How to calculate bellows extension from the bellows factor?

[aduncanson]

Cor, I am trying to understand your question, but I am having trouble figuring out how your Palm software comes up with a bellows factor of 3.2. Among many other useful identities discussed above, Bellows Factor = (1+ Magnification)^2. So a Bellows Factor of 3.2 means the Magnification is 0.79x. That implies that the field of your head & shoulders portrait is an awfully tight 9.8 inches by 12.4 inches. Is that the kind of cropping you are looking for? What did you enter into the Palm that it used to determine magnification or field size? Thanks

[Cor]

Cor, I am trying to understand your question, but I am having trouble figuring out how your Palm software comes up with a bellows factor of 3.2. Among many other useful identities discussed above, Bellows Factor = (1+ Magnification)^2. So a Bellows Factor of 3.2 means the Magnification is 0.79x. That implies that the field of your head & shoulders portrait is an awfully tight 9.8 inches by 12.4 inches. Is that the kind of cropping you are looking for? What did you enter into the Palm that it used to determine magnification or field size? Thanks

I I use pCAM for a preview of a head/shoulders portrait I get a distance of 1.2 meter, and a "frame" around the subject of horizontal 29 cm and vertical of 37 cm.
(still talking about that 480 mm lens on 8*10).

If I use that distance of 1.2 m in the Vademecum (DOFu) I get a Bellows factor of 2.8 (less than I said first, probably I choose a tighter crop, only the head the first time)

Oh the magnification is 0.66 than, so you were correct, that 3.2 is too tight.

It's all armchair LF but in practice these figures usually fit nicely, and at the moment I am preparing a top hat construction for the extension of my bellows,

best,

Cor

[Drew Bedo]

Hello Cor,

Every focal length of extension beyond infinity requires two stops of additional exposure. It follows then that 3.2 stops is equivilant to 1.6 focal lengths beyond infinity. If the lens is 480mm, the extension beyond infinity will be 760mm and total extension will be 1248mm.

Did I do that right? I use a verry small 4x5 and my longest lens is a 210: A meter and a quarter(50 inches) of bellows seems like a lot to me.

[Drew Bedo]

Ok, 2.8: That means 672mm beyond infinity and 1,152mm total.

Post a few shots when you are done? I'd like to see the Top-Hat mount and the portrate.

[aduncanson]

Drew, Cor is expressing Bellows Factor as a multiplier applied to exposure time rather than as a number of stops compensation. [I'm not sure how I picked up on that, but post no. 8 in this thread is pretty clear.] This seems to be one common convention, but clearly care must be taken to avoid confusion about how bellows factor is being expressed.

Using that convention, a Bellows Factor of 2.8 implies 1.5 stops compensation and the total extension required is 803mm.

- Alan

[Brian Ellis]

No need to be that precise unless you're using slide film. All I've ever done is look at the subject and how much the bellows looks like it's extended. If he/she/it looks like they're closer than 8 times the focal length of the lens I don't worry about bellows extension. If the bellows looks like it's extended a lot more than usual for a particular lens I open up one stop. If it's really sticking out there I open up two stops. Works just fine for negative film.

[Cor]

I see Bellows Factor as either a time or a F stop correction ( I believe that is the consensus), so a Bellows Factor of 4 means 4 times longer exposure or open the lens up 2 stops. Bellows factor of 2.8 is indeed almost 3 times more time or 1.5 stops compensation.

I just happen to just finished the tophat construction (from mat board..;-)..to avoid to much strain on the front standard), and I could put theory in to practice on my 11 year old son, and it worked out fine (accept that his head is smaller that that of an adult) so total bellows draw is now 820 mm.

It will take some time to start shooting with this lens, I am curious how it compares to the head/shoulder portraits I made with my 300 mm Symmar-S. Especially if there is a lot of difference in way the face is rendered, a 300 mm close up to a face is supposed to change the perceptive a bit I understood, although I did not notice that too much.

At least the 480 lens is now at a nicer distance away from the subject.

Best,

Cor

[Steve M Hostetter]

use a ground glass gauge like from JBrunner you can down load it free

[Drew Bedo]

aduncanson: Thanks for that clarification.

I do not do portrates myself. Most of my work is still life or table-top compositions. Exposure compensation is always an issue. I usually shoot with a 210mm or 150mm lens. For the 210; every inch beyond infinity (8 1/4 inches) equals 1/4 stop of increased exposure. For the 150mm, each inch of extension equals 1/3 stop exta exposure. Usually this correction is made by opening up the aperature. I rarely actually do naything with correction factors or work out any math.

[Ken Sinclair]

Cor...
I just came across this a 'question' couple of days ago after having recently 'resurrecting' my Palm Pilot that had gone into 'hiding' after a house-move a number of year ago'.
When I was a 'working "Biological Photographer' my Palm with Photographer's Vade mecum was used on a daily basis... I don't know how I could have survived without it since close-up images of specimens were requested with an "I need this done NOW". I managed to find the Vade mecum programme on line and have just reloaded it to my Palm 505... all that I now need is to 're-learn' proper use of the stylus for entering the information required to give me the required 'corrections'

Ken