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xkaes
23-Jan-2018, 17:01
I know I am wading into the swamp. But I've done that since I was a kid, so no amount of MUUUUDDDD will bother me. I've seen it all.

Several people on several board have basically asked a simple question. Admittedly, they asked it in several ways which has just made matters worse -- trying to figure out what they mean.

The question is "Is there a SIMPLE way to determine the amount to adjust the f-stop of a lens if - If - IF you know the magnification?"

First, this question is IF you know the magnification, so there is no need to respond by telling us how to determine the magnification.

Second, this question is how much to adjust the aperture -- not a calculation about how to adjust a SPECIFIC f-stop. In short, +1/3 f, or +2 3/4 f, etc.

Here is a real number chart of how much you need to adjust the f-stop based on the magnification.

173998

You might want to print this out for your use on a sheet of paper or a business card.

It seems like a pretty smooth curve, but when I try to compute the formula for the line/curve the best I can do is a cubic formula.

173999

It's close, but not as close as I would like.

There has to be a better formula to compute f-stop adjustment based on magnification.

Is there one?

Let's stick to this simple question, OK?????

IF I KNOW THE MAGNIFICATION, HOW DO I DETERMINE THE AMOUNT TO ADJUST THE f-STOP????

Bob Salomon
23-Jan-2018, 17:12
1:1 = 2 stops; 2:1 = 4 stops, etc.. 1:2 = 1 stop; 1:4 = stop; etc..

Dan Fromm
23-Jan-2018, 17:34
Just calculate effective aperture from aperture set, pupillary magnification and magnification. m stands for magification, p for pupillary magnification.

For an asymmetrical lens (triplet, Tessar, ...) with the lens facing normally,

effective aperture = aperture set * ((m/p) + 1)

For an asymmetrical lens (triplet, Tessar, ...), with the lens reversed,

effective aperture = aperture set * (1/P) * (1 + P*M)

For a symmetrical lens (dialyte, dagor, plasmat, ...),

effective aperture = aperture set * (1 + m)

I don't think in terms of adjustment, I think in terms of "if this is what I set, that is what I'll get." If I needed to think in terms of "if I work at this here magnification and the meter says f/whatever, which f/stop should I set?" I'd rearrange the equation.

Oh, and by the way, Joe, if you want to learn how to think about closeup work, buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Available at reasonable prices from used bookstores that offer their wares on abebooks.com, alibris.com, amazon.com, bn.com, ... Touted in my list of useful links.

xkaes
23-Jan-2018, 18:07
Oh, and by the way, Joe, if you want to learn how to think about closeup work, buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Available at reasonable prices from used bookstores that offer their wares on abebooks.com, alibris.com, amazon.com, bn.com, ... Touted in my list of useful links.

Yes, I have a copy of Lester's book, but neither you nor he has a simple formula X (f-stop adjustment) for magnification.

xkaes
23-Jan-2018, 18:08
1:1 = 2 stops; 2:1 = 4 stops, etc.. 1:2 = 1 stop; 1:4 = stop; etc..

Great. So what is the formula????????? I've already plotted it out.

I'll be glad to give you the best formula that I have -- it's cubic. I would like to know yours.

xkaes
23-Jan-2018, 18:09
Just calculate effective aperture from aperture set, pupillary magnification and magnification. m stands for magification, p for pupillary magnification.

For an asymmetrical lens (triplet, Tessar, ...) with the lens facing normally,

effective aperture = aperture set * ((m/p) + 1)

For an asymmetrical lens (triplet, Tessar, ...), with the lens reversed,

effective aperture = aperture set * (1/P) * (1 + P*M)

For a symmetrical lens (dialyte, dagor, plasmat, ...),

effective aperture = aperture set * (1 + m)

I don't think in terms of adjustment, I think in terms of "if this is what I set, that is what I'll get." If I needed to think in terms of "if I work at this here magnification and the meter says f/whatever, which f/stop should I set?" I'd rearrange the equation.

Oh, and by the way, Joe, if you want to learn how to think about closeup work, buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Available at reasonable prices from used bookstores that offer their wares on abebooks.com, alibris.com, amazon.com, bn.com, ... Touted in my list of useful links.

I don't care about "aperture set" or "effective aperture". I care about "aperture adjust" amount.

I don't want to rearrange the equation. The question is straightforward and simple -- and many people want to know.

At a given magnification, how much do I adjust the aperture in f-stops? How can I compute this? This is the 21st century afterall.

Tin Can
23-Jan-2018, 18:20
I bought the Lester book on Dan's recommendation some years ago.

