How to calculate time for a bigger print

[macgreg]

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]

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]

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

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

 

[David Brown]

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]

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]

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]

+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]

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]

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

 

[Rick A]

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]

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]

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]

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

 

[pentaxuser]

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]

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]

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]

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]

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

 

[Bruce Osgood]

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]

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.