[pure-silver] Re: correction for bellows extension
- From: hksvk <hksvk@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Tue, 3 Mar 2015 19:18:34 -0500
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
On Mar 3, 2015, at 5:57 PM, Bob Younger wrote:
> I don't recall whether pure-silver allows attachments or not, but here's a
> spreadsheet and a graph I built for my lenses on my 8x10. You can adapt it
> for pretty much any combination you want. If it doesn't come through on
> pure-silver, and you want the spreadsheet then please email me off-list and
> send me your email address.
> Bob Younger
> ryounger@xxxxxxxxx
>
>
>
> On Tue, Mar 3, 2015 at 5:31 PM, <andpph@xxxxxxxxxxxxx> wrote:
> I'll take a shot at it although it has been some years ... you know, the
> trouble with this is that if one is wrong the whole group will laugh! BTW, I
> like logs! OK ...
>
> Assume you have a 100 mm lens on the camera which is a 4x5 one (although
> others would work as well).
> You first focus it at infinity and establish a mark on the back and a mark on
> the front standards. Pick any point. It does not mater if you find the lens
> center or focusing surface.
>
> Now you focus on a nearby (macro?) subject. Let's say the distance between
> marks is now 100 mm greater than it was before. So your extension is 100 mm.
> And your focal length is 100. So the total distance between lens and image
> plane now is 200 mm vs. the original (lens at infinity) 100 mm. You square
> each number and divide smaller into larger. In this case 40,000 / 10,000 = 4
> which means there needs to be a factor of 4 increase in illumination.
>
> So, to find the actual increase in exposure in stops you now find the log of
> 4 = .6 and divide that by .3 (a one stop change) and you end up with an
> increase in exposure required of 2 stops.
>
> This just goes to point out that if you are making photographs at life size
> then the image and subject distance are the same and the lens needs to move
> one focal length away from the sharply focused image plane. And you lose 2
> stops of light. Or a factor of 4.
>
> So now what if the extension is not 100 mm? Let's say at the macro setting
> you only need to increase the lens to image distance by 50 mm so a total of
> 150 mm.
>
> 150 squared = 22,500
> 100 squared = 10,000
>
> Factor increase needed = 22,500 / 10,000 = 2.25 X
> Log of 2.25 = .35 divided by .3 (per stop) = 1.17 stops
>
> How about if you use the lens at infinity? Well, then
>
> 100 squared = 10,000
> 100 squared = 10,000
>
> Factor increase needed = 10,000 / 10,000 = 1 X
> Log of 1 = 0 divided by .3 (per stop) = 0 stops
>
> This seems to work but dang! there must be an easier way to do it!!
>
> Andy
>
> -------- Original Message --------
> Subject: [pure-silver] correction for bellows extension
> From: <mark@xxxxxxxxxxxxxxxxxx>
> Date: Tue, March 03, 2015 1:20 pm
> To: pure-silver@xxxxxxxxxxxxx
>
> Well its been ages. IIRC there is a formula to apply to an f stop correction
> for a bellows extension. I never had to worry about it before because I had
> a gadget that would read the exposure off the ground glass. Now for what I
> am planning on in studio, I don't think its going to be much of an issue
> unless I try some macro work with it.
>
> Still I should understand, and in all honesty some of the articles I have
> read could make push the shutter button sound complicated. There probably
> are some simple explanations if anyone has links or would care to share. I
> can always get my gadget out again, but really no excuse for not
> understanding.
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> <Bellows Extension Compensation.xls>
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