"Pushing" is amplifying or multiplying or "cranking up the gain of" whatever base image is there. You start with a captured brightness range of between 0 and x, and if x is still too dark, you can multiply the whole range of brightnesses in the picture by, say, 4 (the "push"), and get a brightness range of between 0 to 4x (any number times zero is still zero, of course).
Result - a steeper line connecting 0 and the maximum (higher "gamma") = more contrast, plus a stronger maximum and mid-range - a picture you can see.
Thus all of the following do roughly similar things (assuming, in the case of a digital capture, that the sensor is "ISO-invariant"):
- underexpose a sensor by setting the camera to a higher ISO than base, and letting the camera "push" or multiply the image values for you
- underexpose a sensor by leaving the camera ISO setting at base ISO, but using a hand-held metered exposure (or exposure-compensation dial) to feed it too little light, and then doing the amplification in LR or Photoshop or whatever.
- underexpose film, and amplify the silver in it by extended development (pushing it chemically)
- underexpose film, and give it normal development, and then scan the film, and use LR to "crank up the gain" just like any other digital image.
The main obvious difference will be this: chemical "pushing" tends to increase the "grain/noise" in the highlights, by creating more and more silver in the only place there is any exposure to develop. Digital pushing tends to increase the "grain/noise" in the shadows, because that is where the signal/noise ratio is worst. *
Pushed film detail - Pushed (high ISO - 16000) digital -
clean shadows, noisy bright tones noisy shadows, smoother highlights
Digital pushing is rather like recording sound with a weak mic or preamp, and then trying to recover the original sound by cranking the primary amp or playback volume up to 11 - you can hear the song, but also every tiny pop and scratchy bit of static noise.
There may be other differences, just due to the fact that the whole "film" thing tends to be non-linear, especially at the extremes of exposure (toe and shoulder of the film's response to light = "characteristic curve" ** ), and in development (where to compensate for a whole stop of underexposure (half the needed light) you give 1.4x (root-2) the development. You may find that playing with a "curve" in LR better replicates chemical pushing than an overall exposure bump of "+1". Or some combination of curves, exposure, contrast, shadows or highlights, etc.
* the one thing that frustrates me about digital - after 40 years of seeing pushed film pictures with (usually) grainy highlights and smooth empty shadows, the noisy shadows and "smoother" highlights of high-ISO digital just look - wrong - to me.
** http://www.sprawls.o...cteristic Curve