Without looking at the EXIF, do you know what sensor format I used here?

We all hear about “full frame” and “crop” sensors, and usually there’s a certain air of smugness about having a larger sensor. Nikon currently has three sensor formats (outside of the Coolpix line): FX (24 x 36mm), DX (16 x 24mm) and CX (13.2 x 8.8mm). Let’s take a quick look at the pros and cons of different sensor sizes in today’s digital photography. 

Resolution (Megapixel Count)

Resolution is simply the number of pixels (photosites) on your camera’s sensor. Most modern cameras offer between 10 and 24 megapixel resolution. Nikon’s CX-format cameras (1-series) are 10.1 megapixels. The DX-format D7000 is 16 megapixels, and so is the newly announced D4 (FX format). The 24 megapixel D3x is FX format.

So what’s the difference? More resolution means the potential to capture more detail in an image. I say potential, because you need good lenses and good technique to maximize image quality. A blurry 24 MP image isn’t useful, even if it has 24MP. The more pixels you have, the easier it is to print large or crop into the image and still have good detail. It really doesn’t matter what format you have, strictly speaking, when it comes to the number of pixels.

Noise and ISO

What does differ with larger sensors, however, is the size of the photosites. For any given resolution, a larger sensor will always have larger photosites than a smaller sensor. Here, you’ll see sometimes profound differences in the noise level of high ISO images. The 12MP Nikon D3s is practically noise-free through ISO 3200. On the other hand, the D300s (DX) with nearly equal resolution produces noticeable noise (albeit not really bad) above ISO 1000. But there are other things to consider when judging noise, most importantly is the size of the final “print.” I’ll use the term “print” loosely here, as sometimes the final image is a JPEG posted to the web. For very small images (less than 1200 pixels wide), the simple act of downsizing the image reduces noise to the point where it might not be noticeable.

Depth of Field

The larger your imaging sensor, the easier it is to get very shallow depth of field, even without f/1.4 lenses. Many portrait photographers prefer the look of an isolated subject, and bigger sensors (and fast glass) is the best way to get that look. However, there are times when shallow depth of field isn’t always ideal. Macro photographers are always trying to get maximum DOF, to the point where “focus stacking” becomes an option. Here, a smaller sensor has the advantage of producing an effectively greater DOF. When you’re focusing on moving subjects, especially ones coming directly towards you, having a little extra DOF can also help get sharp shots. Photographers using a “full frame” DSLR for the first time might think they’re having focus problems, when in fact it’s the reduced DOF creating more apparent softness in their images.

Lenses and Gear

As sensors get larger, the size of the image circle required to cover it also increases. That means bigger sensors need bigger lenses. While it’s relatively easy to go “wide” with a large sensor, if you want to use telephoto lenses you’ll also need significantly longer focal lengths to get a comparable angle of view to a crop-sensor camera. A 300mm lens used on FX format is a medium telephoto, while that lens is the equivalent of a “super-tele” on a DX body. You’d need to use a 450mm lens on a Nikon D4 to get the same sensor coverage as a 300mm lens on a D300 or D7000. Conversely, if you like wide-angle shots, then it’s far easier to use a larger sensor. A 24mm lens is fairly wide on a D3x, but it’s like using a normal lens on a D300.

The side-effect of larger sensors is that your lens bag gets bigger and heavier. I can carry the equivalent of a 27-80mm and 80-300mm zoom kit in a belt pouch designed to hold a single pro DSLR body when I’m using my Nikon 1 V1 (CX). The whole kit with both lenses weighs a mere 527g (1.16 lbs). A similar kit with my D3s (24-70mm + 70-300mm) would not only take up more space, but would weigh in at 2.8kg (6.4lbs)… and that’s without the battery! And don’t get me started on the price of those kits…

Choosing what’s best for you

Clearly, there are some things that large-sensor cameras can do better than crop-sensor ones. If you make your living shooting high school sports in poor light, then there’s no doubt that a D3s (or D4) is going to give you a distinct advantage in terms of cleaner shots. But there’s also something to be said about a small, lightweight kit that won’t necessitate a visit to the chiropractor after you use it, or make you stand out as a “pro” while on vacation. In a world where fewer people are printing and most images are shared via Facebook or Flickr at (relatively) low final resolution, the need for high-resolution, high-ISO cameras isn’t nearly what we thought it was a few years ago. For the casual shooter, just about any DSLR (or mirror-less compact) will deliver image quality unheard of to those of us who grew up shooting film. Of course, Nikon and Canon (and others) will continue to push the technological limits of sensor technology, just feel good knowing that by knowing your own true needs you’ll be able to make the best choice as to what kind of camera you should get.