Very interesting, as they say.

I calmly read it.


Just calculate effective aperture from aperture set, pupillary magnification and magnification. m stands for magification, p for pupillary magnification.

For an asymmetrical lens (triplet, Tessar, ...) with the lens facing normally,

effective aperture = aperture set * ((m/p) + 1)

For an asymmetrical lens (triplet, Tessar, ...), with the lens reversed,

effective aperture = aperture set * (1/P) * (1 + P*M)

For a symmetrical lens (dialyte, dagor, plasmat, ...),

effective aperture = aperture set * (1 + m)

I don't think in terms of adjustment, I think in terms of "if this is what I set, that is what I'll get." If I needed to think in terms of "if I work at this here magnification and the meter says f/whatever, which f/stop should I set?" I'd rearrange the equation.

Oh, and by the way, Joe, if you want to learn how to think about closeup work, buy a copy of Lester Lefkowitz' book The Manual of Closeup Photography. Available at reasonable prices from used bookstores that offer their wares on abebooks.com, alibris.com, amazon.com, bn.com, ... Touted in my list of useful links.

xkaes
23-Jan-2018, 18:23
Let's stick to this simple question, OK?????

IF I KNOW THE MAGNIFICATION, HOW DO I DETERMINE THE AMOUNT TO ADJUST THE f-STOP????

Can we?

Dan Fromm
23-Jan-2018, 18:31
Joe, most LF lenses are symmetrical. For them, if the meter says "use f/x", calculate the stop to set as aperture to set as aperture recommended/(1 + m). That's all. If the meter says use f/16 and magnification is 1, aperture to set = 16/2 = 8. You can do the arithmetic in your head.

If you don't want to do this kind of arithmetic, get a Horseman meter. For 4x5 a 4x5er will give better results than a 2x3er in an adapter.

If you must do things your way, learn about logarithms. AND DON'T YELL AT ME!

Bob Salomon
23-Jan-2018, 18:38
Can we?

That is what my response told you!

Corran
23-Jan-2018, 18:44
Something that I read on this forum, and is much easier for me, is to simply take the total extension from the center of the lens (non-tele) when focused and divide it by the absolute aperture size (lens focal length divided by f/stop). This gives the "corrected" aperture, which you can use for exposure calculation, or if you want to shoot at a given aperture, you can easily convert in the opposite direction.

IMO, faffing about with the magnification is overly complicated, but that's just me. Unless you have to have a specific magnification for scientific applications or something.

ic-racer
23-Jan-2018, 19:16
IF I KNOW THE MAGNIFICATION, HOW DO I DETERMINE THE AMOUNT TO ADJUST THE f-STOP????

stops = log((1+M)2)/log(2)

That is clever you tried to figure it from the curve fit, but you used the wrong curve fit. Maybe your software did not give you enough different choices.

Dan Fromm
23-Jan-2018, 19:36
stops = log((1+M)2)/log(2)

That is clever you tried to figure it from the curve fit, but you used the wrong curve fit. Maybe your software did not give you enough different choices.

Hmm. I posted that some time ago.

xkaes
24-Jan-2018, 06:38
stops = log((1+M)2)/log(2)

That is clever you tried to figure it from the curve fit, but you used the wrong curve fit. Maybe your software did not give you enough different choices.

The software I used (SPSS) offers over a dozen approaches/formulas -- and tests each for significance. I applied them all. One of them is logarithmic, quadratic, etc. but the best fit to the curve, by far, was cubic. But as I showed, it's close, but not as close as I would like. I have some other "curve fit" software that I can fire up as well, but I doubt that they have more options than SPSS.

Thanks for the formula. I'll give it a try.

xkaes
24-Jan-2018, 09:29
stops = log((1+M)2)/log(2)

That is clever you tried to figure it from the curve fit, but you used the wrong curve fit. Maybe your software did not give you enough different choices.

Do you mean:

stops = log((1+M))/0.30102999566

or something else?

Ken Lee
24-Jan-2018, 10:26
Some of the formulas are the same, but you might find this short article helpful: Formulas for Bellows Extension and Compensation (http://www.kennethleegallery.com/html/tech/bellows.php)

It's written for view camera dummies, by a view camera dummy :-)

xkaes
24-Jan-2018, 10:49
Some of the formulas are the same, but you might find this short article helpful: Formulas for Bellows Extension and Compensation (http://www.kennethleegallery.com/html/tech/bellows.php)

It's written for view camera dummies, by a view camera dummy :-)

Thanks. That's not exactly what I was looking for -- it's BETTER than what I was looking for.