Photons Missing In Action

Started Nov 24, 2009 | Discussions thread
jhinkey
jhinkey Senior Member • Posts: 2,813
Re: Part 1: Lens T-stops?

Bob GB wrote:

Marianne Oelund wrote:

AF-S 200-400mm f/4G VR: 61%, T/5.11 (meas. at 300mm)

I suspect that last entry may cause some shock, but yes it's true, our $6K f/4 lens is only T/5.1

Interesting thread of yours

The only question I have so far is if you have measured the pupil and the focal length of these lenses? I suspect that the nominal f/stop and focal length for a given lens is just approximate values from the vendor.

If the 200-400 is f/4.2 in reality your percentage would be quite different.

I know it is more work.

Yes, this is good stuff - very interesting. Also, maybe I missed it, but at what focusing distance was the measurements taken? Doesn't focal length and effective aperture change with focusing distance with some lenses?

John

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