Reciprocity failure

[Leigh]

When I see an increasing rate of change, I think of differential calculus not multiplication.

Oh WOW! Calculus! Is that supposed to intimidate us? Sorry... it doen't.

Who mentioned rate of change? Nobody but you.
This is not a rate of change problem in the first place, which is why nobody else mentioned it.
Your comment just emphasizes your total lack of understanding of the underlying subject.

I should go out a shoot some film, and not waste my time with you lot.

That's a marvelous idea. In the process, you can quit wasting our time.

- Leigh

[Helen Bach]

The book in your link is out of stock. I know what reciprocal law failure is, E. by the way; I have bothered to look.

Anyway,
Of course, the "Law" is not even a good suggestion at very long exposures(which is the topic of this thread). And most light meters don't work reliably there anyway(the Luna Pro SBC being a notable exception).

Add to that, differences between films, say when Acros 100 becomes faster that a 400 film. The law is useless for determining what is an equivalent time for say an f/11 8 minute shot on Acros compared to HP5+, for example. E., how long would you time that HP5 for, using the Law?

The reciprocity law is a good and very useful law over the range of exposure times for which it holds* - virtually all exposure meters are based on it. Here we are talking about the failure of the law - ie long exposure times for which the law doesn't hold (in this case, strictly speaking it is the low intensities or illuminances for which it doesn't hold: LIRF). You can't expect a law to be much use once it has failed.

*Between 1/10,000 s and 1 s for the Portra 160 used by the OP, for example.

Best,
Helen

[Heroique]

You can't expect a law to be much use once it has failed.

I think a law that fails is intuitively troublesome to many people.

A law is supposed to “stay” true, they deeply feel.

A legislative law that fails is easier to accept. ;^)

[mandoman7]

Rarely is sensitivity fully linear. Just ask my ex-wife.

[Heroique]

It tends toward logarithmic in this forum.

Or maybe it’s geometric – whichever is worse.

Easy to mix up.

[Andrew O'Neill]

Was my suggestion to the OP to experiment(because reciprocal values no longer work), and figure out usable apertures and shutter times offensive?

Richard, this statement is not offensive at all. It is exactly what all photographers should do if they truly want to understand their film of choice. One doesn't have to be as thorough as I was about testing. One could easily start with the manufacturer's data (which I find to be a bit on the long side exposure-wise ), try it in the field and see what happens. Adjust accordingly. Keep notes, etc.
Understanding your tools and controling as many variables as possible, makes life so much easier in the field.
Now it's back to ripping up plywood on my deck (which is the roof to my darkroom) for the vinyl guy on Monday.... Man, that's hard work!

cheers
andrew

[Jody_S]

As someone who's done a fair bit of night photography, in all formats (and studied math): experiment with a given film, and then never change films. Fill your freezer if there's any doubt. Reciprocity tables are virtually useless for night photography unless you're in a brightly-lit city (IMHO).

That being said, any time I am forced to use a new film, I do find it useful to use teh Googles and try to find some hints before doing my own tests.

[Helen Bach]

I think a law that fails is intuitively troublesome to many people.

A law is supposed to “stay” true, they deeply feel.

A legislative law that fails is easier to accept. ;^)

Fortunately almost every film photographer is quite happy to trust and make extensive practical use of the reciprocity law without it troubling them one little bit. The entire method of film speed and exposure determination would have to be very much more complicated if the reciprocity law could never be trusted.

It isn't unusual for things that are generally called laws to have specific conditions that define the range of their applicability.

Best,
Helen

[Helen Bach]

Is it worth mentioning that the reciprocity law has nothing to do with the shape of the H-D curve? The H-D curve could have no straight-line section and the reciprocity law could still hold. What the reciprocity law means is that if H (the light energy arriving at the film surface) is constant, the resulting density will be constant. If the reciprocity law fails, that no longer holds: H could remain constant yet D could vary.

In practice I work almost entirely by experience for long exposures at night - knowing f-number and time combinations that work for a particular film in particular circumstances. I have a terrible habit of falling asleep at night no matter how sober, cold and uncomfortable I am, so many of the exposures I have made have been timed by how long I sleep for. It's amazing how many have turned out OK.

Best,
Helen

[Jerry Bodine]

It isn't unusual for things that are generally called laws to have specific conditions that define the range of their applicability.

I certainly agree with that, and I have to say that it made me chuckle as it reminds me of an engineer I worked with many many years ago who always referred to Ohm's Law as Ohm's Opinion.