You can look at it this way for a reasonable appoximation. Moving the
lens elements out a certain distance, say 12mm, will make the focus
depth (with infinity set on the lens) equivalent to a reproducition
ration of 12mm/35mm. So in this case that is around 1/3 life size,
which would be pretty darn close to the front of the lens, I believe
around 3x35mm=105mm, which is about 4 inches from the front of the lens.
These are just approximations, but probably are close enough for what
you are doing. If you stop *way* down, you might get enough depth of
field to have a sharp nose and reasonably sharp eyes (you would have to
check a DOF chart or calculator for the real figures, though).
Of course focus will get lots closer as you zoom toward the 20mm end
(you can do the math to find out if you want).
I have seen a 7mm tube available for some mounts, but I don't remember
which. Your best bet might be to simply shoot at around 20mm at closest
focus and a high f-stop (without the extension tube) and rely on depth
of field to cover you when the dog gets closer than the 14 inches.
Mike
Jim Davis wrote:
>
> Tom, thanks for the answer, maybe you can give me some more info here.
> I want to focus a bit closer than the minimum focus, which is about 14
> inches. Which tube would give me, say 8-12 inch minimum focus? It sounds
> like I want autofocus, I think the 12mm tube would do what I want - possibly
> the 25mm tube would not allow me to focus far enough out. How close and how
> far out can the 25mm tube focus? I need a decent DOF from about 8 to 36
> inches. (That's about the length of a dog nose to tail mostly :-)
> Sorry not to email privately, don't have your address. In any case, this
> is definitely EOS stuff, maybe someone else is interested too.
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