# Exposure compensation

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• 08-21-2008, 05:03 AM
emanded
Exposure compensation
I'm new to the darkroom and after much advice here I'm up and running with a Durst M670. The results are beyond my expectations - far better than my digi work (done with full frame Canon & L glass). The prints have far more depth and 'liquidity'- if that's the right word for it,
Now, my question is, I've done some prints on 10x8 that are worthy of going to 12x16 - what compensation do I need on the exposure for pulling the light further away, bearing in mind there will also be some dodging & burning. Or do I have to start again with test strips?
Joe
• 08-21-2008, 05:43 AM
Steve Smith
If you keep everything else the same, it is exponential based on length of print or linear based on area of print. So doubling the dimensions from 10x8 to, for example 20x16 gives you four times the area which the same light is covering so it is getting a quarter of the strength. Therefore multiply the exposure time by four.

10x8 and and 12x16 are not the same aspect ratio so you could do a bit of maths to work it out. If we assume that the same linear length of image which fits the 10" side of the 10x8 is also going to fit on the 16" side of the 12x16 then we can do this:

Exposure multiplier = (large size squared)/(small size squared) = (16x16)/(10x10) = 256/100 = 2.56

So about two and a half times the original exposure will get you in the right sort of area.

Steve.
• 08-21-2008, 08:05 AM
pentaxuser
I have a site provided by an APUGer in which you simply put in current exposure time and enlarger height plus new enlarger height and a hidden excel prog gives you the new time.

I cannot give you the URL for it as nothing shows up. I simply saved it from whatever was given on the post. Sorry to be so vague. Someone might refer to it here or the original poster will respond but if not do a search on enlarger posts and it should come up.

pentaxuser
• 08-21-2008, 09:48 AM
David Brown
Do another test strip.
• 08-21-2008, 11:29 AM
ic-racer
This comes up all the time. Do as David Brown suggested and make a new test print.

If you like math the equation is:

new_time = old_time x (new_M +1)^2 / (old_M+1)^2

where M = new magnification (print/neg) and m = old magnification (print/neg)

The exposure time factor would be:

Factor = (M + 1)^2 / (m + 1)^2

The equation does follow the inverse square law for light but, since you re-focus the image, you also have to account for the change in your 'effective aperture.' Similar to 'bellows factors' used in macro photography. When doubling the size of the print, you will be about one stop off if you use the 'inverse square' equation alone.
• 08-21-2008, 02:22 PM
Nicholas Lindan
Quote:

Originally Posted by emanded
what compensation do I need on the exposure for changing the print size

There is an f-stop print size exposure adjustment ruler on the Darkroom Automation web site.

It works with "f-stop timing", which is a good technique to start using early on in your darkroom work. To get started with f-stop timing and your present timer, an f-stop timing dial for a GraLab timer and f-stop timing table for digital timers are provided the Darkroom Automation web site. A true f-stop timer is the best way to control enlarger exposure.

The other way to compensate for changes is to use an enlarging meter. The Darkroom Automation Enlarging Meter will read the exposure change in stops. You can also use an Ilford EM-10 exposure comparator which will let you use the lens aperture to adjust for changes in print size.
• 08-21-2008, 02:49 PM
Ben 4
Do Both!
Because speed and contrast of different batches of paper are seldom exactly the same, you'll want to do a test strip (I'm assuming here that you'll be working from different boxes of paper). But you can use the formulae cited to get yourself in the ballpark. That's what I do!

--Ben
• 08-21-2008, 06:48 PM
ic-racer
Quote:

Originally Posted by Nicholas Lindan
There is an f-stop print size exposure adjustment ruler on the Darkroom Automation web site.

This looks like a good idea. I consider myself easily in the 'double digit' IQ range but frankly, I don't understand how to use it. :confused:
• 08-21-2008, 07:14 PM
pentaxuser
Quote:

Originally Posted by ic-racer
This looks like a good idea. I consider myself easily in the 'double digit' IQ range but frankly, I don't understand how to use it. :confused:

What! Don't know how to use it. Well let me explain.... that I am in exactly the same boat.:D

I have some vague idea about how it works but an amplification from Nicholas would be gratefully received. It looks great, avoiding as it does, I think, any calculations other than simple subtraction, I think, to arrive at the fstop increase/decrease which then of course if you haven't got a fstop timer has to be translated into actual time so a fstop table to time conversion is also required. Again I think but don't know.

pentaxuser
• 08-21-2008, 08:51 PM
Nicholas Lindan
Quote:

Originally Posted by Nicholas Lindan
There is an f-stop print size exposure adjustment ruler on the Darkroom Automation web site.

To use it is simple:

Measure the size of the projected image and note the numbers. You don’t measure from zero but from ‘BASE’. The difference in the numbers is the number of stops to add/subtract to the exposure time.

You can also use it to measure the easel-lensboard height if this is easier. The ‘BASE’ is then the base of the yard-stick.

Adjusting the exposure is most easily done with an f-stop timer. Dials and tables for use with analog and digital and seconds timers are available on the web.

The ruler is in 0.1 stop increments because Darkroom Automation equipment works in a uniform system of decimal stops. If you have a timer that works in fractional stops you can make a ruler in any increment you wish. The equation for distance for each stop ‘f’ is:

Inches = length of ruler * 2 ^ (-f / 2)
= length of ruler * 10 ^ (-f / 6.6439)

The 6.6439 converts from base 2 to base 10, and makes calculations easier in some spreadsheet programs.

The theory:

The ruler, like everything in f-stop land, is logarithmic in base 2.

The ruler is made ‘inside out’ - you start with the f-stop and find the distance for the mark for that stop. As a result the equations use inverse functions: exponentiation and square roots rather than logarithms and squares. Subtracting the distances (logarithms) accomplishes division. Division by 2 “(-f / 2)” when calculating the position of the marks on the ruler takes the square root.

Subtracting the two numbers gives the result of:

R = log base 2 [(x ^2) / (y ^ 2)]

where x and y are the linear distances at the two magnifications.

If the calculations started with distance and calculated the stops the resulting ruler would be filled with numbers that are hard to work with: 27 inches would be marked 0.642; 7 inches would be marked 4.53. The present method puts the marks at even stop intervals.
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