Multiple flash exposures.

marciofs · Jan 16, 2015

I need help in a thinking because I am not the most inteligent guy you will meet.

I was reading about Ilford MGIV paper being equivalent to about ISO 25.

I metered an object to be photographed using a flash on its full power at ISO25. The apeartire given was f2.8.

If my pinhole camera has an f248 does it means I will get the right exposure if I fire the flash 4 times?

Because if I fire once and it is f2.8 it would mean that if I fire twice it would be f11, if I fire 3 times it would be f45, and finally if I fire 4 times it would be about f181 (which is about one stop from f248). Does it make sense?

Also, looking at different f-stop charts, some starts from f1 (0) and some starts from f0.7 (-1). I supose the correct would be to start from f1 right?

sandholm · Jan 16, 2015

The full f-stop scale is
2.8 4 5.6 8 11 16 22 32 45 64 90 128 180 256
Where between each f-stop 1/2 of the light is let through, so the light needs to be doubled between each to keep the same exposure

so to go from 2.8 -> 256 shouldn't that be 13x of light needed at 256 as in 2.8? Its friday and I am a tired so maybe I am thinking in the wrong way.

cheers
(if you have a light meeter with accumulate metering its pretty easy to figure out)

Ian Grant · Jan 16, 2015

Your estimated speed is way out, MGIV has an EI of around 3. Also when using multiple flash exposures it's not a simple multiplication as reciprocity becomes an issue, so when you think 8 flashes may be enough for a 3 stop increase it may in fact need 12 or even 16.

Ian

marciofs · Jan 16, 2015

F1.0, f1.4 and f2 are not full stop?

Let's put in an other way.

If my mtter indicates f5.6. From f1 to f5.6 there are 6 full stops. If i fire twice i would add an other 6 full stops Meaning f45. And if I fire three times I would add an other 6 full stop, meaning f362, which is also only one stop above my pinhole fstop.
Would it not mean I could fire three times and get the right exposure?

marciofs · Jan 16, 2015

Ian Grant said:
Your estimated speed is way out, MGIV has an EI of around 3. Also when using multiple flash exposures it's not a simple multiplication as reciprocity becomes an issue, so when you think 8 flashes may be enough for a 3 stop increase it may in fact need 12 or even 16.

Ian

You are saying that there are reciprocity failure when using multiple flash light?

Ian Grant · Jan 16, 2015

marciofs said:
You are saying that there are reciprocity failure when using multiple flash light?

Yes, it's a well known issue and it doesn't matter whether you are using film or paper. It also occurs when you use a number of multiple exposures in daylight to build up an overall exposure.

Ian

marciofs · Jan 16, 2015

Ian Grant said:
Yes, it's a well known issue and it doesn't matter whether you are using film or paper. It also occurs when you use a number of multiple exposures in daylight to build up an overall exposure.

Ian

I know it is a well know issue. What I didn't know is that would happen with flash light or multiple exposures. I thought it was a long exposure issue.

Ian Grant · Jan 16, 2015

marciofs said:
F1.0, f1.4 and f2 are not full stop?

Let's put in an other way.

If my mtter indicates f5.6. From f1 to f5.6 there are 6 full stops. If i fire twice i would add an other 6 full stops Meaning f45. And if I fire three times I would add an other 6 full stop, meaning f362, which is also only one stop above my pinhole fstop.
Would it not mean I could fire three times and get the right exposure?

If your meter says f5.6 and that requires 1 flash then f8 requires 2, f11 in practice probably needs 6 flashes and the more you stop down the greater the number of additional flashes needed.

Ian

Steve Smith · Jan 16, 2015

marciofs said:
If i fire twice i would add an other 6 full stops Meaning

If you fire twice that is twice as much light - so one extra stop.

Steve.

Ian Grant · Jan 16, 2015

marciofs said:
I know it is a well know issue. What I didn't know is that would happen with flash light. I thought it was a long exposure issue.

I meant it was well known when using multiple flash exposures. I shot underground with a group of photographers we had 4 or 5 large hammer head flashes painting the cavern, I wanted 32 flashes from each gun, everyone else closed their shutters after about 4-8, I got my image they had almost blank film.

