Sorry I have this bookmarked under Print Adjustments. I got it from an earlier thread where someone kindly directed posters to this site. I simply copied the address from the site but I have just tried it by clicking on the copied address and it said not found.
I am not computer literate enough to know what to suggest now. Maybe it has simply disappeared
I certainly do hope so.
Originally Posted by rshepard
As the subject of threads seems to be pre-ordained
by the stars and planets, from a recent post on the Pure Silver list:
The easiest solution is to use an enlarging meter and
Measure the difference in light with the
meter [the meter has a special function for
doing just this] and add the meter
reading to the f-stop timer setting. You
are done ... accurate to 1/10th of a stop
And all the dodges and burns made by the timer
automatically track the change.
I do confess I make f-stop timers and enlarging
Another solution is to use an enlarging comparator
(Ilford EM-10 or an Analyte) and open/close the
lens so the same amount of light falls on the easel.
However, you are no longer using the lens at optimum
aperture and high-end lenses have fixed apertures
so the method is a non-starter.
Excel formula for scaling prints
I made an Excel spreadsheet to do the calculation for me. I have cells labeled, (in the following order):
Original exposure time: Original dimension: New dimension: New exposure time.
These are all on row 5 of my spreadsheet and in columns A,B,C and D respectively.
The formula in the D5 cell is the following (if you use other rows or cells, you must compensate accordingly, of course).
=SUMME((C5/B5)*(C5/B5))*A5 (Note: my software is in German, SUM is probably correct for English versions)
This is based on the formula: To(Ln/Lo)2=Tn
To is Original exposure time
Ln is New dimension (Length)
Lo is Original dimension (Length)
Tn is New exposure time
As mentioned above, this is not completely accurate, but will get you in the ballpark. There are always small changes in exposure/contrast to be made when scaling a print up or down anyway, since the difference in size changes our perception of detail, contrast, tonality. It does save time, however, to start here instead of making test strips.
Hope this helps,
I always use this method because any resizing / cropping of the image does not effect the calculation. By the way, stating the obvious, but it also works in reverse.
Originally Posted by DannL
My Excel program works thus:
NEW TIME = ((NEW DISTANCE / OLD DISTANCE)^2) X OLD TIME
This works for either enlarging or reducing size by distance and is is based on the NASA explanation of the Inverse Square Law.
What is lacking in this is the fact that contrast is affected and we do not believe it should be, or at least I don't. But it is and I don't know why.
When enlarging from 8X10 to 11X14 there is an ever-so slight reduction in contrast. I thought there was a problem with the math but the math is correct.
I'm not too concerned with this mystery as the results are generally quite pleasing.
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-- oops, wrong thread, this is/was a dupe --
New thread started...
Many years ago, Unicolor offered a series of workshops on the use of their products. One of the handouts you received was a card that showed the approximate adjustment in printing time as you moved from one print size to another - 4x5 to 5x7 to 8x10 to 11x14 to 16x20, etc.. While I haven't printed in color for many years, I still keep this card hanging in my darkroom.
It doesn't guarantee a perfect print, but it gets me into the ballpark so that the first workprint is close.
I would offer to scan it, but there are those copyright issue. But you can generate a similar card/table from the formulas that others have suggested.
"The working focal length of the lens must be taken into
account. As the lens is focused for larger prints it's focal
length is lessened and the lens approaches it's rated speed.
The lens becomes faster. The reverse holds true when
increasing the working focal length."
This may well explain why the inverse square law calculations quoted here have never worked that well for me, I have always had to make adjustments to time but have not been able to incorporate the differences into a new model.
The exact formula is somewhat more complex. I've never
Originally Posted by John Roberts
made a point of having it at hand and do not use it. Likely
it is some where on the Web. The exact exposure will be
some little shy of that indicated by the inverse square
law on greater magnification and the reverse when
down sizing. Dan
[FONT=Times New Roman][SIZE=3]You are correct, I believe the effect is an optical illusion[/SIZE][/FONT]
Originally Posted by Bruce Osgood