There are significant advantages to D-SLRs that are designed around sub-full-frame image sensors. Before you decide that only full-frame will do, consider all the angles.

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For the better part of the last three years, one of the biggest buzz topics for pro photographers going digital was the full-frame camera—an SLR with an image sensor that’s physically the same size as a frame of 35mm film. Along with the implication of higher resolution, these cameras have the added benefit of not requiring users to apply a magnification factor to their lenses in order to determine the apparent focal length. In the past few months, however, several advancements call into question the superiority of the full-frame sensor.

We touched on this in “Clash Of The Titans,” Digital Photo Pro, May/June 2005, but with so many professionals contemplating the pros and cons of a new full-frame image sensor, the topic merits a more in-depth discussion.

Background

When we’re talking about fullframe D-SLRs, we’re talking about the image sensor. While commonly referred to as the heart of a digital camera, the image sensor is one of many components that contribute to making the final image. The sensor is by no means the only element that needs to be considered, however. As the heart of a camera, the sensor is the single most well-known component and the one that photographers are most apt to discuss when kicking around the features and specs of a given camera.

Since digital SLRs became more mainstream, two types of image sensors have formed the great polarizing debate among photographers: full-frame sensors and sub-full-frame sensors. While these aren’t industry-standard terms necessarily, they’re descriptive. When we say “full frame,” we’re referring to the relative size of the sensor vis-à-vis a frame of 35mm film. A full-frame sensor is the same physical size as a frame of 35mm film.

Sub-full-frame sensors are somewhat more difficult to pin down in terms of their precise size. Rather than list a specific measurement, we tend to discuss the size of these sensors in terms of their magnification factor. This is done for purely pragmatic reasons. The actual sensor size is less important to a photographer than the effect that sensor will have upon the way the photographer uses the camera. A 1.5x sub-full-frame image sensor, therefore, is one whose physical size creates a 1.5x magnification factor for the focal length of a particular lens. Most sub-full-frame sensors are in the 1.5x to 1.7x range. One exception is the Olympus 4/3rds sensor in the E-1, which has a magnification factor of 2.0.

The Magnification Factor Myth

If you’ve been following digital camera technology for a while, you’ve no doubt come across the phenomenon of the magnification factor. You might have seen it expressed or explained in any number of ways. Typically, you’ll see comments like “…camera X has a magnification factor of 1.5x, so your 100mm lens will really be a 150mm lens on this model.” This is preposterous, naturally. The focal length of a lens doesn’t change simply because there’s a different sensor behind it. The laws of physics don’t care a whit about the sensor size. A 100mm lens still focuses at infinity when it’s 100mm from the image plane (okay, this assumes we’re not talking about a lens with a telephoto design, which can be made to focus closer to the image plane).

The term “magnification factor” was adopted to explain why a photograph taken with a 100mm lens on a film camera doesn’t look the same as a photograph taken with that same lens on a camera with a sub-full-frame image sensor. Magnification factor is better than saying “35mm equivalent,” but it still isn’t correct. “Crop factor” would be a more accurate way of describing what’s going on as the image is being formed on the sensor.

All lenses generate an image circle of a specific size on the image plane. That image circle doesn’t change no matter what kind of image sensor or film is being used. The sensor (or film, for that matter) simply determines how much of that image circle is creating a photograph. The rest of the circle becomes non-imaging light. So, essentially, the sensor or film is cropping a portion of the image circle to create the photograph.

Taking the discussion a step further, consider the various formats—35mm, 2 1/4 (or medium format), 4×5, 8×10, etc. If you’ve shot with these formats, you know that you’ve never contemplated magnification factors with any of them. You simply got used to the way a 300mm lens looked on a 4×5 versus a 35mm camera. On both cameras, the lens focuses at the same distance from the image plane—300mm.

The point here is that cameras with sub-full-frame image sensors tend to take a beating in professional circles because the magnification factor implies that a device is in some way inadequate. That reputation isn’t deserved.

