What's between 1.8 and 2.8 on my 85/1.8?

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My FD 85/1.8 is engraved with 1.8, 2.8, 4, 5.6, 8, 11, and 16, with click-stops betwen each engraving. What I can't figure out is, what aperture does the click-stop between 1.8 and 2.8 select? 2.0? 2.5? Something in between?

-- Andy Heffernan (ahh@juniper.net), December 12, 2000


Andy, I believe it will be 2.0. Each full stop is a multiple of 1 or 1.4, except for the wide open stop on certain lenses, such as the 1.2, 1.8, or 2.5 lenses. Canon built them with just a bit more maximum aperture than a "standard" stop or half-stop. I recently learned that there is a 50mm f/2 that was built for the T-series. It is the 50/1.8 rounded to f/2.0. As for the other half stops on the lens, they run: 1.4, 2.0, 2.8, 3.5, 4.0, 4.5, 5.6, 6.7, 8.0, 9.5, 11, 13, 16, 19, 22, 27, 32.

-- Alan Swartz (swartz@tyler.net), December 12, 2000.

What confused me is that I understand that they will mark the full stops on the lens as well as the maximum aperture (since it often doesn't fall on a full stop and is useful to know), but there should be two clicks between 1.8 and 2.8: one for the full stop (2) and one for the half-stop (2.5). But 1.8 isn't a half-stop, it's a 2/3 stop, so I figure Canon decided that it was close enough to 2 that they could skip 2 and so the next click stop would be the half-stop 2.5.

-- Andy Heffernan (ahh@juniper.net), December 12, 2000.

I don't have an 85/1.8, but my 50/1.8 SC (you know, that ultra-rare lens) appears to go 1.8, half-stop between 2.0 and 2.8, 2.8, etc. I don't observe a detent for f/2.0.

I'm basing this on how much rotation is between the detents. Going from 2.8 to the next (wider-open) detent feels like all of the other half-stop clicks, and going from there to 1.8 feels like a larger rotation.

-- Matthew Hunt (mph@astro.caltech.edu), December 14, 2000.

This is what happens when I try to think late at night! 2.5, as the half stop between 2.0 and 2.8, didn't even enter my mind or my message. I now vote for 2.5 as the detent between 1.8 and 2.8. I tried an experiment with my A-1, my incident light meter, and my 85/1.8 and 100/2.0. Although it wasn't completely repeatable, I did get almost identical readings with both lenses stopped down to the mystery stop. That would suggest that it is indeed f/2.5. It's kind of funny--I've had that 1.8-2.0-2.8 mindset for some time. Thanks for setting me straight!

-- Alan Swartz (swartz@tyler.net), December 17, 2000.

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