focomat V35 enlargement index question

[ericzhu]

A little question about focomat V35 enlargement index. For example, while using the index 8, the exposure time is 10 seconds, after lifting up the arm to the index of 12, what's the new estimated exposure time (keeping the same contrast/highlight as the 1st exposure)? Is it 12/8*10 ? Thanks.

 

[mmcclellan]

I just always test strip it when I change the enlarger height. That's the best way and will give you best results that you can verify visually.

 

[Nicholas Lindan]

To the best of my knowledge, the numbers are the approximate enlarging ratio. The exposure correction would be (old number / new number) ^ 2.

Another way to find the exposure correction for changes in magnification is the special ruler provided by Darkroom Automation
http://www.darkroomautomation.com/support/stopsruler.pdf.
The results of using the ruler are very accurate.

You simply use the ruler to measure the size of the image before and after the magnification change. The difference in measurements is the exposure correction in stops.

Example: the original size measures 5 with the ruler and the new size measures 3 -- the exposure change is required is 2 stops - or 4 times more exposure on the timer setting.

Charts of stops <-> seconds and a stops dial for GraLab and large Time-o-lite timers are also provided on the Darkroom Automation web site.
http://www.darkroomautomation.com/support/grastops.pdf
http://www.darkroomautomation.com/support/stopstable.pdf

An enlarging meter is another way to determine exposure correction.

The Darkroom Automation meter will read out the exposure correction directly - you can either adjust the timer by the given amount or adjust the lens aperture to compensate http://www.darkroomautomation.com/em.htm. The meter will also determine base exposure, paper grade, dodges and burns, and is a precision projection densitometer.

Ilford makes a budget meter that allows compensation by adjusting the lens aperture. http://www.bhphotovideo.com/c/product/24631-REG/Ilford_1470279_EM10_Exposure_Monitor.html

 

[ericzhu]

Thanks for kind instruction.

 

[waileong]

A^2/B^2 = Ta^2/Tb^2

Just solve the equation where A and B are the sizes of the longest sides of the two prints, and Ta and Tb are the respective times. So going from a 8x10 print to a 16x20 print, A = 10 and B = 20. So if you took 30s for the 8x10, you'd need 4x30 = 120s.

 

[Lee L]

I believe Nicholas is correct about the approximate enlargment factor on the scale. However, he inadvertently (he's a very intelligent man) switched up the numerator and denominator, as the factor should be (new number/old number)^2 . Going from 8x to 12x means a longer exposure because of the greater area covered by equal light. 12/8=1.5 and 1.5^2=2.25, so extend your initial time by a factor of 2.25. For 10 seconds at 8x adjust to 22.5 seconds at 12x. It's necessary to square the linear enlargement ratio because you're looking for the ratio of the difference in square inches (or other units) covered by the same light output.

Lee