A return to the slide film vs. print film thing

I wrote a blog entry 2 years ago in response to Nikolai's post on the subject and I kinda wanted to return to some of the things covered, since I've done a lot more shooting and understand some of the numerical figures much better.

Years later, I'm still mostly shooting slide film when I shoot color.

Now, I've already talked about the workflow advantages of shooting slide film and how you really need to experience the sensual joy of slides. But there's also some nice points to be made about dynamic range.

First, we need to talk about some basic terminology. Stops, density, and gamma.

Your eye responds logarithmically to light, so all of the units from sensometry correspond to this. Stops are a base-two logarithm. Density is a base-ten logarithm. Depending on the situation, one unit or the other is used. Generally stops make the most sense for the photographer.

When light hits the film (or digital sensor) it doesn't necessarily react exactly the same. So you may add one stop of light but only gain a fractional stop of added density. This is called gamma. The gamma of print film is usually around .6, so for every stop of light we add, the film will change density by .6 stops. For slide film, it is often time somewhere between 1.2 and 1.5.

Now, there are fairly hard limits to how dark or light the film can get, which is pretty hard to get around. The film base is usually not perfectly transparent and film will always have a certain amount of "fog". And even drum scanners can only delve so far into the shadows of a scene, even if it was chemically possible to produce a film that could get that dark.

The range of density that print film can produce is much less than slide film. However, the contrast of a print film is also less than slide film, so if you develop normally, slide film will always have less dynamic range than print film. Therefore, print film has a greater dynamic range than slide film. This is an unarguable fact.

Room for screwups

Now, at the first glimpse, this seems like the end of this discussion. Print film is superior because it has more room for screwups. Right?

Let's say that you just got your exposure value off. Slide film's going to be OK within a half stop or so in either direction. Once you are off by a stop or so in either direction with slide film, it's pretty much guaranteed that you'll ruin the shot.

Print film's going to be OK within a stop or so in either direction without looking really grungy. Two stops off and you'll start to get images that are still useful, but it's going to be really grungy. This is fine if that's what you are going for or for fun snapshots where it's only important to preserve what happened, but if you shoot somebody's wedding and are off by two stops, you aren't going to want to work in that town ever again.

So, print film is better, right? You get twice as much room for slop, don't you?

Ah, but what about how I usually blow my exposure? I take a picture out on a sunny day and discover that the sky is several stops brighter than everything else. With slide film, the sky's transparent white. With print film, you can barely see a little bit of white detail in the sky that you can recover with some editing. Except that even print film will run out of dynamic range, so if you were already exposing too high, it's going to be blown out anyway.

Now, there's a lot of cameras out there with crappy exposure meters and crappy shutters. Disposable cameras have one exposure settings. Holgas may have two, if you fix a design error, otherwise there's one. Older cameras often don't have especially accurate meters or shutters.

In these cases, you probably do want to use print film.

Say you have two scenes with the same light range...

Now, when you print a normal-contrast print film on normal printing paper, you'll get a six stop range. I've written about this in the past. Coincidentally, that's usually what folks call the dynamic range of slide film.

So, let's simplify this down to a scene with a six stop range, shot with two pieces of film, one a slide, one on negative film. The slide is going to show this range of about 9 stops on the final piece of film, wheras the print film is going to show it as about 5 stops on the final piece of film.

Scanners tend to get noise from heat and cosmic rays and stuff. There's a hard limit to how far they can peek into shadows. If you get 10,000 normal photons and about 100 "noise" photons in a sensor, you won't notice if a few more or a few less noise photons arrived at your neighbor pixel. But if you go down a few stops and get 100 normal photons and about 100 "noise" photons, if a few more or a few less noise photons show up to your neighboring pixel, it's going to start causing problems. So being able to peer into almost-black film, slide or otherwise, comes at a cost.

Now, what if we've bumped up the contrast on print film. Now we have 10,000 normal photons for one exposure level and 10,100 normal photons for one stop brighter. So, being able to take a low contrast image and turn out a high contrast result also comes at a cost.

Let me put this another way: Unlike film, scanners are linear devices that have to act like logarithmic devices

This results in two things. First, there's usually image hidden in the shadows of slide film that requires a really good low-noise scanner to get at. Second, if you are only capturing the range covered by a high contrast slide film, it's better to just shoot slide film instead of bumping up the gain of print film.

In other words, you can only increase contrast so far.

So what if you made a slide film with a contrast ratio of .6?

A common question folks want to try is pulling slide film. Or they want to buy slide film with a wider dynamic range (meaning, a low-contrast slide film).

First, pulling slide film doesn't give you any appreciable amount of increased dynamic range. Sure, it lowers the contrast by some, but it will also screw up your colors and reduce your DMax so any real change in dynamic range is going to be negligible. It can be used to compensate for overexposure, but even there it's fairly marginal.

Second, removing the contrast of slide film ruins the cool properties of slide film. Remember, a print made at normal contrast covers the same dynamic range as a slide covers. If you reduce the contrast of a slide, you'll end up with your normal shot looking flat and you'll probably want to build up contrast later... at which point you might as well use print film because it looses less with each generation.

Furthermore, you need to consider what you need to protect and what you don't. Digital cameras tend to have nasty crossover and other objectionable artifacts if you overexpose. Film has a shoulder and then it goes white, so that's going to let you capture the sparkly highlights of the sun bouncing off the ocean without looking bad. If you look at a good print by Ansel Adams, you'll notice that the magic wasn't just that he knew how to capture every single stop of detail between highlight and shadow but also that he would allow just the right amount of highlight and shadow to go full black or full white.

So, my conclusion?

If you have no control over exposure, you have a hard time shooting slide film, because you don't have any room for slop. This is what makes disposable and cheap toy cameras possible, although you can shoot slide film in a Holga if you know what you are doing. You can be off by quite a few stops with print film and most folks won't notice too much.

But if you are good enough at controlling exposure, both by metering correctly and also by effective use of off-camera lighting and filters, there's no point in preserving room for slop that you don't actually need given that they come at a cost.

You don't need a spot meter or matrix metering or an electronic shutter. Just a brain. I've got mechanical shuttered cameras and I've done plenty of shooting with a center-weighted meter. I might be pointing the camera up at the sky or down at the ground to make sure I understand the tonal range, but I get plenty of properly exposed slides with no bracketing.

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Sidebar: Overview

This is a multi-part series of articles that explains my views on films. There have been many other guides, but a lot of them are years old now and written by folks who have gone digital.

Sidebar: Another thing

People love to go on about how wonderful print film is about having better dynamic range than digital. This is partially mistaken. If you overexpose digital by a stop, it's going to look awful. But if you underexpose, it'll be fine for at least a stop, maybe more.

Print film is very much the other way around. You can only underexpose by a stop, but you can overexpose like crazy.

I've seen some arguments that the room for slop with a good RAW converter is bigger than the slop room for film, as long as you don't blow out the highlights. I've also seen a bunch of owners of the Fuji SLRs with high dynamic range going on and on about how their cameras beat film because of the extended highlight room.

Most of these people don't understand photography. The reason why people shoot film has very little to do with dynamic range and very much to do with other things.