Film Exposure - A Primer to Over/Under Exposure and Push/Pull Development
When I first started dealing with film, it was a fairly straight forward ordeal. Buy the film with the right speed and set your camera to that ISO, and off you go. Once the roll was done, wind it back up and send it off the lab and get some scans and negatives back.
It was all easy and fun until I tried, on a whim, a suggestion of a clerk as I was buying some Ilford Delta 3200. I'll paraphrase the conversation, but basically he suggested I shoot the film at ISO 1600, as he liked the look of the film at that speed. So, being new and curious to the world of film, I took his advice and shot my roll at ISO 1600. I happily wound it back up and sent it off to the lab, eagerly awaiting my scans and negatives. It was a disaster but the fault was all mine. Let's pause this story here for now (I promise I'll finish it by the end of this article!). You see, in the world of film people often talk about pushing, pulling or otherwise over or underexposing film. It's such a common practice but the knowledge gap caused my own first experience with shooting film off box speed to scare me off until I was brave enough to try again. Ultimately, the concepts aren't that hard to understand but there wasn't any good introduction to the world of film exposure and the many ways you can twist and turn it. So I decided to write one. We'll get back to explaining my error in the story I've started.
Before we get started, let me just caveat this article with saying that the terms I use here are ones that I've found helpful and have picked up in my own reading on the topic. That being said, I think I've done enough reading to offer this quick intro. Keep in mind there is a ton more information out there and much more knowledgable people than I, I'd be happy to point you towards some if you read all of this and want more.
The best way to unlock knowledge about advanced film exposure techniques like pushing, pulling and over/under exposure is by contrast to digital. Many people these days start with digital and are familiar with the basics which they then carry over to film. It goes something like this: the more sensitive your recording medium is, the less light you need to make an exposure. The trade-off is noise in your amplified digital signal, or more visible grain in your film. So with digital you set the CMOS sensor to a higher ISO, while on your camera you buy faster rated film and set your meter to the same. Here is the key to all these advanced exposure techniques!
With digital cameras, your photographic medium's ISO is the same as your camera meter's ISO. That is to say, when you change the sensitivity of your sensor, the camera takes that setting and also applies it to the camera's built-in light meter, so it can tell you the correct exposure (exposure compensation dial notwithstanding...). With film, these two "ISO" settings are not the same. So for example, I can load some 400 ISO film (Kodak Tri-X 400, for example) but I have a choice with film. I can either tell my camera's meter the box speed of 400 ISO or I can tell it something else.
Before we carry on, let me define some terms that I use to help untangle these settings from each other.
Box speed means the speed of the film as stated on the box
EI, standing for exposure index, is the speed I tell my camera the film is
So in my scenario with Tri-X, I have a few choices if I don't want to set my camera to box speed. I can set the camera meter to something higher, say 800 ISO, or I can set it to something lower, say 200 ISO. These values would be my exposure index, the ISO I shot my film at. Sometimes, people do this for a few reasons. If you're shooting in low light and need a faster film speed to make proper exposures, then you probably want to set your EI higher. As an example, I usually shoot concerts on black an white flim at EI 1600. That is, I am shooting the film at this higher rated speed. In other cases, we want to set the exposure index to a lower value for some aesthetic effect. A good example of this is when folks take photos with Portra 400. Often times, you'll hear people say they overexpose Portra 400 for the pastel palette, and now you can too! Just load Portra 400 into your camera and set your exposure index to 200 or even 160. Because we are telling the camera you have a 100 or 160 ISO film (ie, less sensitive to light) the end result is that you are over-exposing the film on purpose (with an end goal in mind of course!).
So far, we've talked about film and exposure index while shooting. That's all well and good, I've got my roll of Tri-X shot at exposure index (EI) 1600 and my roll of Portra 400 shot at exposure index (EI) 200. Now what? Well this is where the language I've been using is helpful for unravelling these advanced film exposure techniques. Again, here there is a helpful contrast with digital cameras, so let's start there.
