Hi Take,
> So, my belief is that
> if I use 1.4x TC on my f4 lens, the open aperture value will
> be 5.6.
Correct.
> Does this apply
> to all the f value? I mean, with 1.4x TC, if I choose, say,
> f8 on the lens, the f value will be 11?
Correct again. The reason for this multiplication is that the
teleconverter "stretches" the image in order to make the central portion
cover the entire 35 mm frame. When you stretch the image, however, you are
distributing the same quantity of light onto a biger area. Hence the
entire picture is darker.
For example, when you use a 2x converter, the "original" image is twice as
big. To achieve that, the image needs to be stretched to 4x times its
original area. This makes every "pixel" of the image only 1/4 so bright as
before. And 1/4 is exactly the same as two stops underexposure. Quite
naturally, this happens not only at the largest aperture, but at ALL
apertures.
A similar thing occurs when you focus a lens to a subject that is very
close. To focus on this close-by subject, the lens needs to move further
away from the film plane. But as the lens moves away, it produces a larger
and larger image circle. And of course, to cover more area with the same
amount of light, you need to make the entire image darker. For "standard"
lenses this is not really an issue (plus TTL metering takes care of it
automatically), but for Macro lenses or for large format, this is something
that should always be taken into account.
Well, I should stop now. Somehow this turned into a lecture... :-)
Cheers,
Boz
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