Ian

ME Super · Jan 16, 2015

Each stop is a doubling or halving of exposure. Since you're going from large aperture to small, and sensitivity and shutter speed are not changing, only aperture, you'll need double the light for each stop... If one flash is used for f/2.8, 2 gets you to f/4. You'll need four flashes to get to f/5.6, 8 for f/8, 16 for f/11., and so on. By three time you get to f/256, you'll need 213 (that's 4096) firings of your flash, and I haven't even taken reciprocity failure into account. Hope you have lots of batteries and patience! You're provably just going to have to crank up the lights and do a several hour exposure if you want to do this with a pinhole camera.

baachitraka · Jan 16, 2015

I rather use continuous light if possible...

marciofs · Jan 16, 2015

ME Super said:
Each stop is a doubling or halving of exposure. Since you're going from large aperture to small, and sensitivity and shutter speed are not changing, only aperture, you'll need double the light for each stop... If one flash is used for f/2.8, 2 gets you to f/4. You'll need four flashes to get to f/5.6, 8 for f/8, 16 for f/11., and so on. By three time you get to f/256, you'll need 213 (that's 4096) firings of your flash, and I haven't even taken reciprocity failure into account. Hope you have lots of batteries and patience! You're provably just going to have to crank up the lights and do a several hour exposure if you want to do this with a pinhole camera.

Ok I got it. Thanks.

Ian Grant said:
I meant it was well known when using multiple flash exposures. I shot underground with a group of photographers we had 4 or 5 large hammer head flashes painting the cavern, I wanted 32 flashes from each gun, everyone else closed their shutters after about 4-8, I got my image they had almost blank film.

Ian

Since it is my first attempt it is very new to me.

baachitraka said:
I rather use continuous light if possible...

It is what i have being doing. But I need a stronger light power than the 150w I got and it is not easy to find stronger light where I live. It may also be much more expensive.

Any way, I am just trying to see if I can find out a way to do pinhole portraits without the sitter have to stay still for more than a minute or two.

Ian C · Jan 16, 2015

In reply to post #5

You are saying that there is reciprocity failure when using multiple flash light?

Echoing what Mr. Grant has said, reciprocity failure occurs when the light intensity at the film or paper falls below some threshold value. We usually associate this threshold with long exposure time for continuous-light exposure. The same thing happens when many individual doses of light are requiredsuch as in using many successive pops of flash. The light intensity of each single pop of the flash at the film or paper has fallen below the threshold (has fallen off of the straight-line portion of the characteristic curve for the material).

For example, if you measured the flash and decided that you need 8 pops of the flash for correct exposure, but the light value reaching the film or paper had fallen to the point that you need, say, 2/3 stops of additional exposure to compensate for reciprocity failure, youd give the film or paper 12 pops instead of 8. If the reciprocity failure was one stop, then youd need 16 pops instead of 8.

marciofs · Jan 16, 2015

So I guess the only solution for me is to stick with film negatives at high ISO with continious light.

Ian Grant · Jan 16, 2015

marciofs said:
Since it is my first attempt it is very new to me.

Fair comment, we are here to help each other

I bought some modelling lights recently at a camera fair here in the UK they are 220v 250W and very bright, they fit my Elinchrom flash units but are good on their own. They were about £4-£5 a bulb this is the code on them - FR JDDE27

Another option is the builders type halogen lamps, these come with one or two flood lights per stand, mine were about £20 each, I bought a pair (2 lamps on each) a few years ago as video lighting, they've been very useful. These are the brightest low cost continuous lights and might be an option with paper

Ian

marciofs · Jan 16, 2015

Ian Grant said:
Fair comment, we are here to help each other

I bought some modelling lights recently at a camera fair here in the UK they are 220v 250W and very bright, they fit my Elinchrom flash units but are good on their own. They were about £4-£5 a bulb this is the code on them - FR JDDE27

Another option is the builders type halogen lamps, these come with one or two flood lights per stand, mine were about £20 each, I bought a pair (2 lamps on each) a few years ago as video lighting, they've been very useful. These are the brightest low cost continuous lights and might be an option with paper

Ian

Thank you. I will look for it.

ic-racer · Jan 16, 2015

marciofs said:
It is what i have being doing. But I need a stronger light power than the 150w I got and it is not easy to find stronger light where I live. It may also be much more expensive.

I'd certainly go with continuous lighting. My simple Smith-Victor hotlights accept up to 500W screw-in lamps.

Isn't Osram a German company? They make 500w lamps.

marciofs · Jan 16, 2015

ic-racer said:
I'd certainly go with continuous lighting. My simple Smith-Victor hotlights accept up to 500W screw-in lamps.

Isn't Osram a German company? They make 500w lamps.
View attachment 101473

I think it is because I am in a small city far from big centers, the specialized shops here sell now lamps stronger than 150w. It looks like I will have to buy from internet.

David Allen · Jan 16, 2015

ic-racer said:
I'd certainly go with continuous lighting. My simple Smith-Victor hotlights accept up to 500W screw-in lamps.

Isn't Osram a German company? They make 500w lamps.
View attachment 101473

500w lamps are no longer legal in the European Union as part of wider measures to improve the efficient use of resources and protect the environment.

