Pushing 1 Stop.. will really yield 1 stop more sensitivity?
Posted 20 November 2007 - 04:01 PM
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.../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...
Posted 20 November 2007 - 04:30 PM
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.
Posted 20 November 2007 - 07:34 PM