How to calculate time for a bigger print

macgreg · Sep 25, 2014

I want to start printing on 8x10 papers, so far I print only 5x7 and I wonder if there is a method to calculate the exposure for the bigger size print.

Let's say-for example-that I need 10 seconds for a 5x7. If I want to go to 8x10, keeping the same Grade filter and f stop, what will the time be ?
I think I had read somewhere about a math type that could be used, but I'm not sure. Is that the case ?

Of course I know that I could make test prints, but 8x10 papers are pretty expensive and I don't want to start "experimenting" with the time, as I do with the 5x7. Test printing the same photo small and then bigger would be cost effective.
That's why I wonder what I could do.

edit : I am sorry I posted on the wrong subforum !

LikeAPolaroid · Sep 25, 2014

The factor is: (new head height/old head height)^2.
If you double the head-paper distance you need to 4x the exposure.
But I'd always do a test strip to verify. Things are not always as linear as the math behind them.

Mainecoonmaniac · Sep 25, 2014

Take a look at Ralph's templates. His book is worth buying also.

http://www.waybeyondmonochrome.com/WBM2/Library_files/TemplatesEd2.pdf

David Brown · Sep 25, 2014

You do not have to use a whole sheet of paper for a test print. Cut the paper into 3 or 4 pieces for an exposure test. (Just have the image set up and focused for the 8x10)

cliveh · Sep 25, 2014

David Brown said:
You do not have to use a whole sheet of paper for a test print. Cut the paper into 3 or 4 pieces for an exposure test. (Just have the image set up and focused for the 8x10)

+1, as you will always achieve better results using a visual reference rather than trying to print by calculation.

David A. Goldfarb · Sep 25, 2014

5x7, 8x10, 11x14, 16x20--recognize that sequence of numbers (5, 8, 11, 16)? They're about one stop apart, so use that as a starting point when sizing up.

You might find you need to adjust contrast and exposure, because as you go up, different elements of the image may become more important than they were at a smaller size, and having the enlarger head higher might increase reflections from your darkroom walls, depending on how you've got it arranged, reducing contrast slightly.

Colin Corneau · Sep 25, 2014

cliveh said:
+1, as you will always achieve better results using a visual reference rather than trying to print by calculation.

+2.

I use a thin strip, and set my timer to 2 seconds -- then I simply block off everything but a small patch for 2 seconds; next time I uncover a little bit more for another 2, then a little bit more a third time and so on...by the time I'm done I have the full gamut from 2 seconds to (usually) 10 or 12 seconds. I can narrow it down to the second from that.

pentaxuser · Sep 25, 2014

There was once a link, kindly given by an APUGer whose name escapes me, to a site he had made himself where you in-putted the old height of the enlarger column, the old exposure time and the new column height and it supplied the new time.

This was very useful and whenever I used it always seemed accurate. I bookmarked it but the site is no longer recognised which is a pity

Anyone remember who it was?

pentaxuser

toro_mike · Sep 25, 2014

If you know the site address, you may be able to find it in the WaybackMachine - http://archive.org/web/web.php.

Rick A · Sep 25, 2014

Purchase an Ilford EM 10 exposure monitor or similar. You need to take a light reading of your negative as set for whatever the original size, move the enlarger head, focus then adjust the lens aperture to obtain the same light for the new setting.It's actually easier to do than explain.

bsdunek · Sep 25, 2014

David A. Goldfarb said:
5x7, 8x10, 11x14, 16x20--recognize that sequence of numbers (5, 8, 11, 16)? They're about one stop apart, so use that as a starting point when sizing up.

You might find you need to adjust contrast and exposure, because as you go up, different elements of the image may become more important than they were at a smaller size, and having the enlarger head higher might increase reflections from your darkroom walls, depending on how you've got it arranged, reducing contrast slightly.

This is true. I generally go by area of the print, i.e. 8X10 is roughly twice the area of a 5X7, hence twice the exposure, etc. I still do a little test strip of an important area.

Old-N-Feeble · Sep 25, 2014

bsdunek said:
This is true. I generally go by area of the print, i.e. 8X10 is roughly twice the area of a 5X7, hence twice the exposure, etc. I still do a little test strip of an important area.

The enlargement is actually a bit more than double... a half inch cropped from a 5x7.5 (to make 5x7) vs. 2 inches cropped from an 8x12 (to make 8x10) so exposure time will nearly triple. If making a 10x15 print the exposure (in theory) should be exactly 4x for the larger print (inverse square law).

ic-racer · Sep 25, 2014

Sticky has been up for a few years: (there was a url link here which no longer exists)

pentaxuser · Sep 25, 2014

I may not be experienced enough to search the archive machine but when I put in the URL it could find nothing. The site's compiler seems to be dwilder57. Does anyone know if he is still active?

Thanks

pentaxuser

Gerald C Koch · Sep 26, 2014

Pick up a copy of the Kodak Darkroom Data Guide. It contains a very handy circular slide rule to bring together f-stop, time, print ratio, paper speed, etc into one handy calculator. There is also a lot of other useful information about Kodak films, papers, chemicals.

Bruce Osgood · Sep 26, 2014

pentaxuser said:
There was once a link, kindly given by an APUGer whose name escapes me, to a site he had made himself where you in-putted the old height of the enlarger column, the old exposure time and the new column height and it supplied the new time.

This was very useful and whenever I used it always seemed accurate. I bookmarked it but the site is no longer recognised which is a pity

Anyone remember who it was?

pentaxuser

That might have been me. I shared an excel sheet I had built for this purpose.

I still use it and willing to share it again if anyone can use it. But as always, test it to make sure you like the results.

Bruce Osgood · Sep 26, 2014

I forgot to say the Gray boxes need your input and the Green boxes deliver the answer.

Also, the column of my enlarger is calibrated and those are the number used in measuring. It doesn't matter what they are, inches, cm, mm or potatoes, it's the distance that matters.

Old-N-Feeble · Sep 26, 2014

Enlarger height will work for calculation but only if you use the same identical lens... or at least the same focal length.

Bruce Osgood · Sep 26, 2014

Right.
I think that if you're making an enlargement that requires a different lens, you're making a different photograph and requires a new start.

rorye · Sep 26, 2014

Mainecoonmaniac said:
Take a look at Ralph's templates. His book is worth buying also.

http://www.waybeyondmonochrome.com/WBM2/Library_files/TemplatesEd2.pdf

+1, a great book.

How to calculate time for a bigger print

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