Light / lumens question about photography exposure times

[freedda]

TL;DR Summary

For photography: changing lumens changes exposure time by how much?

This question is part photography, part about light (and exposure times), and I assume also involves physics (or the science of light):

There is a technique in film (analog) photography called 'flashing' where you pre-expose a piece of photo paper, which cuts down on the exposure time (the time it's exposed in an enlarger with a negative above it), and it also cuts down on the contrast, giving you better midtones.

The instructions I have say that if I pre-expose (flash) the photo paper (in this case, it's for Harman Direct Positive photo paper) for 20 seconds using a 1000 lumen flashlight (or 'torch') it will cut the exposure time by half. So ... instead of exposing the paper and neg for two seconds I'd do it for one second.

However, I also have a smaller 500 lumen flashlight (which is much easier to travel with). Someone told me that if I pre-expose the photo paper for 40 seconds, it will give me the same results of halving my paper-negative exposure time.

On paper (pun intended) this seems reasonable, at least to me - who doesn't know anything about the physics of light or how lumens are measured: halfing the lumens (500 instead of 1000) needs twice the exposure time (40 instead of 20 seconds). But for some reason I have this nagging feeling that lumens and light may not be so 'linear', etc.

Any help would be greatly appreciated.

Thanks, d.

 

[Vanadium 50]

I would step back a bit - is the variable you want lumens? Or lux? Or candelas?
Then I think you are right - there could be significant nonlinearities in your emulsion response.

 

[jrmichler]

This brings back old memories of my time in a photo lab making black and white halftones, where we used flash and bump exposures to adjust the contrast. My experience was with a large copy camera. We did the flash exposure by photographing a white surface. This made for uniform illumination, and was easy to control the exposure by using the camera controls. You need uniform illumination over the entire film to make this technique work. You can measure light intensity variation using a cell phone light meter app.

Lumens is the total amount of light that a flashlight emits. Lux is the intensity of light at a particular point. The intensity of light (in lux) on a surface depends on how far away the flashlight is, and whether it is in the center of the beam or off to the side. Every flashlight I have ever seen has much higher brightness in the middle of the beam, than outside that area.

You will need to experiment. First, find a uniform light source. Second, find the best exposure for a particular image. If you guess the best flash exposure is 10 seconds, expose part of the image for 5 seconds, a larger part for 10 seconds, and the last part for 20 seconds. Then iterate from there.

 

[freedda]

I would step back a bit - is the variable you want lumens? Or lux? Or candelas?
Then I think you are right - there could be significant nonlinearities in your emulsion response.

As I said, I'm talking Lumens, as in the power /output of the flashlights I'm using. I don't know what you mean, that 'there could be significant nonlinearities in your emulsion response'. I'm talking about the linearity or not of the lumens, the light source.

D.

 

[freedda]

Lumens is the total amount of light that a flashlight emits. Lux is the intensity of light at a particular point. The intensity of light (in lux) on a surface depends on how far away the flashlight is, and whether it is in the center of the beam or off to the side. Every flashlight I have ever seen has much higher brightness in the middle of the beam, than outside that area.

You will need to experiment ....

I am talking lumens, as that is what the flashlights are rated in, and that's what the instructions say.

One detail I didn't get into: I'm doing this 'flashing' with a Pinstra pinhole camera which has a diffuser setting made just for this sort of thing. That means the light from the flashlight need to be 'focused' on an area that less than a half inch wide.

I am of course going to experiment, but I wanted to know if what I said about half the lumens needed twice the exposure time is correct. (So, instead of a 20 second 'flash' exp. at 1000 lumens, I'm looking at a 40 second exp. time with 500 lumens - to reduce the image exposure time by half).

Without any verification of this, I suppose I'll start my tests at maybe 20, 30, 40 seconds and see what happens.

 

[jrmichler]

Twice the lumens will be half the exposure IF both flashlights focus to the same area. So you will need to experiment.

Hint: Whatever your starting flash duration, make the second try double or half until you overshoot. Then split the difference until you get what you want. That should get you dialed in with minimum waste of film.

 

[Vanadium 50]

As I said, I'm talking Lumens,

I get that. My question is whether that is what you should be talking about.
Lumens refer to light in all directions, not just the direction of interest.

