photography - reciprocity failure law

[Penny]

I have an assignment for college that's all about colour transparency films.

it says...

'you must implement the reciprocity failure law in at least one of your images'

i've read about what the RFL is, but i just can't take it in and understand it at all, so can someone please tell me...

how would I implement the RFL in my image? ..what would I have to do?

thanks

[Zen]

http://silvergrain.org/Photo-Tech/reciprocity.html

[Buenchico]

(Posted in 2 parts):

I've just looked at that link which Zen provided and it had my head spinning! Now, to put that into some sort of perspective, perhaps I ought to say that I've got a degree in maths, I've been a member of several photographic societies (including the RPS), I've made money as a freelance photojournalist and I collect books on photography. So, if it baffles me, you shouldn't be too bothered if it baffles you as well!

I can think of no college course which would expect you to completely understand the science and mathematics of reciprocity failure. What they require you to know is this:

1. The usual rule of "close down by one stop and double the exposure time = the same density of image" cannot be relied upon when lengthy exposures are involved (i.e. in low-light situations).
2. Because of this, readings from light-meters need to be corrected in low-light situations.
3. The amount of correction varies between film types.

So what should you do? As I've suggested, nobody expects you to start doing degree-level calculations. What you need are some correction tables. Assuming you use Kodak film, go here:
http://www.kodak.com/global/en/professional/support/tech Pubs/e31/e31.jhtml
(If you use another brand, you'll need to go to the film manufacturer's website and look for the appropriate tables).
Assuming that we're sticking with Kodak, print out Tables 5 & 6 (which are the relevant tables for colour reversal film). Remember to include these when you submit your college work.

[Buenchico]

(2nd Part):

Now seek out a photographic opportunity, for your chosen film, which avoids those parts of the table labelled 'None/No filter' or 'NR'. e.g. if you're Ektachrome 100 Plus Professional, find a low light situation and adjust your aperture until the meter indicates an exposure time of approximately 10 seconds. (Don't forget the tripod!) The table shows that you should open the lens by 1 stop, so do this. (The table also shows that, for correct colour balance you should also use a 25-Red filter so, if at all possible, do this as well). Take your picture and process it in the usual way. When you present your work, give a written description of what you did, including something like this:

"An initial aperture of f16 was selected and a meter reading for this aperture was taken. The meter indicated an exposure of 10 seconds but I was aware of reciprocity failure in low-light situations. I therefore referred to the accompanying table which is produced by Kodak. This states that an exposure increase of one stop is required. So I changed the aperture to f11 while retaining the 10 second exposure time. Also, in accordance with the table, to ensure both correct exposure and colour balance, I used a 25-red colour-correction filter".

This should be sufficient to convince your tutors that you know how to take account of reciprocity failure. (It's all theory anyway: In 'real life' any decent photographer wouldn't rely on just one shot in a situation like this. He'd take several frames, ensuring that he 'bracketed' his exposures).

Chris

[Penny]

yeh...we have to do bracketed shots too.

Thank you so much for that ^ ...its really helpful!! :)

[123paul]

When they say implement RF don't you think they mean actually take a shot and allow RF to take place so you end up with the unusual colour hues / balance etc. It can be quite effective. Try a shot of the moon over a town skyline; for some reasons it always turns out purple !

[Penny]

well...the whole assignment is about different colour casts under different lights (we have to have a cast in all of our photos), and it just said about having the RFL in at least one of my photos. so...not quite sure what this means. i would just ask the teacher but i havent seen him since he gave it to us