Magnification Equation:
M = x/f, where:
M = Magnification
x = total lens extension
f = lens focal length
Note that the total lens extension is the length of bellows, or extension
tube, *plus* any additional extension added by rotating the lens' focus
helical from its infinity position. If you're working with a zoom that has
internal focusin (doesn't use a focusing helical to extend the lens cell),
it is more difficult to determine magnification. In practical use, for the
purpose of determining exposure compensation when using fixed output studio
lights, I sometimes calculate the magnification directly by puting a ruler
at the critical focus distance and measuring either the horizontal or
vertical dimension of the field.
The longer the lens, the more extension is required to achieve the same
magnification. All a longer lens does is allow farther standoff distance
from the subject material for the same magnification level.
For exposure correction, see my web page:
http://johnlind.tripod.com/science/scienceexposure.html#Macrophotography
Those that use TTL metering to set/control exposure need not make
corrections, but the same equations can still be used to find how much
light loss there will be and how much more light the TTL controlled flash
will have to put out for an exposure.
-- John
At 08:01 PM 10/5/03, Julian wrote:
I don't believe you'd see anything much through the viewfinder, to start
with. Even at 65mm, the tube would have more than doubled the back focus,
and therefore the image area would be more than four times the normal size.
According to my maths (often questionable), that's nearly 3 stops of light
loss. From f5, that's dim!
The 80 and 135 macros are designed with this extended back focus in mind.
Julian
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