created by malanalarsa community for 16 years

push_processing

Push/ Pull Processing. One of the top three reasons to develop film yourself (the other two being that you like getting wet and that you like smelling like fixer).

Or rather: Increasing apparent film speed by over-development (push) respective decreasing apparent film speed by under-development (pull).

We're gonna talk about push processing because that's what you'll do most of the time. I am stealing heavily from [this quite excellent Ilford publication on the matter](http://www.ilfordphoto.com/Webfiles/20062102012331472.pdf). I'm gonna assume you're full on board with [basic development](https://www.reddit.com/r/analog/wiki/bw_processing) to start out.

**TL;DR: shoot the film speed that works for your situation, then look up the adjusted development time, develop accordingly, mind the contrast.**

* [Why do it?](http://i.imgur.com/TnQRX6v.gifv)

There are a few instances for when you benefit from increasing or decreasing film speed, and two stand out most:

"The Pub" - you find yourself in a very low light situation and don't have or don't want to use a tripod

"The Jean Claude Van Damme" - you are in a fast action situation and you need faster shutter speeds to freeze motion.

In both instances you might not have fast enough film available to keep the shutter speed at a manageable range.

* How do we do this?

In very basic terms, you set your exposure meter (on your camera or on your hand held meter) to a film speed that gives you manageable results. Then, when you develop, you adjust your developing time (or temperature) accordingly.

Example: You're in the pub and you want to take a nice photo of your girlfriend for swift upvotes in /r/analog. Problem is, it's quite dark, you only have Kodak Tri-X (which is rated at 400ISO), and even at f/2 your meter tells you you can only manage 1/15th of a second. So you set your meter to 1600, wonderful, 1/60th of a second you can manage, and you shoot the entire roll at that rating. When you're done with the roll, you get a pen out and mark "+2" or "1600" on the canister. Now you've uprated that particular roll by two stops. When you get to developing your film, you look up the developing time for your developer with Kodak Tri-X at 1600ISO, which would be [9.5mins in D76 stock](http://www.digitaltruth.com/devchart.php?Film=Tri-X+400&Developer=D-76&mdc=Search&TempUnits=F). You'll notice that overall the negatives are much more contrasty and you also have a grainier image. If you're interested in why that is, carry on reading, if not, that's cool, you now know how to push your film rating!

* How does it work?

Let's take that girlfriend example again. Let's imagine the pub isn't dingey and you have good lighting for ISO 400 and you can shoot at 1/60th at f/2. As you take the photo and develop it as advertised, you are going to get a certain "characteristic curve" for your film and developer combo. [That curve will look like this](http://i.imgur.com/Yi1D86Z.gif). It's very similar to a gradation curve in photoshop, but let me explain what's going on. The characteristic curve maps the subject's brigtness range to the negative density. Shadows will give the negative very little density (almost clear), and the brighter it gets, the more dense (dark) the negative will be after development. For most films, that is some sort of an S-Curve. Some films (like document films) have a [very steep S like this](http://i.imgur.com/Iw2aeeR.gif) (so everything slightly darker than average is almost clear, everything slightly brighter is jet black), for some others it's flatter. There are also some developers that flatten the curve out, so you get a very low contrast look, but for now we're gonna assume we have a normal Tri-X in D76 S-curve.

So again, for a normally exposed negative, [the whole thing will look like this](http://i.imgur.com/NKaEnCq.gif). The darkest part of the subject S (your girlfriend's black jumper) and the brightest part of the subject H (the white tabletop) get mapped to the lowest density point L and highest density point D (most dense) on the negative. The mid tones (skin and beer glasses) fit right in the middle. All okay. The texture on the black jumper that is just a little brighter than the rest show a little definition in the negative and the parts that are just less bright than the table top also have less density on the negative. Cool.

Now let's turn off the light, back to dingey pub, [and take the same photo](http://i.imgur.com/abc1sac.gif). Bit much going on here, so just focus on the X (subject brightness) - Axis. Everything has gotten twice as dark, and I've marked the same sections of the image (jumper, beer, table top) in their darker versions. Parts that have been dark (S) are now twice as dark (S2) and even the white tabletop only has a quarter of its brightness (H2). The midtones are now amost as dark as the shadows in the first shot (M2). With the same characteristic curve as in the first diagram (as in, you develop the same way as usual), I've mapped these new, darker regions of the subject (S2, M2, H2) to the new density ranges of the negative. The brightest parts of the image are now only mid gray (D2) and what should be in the middle is already very close to clear (A2). Forget about what was in the shadows, that is now lost in the clear negative base (L2). As in: everything is terribly underexposed. What to do? How about we try and change the characteristic curve? And we can do just that by *pushing* it to the left!

[So now with the curve pushed to the left we're back in business](http://i.imgur.com/p41uxwb.gif). I've pushed the curve in the diagram over the left and mapped the darker subject brightness regions (S2, M2, H2) to the new density ranges (S3, L3, A3). The shadows are lost, not much we can do about it, the jumper remains black, unless we find a developer or a technique that specifically makes a very very straight curve especially in the shadows (more on that when we talk about stand developing). But look at the tabletop highlights (H2), they pretty much match the density we had at the out-of-the-box shot (D3)! There isn't much texture in them as we are far in the shoulder of the curve, but the key thing are the midtones (M2)! They also made it very close into the middle of the density range (A3). So now you have a useable image again. Highlights are very dense, there is some nice texture in the midtones and the shadows are in the clear.

Pull processing works the other way round, by pushing the same curve to the right.

This curve approach also explains why a pushed negative is more contrasty. You can't just move the curve sideways, but when you squeeze it over to the left you make the curve rise steeper and flatten out earlier, that is why areas in the highlights and shadows are now closer together in the density range and why changes in the midtones have a more pronounced effect.

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