Wondering about an old article.

[wogster]

Back in the 1970's I had a magazine article on printing, where if you had the time on a 4x5 print from a 35mm negative, you could calculate the time on an 8x10 print of the same negative, given the same F/stop. It was all formulae for use with a scientific calculator. Anyone remember the article, or the formula, I think one could build a spread sheet of values so you could read it off a table. Might be handy for a lot of wet printers. If someone has the formula I can make a spread sheet from it and print it to a PDF for others,,,,,

 

[Konical]

Good Evening, Paul,

There was a recent thread on the topic here on APUG which had lots of participants. My own standard is (NEW DISTANCE/ORIGINAL DISTANCE) squared, times ORIGINAL EXPOSURE time. I use the scale on my enlarger standard for the measurements. It's easiest to figure using a Reverse Polish calculator. There are various ways to go about it, but it's all based on the idea of light falling off as the square of the distance.

Konical

 

[trexx]

A simple rule of thumb is one stop for each standard size increase.
4x5 +0
5x7 +1
8x10 +2
11x14 +3
16x20 +4

They are not exact but close enough to get a good starting poing and not calculator needed.
I am sure there is a reason , lost in the past, that the sizes are this progression to take advantage of these one stop differences.

TR

 

[wogster]

A simple rule of thumb is one stop for each standard size increase.
4x5 +0
5x7 +1
8x10 +2
11x14 +3
16x20 +4

They are not exact but close enough to get a good starting poing and not calculator needed.
I am sure there is a reason , lost in the past, that the sizes are this progression to take advantage of these one stop differences.

TR

I always thought the sizes were what they are, because originally all prints were contact prints, and large format cameras were those sizes. I think it's most likely they picked those sizes because they are sizes for painting canvases and that meant that originally photos could be displayed in existing frames, rather then needing to design new frames that were different sizes.

 

[Larry Bullis]

You can either use the √2 as a factor on the distance from the lens to the paper, or you can simply use the multiplier of the # of square inches. Now I'm no mathematician, but I do know the basic f/stop cipher. If, for example, you have a light at 8' from the subject and move it to 11' from the subject, your exposure would be twice as long, etc. It is great for figuring bellows draw, etc. Same thing with the enlarger. If your pointer on your tape scale on the column or however it works is pointing at 11" and you raise it to 16" you're going to add a stop. I just do this. You are going to have to verify with a test (usually just one) anyway, so why not keep it simple. The less I have to do with a calculator, the better I like it. I use the span of my fingers, on bellows draw, for example. It's 8". 11"? One stop. 16"? two stops.

I'm always amazed at how few photographers, even professionals, know this.

 

[Frank Szabo]

Try this out for an explanation:

http://en.wikipedia.org/wiki/Inverse-square_law