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
Tom@xxxxxxxxxxxxx wrote:
Mirror lenses lose almost a stop of light from the central 2nd mirror, so that
f/8 becomes an f/11
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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