I'm trying to work out if I have my exposure and filtration concepts correct?

I want to shoot a room interior. The main lighting will be fairly low daylight - about 12 - 15 seconds @ f32 say. There will also be some regular household incandescent lights in the shot. These light their immediate area about 2 to 3 stops brighter. So, with a normal exposure, they would burn out the immediate area.

So, can I do 2 exposures, one for the daylight without the incandescent lights on, then one shorter one for the incandescent? In calculating the exposure, do I just base it on the longer main daylight exposure? Or do I add up the total time of the two exposures and divide proportionally? My math always gets confused on these things!

Secondly, Is it worth trying to reduce the colour cast of the incandescents a bit by using an 80a/b filter (the only blue ones I have right now), just to cut the warming effect a bit, but not totally. And how would this effect the exposure above, given the effective film speed is now around 32 or 25 from 100?

I am stuck with using transparency film - probably Fuji100F, because I need to get the images back in 5 days or so. As I am 1500 miles from the nearest lab, that means transparency, as i don't have time for sending c41 down to be processed, get contacts back, and send negs down to be printed and sent back! (I would have used c41, but I'm stuck on this one).

Thanks

Tim A

-- tim atherton (tim@KairosPhoto.com), October 24, 2000

Answers

Tim,

I'm a natural light freak and rarely use extra illumination. Therefore, I would handle your problem by making a double exposure. Exposure 1: for the incandescent lights, Exposure 2: with the incandescents turned off, for the room in ambient light. If the exposures are far enough apart, you won't need any exposure compensation for the second exposure. If they are close, subtract the first exposure from the second and expose accordingly. I use the Zone System and calculate exposure differences using a system of exposure units. Zone I is 1 unit, Zone II, 2 units, Zone III 4 units, Zone VI 8 units, Zone VI, 16 units and so on, doubling for each Zone. Then you can subtract arithmetically. For example, if your initial exposure for the incandescent lights places the rest of the room on Zone III, but you want it on Zone VI, you subtract 4 EU (exposure units for Zone III) from 32 EU (Zone VI). This gives you 28 EU, which is about a third of a stop down from the normal Zone VI exposure. As you can see, unless the two different exposures are within a couple of Zones, the difference is slight. With B&W I usually just ignore it, but with color transparency film you probably want to make those 1/3 stop adjustments. Bracketing the second exposure towards a slight underexposure will help till you get used to it. As far as using a filter (base the above calculation on the exposure before you apply the filter factor or it will be off by the filter factor!), I would probably try an 80B filter, but why not make several, some with, some without and see which you like best?

One tip: do not replacing the dark slide between exposures. Also, make sure your camera is rock solid so that you don't move anything out of registration when you change shutter settings. Take several exposures and you'll increase your chances of getting a keeper. I'd be happy to expound further if you need.

Regards, ;^D)

-- Doremus Scudder (ScudderLandreth@compuserve.com), October 25, 2000.

Have you thought about replacing the existing bulbs with lower wattage ones? D.S. advice about not moving the camera if you go the double exposure route is right on the money. Absolutely don't touch the camera between the exposures unless absolutely necessary.

As far as establishing the correct mix of exposures (in terms of light brightness, not color) that is what Polaroid is for. Since you are so far from processing I suggest you bracket heavily with at least three exposures made of each exposure combination. Make sure you take into account the possibility of reciprocity failure from such long exposures and make exposures that are 2x, 3x and 4x of what is indicated by your meter readings and polaroids.

-- Ellis Vener (evphoto@heartstone.com), October 25, 2000.