"clean" isos and focal length applications

[Steve Oakley]

Originally Posted by Daniel Browning

70-200 f/2.8 L IS (Mark I for $1900) @ f/4 vs 70-200 f/4 L IS ($1200) at f/4 - the slow lens is slightly sharper and much cheaper.

70-200 f/2.8 L ($1300) @ f/4 vs 70-200 f/4 L ($700) @ f/4 - the slow lens is softer but much cheaper.

24mm f/1.4 L Mark I ($1200 used?) @ f/2.8 vs 24mm f/2.8 ($300) @ f/2.8 - the slow lens is a little sharper and much cheaper.

let me add into the mix the Tamron 70-200 2.8. optically better then the original canon 70-200 2.8, and close enough with the V 2 of the canon that you couldn't tell them apart optically looking at images.

Tamron 70-200 2.8 vs Canon 70-200 2.8

and its a $700 lens, or 1/3 the price of the canon V2 which is something like $2300 or so ?. its internal zoom & focus as well, and I love shooting with this lens. However, I don't think price has really been much of a consideration here, unless maybe we were talking megabuck glass.

Originally Posted by Daniel Browning

Let me illustrate it. f/10 on a 10mm optic is a very small aperture stop (1mm in diameter), while f/10 on a 2500mm optic like my telescope is much larger (250mm). The 10mm weighs 1 pound and the 2500mm weighs 50 pounds. But when they're both used on the same camera, the diffraction in image space (what photographers care about) is the exact same, because the f-number is the same.

well the question is : is diffraction in a lens a function of the iris size ratio in relation to the overall size of the lens, or is it a function of the physical size of the light wavelengths passing thru the hole of an absolute size, like how a diffraction grating works.