Huh?!?!?!? That's *way* too much math for my puny brain! :-)
What the little sheet says is that with a 50mm lens and a 15mm extension
tube the "required exsposure increase" is 1.65. On the other side of the
sheet it says "exposure compensation can also be computed by
determinging the effetive f value (Ef) as contrasted with the indicated f
value (If) by using the formula: Ef = If x (focal length + extension / focal
length)." Thus, a 50mm lens and a 15mm tube would work out to
indicated f value times (50 + 15 / 50) or a factor of 1.3 or thereabouts.
Anyway, all this is mute as my trusty Oly-10 and Oly-2 will sort it all out
for me automagically. Thank God!!! ;-)
Chris
> 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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Chris O'Neill (coneill@xxxxxxxxxxx)
Web: http://www.nucleus.com/~coneill
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