I have a Beseler 23C XL II, printing 6x7 cm with an 80mm Rodagon.
Does anyone know what the "factors" are for 11x14, 16x20 & 20x24?
Thanks.

For photographic purposes, it is conventional to use the linear magnification factor, so the magnification factor would be the size of the print along one dimension divided by the size of the negative along the same dimension. So if 6x7cm is nominally 2-1/4x2-3/4", the magnification factor for an 8x10" print could be computed as 10/2.75=3.6X magnification.

I do what David suggested, only in millimetres!

For photographic purposes, it is conventional to use the linear magnification factor, so the magnification factor would be the size of the print along one dimension divided by the size of the negative along the same dimension. So if 6x7cm is nominally 2-1/4x2-3/4", the magnification factor for an 8x10" print could be computed as 10/2.75=3.6X magnification.
Just a thought; shouldn't the "size of print" above be replaced by "size of enlargement". If printing full frame they are of course the same, but not with a selective enlargement.

IMO the easiest method is to ignore image size and use enlarger lens to baseboard distance. This way selective enlargement or not, you are always measuring the same thing. You are never going to get caught out by not noticing that you have masked off the image etc.

Say you do a 10x8 ish print and the distance is from lens to baseboard is 50cm
Then you want to do a bigger print and having racked up the column and getting it how you want it the distance is now 70cm

70 squared (4900) divided by 50 squared (2500)= 1.96

If your exposure was 10 seconds for the first print it will be 19.6 for the second.

Simple - just keep a tape measure and calculator next to your enlarger. Takes a few seconds and never fails. Bear in mind that if exposures change a fair bit the relationship is not quite linear because lamps get hotter ie if the factor is 4 so goes from 5 seconds to 20, I find that with my enlarger the second time would be somewhat less than this, maybe 19 seconds. To fix this I tend to make small images at say f11 and the bigger ones at say f8 or f5.6. This way I can use aperture changes to help keep exposure time close. You can of course use a combination of both. Say the factor is 2.6 and you originally used 10 seconds at f11. This would be 26 seconds at f11......or you could use 16 seconds f8.

Sometimes I confuse myself too.

No confusion Tom, that is the system that I use, with the slight difference that I measure from lens to easel surface to the nearest centimeter.

print image distance / negative image distance = magnification.

be sure to measure between the exact same two points on both the negative and the print. I will measure a particular distance on the negative before I place it in the carrier, then i can calculate the magnification easily when enlarging by measuring the same two points on the easel.

example: 8" on the easel / 1" inch on the negative = 8x magnification

One more way to skin the cat - measure the size of the neg holder opening (always stays the same). When you get the magnification you want, measure the uncropped image at the easle plane. Should give accurate image enlargement ratio, I think(works for me). If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.

........If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.

Yes, it’s the Inverse Square Law relating to light spread; hence the squaring referred to in Tom’s post above.

No confusion Tom, that is the system that I use, with the slight difference that I measure from lens to easel surface to the nearest centimeter.

Same....I meant easel !

Foolproof and very accurate as the distances from lens to easel are big enough to make measuring them accuarately easy.

davetravis,
if you like to know the exact magnification ratio of an arbitariy negative clip to an arbitraty print size, simply replace the negative by a transparent ruler and measure the projected scale.

This is one thing I have never thought about. I just adjust height for the crop on the print size. Magnification factors have never been a concern to me. Than again I havent done anything under 4x5 for years.

Magnification adjustment factors are only of interest if you wish to change your enlargement size without producing a second test print(s). Negative size is irrelevent.

vet173,
this could be of interest for several reasons, which do not necessarily have to do with enlarging itself. It could be necessary to know the scale of a reproduction or to produce an exact 1:1 reproduction. Of course, you'll have to know the exposure mag ratio in this case as well.

Thanks for the info. I use an ilford exposure meter for exposure determination. It's only an obsession with me not a job, so I don't do any product photography. I can see now where someone might need that kind of information.

I needed to know the mag factor so I could decide if I wanted to switch to an APO lens. My Rodagon 80mm is best around 6X. My 16x20's come to around that. My 20x24's are around 9X. The APO Rodagon is best around 10X.
Since the detail and contrast that I'm getting now printing on Ilfochrome is so fantastic, I have decided that the APO would be a waste of money for my current format. Besides, Ciba doesn't need any more contrast!
Thanks to all for responding. :)

Would the same calculations aplly to each exposure of a split contrast black and white print?

For example if I make an 8x10 print at f11 with 6 seconds Magenta 200 and 4 seconds Yellow 200; and then I want to convert to 16x20 I should do the arithmetic and apply it to each exposure?