Emmm, hrrrr, hummphh....
Originally Posted by John Roberts
centimeters, it's centimeters -- change the 'mm' to 'cm'.
OK, I realize that I am in the minority here, but I have made a choice and simply stated my opinion (just like everyone else) :rolleyes:
Originally Posted by Nicholas Lindan
I own and have used:
The old Kodak "computer" in their B&W dataguide.
Mr. Linden's ruler
an Ilford EM10
the enlarging attachment for a Gossen Luna pro
the algebra formula being discussed.
All of these got me close. "Close!" I still had to fine tune, and - in my experience - a test strip and/or a "work print" or two ended up being made anyway. Starting with a test strip is, for me, the quickest, cheapest and most efficient method.
Even with the EM-10 I still do a test strip...... have you priced cibachrome materials.... I would never just toss in a bigger sheet of paper and run a print like it would be a final.....
Originally Posted by David Brown
It should be possible to tweak the formula to be "spot on" at any enlarger height.
Things that probably aren't being taken into account are paper reciprocity, actual lens-to-print distance (not the hypotenuse of lens-to-print distance and chassis stalk angle), and the aforementioned contrast loss due to light scatter with increase in magnification factor.
Time for a real world test ...
Conditions: Beseler 45MX, condenser light source, Stouffer 4x5 31-step tablet, 150mm Rodagon, Ilford MGIV RC glossy, #2 1/2 filter, D-72 70F 2 min
Prints were made at 4x5 (1:1), 5x7, 8x10, 11x14 and 16x20.
In each case the #1 step was metered with the Darkroom Automation meter. Time was controlled with a DA f-Stop timer. A total of 10 stops of exposure (meter reading + timer setting = 10) was given in each case.
The results are as consistent as I could ever do with test strips. All the prints were made with the same contrast filtration - results would be a bit closer if contrast were tweaked.
Time to find new exposure and set it into the timer after changing magnification - less than 5 seconds.
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Why am I not surprised to find such thorough and thoughtful insight on such a niche APUG forum topic. There's a lot here that I need to still digest. It looks like making a test strip is probably the best bet, but at least the calculations will be useful to even get in the ballpark on the test strip. I find that sometimes I am way off base especially if proper exposure time goes from say 12s to 45s or so, the usual 3s test strip just isn't going to work at the longer exposure, so I will now know where to start with a certain base exposure.
Comparison of methods for determining the exposure correction required for magnification changes using a 50mm lens, Beseler 45 enlarger and condenser lamphousing
The ruler and the magnification formula provide the same result - the previous difugalty was due to not correctly locating the lens nodal point. In this case the point is 1.1" back from the front flange of a 50mm f2.8 El-Nikkor, FWIW.
However, although they agree, they both provide the wrong exposure correction - enough to create a shift of about 1/3 of a Zone in some cases. Intriguingly the error doesn't seem to follow any sort of pattern.
The error doesn't seem to be due to the variation in the distance from the lens to the condenser. Changing the distance by 1", a bit more than the lens travel in the above test, changes the light intensity by less than 0.005 stops. It might be interesting to repeat the test with a diffuse light source.
So far, it seems only a good meter will provide accurate compensation.
A lot is/has been said about Kodak but the Data guides both BW and Color have probably helped more people than can be counted. In fact the data from the papers you could order were invaluable.
The old Kodak "computer" in their B&W dataguide.
Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand
In all practicality, you are fine with an estimate. You will never be spot on via formula anyhow, so will end up wasting a full test print, when you could have just done a test strip. Going from one standard print size to the next (say from 8x10 to 11x14), I would add one stop of exposure as a general adjustment. Going to 16x20? Add two stops versus the 8x10. This is making the quite general estimate that each standard size contains twice as much surface area as the next smallest. This is not exactly true, but it is close enough to make a good test strip. One thing that is true is that when going from 4x5 to 8x10, or from 8x10 to 16x20, you exactly quadruple the surface area, therefore you end up with only one quarter the light. This means that you can theoretically compensate by increasing the amount of light by a factor of four, also known as adding exactly two stops of exposure.
Last edited by 2F/2F; 12-14-2008 at 03:08 AM. Click to view previous post history.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."
- Rob Tyner (1944 - 1991)
That's really interesting and something I had never realised. However, trust us pesky Europeans to come along with our 12 x 16 paper and ruin the sequence!!
Originally Posted by David A. Goldfarb