It is rather hard to get one's head around the thing, but the 'effective aperture' concept is the same-old square law compensation in different clothing. A thought experiment is to imagine the square-law experiment with a point source: if it works for one point source, then it will work for two point sources next to each other; ditto three, four ...; and finally, ditto a circular aperture. This is why measuring lens to easel distance works for determining exposure compensation.
Originally Posted by ic-racer
The biggest systematic error in a geometrical approach to print size compensation is because of the change in condenser "efficiency" - not the right word but I can't think of a better one. The distance from the condensers to the lens has to be held constant for a geometrical compensation scheme to really work well. You can do this on a Beseler 45 series enlarger (and on the 23 series). However, I have always used an enlarging meter when things get critical. Beseler has never mentioned this feature of their enlargers (and may not even be cognizant of it) - the condenser gauge on a Beseler should, ideally, be from the lens stage to the condenser and be fixed at the condenser focus. It's factory location, from the negative stage to the condenser, is wrong. A perfect diffusion enlarger should have no error due to diffuser-lens distance.
In practical work if the required compensation is less than 2 stops then a geometrical approach to adjusting exposure for magnification changes is more than accurate enough. With larger changes, errors due to stray light and flare become meaningful and it's test strip time.
As is often mentioned, the aesthetics of the print change with print size and often a much larger or much smaller print looks better if it is made lighter or darker or with more or less contrast than the original.
Last edited by Nicholas Lindan; 08-22-2008 at 12:49 AM. Click to view previous post history.
Personally I have no desire to turn a darkroom session into a maths session, so a test strip for me too. Papers can vary in batch, so a test will also give you the opportunity to check contrast also.
For reference, take a note of how much extra exposure was required and refer to it next time..that should spped things up a bit. Glad to hear you are noticing the qualities of darkroom printing.
Originally Posted by Gary Holliday
Neither do I and I personally would not do the equations other than very roughly in my head to get close. Then do a test.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.
I'm with Steve
I do the maths - to get close to the size corrected exposure
But because paper varies batch to batch and with age, I do another set of test strips based bracketed around the size corrected exposure
That's the whole reason for the ruler - there isn't any 'maths' to using it.
Originally Posted by Gary Holliday
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Some examples may help. There is a table of stops to percentages below:
Originally Posted by Nicholas Lindan
You want to increase the size of a print by a few inches:
- Use the ruler to measure the size of the negative carrier opening projected on the easel - read 3.0 (~12 inches).
- Change to the new size and measure again - read 2.5 (~14 inches).
- The difference is 3.0 - 2.5 = 0.5, so you increase the exposure 1/2 a stop, about 40%. If you are using an f-stop timer you just change the exposure by 0.5.
You want to decrease the size of a print by a small amount:
- Read 1.8 (~18") before
- Read 2.0 (~17") after
- The difference is .2 stops, about 15% or between 1/8 and 1/4 stop. If the old time was 20 seconds then remove 15% (3 seconds) and the new time is 17 seconds.
You want to go from a small work print to making a large final print:
- Read 4.7 before
- Read 0.4 after
- The difference is 4.3 stops - increase the setting on the f-stop timer by 4.3
- If you have a linear seconds timer, then from the table below, a 4.3 stop adjustment requires 19.7 times the exposure. If it was 10 seconds it is now about 197.
- Because the size change is so large, it is a good idea to make a new test strip around the new time.
In the last example, the new time is too long. Instead you can open the lens up 2 stops, so now the time exposure only needs to change by 2.3 stops. From the table below you can see a 2.3 stop increase requires 4.9x the time, so set the timer from 10 seconds to 49 seconds. Or add 2.3 stops to the f-stop timer - less maths that way.
A table of stops to the time adjustment.
- To add 0.2 stop, multiply the time by 1.15 or add 15%
- To remove 0.2 stop divide the time by 1.15 or subtract 15%.
Use a calculator! You lot do make life difficult for yourselves.:o
It really is level one math.
The new exposure time is the old exposure time divided by the old height, divided by the old height again, multiplied by the new height, and multiplied by the new height for a second time.
I measure the height from the paper to the lens and find the method simple, quick, and very accurate.
You may use rods, poles, perches, feet or inches for measurement, I use centimeters since they feed into my calculator easily.
It's commonly known as the "Law of Inverse Squares" (a helluva lot simpler than Simpson's Rule), if age hasn't addled my mind too badly. It works well with setting studio light, both flash and continuous, calculating bellows extension factors, and the very problem before us.
However, when I had a wet darkroom, I always had a 4X5 box of whatever paper I was using laying around and would just throw a sheet onto the easel and check exposure. Trying to apply a definite (math) to a subjective (photograph) is very much like trying to mix apples and oranges.
Just guess and do a test print. Have a beer while you're at it.
"Beer is proof that God wants us to be happy."
What a lot of math. How about "subtract - enter value into timer". That sounds a lot easier to me.
Originally Posted by Dave Miller
Originally Posted by Frank Szabo
"When mixing apples and oranges be sure to add a beer."I'll drink to that.