How to make an eight hour exposure...

Mr Jones It is not often that I get to rural Mo. What do you charge for Koolaid..I am dieing to know?

Are you sure that an 8 hour exposure is needed to eliminate people. I seem to remember some photos doing the same with an exposure of 40 min.

I finally found the time to make some tests and here are the results:

Shot Fortepan 200 with an IR filter (the one that is too dark to see anything but the sun through it) at f8 and an exposure of 3.25 hours gave excellent results (rodinal 1+50, 11 min).

Shot Ilford Ortho with a deep red filter and f32 for 30 minutes and the results were OK too. Developed it toghether with the Forte, but it actually needed less time in the soup (8 minutes would have given great highlight detail, now they are dense).

All tests were made at bright sunlight, outside.

I guess that if you want to shoot indoors, you should make some trial exposures, but I am sure you're going to find a way to expose for the right time. My guess is that 30 minutes are more than enough in order to avoid any moving object from appearing on the film.

David said:

When I was doing the really long Cathedral exposures I couldn't find anything in the way of good information. The times were just too far out of the norm. You just learn by doing. What I did find through experimentation was that with the really long exposures it was almost impossible to blow out the highlights - something that was both unexpected and counter-intuitive. I develop by inspection but the decelopment times were pretty close to normal (whatever that is).

Click to expand...

Hi David,

Different films have different reciprocity curves and I hope you'll accept that there is a point with all films at which no amount of reciprocity failure can offset the lack of light. On a graph of time against negative density this would show as flat. At the other 'normal' end we have the usual nicely graduated S curve ranging in density from fb+fog up to the dmax of the film/developer combination. As we approach an infinite reciprocity factor we tend towards the former and at some point the threshold becomes more critical and we lose the ability to distinguish between anything but either the exposed or unexposed clumps - ie we lose tonality. This is the point I was trying to make however I admit it didn't exactly come out as such (and may still be somewhat confusing). A similar situation occurs when you overrate film to near its limits.

I agree that with some films an eight hour exposure is viable and you can end up with a negative with reasonable/good tonality. Ilford doesn't appear to publish any reciprocity information on SFX so it may or may not suffer badly from this type of failure.

Roger.

Sorry that I didn't think to post this earlier. I had forgotten that SFX was in a test group of potential astrophoto films. Robert Reeves has tested SFX along with other B&W films for reciprocity failure and use for astrophotography. See: http://www.robertreeves.com/b&w.htm for the full scoop. He has the Schwarzschild formula and the Schwarzschild factor for SFX posted there, so you can do some calculations. SFX loses 2.67 stops in a two minute exposure when adjusted from a short exposure with no reciprocity allowance, so you can think of the ISO as dropping to about 32 in a two minute exposure, and even lower for longer times.

There are several threads on APUG on reciprocity failure that will give you a few formulae and references on how to use them. Those formulae and the Schwarzschild exponent from Reeves' test might help you do a better estimate of where you reach the point of total failure. However, I'd bet you'll still have to test to be certain of usable results. Look at his testing setup to get ideas of how to make some long exposure tests of your own.

Hope this helps.

Lee