[pure-silver] Re: correction for bellows extension
- From: "bobkiss @caribsurf.com" <bobkiss@xxxxxxxxxxxxx>
- To: "pure-silver@xxxxxxxxxxxxx" <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 4 Mar 2015 08:26:27 -0400
DEAR MARK,
I think the quick disc does the same thing as the formula using
magnification rather than bellows extension to calculate bellows factor.
Same results.
The formula is Factor = (M + 1) squared where M is the magnification.
This is very easy using a view camera. Measure any dimension of your
subject Ls. Then measure the same dimension in the image on the ground
glass Li. Now M = Li/Ls Using the example given, if M =1, M +1 = 2 and (M
+ 1) squared = 4. That is a FACTOR of 4, NOT 4 stops.
*****You can go through logarithms or you can use the Shakespeare's MacBeth
method, "Double, Double, Toil and Trouble". By this I mean that opening up
each stop doubles the intensity of light on the film plane which means,
more simply, opening up one stop = a factor of two...double. So a factor
of 4 equals two stops. Going in the other direction (closing up each stop)
it is "Half, Half..." but I have had a harder time finding a good quote to
represent it. Perhaps, "To half and half not". Weak... Perhaps "Half a
league, Half a league, Half a league on!" Dunno, you choose...
The reason it is good to know both the factor and the equivalent # of
stops is that you might choose to change the shutter speed instead of the
f# and then you simply multiply your shutter speed by, in this case, 4.
E.g., if you were shooting at 1/60, 4 X 1/60 = 4/60 = 1/15.
I hope this hasn't got "Muddy Waters" playing in your head...
CHEERS!
BOB
On Wed, Mar 4, 2015 at 1:56 AM, <mark@xxxxxxxxxxxxxxxxxx> wrote:
> well I understand the why, but I haven't done any algebra since college or
> trig since high school. [image: Cry] Oh I can relate to and follow the
> formula, but college was over 30 years ago. The trig I had to use since
> has all been done with a whiz wheel or a calculator that did the hard part,
> and that was more geometry than trig. The 11 inches = 1 stop 16= 2 stop I
> can understand. Wonder if that on the ground glass attachment to my flash
> meter still works?[image: Smile]
>
> Funny after a time of using cameras that use something other than film,
> its easy to forget that using film has a different set of habits to be
> effective. I didn't use a 4x5 that much back then anyway. Reviving the
> old skills I had developed and taking and learning new ones is becoming
> quite the challenge, and is a lot of fun.
>
> -------- Original Message --------
> Subject: [pure-silver] Re: correction for bellows extension
> From: <andpph@xxxxxxxxxxxxx>
> Date: Tue, March 03, 2015 6:33 pm
> To: pure-silver@xxxxxxxxxxxxx
>
> I think that is what I derived :)
> but Chauncy's approach is very neat!
> I like shortcuts! Reasons are good too!
>
> Andy
>
> -------- Original Message --------
> Subject: [pure-silver] Re: correction for bellows extension
> From: hksvk <hksvk@xxxxxxxxxxx>
> Date: Tue, March 03, 2015 7:18 pm
> To: pure-silver@xxxxxxxxxxxxx
>
> I can't find the reference at the moment, but I've always used this,
> programmed into a pocket calculator:
>
> bellows extension squared divided by lens focal length squared=bellows
> factor
>
> then, log bellows factor divided by 0.3=lens opening required in f/stops
>
> Harry
>
>
>
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