developing – Overexposing and pushing in a roll of film, can they compensate each other?

If we accidently, or on purpose, err as to exposure, likely the outcome will yield a satisfactory image. This is because the negative / positive method is very forgiving. Think of the negative as just a means to an end. Nobody holds up a negative and says “look at Aunt Sadie, doesn’t she look grand”. We must print the negative to get a positive print. This printing step is tantamount to a retake of the original scene. In other words we get a second chance to expose the original scene, this time on paper. During this second exposure we are able to work our magic and make corrections. This second exposure (printing stage) is the chief ingredient in what we call “latitude”.

We can push the film during the initial developing. As a general rule of thumb the factors to use are: 1.5x developing time doubles the ISO (1 stop). 2 stops = 2.25x and 3 stops = 4.5x the regular time.

That all being said, let’s look again at your question: You wish to push process 100 ISO to 400 ISO — that’s two f/stops. Further, you said you were going to over-expose. If you over expose 2-stops, this is tantamount to re-rating the 100 ISO to 50 ISO. This will result in an over-exposure. Now you need to compensate by “pulling”. We pull by shorting the developer time. We are talking about a 20% thru 25% reduction in developer time.

About film exposure latitude: We are talking about the range of exposure levels that will yield an acceptable image. For negative materials, exposure latitude is greater in the over-exposed direction. We express these in terms of f/stops. An f/stop is a 2x change, a doubling or halving the normal rating.