Hi James,
I might help you in the direction of a solution to your problems.
I don't know about light loss (perhaps somebody can explain the principle of
that), but I can help with the
magnification riddle:
These formulas are the basic ingredients:
The direct connection between focal length (f), distance from the object to the
optical centre (o) and image
distance from the optical centre (i) is given in:
1/f = 1/i + 1/o
Normally, the distance scale (d) on a lens is measured from the object to the
film plane:
d = i+o
Now we should know that the magnification ratio (N) can be defined as:
N = i/o
Some solid handwork shows how we can express o as the result from f and d:
o = 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f))
We also learnt that i = d - o , so:
i = d - ( 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f)) )
Now, the idea was to express N as a finction of f and d:
N = i / o =
N = (d - ( 1/2 * f * (d/f - SQRT(SQR(d/f) - 4d/f)) )) / (1/2 * f * (d/f -
SQRT(SQR(d/f) - 4d/f)) )
Nope, I haven't checked this. Try it, a 90mm (f) lens should do 1:1 (N) at a
working distance (d) of 360 mm.
You do the math :)
One additional nastiness is that many lenses (esp. zoom lenses) use floting
alements while focusing. Efectively,
this means that the focal length of the lens depends on the distance you focus
on. This effect is not present when
you use a bellow of course.
Or you could put a ruler in your composition and see how many mm's on the
subject side fit into the 36 mm's that
are resembeled by your focusing screen (allright, multiplied by 98% or
whatissit).
Or follow Chuck's advice, and forget about the math and figure it out in an
emperical way. :)
Good luck!
Bart
Citeren "jking@xxxxxxxxxxxxxx" <jking@xxxxxxxxxxxxxx>:
>
> ok, I may be about to open myself up to calls of idiot but here goes
> anyway....
>
> I would like to calculate how much light loss there is depending upon the
> lens focusing distance and lens focal length. This is bcause I mainly use
> an incident light meter and want to be able to compensate for the light
> loss at close focusing distances e.g. macro but also taking photos of
> statues at close range etc.
>
> I found a nice equation that can calculate the light loss based upon the
> magnification ratio and the tamron 90mm macro lens has course
> magnification marks on it. Fine. I converted the light loss ratio into
> stops of light loss, ploted and plotted a graph. I found that until you
> get to about 1:8 the light loss is less than 1/3 of a stop. By 1:1 its two
> stops and very significant. Great what about my other lenses that only
> have focusing distance and not magnification ratios marked on them...
>
> Intuition says to me it must be possible to get the magnification ratio
> from the focal length and the focusing distance but everything I tried did
> not work. Part of the problem was that I had little additional information
> about my other lenses other than minimum focusing distance and for some of
> them the magnification ratio at that distance so I was playing with three
> variables, focal length, minimum distance and mag ratio. I got out my copy
> of ansel adams the camera and used his equations for working out image to
> nodal point vs magnification and subject to nodal point vs magnification
> and I could find no match between them and the markings on the tamron!
> HELP
> am I crazy? (probably) can it be done... I guess so since macro lenses
> have magnification ratios on them as well as focusing distances...
>
>
> James
>
>
>
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