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Marc Roessler

Pushing 1 Stop.. will really yield 1 stop more sensitivity?

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I thought so - until i read this:

 

With longer development times, such as those used in push processing, EI values actually do increase, but only slightly. Underexpose a film by two stops and give it a two-stop push, and the real film speed will typically increase by perhaps a half stop. This means that the film is really underexposed by only 1 ½ stops, not two stops. But it is underexposed.

 

(from: http://www.kodak.com/cluster/global/en/pro.../o3/O3wp3.jhtml )

 

Am I getting something wrong here? Pushing 2 Stops does not really give you 2 stops?

Why is it called "pushing 2 stops" then?

I'm slightly confused...

 

Greetings,

Marc

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Pushing doesn't change sensitivity. There is some theory that very low levels of flashing (similar to latensification) can improve low-end sensitivity by giving those silver halide grains the few more photons needed to jump to an energy level that makes the molecule "developable" into silver.

 

Pushing -- i.e. extended development -- just increases the density of the negative, usually to compensate for a potential lack of density from underexposure if developed normally.

 

For this reason, there is also an increase in contrast -- you're taking less information captured and sort of forcing it to cover a wider brightness range.

 

Now when people say that a 2-stop push does not give you two more stops worth of density, they mean that pushing is not an exact science and there is often not a precise gain of extra density to perfectly match the amount of underexposure (exposing is not an exact science either afterall.) Sometimes when you measure the density of the film that got a 2-stop push, you find it only got a 1 2/3 stop increase in density. Or in other words, if each stop on a printer is equal to 8 points, let's say (I can't remember if it is 8 or 6) and a normally exposed image prints at 25, 25, 25 at this particular lab, then in theory something underexposed by two-stops and pushed by two-stops should also print at 25, 25, 25. But often it doesn't, it may be more like 24, 24, 22 (I'm just throwing out these numbers.)

 

This is why it is safer to rate the stock a little slower than the amount you are pushing for -- in other words, if you are going to push 500T stock by two-stops, rate it at 1600 ASA instead of 2000 ASA, if not even 1280 ASA. But you still ask for a two-stop push from the lab.

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Could that theoretical half-stop increase come from the increased contrast? I doubt most films increase contrast in a perfect linear way when they are over developed. Perhaps that big a push causes middle grey to fall a bit higher on the curve and, since finding a normal middle grey density is how EI is determined, that causes the slight EI increase.

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