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."
Moose
P2CON@xxxxxxx wrote:
Someone said:
<< > Don't forget that mirror lenses lose about a stop to the middle mirror,
so
> you wouldn't need an Neutral Density filter in daylight unless your ASA
was over 400, even with 1/1000. >> , also several have commented on the
horrible OOF characteristics of the mirror lens.
I think we are giving the mirror lenses a bum rap in general.
Yes, they do exhibit doughnut OOF shapes with some backgrounds, but not all,
and when they do you can see it clearly before the shot is taken giving you a
chance to alter the viewpoint or composition to get the best display. They
are truly portable, mostly, and with the simple addition of a monopod or
common walking stick, can be stable down to at least 1/250. My lens is a
500/f8 Tamron, which makes it handy as I can mount it on my OMs, or if it is
really bright sun outdoors, I can put it on a camera with a faster shutter
speed.
I do not use it often, but it is nice to have for those occasions when going
light is more important than being completely prepared for the long shot with
big glass and a heavy tripod.
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.
Regards,
Paul Connet
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