I've been covering a range of film-related topics in the last few months—from why every photographer should shoot film, to how to develop it at home and how to scan it—so I thought today we'd take a look at the technique of pushing and pulling film. It can be an intimidating topic to get into, but it doesn't have to be. I'll try my best to make it accessible.
Every roll of film has a recommended ASA/ISO value written on it. It's commonly referred to as the "box speed" of that particular film, because, well, it's written right there on the box you buy it in. It can be a number between 50 and 3200, although the most common film speeds are 100, 200, 400 or 800.
Film speed is a measure of sensitivity to light. A complex science of sensitometry is required to determine a film stock's optimum performance level—a delicate balance of factors like color accuracy, dynamic range and granularity.
Although, as we've covered before, film can be a very forgiving medium for photography, so it's not uncommon for photographers to stray away from a manufacturer's recommendations and start experimenting a bit. Most negative film has enough latitude that you can safely shoot it below or above its box speed to achieve different effects. Even positive (or slide) color film can be flexible, although much less so than its negative counterpart.
To shoot a roll beyond its box speed is referred to as "pushing" film. Shooting below the box speed is "pulling" film. Pushing/pulling film also affects processing times during development, so it doesn't end with simply adjusting the ISO/ASA dial on your camera.
When To Push Film
Pushing film basically underexposes it. Why would you need to underexpose? Well, the most common reason people push film is because they want to be able to shoot handheld in low light situations. Say you're shooting an indoor birthday party and all you have is some Kodak Tri-X 400 and the maximum aperture on your 50mm lens is f/1.8. In order to keep your shutter speed over 1/60th to prevent blurry results, you may have to push the film two stops to ISO/ASA 1600.
After you shoot your roll, you make a note for your process lab that it should be developed not at the box speed of 400, but at 1600, to compensate for your underexposure. This will likely result in more contrast and more grain, but it's a good look for black and white negative film—the highlights are brighter while still retaining detail, while the shadows remain dark. This striking look is a common reason why people push Tri-X or Ilford HP5 even when they don't have restrictions on their shutter speed.
Color negative film can also be pushed to achieve more contrast, although color shifts can occur as you go beyond a couple stops. Kodak Portra 400 is commonly pushed one or two stop to pleasing results. Film grain will be more pronounced, but again, that granular look grows to be quite pleasing to a film photographer's eye.
When To Pull Film
As you may have figured out by now, pulling film overexposes it. Unlike pushing, pulling film decreases contrast and pulls more detail out of the shadows. A good time to pull film is when you want to retain as much detail as possible from a scene with contrasting light. Film is quite forgiving to overexposure, and is able to retain detail in the highlights a lot easier than it is in the shadows, so the pulling technique works great.
Pulling film works really well in portraiture or wedding photography because it softens the shadows and tames highlights, resulting in more even skin tones. It's also used in landscape photography when shadow details need to be retained across a scene with uneven lighting. If the light in the scene is already a bit flat, pulling film can lead to a rather dull negative though, so use caution on grey days.
How To Push/Pull Film On Your Camera
Now that you have an idea of what pushing/pulling does to your photos, it's worth mentioning how to actually set up your camera to do it. Most film cameras with an internal light meter have a dial for adjusting the ASA/ISO, and this is definitely the easiest way to adjust film speed. All you have to do is set it to your desired value and shoot as per usual. Similarly, if you use an external light meter, just adjust the film speed on it to your desired value and meter the scene as usual.
If your film camera determines film speed by its internal DX reader, there may be a feature on the camera to override the reading. If not, you may have no choice but to alter the film canister so that the camera will read it automatically at a different speed. There are detailed tutorials on this practice online.
Just remember to let your film lab know that you shot your roll at any other speed but the box speed. They will have to make adjustments to the developing time to complete the push/pull process. Otherwise, you may end up with under or overexposed photos.
There you have it—a primer on the wonderful world of pushing and pulling film. I encourage everyone to give it a shot when the opportunity arises. It may just take your film photography to the next level and help you find your own style.