Ken Durling wrote:
> Hi all -
Hi Ken,
> I'm assuming that the EF-25 allows you to get twice as close as the
> EF-12? Probably also with an equivalent loss of light.
Whatever loss of light there is will automatically be compensated for by the
meter. The spec sheet for the 12 and 25 tubes says nothing about this; however,
according to John Shaw in "Closeups in Nature," the light loss varies with focal
length: "a 25mm extension tube on a 50mm lens yields 1/2X magnification and
costs about one step of light. The same tube on a 300mm lens gives 1/12X and
takes roughly only one-third stop of light." According to Shaw, the only
practical way to get a chart showing the light loss for different lens and tube
combinations (if you wanted to use a hand-held meter, for example) is to make
one yourself, by testing each combination using the camera's built-in meter. In
other words, there is no hard-and-fast rule. BTW, I highly recommend this book
if you're interested in close-up photography.
Focusing distance does not simply multiply when going from 12mm to 25mm. For
example, with a 50/1.8 lens, the focusing-distance range is 8.2-8.7 inches
(207-221mm) with the 25mm tube, and 9.8-12.8 inches (249-324mm) with the 12mm
tube. Again, the exact figures vary with the lens being used--figures for the
300/2.8L are 6.6-25.7 ft. with the 12mm tube and 6.3-12.5 ft. with the 25mm
tube.
There's another issue involved. For reasons that are left unexplained, some EF
lenses, it appears, require some exposure compensation to be dialed in when
extension tubes are used, at least when using an EOS 1, 1n, or 5. The lenses
concerned are the 50/1.4, 50/1.8, and 85/1.2. There is a small chart on the
instruction sheet that comes with the tubes. The sheet also includes a large
table showing magnification values and focusing distances for a number of
lens-tube combinations. You might try obtaining a copy of this sheet from Canon
if this information is of interest. The publication number is CT1-7715-009,
"Extension Tube EF 12/EF25 Instructions."
fcc
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