Exposure mathematics

[Calamity Jane]

Howdy!

I have been thru my photography books but they don’t go into the mathematic relationship between f-stop, shutter speed, ASA, illumination level

The problem:
I am going to be doing some tintypes where the manufacturer’s recommended exposure is F16 @ ½ Second in bright sunlight, which puts the ASA below 6 and off-scale on my best exposure meters.

If I can calculate the ASA of the tintype emulsion, I should be able to use a multiple of the ASA and adjust the shutter speed by the same factor to use my standard spot meter to determine the exposure for the tintypes.

Can somebody point me to the appropriate formula (formulae)?

Thanks!

[Jim Rice]

One stop=one shutter speed (assuming modern speeds) and = either 2x or 1/2x ASA, depending upon direction. Someone who is much smarter than me will probably elaborate.

[Ralph Barker]

Assuming an ASA/ISO of about 6 for the tintype material, if you set your meter to ISO 25, and open up two stops from the middle gray exposure it suggests, you should be close. Opening up one stop would rate the material at ISO 12 (1/2 the ISO 25 setting), and two stops would drop it to 6.

[Larry Gebhardt]

It sounds more like an ISO of 2 if you use the Sunny 16 rule. This is an additional 1 and 2/3 stops less than 6. So setting your meter to 25 and open up 3 and 2 third stops.

[Joe Smigiel]

>>>I have been thru my photography books but they don’t go into the mathematic relationship between f-stop, shutter speed, ASA, illumination level <<<

The relationship expressed by the Exposure Formula (according to Ansel Adams somewhere in the book "The Negative" I believe) is:

the proper exposure is the reciprocal of the luminance in candles per square foot for the shutter speed at an aperture that is the square root of the ISO of the film.

From this relationship the Sunny 16 Rule can be derived. For example:

using ISO 64 speed on a normal sunny day and metering the northern clear blue sky 45 degrees up from the horizon the luminance will be found to be approximately 256 candles per square foot. The proper exposure will then be:
1/256 second @ f/8
= 1/125 second @ f/11
=1/64 second@ f/16 (or practically, 1/60 @f/16)

In this situation the Sunny 16 Rule states the proper exposure is 1/ISO seconds @ f/16 for a normal frontlit subject, or in this example, 1/64 @ f/16.

You have ISO 2 as Larry has related. But, I'd forget about using ISO 6 and compensating from that reading as a different ISO number will make calculations easier. At ISO 4 compensate by exposing 1 full stop more than the indicated meter reading (or giving 2x the shutter speed), at ISO 8 give two full stops more (or 4x the shutter speed) , at ISO16 use three stops more (or 8x the shutter speed), etc. Personally, I'd go with ISO 8 given the range on your meter.

[Tracy Storer]

Two things I remember from playing with the Rockland Colloid "tintypes
1. it's an enlarging emulsion, so it's not panchromatic-different sensitivities to different colors of light.
2. also because it's an enlarging emulsion, it expects exposures in the "many seconds" range....it doesn't suffer reciprocity failure at the same rate as fast film-emulsions.
Have fun, they can be nice.

[jnantz]

bob at rockland colloid has also suggested to me ( using liquid light for dry plates )
to use a blue filter over you light meter so it will "see" the right kind of light
to meter.

good luck!

[Conrad Hoffman]

I found the following site to be pretty good with the math details on exposure: http://johnlind.tripod.com/science/scienceexposure.html

[Donald Qualls]

Jane, the exposure you quote, 1/2 second at f/16, corresponds to ISO 0.5. You would be correct in setting your meter to ISO 16 and opening up five stops (or ISO 32 and opening up six stops) in any combination of aperture and shutter -- but be aware that the Rockaloid emulsion almost certainly suffers from reciprocity failure at some point, increasing contrast (because the lower brightness areas will no longer be proportionally exposed relative to the brighter ones) and requiring still more exposure and reduced development to compensate. If you can shoot in daylight, say no dimmer than "cloudy dull" conditions, you'll be fine with just opening up the lens a stop or two from the above. If you have to increase exposure time, you might have problems; it's always best to test to be sure.

If you're shooting outdoors anyway, however, it would be much more "in period" to shoot by "sunny 16" rule than to use an electronic meter of any kind. Given the above exposure, "hazy" sun (shadows less sharp) would require f/11, "cloudy bright" (shadows very fuzzy) would go to f/8, and "cloudy dull" (almost no shadows at all) would require f/5.6, all at the same 1/2 second exposure and without requirement for any further reciprocity compensation. No meter needed -- and this is the way I shoot almost all of my images in my smaller format cameras; I rarely even carry a meter unless I've got my large format bag with me (or I'm shooting one of my two cameras with built-in metering).

And, of course, to be in period, you should make exposures using the lens cap, rather than a mechanical shutter...

[Calamity Jane]

Thank you everyone for the great information. Thanks Conrad for the link - I'm going to bookmark that one!

Thanks Donald for the "rules of thumb".

Although the spot meter isn't "period correct", I'd like to give customers the best possible portraits so maybe I'll wrap the spot meter in a fringed leather case ;-)

As soon as the last of my supplies arrive, I will do some experimenting. Maybe, with the really low-speed emulsion there will be greater latitude in the exposures.

Thanks again.