Similar rules have led photographers having to take drastic measures to source certain beloved chemicals for making their own developers.

Bests,

David.
www.dsallen.de

MattKing · Jan 16, 2015

marciofs said:
I know it is a well know issue. What I didn't know is that would happen with flash light or multiple exposures. I thought it was a long exposure issue.

Reciprocity failure is actually due to the light being of very low intensity (at least when we are dealing with low light levels). We try to compensate for those low levels by increasing the exposure time, but due to the fact that the emulsion does not respond linearly to those low light levels, doubling the time, or doubling the flashes, does not fully compensate for reducing the light intensity at the film by one half (one stop).

There is another type of reciprocity failure that happens when the flash duration is too short. Don't let any discussions of that confuse your consideration of this problem.

wiltw · Jan 16, 2015

sandholm said:
The full f-stop scale is
2.8 4 5.6 8 11 16 22 32 45 64 90 128 180 256
Where between each f-stop 1/2 of the light is let through, so the light needs to be doubled between each to keep the same exposure

so to go from 2.8 -> 256 shouldn't that be 13x of light needed at 256 as in 2.8? Its friday and I am a tired so maybe I am thinking in the wrong way.

cheers
(if you have a light meeter with accumulate metering its pretty easy to figure out)

+13EV of light is 8192 times as much light (each time it DOUBLES). So instead of one pop of the flash, you need 8192 pops of the flash to increase exposure by +13EV

Reciprocity applies to very low levels of light, and the accumulation of exposure. One might say that at f/256 the admitted light to the film is very little amount, so that doubling the light does not increase exposure by a normal geometric progression. because we are still on the toe of the S-curve of the film rather than up on the linear portion of the sensitivity. I'd say that the light striking the film at f/256 is weak enough to need to consider reciprocity failure and may even need an additional 2-3 EV of exposure accumulation.

M Carter · Jan 16, 2015

Some thoughts, if you need to actually light stuff at those tiny apertures:

Here in the US, there's a lot of older generation speedotron black line stuff available, 70's era - but they're workhorses, I have several packs from the 70's and use 'em all the time. The 2401A (2400 watt second) pack was pretty ubiquitous and shows up on eBay/etc often. Expect to pay a couple hundred bucks, but that and two heads = a lot of light. The 1201 A is also a great basic pack with some oomph.

There's lots of pack & head gear on the used market, but not much at the power levels Speedotron offered, that will still be running like new after decades.

For hot lights, you can still buy professional tungsten lights with a lot of power. The Source Four 575 watt par (copied by Altman, and MTB makes one for the disco market called the flexipar, basically the same fixture for about 100 bucks) is a 575 watt light with 4 different lenses (wide, spot, etc). That's one insanely efficient light fixture; you'd probably want to bounce it off a ceiling or white sheet for soft light. It's got pretty amazing output.

Used theatrical open-faced lights (the rectangular style, sometimes called "nook lights" or "cyc lights") use the same style bulbs as work lights from the hardware store, but can be lamped with 100, 200, 350, 500 and 650 or 750 bulbs. They're basically "work lights" but have real barn doors and yokes. They usually have a ceiling clamp, which you replace with a stand adapter (called a TVMP). A 650 or 750 puts out some lumens. Again, you'd use these for bounce or through diffuse fabric.

Used theatrical and movie fresnels (there's all kinds of new fresnels out there, I'm talking on-the-cheap) are pretty great when you actually need to point light directly at things or people. Range (in smaller use sizes) from 2" or so to 8" lenses, from 100 watts to 1000. There are dozens of current and discontinued brands out there.

You can get a lot of control over tungsten lights with dimmers - (in the US) regular household dimmers max out at about 600 watts and they'll burn out with big lights. Router speed control boxes ($25 at Harbor Freight) make great dimmers for up to 1K or so.

There's all kinds of pro stuff out there, but I'm guessing when you're ready to spend $2000 and up on a light you'll find it... I still have quite a pile of used theatrical lights I use for paying work - started out with 'em, too good to replace!

cliveh · Jan 16, 2015

marciofs said:
If my pinhole camera has an f248 does it means I will get the right exposure if I fire the flash 4 times?

If you are using a pinhole camera you need daylight. A flash will make no difference. You need bright sunshine.

ME Super · Jan 16, 2015

wiltw said:
+13EV of light is 8192 times as much light (each time it DOUBLES). So instead of one pop of the flash, you need 8192 pops of the flash to increase exposure by +13EV

Whoops! I was off by a factor of two. Sorry about that! And yes, you'll need to take reciprocity into account, so I'd definitely recommend continuous light and not a flash for this. Talk about a battery killer!

Multiple flash exposures.

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