Changing Paper Size.

[Andy K]

I have only been making my own enlargements ('print' just doesn't feel like the right word?) for a couple of months, I am almost completely self taught from books and advice given here on APUG.
At the moment I am enlarging mostly onto 5"x"7, I have made some 8"x10". I now have several prints I would like larger, not much, maybe up to 6.5"x8.5". Is there a ratio I could use to calculate the new exposure time? I always catalogue print exposure time (including dodge and burn details), enlarger head height, f stop and filtration, so I have a point to work from.
The reason I ask is I do not want to use too much paper. I don't mind using a couple of sheets cut into test strips, but if there is a way to get close to the correct exposure time without several tests, I would be grateful to hear it!

Thanks in advance,

Andy.

 

[CharlieM]

I think the short answer to that I'm afraid is "no" (in my experience). Whenever I try short cuts I end up using even more time/paper and have to do a new test-strip anyway...

 

[edz]

I have only been making my own enlargements ('print' just doesn't feel like the right word?) for a couple of months, I am almost completely self taught from books and advice given here on APUG.
At the moment I am enlarging mostly onto 5"x"7, I have made some 8"x10". I now have several prints I would like larger, not much, maybe up to 6.5"x8.5". Is there a ratio I could use to calculate the new exposure time?

No. But the difference you have selected is small enough that the following will be sufficient:

(1+M0)^2
---------------
(1+M1)^2

Say with the 5x7" you enlarged from a KB (24x36mm) negative using a masking frame and an enlargement ratio of 5.5. Then to print onto the larger paper you use an enlargement factor of 7 then:

(1+7)^2 = 64
-------------------- = 1.5
(1+5.5)^2 = 42.25

So if the exposure took 20 seconds the new exposure should take 30 seconds.
(the are, however, differences between emulsions batches and unless they are from the same master roll and have been identically storaged you will see variances).

P.S.: The above also assumed they you wil be using the same enlarging objective.

 

[David A. Goldfarb]

5x7, 8x10, 11x14, 16x20--Do those numbers look familiar? All things being equal, you should find that you have approximately a one stop difference as you move up by standard paper sizes. Things not being equal, you might find that you need to make adjustments in contrast as well as you go up, but that rule should put you in the ballpark for a first print at a new size.

 

[Rlibersky]

I've tried this with the same results as stated above. Will end up starting over anyway.

Randy

 

[Nige]

there are formula's and rules of thumb. The rules of thumb are often based around moving from one fixed size to another, which I don't seem to do when increasing the size of an enlargement.

I use one based on recording the time and the image size on the baseboard and the new size you want. It's quite accurate although when printing bigger I find I regularly will go up in contrast a little to get the print I want. I think that's a visual thing. For the formula, I have a PDA that was given to me with a spreadsheet I key in the parameters but written down it's...

New Time = OLD Time * (New Magnification Factor + 1) ^ 2 / (Old Magnification Factor + 1) ^ 2)

I measure & record the size of the image on the baseboard and let the spreadsheet divide that by the size of the neg in mm to work out the magnification factor.

Other methods include using an Ilford EM10, of the Kodak B&W darkroom Guide (which has a little wheel thingy that estimates the new time)

 

[modafoto]

(1+M0)^2
---------------
(1+M1)^2

The rule above gives you a fair starting point from which you can do your testing. At least you will have a hint...I usually do trial and error with this not calculating, but only guessing a bit when I change print size. I normally do some prints on 5"x7" before doing a 18x24 cm print or 8"x10" or larger. When I have found a good way of dodging and burning I move the head up and do some tests. Normally the exposure is within 15% on either side. So a good starting point it is to calculate.

 

[haris]

Andy, I have enlarger exposure meter/timer, and even with that from time to time I must use some "old fashion" work. As CharlieM said, there is no real shorcuts to fine printing...

 

[Andy K]

I figured as much. Still, it was worth a punt, lol!

Thanks to all who replied. Every time I ask a question, the replies just show I am still only scraping at the very surface of photography and that there is more to be learned all the time!

 

[Gerald Koch]

Invest in Kodak's Black & White Darkroom Dataguide. It contains a circular slide rule which allows you to recalculate exposure time based on changes in magnification, changes in f-stop, paper speed, etc. Lots of useful information and other calculators. Well worth the price particularly for someone starting out.