Advantages Of A Sub-Full-Frame Sensor

Cost. One of the primary benefits of sub-full-frame sensors is that the sensors themselves are relatively easy and inexpensive to produce—at least compared to the full-frame bodies. The cost savings translates into a less expensive camera. And it’s not just the sensor that impacts the price. The other key internal components in the camera can be less expensive because they don’t have the same loads that a full-frame sensor demands. The ability to use lower-priced components cascades throughout the camera.

Speed. A sub-full-frame sensor is generally faster than its full-frame counterpart. The sub-full-frame sensor tends to be lower resolution and generates a smaller image file. These factors combine to make the camera faster when you’re shooting and faster when saving to the card. Consider the Canon EOS-1D Mark II N, a brand new camera that refines and improves upon the original EOS-1D Mark II. Canon is building this camera despite the fact that it now has two full-frame SLRs in the lineup. The camera is built for users who place a premium on speed versus resolution. You know who you are.

Workflow. Images generated by smaller image sensors generally make for a faster workflow because the image files themselves are smaller. You’re not taxing your computer as much, the images open faster, they save faster, they can be batch-processed faster—all in all, efficiency is improved across your workflow.

Resolution. What’s this doing under advantages? I’m glad you asked. We’ve already discussed how a lower-resolution image sensor can be an advantage, but what if you need higher-resolution images? The clear advantage that full-frame sensors had in this arena is eroding. The Nikon D2X was introduced this past spring with a sub-full-frame sensor that sports 12.4 megapixels. That’s only about 25% fewer pixels than the current king of resolution in this class of camera, the Canon EOS 1DS Mark II (16.7 megapixels). If you need resolution, you don’t necessarily have to go full-frame.

What’s The Catch?

The single biggest disadvantage of a sub-full-frame sensor is the crop of the image circle. If you bolt your trusted 20mm to a body with a socalled 1.5x magnification factor, you’ll get an image that doesn’t look nearly as wide as you were expecting. The appearance would be closer to what you’d expect to see from a 30mm lens attached to a 35mm film camera. This clearly is a disadvantage at the wide end, but it’s not a fatal flaw. Because of the ever-increasing demand for widerangled lenses, manufacturers have stepped up and designed lenses that are specifically made for these cameras. The professionally oriented lenses are excellent performers and capable of professional results.

Going Big

Okay, so you’ve decided that you simply need the full-frame. Nothing else will do. Obviously, these cameras have some definite advantages.

Resolution. First and foremost, resolution is the primary advantage of a camera equipped with a full-frame sensor. If it’s an absolute necessity that you have the most resolution possible, a full-frame sensor is your tool. The 16.7-megapixel Canon EOS-1DS Mark II has pixels to spare.

Noise. A key aspect to image sensor design is the size of the individual photosites. A large, full-frame sensor The EOS 5D features Canon’s latest full-frame image sensor. The camera is can accommodate large photosites with plenty of space in between them. As electronic components within a box full of other electronic components, the sensor generates heat, and it’s surrounded by hot components. Heat creates noise and noise degrades image quality. To reduce noise, it’s important to have the ability to reduce heat, and to reduce heat, the sensor must be able to dissipate that heat. Larger photosites with space between them keep heat and, therefore, noise from becoming a problem.

Dynamic Range And Color. The larger photosites on a full-frame sensor are analogous to buckets filled with photons instead of water. The more photons you can capture, the better the overall image quality. Just like buckets left outside in a rainstorm, a larger photosite can capture more photons than a smaller photosite. With more photons, the dynamic range and color fidelity are superior. Smaller image sensors that have the photosites crammed together to get a higher-resolution sensor are more likely to have dynamic range and color problems than the larger full-frame sensors. Of course, as engineers cram ever increasing numbers of photosites onto the full-frame sensors over time, those sensors will have the same inherent problems, but currently full-frame sensors have an advantage over sub-full-frame models.

Comparing Cameras

We can’t emphasize it enough: there’s more to a camera than the image sensor. Still, the image sensor is at the very heart of the camera, and it’s a key element in the chain to produce your finished image. Image sensors should play a large part in your decision-making process when you’re planning on buying a new camera, but there are plenty of other important elements to consider as well. In the end, no one camera out there is “the best.” It’s all about finding the best camera for you.

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