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Re: [OM] Help!

Subject: Re: [OM] Help!
From: Moose <olymoose@xxxxxxxxxxxxx>
Date: Thu, 20 Nov 2003 18:25:37 -0800
This is the lie that just won't die! (Well... at least one of them  :-)  )

Take a look here <http://brashear.phys.appstate.edu/lhawkins/photo/mp-cat-tests.txt> where Modern Photo test results for most mirror lenses in which we might be interested are put in one place. Notice the measured "F" stop. The Tamron is 8.0 and the others are all close to the marked stop. In fact, it appears that mirror lenses are marked in "T" stops, but not so noted to avoid confusion for the many who have no clue about the difference.

Moose

ll.clark@xxxxxxxxxxx wrote:

In <3FBC5913.8020800@xxxxxxxxxxxxx>, on 11/19/03 at 10:02 PM, Jim Couch <jamesbcouch@xxxxxxxxxxxxx> said:

So your f/8 may in reality be f/11 or even worse as far as light transmission 
goes.

I thought the Zuiko was an exception to this "rule."

If you want some further, potentially eye-glazing, material on the subject, I offer a post I made in June of '02 (which seems to be a repost from 1/02, I told you it wouldn't die!):

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This keeps popping up, and is theoretically about half true, but I believe it is not true in practice.

Back in Jan,. Paul Connet wrote: "Interestingly enough I just put the lens on my camera and took a reading on the green grass in my back yard evenly illuminated by a low angle sun. On Auto it said 1/250, (ASA 400). I then changed the lens to a 50mm, set at f8 and the reading was 1/250, (ASA 400), certainly no indication of a one-stop loss in light due to the mirror construction."

I then wrote: "I know Gary's test of the Oly 500/8 commented that it's T-stop was close to f8.

For anyone unfamiliar with the term 'T-stop': F-stop is simply a mechanical ratio of the focal length of a lens divided by it's clear aperture. Wide open, the clear aperture for most conventional (not fisheyes, for example) lenses is essentially the visible diameter of the front element. Thus when I measure the clear opening through the front element of a 50/1.8 at about 27.5mm and divide the 50mm focal length by 27.5, I get 1.8, the 'speed' of the lens. A T-stop is a measure of the light transmittance of a lens expressed as an equivalent f-stop. The difference between f-and t-stop for most modern multi-coated prime lenses is quite small. I seem to remember that it could be significant for some old SC zooms with many elements (and perhaps less than ideal internal design?). I don't know what the story is for modern MC zooms. In any case it only matters in this era of TTL and OTF metering for those using hand held meters, and then only in special circumstances..

Being a curious type, I just took a closer look at my Tamron. A 500mm f/8 lens, by simple calculation of 500/8, needs a clear aperture of 62.5mm. The clear outer aperture of the Tamron is about 78mm! The diameter of the center secondary mirror holder is 35mm. Subtracting the area for a 35mm diameter from that for 78mm gives an area equivalent to a 69.7mm diameter, larger than the theoretical 62.5. It looks like the designers not only provided a clear aperture equivalent to f/8, but added additional area to adjust for internal losses. When they say f/8, it appears they mean "equivalent to an f/8 conventional lens."

The Modern Photography lens test to which you provided a link show the actual, measured focal length and speed of the tested lenses. Although it is not so stated, these are clearly T-stops, both because a measurement of the simple focal ratio is meaningless in context and because that was part of how Modern's regular testing procedure. Further proof is in their measurement of the Tamron 500/8 at an actual 508mm and f8.0. My above physical measurements of this lens give a mechanical focal ratio (F-stop) of f7.3 (508/69.7) using the effective clear aperture and f6.5 for the clear diameter of the front element. Clearly, Modern was not measuring any mechanical ratio.

Thus, the actual difference between the nominal FE clear aperture and the true physical focal ratio, adjujsted for the secondary mirror, is f6.5 vs. f7.3, or 1/4 stop, and other losses in the lens make the total difference f6.5 vs. a measured t8.0, or 1/2 stop. However, the manufacturer has simply marked the lens f8, correctly indicating it's effective useful speed.

I just took a quick look at similar measurements for other mirror lenses and they aren't as clear as for the Tamron 500/8. So I tried another test, I just took exposure readings of a blank wall with a 50/1.8, Sigma 600/8, Meade 1000/11, and Tamron 500/8. I used an OM-1 so I could see an exact needle reading with no digital rounding. The 50/1.8 set at f8 and the 3 f8 mirror lenses all gave identical meter readings. With speed set one stop slower, the Meade also gave the same needle reading. I really think one can trust the marked speeds on mirror lenses to be at least as accurate as those for prime conventional lenses and probably more accurate than those for complex zooms.

Moose
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