When you've got a RAW file, you pop it in Lightroom to make adjustments. One of the adjustments you can make (and probably do often) is adjusting the exposure. This post-processing is you deciding that maybe a photo is too dark or too bright and adjusting it in post. In effect, with the RAW file, you are changing the exposure index of the final output image. Well, in the case of film, development is the very same.
When you develop a roll of film, you develop according to an exposure index. When you shoot film at box speed, it's easy. The rating is on the box! But with film, you get some freedom and so let's return to my two rolls of film, the Tri-X shot at EI 1600 and Portra 400 shot at EI 200. When I develop these rolls of film, I can tell the lab what outcome I want. Pushing and pulling are short hand for "push" and "pull" development - that is, when you leave the film in the developer for longer or shorter than normal. This is like adjusting the exposure in Lightroom with a RAW file in many ways. The analogy isn’t perfect, but it’s one way to start to understand the difference between over/under exposure and push/pull development. Let's dive in to an example, my Tri-X at EI 1600.
Tri-X is a 400 box speed film, but I've shot it at +2 stops (400 to 800 to 1600). What this means is that if I were to develop this film at EI 400, there would be very little information on the negative and therefore, the shots would be heavily underexposed. Maybe I want this look, maybe not. If the lab had developed the roll at EI 400, the entire roll would be underexposed and a disaster. But, if I tell me lab that I shot the roll at EI 1600, well then that changes everything. What I've asked my lab to do, in effect, is to "push develop" the roll two stops. That is, develop the roll as if it were 1600 speed film to begin with (the practical effect is that the film is kept in the developer longer, thus giving the chemicals more reaction time to form an image). So, it's kind of like boosting the exposure in Lightroom after the fact.
The second example is my roll of Portra 400. Here, because we wanted to overexpose the film, we can just tell the lab to develop it at box speed. The difference here is that we intentionally over-exposed the film for some color effects, but those color effects only happen if the film is overexposed. That is, if we compensated in development for the over-exposure in-camera, we should have not even bothered with the intentional overexposure to being with. In this way, the film stays "over-exposed" because we developed the film at its rated speed of 400. People do this because if you over-expose the emulsion, there is more data (aka silver and dyes) on the film which can result in some changes to the colors, sometimes desirable. There are plenty of examples online, so I'll spare you mine.
In this way, it's helpful to think about film ISO in three, related ways.
ISO of the film as rated by the manufacturer
ISO of the film as shot (what I call exposure index)
ISO of the film as developed (also exposure index, but can be different than what you shot it at, as in our Portra 400 example)
So as a quick recap:
In our Tri-X example, the box speed was 400, the in-camera EI was 1600 and the development EI was also 1600
In our Portra 400 example, the box speed was 400, the in-camera EI was 200 and the development EI was 400.
In the case of the Delta 3200, the box speed was 3200, the in-camera EI was 1600, and the development EI was 3200.
You can under-expose and "pull" films too! So you could shoot Portra 800 (box speed 800, duh) at EI 400 and develop it at EI 400. Heck, you could develop it at EI 200 if you wanted, but you better be prepared for some weird stuff. This is kind of what happened in the opening example I gave. I bought box rated 3200 film, shot it at EI 1600, but then the lab developed it as it were 3200 film due to my error resulting it painfully over-exposed negatives. In this case, I had only half "pulled" the film, as I didn't instruct my lab to compensate in development.
So, when you're shooting film, keep these points in mind. To over or under expose film, it's implied that you changed the exposure index in-camera, but developed the film at whatever speed it is rated at by the manufacturer. To push or pull film, it's implied that you change the exposure index in-camera, and compensated in development as well for the change. Just keep in mind people often use the same language when talking about these two different concepts! So "Portra 400 +1" can mean either the film was pushed or overexposed 1 stop, and it'll take time to recognize whether something was push developed or over-exposed on purpose.
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