[pure-silver] Re: correction for bellows extension
- From: `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Wed, 04 Mar 2015 13:39:23 -0800
On 3/4/2015 12:30 PM, mark@xxxxxxxxxxxxxxxxxx wrote:
By factor of three, you mean three stops?
Not sure who this question is meant for but if its the formulas I
posted its the ratio not stops. Remember one stop is a ratio of 2, i.e.
half or double the exposure. The light transmission is dependent on the
area of the aperture, i.e. the square its aperture. Since, for example,
f/4 is double the diameter of f/8 it admits four times the light. In
this case f/5.6 is approximately the mid point, that is twice the light
of f/8 or half the light of f/4 but not exactly. So, if we are set up
for a unity magnification, that is image the same size as object, the
effective f/stop will be two stops larger than the marked stop because
we need four times the light to overcome the inverse square law loss.
The exposure _factor_ is 4. This makes sense because it can also be
used if we want to change the time of exposure. For the marked f/stop
the exposure is four times as long.
I have a couple of Kodak Professional Data Guides, they have a
dial calculator that takes into account distance, magnification and
other factors. I suspect these are not too difficult to find on the
used market but are not really necessary.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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