Using multiplier of area to scale print size and adjsting for print density

[keithwms]

I appreciate everybody for comments. Experimentally, I found 8x10 print at 21 seconds can be matched EXACTLY to 11x14 print at 38 seconds. Margin of error is +/- 1 second at 11x14 scale.

Yes, well the calculator I linked gives 37.3 sec. So you're off by 0.7 sec

[tkamiya]

Oh no! I'm off! Waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!!

[Ronald Moravec]

Be careful you do not run into reciprocity error when scaling up or down.

Actually you will be very close if you remember from 35mm only, 3.5x5 5x7 8x10 11x14 16x20 all one f stop apart if you open or close the lens.

You can prove this with an enlarging meter if you take care to get the cell EXACTLY on the plane of focus. It will not work if you drop it op to of the easel.

[Ian C]

With respect to posts #10, 11, 12:

The difference in going from a calculated exposure of 37.3 seconds to 38 seconds is 0.03 f.

In either case, could anyone tell the difference?

[David A. Goldfarb]

The standard sizes are about one stop apart, corresponding approximately to the f:stop series--4x5, 5x7, 8x10, 11x14, 16x20, 20x24. There will be differences, because the proportion of the frame is different, so the composition might change slightly, unless you are printing full frame or changing the proportion of the border to maintain the composition independent of the aspect ratio of the frame.

When you go larger, though, you may also find there is more light scattering from various sources, the lens, the reflection of the image from the paper onto the walls of the darkroom and back, etc., so it's not unusual to find that you need a little more contrast as you go larger, and depending on the exposure time, there may be reciprocity effects, so there's always a little fine tuning to do, and I wouldn't count on being able to calculate the new exposure precisely.

[kraker]

I would use enlarger height to get close, but still use test strips before devoting a whole piece of paper to the enlarged print.

I agree. This is indeed the way I do it. (new height)^2/(old height)^2 gives an estimate of the extra factor, 2log of that gives the difference in f-stops, but that merely gives me a new starting point for test strips, not an absolute truth for a good print. I don't even consider the height of the easel nor the exact height of the lens; I just read the heights from the enlarger column and do the math to give me an estimate; nothing more than an estimate.

[tkamiya]

I didn't expect it to be exactly right based on calculations only. But getting them as close as possibly can with calculations help. For one, I don't think paper response is exactly linear. Also they may be technically identical but different size gives different visual impressions.

I got it pretty close and to my liking - visually that is.

[Nicholas Lindan]

Geometric correction - if done carefully - is within a few hundredths of a stop. From the sticky post at the top of this forum:



Unless the exposure time is greater than 100 seconds there is no measurable reciprocity failure with paper.

The old adage 'open up a stop for each paper size' is good to 1/10th of a stop - as long as you are printing from 35mm negatives, using a 50mm lens, cropping to the central 1 inch/25mm (length) of the negative or so, only changing one print size, and going no smaller than 5x7. For most prints made from 35mm negatives these aren't onerous conditions. More exact results can be had by using 0.9 stops correction per print size. However, with a 4x5 negative/150mm lens the error can be 0.25 to 0.35 of a stop, or a print zone of density.

There is no debating the physics of the matter concerning geometric correction - it is exact. However, exact correction doesn't tell the whole story as the aesthetics of small prints and large prints are different. A small test strip won't uncover the changes needed and a full size work print is needed.

Metering will take into account the effects of changing flare levels. Flare levels don't play much of a part until print size gets really big - over 20x24 in my darkroom. If your darkroom is painted with titanium white and your enlarger leaks light like a sieve then things can be different.