 

[Vanadium 50]

IF both flashlights focus to the same area.

Which is measured in candelas.

 

[freedda]

Twice the lumens will be half the exposure IF both flashlights focus to the same area. So you will need to experiment.

Hint: Whatever your starting flash duration, make the second try double or half until you overshoot. Then split the difference until you get what you want. That should get you dialed in with minimum waste of film.

And for my purposes, half the lumens would be twice the exposure time, correct? The 'benchmark' I have is

• Pre-flashing with a 1000 lumen flashlight for 20 seconds cuts my exposure time by 50%

So, I'm assuming that pre-flashing with a 500 lumen light for 40 seconds cuts my exp. time by 50% - or at least that's a good starting point, and I can dial it in from there.

 

[freedda]

I get that. My question is whether that is what you should be talking about.
Lumens refer to light in all directions, not just the direction of interest.

You are perhaps correct, however, the information I have to work with (and talk about) is what I shared: the flashlight lumen ratings of 1000 and 500 (for my flashlight), and a 'benchmark' of:

• Pre-flashing for 20 seconds with a 1000 lumen flashlight cuts your exposure time by 50%.

I understood another person here, jrmichler, to say that if I half the lumens I double the exposure time, which is the info/numbers I started with and I am asking about.

Best, D.

 

[Vanadium 50]

I think you are just going to have to experiment. I have two flashlights with the same number of lumens, but one makes a small bright spot and the other a larger less bright spot.

 

[DaveE]

I agree, you'll have to experiment with the actual stuff. My darkroom experience (ages ago) was that you need to know the concepts, but the actual parameters were always found by trial and error.

BTW, one of my favorite cheap tools was a pie chart of ND transmission filters for the first test. Easy and quick.

 

[gmax137]

or at least that's a good starting point, and I can dial it in from there.

I agree, that's as good a starting point as any other. If you've been happy with pre flash in the past, might as well try it with your new flashlight. Please let us know how it goes!

 

[gmax137]

It's been years since I tried pre flashing, but (below, stolen from another forum) is how I remember doing it. The idea is to get the paper right on the threshold of printing. Thats why the non linearity @Vanadium 50 mentioned might play into your results. Do you flash the entire paper, or just the areas you would otherwise burn?

Make a series of exposures with no negative, like a test strip, but maybe with 1 second bursts at a small aperture. Develop the paper and see at which exposure you start to see tone (just off paper white).
Pick the exposure time just before the one you get tone, if you need a lot of help with density in your highlights, or if you just need a little boost pick a smaller amount of time

 

[freedda]

I think you are just going to have to experiment. I have two flashlights with the same number of lumens, but one makes a small bright spot and the other a larger less bright spot.

You may be correct. In my situation, the pinhole camera I'm using, called a Pinstra, has a half inch dia. (approx.) diffuser and I hold the flashlight directly over that. My sense is that perhaps the flashlight's light is diffused or spread out over the photo paper regardless of the size of the flashlight's 'spot'. I'll just have to experiment to find out.

 

[freedda]

I agree, you'll have to experiment with the actual stuff. My darkroom experience (ages ago) was that you need to know the concepts, but the actual parameters were always found by trial and error.

BTW, one of my favorite cheap tools was a pie chart of ND transmission filters for the first test. Easy and quick.
View attachment 348350

Interesting! I see B&H has one of these and it's 4x5 in. so I could place it inside my camera, directly over the photo paper. My question is, can I calculate time based on different scales? For example, if I used this and did a minute long exposure, and I found the most properly exposed image at scale 12, would I then be able to use this to determine the best (shorter) exposure time? (I just watched a video of someone using one of these in an enlarger, so it seems I may be able to adapt it to my needs.)

 

[freedda]

I agree, that's as good a starting point as any other. If you've been happy with pre flash in the past, might as well try it with your new flashlight. Please let us know how it goes!

Thanks, but this is all new to me, so I have no past pre-flashes I'm happy with! The lumens/times are from someone else, and I'm trying to adjust them to my 500 lumen torch (flashlight). D.

 

[freedda]

Which is measured in candelas.

I understand. However, the only variables I have to work with are the lumens and exposure times. I've gotten some good advice and places to begin based on these.