In a message dated 3/16/01 1:07:15 AM Pacific Standard Time,
thomas.bryhn@xxxxxxxxxx writes:
<<
Nope, but the difference between f/4.0 and f/4.8 can. That's why I
suggested you try to set the Tamron to f/4.8, press the button or lever to
stop it down and then compare the two in the viewfinder. If you still see a
difference then something is seriously wrong with that Vivitar, but it
doesn't apply to mirror lenses in general. Their f/-numbers are as reliable
and correct as the ones for refractive lenses.
Regards,
Thomas Bryhn
>>
Well, the 450/f4.5 defintelyy still meters lower than the refracting Tamron
400mm f4 at 4.8 or so AND the viewfinder is much dimmer when the Tamorn is
stopped down. I would say with the mirror the viewfinder appears to be
atleast an f8 or slower. I am puzzled.
The confusion I have is that I have read from other mirror users that this
loss of light is common and the f stops are not accurately described on
mirror lenses. I thought then I wasn't alone.
I am going to throw the question out to more people on photo.net.
The 450mm f4.5 mirror is extra clean and I see nothing on the coating to
cause any problems. The only thing I can see possibly causing problem (this
might be huge) is that the lens I have doesn't have the UV filter on the rear
end. I am not sure that it is part of the optical formula as it would sit
right against the last real element anyway. I will have a hard time finding
the 35mm filter it takes and there is some thread damage that will have to be
addressed before fitting. Can this last seemingly meaningless filter cause a
light loss so great?
Also, the t-mount doesn't stop in accordance to the built in-non movable
tripod mount. If it were to be on a tripod I would have to tilt the platform
quite a ways to bring it to "flat".
I have heard that some t-mounts can be adjusted to get this alignment-maybe I
have a bum t mount as well? : )
I will also research this.
Thanks Thomas for your thoughts and any others who have responded or will.
John Raymond
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