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. > ============================================================================================================To > unsubscribe from this list, go to www.freelists.org and logon to your > account (the same e-mail address and password you set-up when you > subscribed,) and unsubscribe from there. > > To unsubscribe from this list, go to www.freelists.org and logon to your > account (the same e-mail address and password you set-up when you > subscribed,) and unsubscribe from there. >
Attachment:
Bellows Extension Compensation.xls
Description: MS-Excel spreadsheet
