Dynamic Range and Latitude Are Two Different Things
I've been meaning to write this one for a while. In fact it's been sitting as an empty draft for months, so it's about time we get on with it. One of my pet peeves is people who act like they know everything when they don't know what the hell they're talking about. A few years ago I wrote one of my favorite blog posts ever called Image Properties (Or how most people talk out of their ass), to tackle one of the common areas of confusion.
Today, in an attempt to help out some more of you we're going to talk about the difference between 'dynamic range' and 'latitude'. Two terms that many people use interchangeably which are actually two different things.
Dynamic Range is a measurement of the size of the range of tones that a camera can record from completely black to completely white. In simple terms, the greater the dynamic range, the more detail you can see the shadows and highlights of a picture. In the old days, different film stocks had different dynamic range properties, especially from type to type. As a rule of thumb traditional black & white film has the most, then color negative film, and finally color slide film the least at about 7 stops. Now most of us use a digital sensor in the place of the film and so it's this sensor that now has the properties of dynamic range. So if you want better dynamic range, you've got to buy a new camera. The good news is that today's cameras are really pretty great in this regard, most within the 12-13 stop range which is as good as all but the best film with the best scanners.
I'd like to take a second and discuss the dynamic range of film and why it's so hard to measure and compare with digital. You see, digital sensors are designed to be very linear, that is, if you add a stop more light the sensor will record the image a stop brighter. The down side is that when the light coming in overwhelms the sensors ability to record more light the highlights in your image "blow out". Digital doesn't do this very pleasantly and it's definitely one of the few things they've really got to work on. Film on the other hand (and this goes for audio recorded to magnetic tape as well) is mostly linear for most of the range. However at the highlight end of the scale the film pushes back, and when you add a stop of light and then another, the film records less and less of an increase. It's got sort of a built-in compression curve which transitions highlights to white very very smoothly. It's very pleasing to the eye, but that same effect also makes it very hard to measure how much dynamic range the film is accurately recording. Do we count those top few stops that get compressed as 'accurately' recorded? I'm not sure.
Either way, let's imagine that there's a ruler that has black at one end and white at the other. And since I live in America and there are about 12 stops of dynamic range in a sensor, I'm going to use a one foot ruler as a visual guide. Imagine that this ruler is broken up into 12 equally sized inches each representing one stop of brightness that the sensor can measure. The 0 on one end equals black and 12 at the other end equals white with grey in the middle. Easy right? Ok, hold onto that, we'll get back to it in a minute.
Latitude is the other term which people use interchangeably when they typically mean dynamic range. "That new camera has better latitude than last year's model", I'm sure you've heard that kind of quote before. What latitude actually refers to is how much a picture can be over or under-exposed while still getting the image you were looking for, and that very much depends on the image you're trying to take.
So for example if you're taking a picture which only has 8 stops between it's darkest and lightest areas, then your camera's 12 stops of dynamic range is overkill. That means that you've got 2 stops on both the shadow or highlight ends which are technically going unused. In fact if you overexposed by a couple of stops and then pulled it down by 2 stops in post, especially if you're shooting RAW files, you should end up with an image that looks just like one which was exposed correctly. In this instance you could say that you've got '2 stops of latitude'.
To use the ruler analogy from above, if the picture you're trying to take is only 8 inches long, you could measure it from the 2" tick to the 10" tick on the ruler, or from 0" to 8", or 1" to 9", etc. As long as it all fits inside the twelve inches of the ruler, then you'll be fine.
Back in the film days, when you dropped off a roll of film at your local lab, a tech or a computer would make these kind of corrections for you when making your prints so that everything looked correctly exposed. It was the fact that film had a lot of dynamic range which allowed people to take all kinds of incorrect exposures and still delivered great looking prints. This fact was used to great effect in disposable film cameras which had a single shutter speed and aperture. They relied on the fact that you could usually get a 'usable' image regardless of how much light actually hit the film, especially when they could just pop a built-in flash.
Of course in the real world there are consequences to incorrectly exposing your image. Even if it fits into the dynamic range of the sensor, a stop or two (or on the odd occasion three), is about all you're going to get back in either direction and you'll likely have some artifacts like shadow noise. We've also been talking about this from the very basic point of view of monochrome luminosity. However real images have three color channels of Red, Blue, and Green each of which have a 'ruler' in them. So you might blow out one of the color channels and not the others, and so forth, which further complicates things like recovering highlights. It all varies from image to image.
So to wrap up, Dynamic Range refers to the size of the range of light that a camera can capture from complete black to complete white, while Latitude is the amount of under or over exposure that a particular image can handle while still fitting into the camera's dynamic range.
Now you know, and knowing is half the battle. Go forth and school some friends.