Any calculation for exposure as enlargement changes?

[Matt5791]

What agreat forum - have only just joined!

I have recenly st up my own darkroom and have been doing quite a bit of enlarging.

One question I have is:

Say I decide that 30 seconds exposure is about right for my 5X5 print I am about to make.

Is there any calculation to enable me to decide on comparable exposure if I increase the size of the enlargement to say 8X8?

I have discovered that as the enlargement is bigger I need to expose for longer, but sometimes I don't want to have to go through the busines of making test strips.

Mabey there is a rough estimation or an actual calculation?

Thanks for any help,

Matt

[Thomassauerwein]

Other than instinct and testing, I've found this Ilford tool really handy. It is an enlrger light meter of sorts called an EM10. really cheap and very accurate. You can measure a highlight then enlarge then re-set to the same spot, and finaly adjust the f-stop till you have the same exposure.

[gordrob]

Try and find an old copy of the Kodak Darkroom Guide for B&W. They have dial calculators built into them for just this purpose as well as a lot of other useful darkroom information
Gord

[Gerald Koch]

Kodak publishes a book called the Black & White Darkroom Guide. Among some very useful information it contains a calculator (sort of a circular sliderule). Once you have made a good print you put the information into the calculator and it will tell you the time for any other size print. Most good photostores will have it or order it online. No one just starting out in the darkroom should be without it.

[Pastiche]

What I do is look at the height of the enlarging head on the chasis. Many chasi's have a scale on one of the posts, or the post, somewhere.. you dont need to be acurate as far as what height the film actually is, just consistent (precise) on how you read the height. On the Beseler MX45, I always read the height of the motor switch (but any part of the machine that goes up or down with the film will do).

If say, I do an enlargment and the (enter your choice of "marker" here) switch is at 8inches, and I then decide to make things larger, say I raise the head so that my marker is now reading 11" on the post, I double the exposure. It's not dead acurate, but it gets me whithin one print of where I want to be.

What it relies on is that the way light falls off is consistent whether it's going into the camera, or comming out of it. As the distance increases, the light falls off at a predictable rate. I just choose to use aproximate f/stop height readings to judge more or less how much to change exposure times. If I started with the enlarger head at 44", and dropped it down to 6", I'd cut exposure by the same # of stops between f/5.6 and f/45 . . . It's harder to write about than to actually do... Just think about the way light is behaving, and what it's doing as you raise and lower the head. If the times get out of hand b/c of raising or lowering the head, I finish the adjustments with the lens aperture control. It's something I've been trying out to cut down on wated paper myself.. and it seems to be giving me workable approximations. YMMV.

[johnnywalker]

Is there any reason why you can't simply calculate it using the square inches of the print? If a 4x5 (20 sq. in) print takes 20 seconds, then an 8x10 (80 sq in) will take 80/20 = 4 times as long at the same f stop.

[Pastiche]

JW, I might be wrong - but it seems to me that the relation between sizes is linear, while the behaviour of light is log... (arithmetic? Im no mathematician.)

without knowing if I'm right, I'd guess that each time you double the size of the enlargement, in terms of total square inches, you have to double the exposure (EV). BUT - I think the trouble comes in where you begin to have to account for the kind of lens, and the angle of view (projection in this case) in each case... obviously a longer lens will have a narrower angle of view/projection... hm... I'm just out of my depth here... I'd better leave that last thought unfinished... better to get a solid answer.

[Pastiche]

just to throw more sand in your eye...

I have put my sekonic 358 w/ incident bulb on the easel a couple of times... nothing worth feeling confident about.. but I THINK (stress THINK) that it should work. All you really care about is total vol. of light after all...

so -

after you have made one sucessful print at size X -
pull the neg. from the carrier but dont change the head height -
set your incident/spot meter on the easel -
take a reading, and adjust the ISO untill the time matches your exposure time for the last sucessful print -
raise lower the head as desired -
take another reading -

I'd guess it will give you the right exposure for the print.
dial in your time, put the neg back in the carrier-
and expose the paper-
if it comes out totaly screwed up -
say my name in vain along with a good many explicative/descriptive terms

[Donald Miller]

This is the formula for changing exposure time for different enlarger heights for the same print:

TO*(LN/LO)^2=TN (the "*" is a multiplication sign, "^2" is "squared")

Where: TO is the original time

LN is the new length

LO is the original length

TN is the new time (at the same f-stop).

Exposure calculations are calculated using the law of inverse square.

[Pastiche]

what DM said , which is exactly the same as bellows factor calcs + time variables (aha! a little light goes on)