Originally posted by jon404 So -- help me out with this -- if "You still get more light through a 200/2.8 than a 18/2.8 due to the 'clear aperture' on the 200 being muuuch bigger..." then -- on the same scene at the same ISO -- wouldn't you be able to use a faster shutter speed at f/2.8 with the telephoto?
What am I missing here?
Originally posted by Woolcott The idea of larger format lenses capturing more light (as they have longer focal lengths in order to achieve the same FOV and therefore larger clear apertures), but that most of that light would never reach the sensor/film as the projected image is larger, is the idea that all those metabones speed boosters and so on work on.
All they do is focus the larger image (with all its extra photony goodness) down to a smaller area, thereby giving you 'extra' light.
I don't know if it helps, but here is a very specific set of examples:
A. Suppose I want to shoot the Full Moon with a 100mm f/2.8 lens and a 300mm f/2.8 lens
My 100mm lens will have an open aperture (entrance pupil) of almost exactly 10 square centimeters (1001.78 square milimeters to be exact) and my 300mm lens will have an open aperture area of 90 square centimeters. Thus for each unit of time, my 300mm lens will capture 9 times more photons from the Moon that will my 100mm lens.
Now, the Moon is an extended object with an angular diameter of 0.5 degrees as seen from Earth. Thus the image formed by my 100mm lens on my sensor will be a circle with an area of 0.6 square milimeters (0.598 to be exact) while the image formed by 300mm lens will have an area of 5.4 square milimeters - that is a 9 times larger area of the image on my sensor.
Therefore, the 9 times more photons that my 300mm lens captures relative to my 100mm lens will also be spread over a 9 times larger area, and thus, exposure times will have to be the same with both lenses.
B. Suppose I want to shoot some stars with a 100mm f/2.8 lens and a 300mm f/2.8 lens
This is to complicate matters a bit, but also to give the full story:
Stars are so distant that we don't see them as extended objects - only as point-like sources of light of varying brightness. Our camera lenses see them the same way and they do NOT form images of stars as extended objects the same way as with the Moon. Rather images are small diffuse dots and faint rings (: diffraction circles)
which size only depends upon the f-number, irrespective of the other lens characteristics.
Therefore, in THIS case my 300mm lens will again capture 9 times more photons from any given star than will my 100mm lens. But the photons from both lenses will form images (diffraction circles) of identical sizes. If I can just and barely form an image of a faint star in one minute with my 300mm lens, I shall need 9 minutes to capture the same star with my 100mm lens. But mind you: This is for point-like sources of light only!