How I Rate My Film (and Why I Overexpose)

If you want to know how to rate film for the way you actually shoot, the short answer is this: take the number on the box as a suggestion, not an instruction. Most of my colour negative gets rated a stop slower than the manufacturer recommends, which is a polite way of saying I overexpose it on purpose. Here is how, and why.

First, the vocabulary. The ISO printed on the box is the film's nominal speed, the sensitivity it was designed around. The EI, or exposure index, is the speed you tell your meter the film is. They do not have to match. When I load a roll of Portra 400 and set the camera to 200, I am rating it at EI 200, and every frame on that roll gets one stop more light than the box would strictly ask for.

Why would anyone do that? Because colour negative film is lopsided in its tolerance. It will take a remarkable amount of overexposure and hardly blink. Two stops over, sometimes three, and a good scan will still hold the highlights. Underexposure is another matter. Starve a negative of light and the shadows turn thin and muddy, the grain climbs, and the colour drifts towards a sickly green-grey that no amount of scanning will quite fix. Given a choice between two ways of being wrong, I would rather be wrong on the side the film forgives.

There is also the look. A generously exposed negative has denser shadows, and dense shadows scan clean. Skin tones come out warmer and smoother, the palette softens, and the grain settles down into something fine and even. Most of what people describe as the Portra look is, in practice, the Portra-at-200 look.

In Queensland there is a practical reason too. Our light is hard and the shadows under it are deep. Midday under a fig tree in New Farm, the difference between the sunlit path and a face in shade can be five or six stops. Rating a stop slow and metering for the shadow side of the subject means the faces are properly exposed and the highlights, which negative film tolerates, look after themselves.

My working ratings, for what they are worth:

Portra 400 at 200. Occasionally 320 in dim interiors, where I would rather keep the shutter speed.

Portra 160 at 100. It is a fine, gentle stock and a little extra light keeps it from going flat.

Ektar 100 at box speed, or at most a third of a stop over. Ektar is a saturated, contrasty film with less latitude than Portra, and it punishes careless overexposure with blown, orange skin.

HP5 at 320 for general work, which gives slightly denser shadows under normal development. When I need speed, I push it to 1600 and tell the lab, which is a different conversation entirely.

Slide film, if you ever shoot it, gets rated at box speed and metered for the highlights. Everything above reverses. Transparency film has almost no highlight latitude, and overexposing it is simply ruining it.

A few rules that keep this sane.

One rating per roll. You cannot change your mind halfway through a roll of 36, because the whole roll goes into the same developer for the same time. If I start at 200, I finish at 200.

You do not need to ask the lab for anything when you overexpose colour negative. Rating Portra at 200 and developing normally is standard practice, and is not the same as pulling. Only tell the lab if you have actually asked them to change development, as with a push.

Meter for the shadows you care about. With negative film the question is not whether the sky will blow. It usually will not. The question is whether there is enough light on the thing you want to see.

Write it down. A strip of gaffer tape on the back of the camera with the EI in marker has saved me more than once, usually on the morning after a long day when I could no longer remember what I had set.

Last, trust your scans more than the internet. Shoot a roll of your usual stock at box speed, another at a stop over, and another at two, in the same light, and look at them side by side. You will learn more from those three rolls than from any number of forum threads, including, I suspect, this one.

It costs about the price of a decent lunch in the Valley, and it settles the question for good. Your own light, your own lab, your own eye.