 3.5-6.5 Tamron zoom be as 'fast'

as a 28mm 3.5 Nikkor prime? I mean at full wide. Cause if so, Why would I bother getting the nikkor if the tamron isn't losing more light at 28mm?
Click to expand...

Same 'speed' but the Nikon would be MUCH sharper and have lower CA.

Regards
JohnG
 
its just a measurement of the maximum size of the apeture in relation to the focal length.

F1.4 for example is a 25mm wide apeture on a 50mm lens, F 2.8 is a 12.5 mm opening... etc. all the way down....
 
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