Chris,
To answer one part of your question, your OM will TTL meter correctly
without having to use these factors . . . because the metering is TTL.
*Don't* adjust anything. Just meter as you normally would using the
camera's TTL metering system.
About the exposure compensation factors on the instruction sheet:
These are more properly called called "bellows factors" (dating to view
camera adjustments for close-ups and macros).
Be *very* careful in using these to adjust an aperture!!!! If they are
labeled "exposure factor," your math in using them on the aperture f-number
was *incorrect*. They represent how much increase in exposure you need. A
larger f-number is *less* exposure. Neither can you divide an f-number by
the factor. That is still incorrect.
You *can* use them directly to adjust a shutter speed:
Assume a hand held metered exposure shutter speed of 1/125th. In this case
you will adjust the shutter speed only. Divide 125 by the bellows factor.
125/1.65 ~= 78
To correct using only the shutter speed, you would have to change it to
1/78th second. Can you actually set a shutter speed of 1/78th second? Not
on an OM. But if the bellows factor is close to some power of two you could.
You *cannot* use them in this manner on an aperture:
Assume a hand held metered exposure aperture of f/4. Divide the *square*
the aperture by the bellows factor, and then take the square root of the
result for the new aperture:
SQRT(4^2 / 1.65) = SQRT(16 / 1.65)
SQRT(9.7) ~= f/3.1
Why? Exposure is inversely proportional to the *square* of the aperture
f-number; i.e. the area of the diaphragm opening, not the diameter (on
which an f-number is based).
BTW, I checked this with my wonderful little bellows factor wheel in my
Kodak Master Photoguide. For a 50mm lens extended by 15mm, 1.65 agreed
with the wheel for the exposure factor, and looking at the aperture scale,
f/4 shifted to just above f/2.8; I would have estimated it at about f/3.
-- John
At 03:06 7/30/00 , Chris wrote:
>Hi, gang! It's "Mr. Confused" again! (Grin!)
>
>I also bought a set of no-name manual extension tubes for my Olympus
>outfit. The tubes came with a very brief instruction sheet that says I need
>to compensate for the aperature when using the tubes. So, for instance,
>with the 50mm lens and the 15mm extension tube, the instruction sheet
>says I need to multiply the aperature by a factor of 1.65 so that f4
>becomes something around f6.6 or so.
>
>Huh?!?!?!?
>
>I thought that the Olympus measures the exposure off the film plane at the
>time the shutter triggers??? If so, do I *really* need to worry about all
>this exposure compensation stuff, or will my faithful Oly stuff figure it all
>out for me???
>
>I hope Oly will do the job for me because my math is really rusty and I
>don't wanna have to carry my laptop with me to calculate exposure
>settings when I use the tubes! (Grin!)
>
>Again, thanks for any help you can offer!
>
>Regards,
>
>Chris
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