Framed and Exposed: Balancing the Lens and Sensor Equation

Ben Long

The semi-pro/pro digital camera market is divided fairly cleanly, right now, along a very simple line: sensor size. Yes, resolution is also a factor, with hardcore pros using SLRs with high-res 12 to 14 megapixel image sensors, while serious hobbyists and less-moneyed pros are using 6-8 megapixel SLRs, but the coveted “full-frame sensor” is the mark of a true high-end digital professional.
What’s the big deal with sensor size? First and foremost, of course, is image quality. Though less-expensive, smaller-sensored cameras like the Canon EOS 20D and the Nikon D70 offer excellent image quality and high-end features, but they use an image sensor that’s roughly the same size as a piece of APS film. High-end, pro cameras that pack more pixels, such as the Canon 1DS Mark II use an image sensor that’s the same size as a piece of 35mm film.
This difference in physical size has a tremendous bearing on image quality. Obviously, more pixels can be crammed onto the bigger sensor, but more importantly, the pixels themselves are larger than what you’ll find on a smaller sensor. Larger pixel size means better signal-to-noise ratio, which means images with better color, less noise, and potentially better detail, sharpness, and low-light performance.
There’s another significant advantage to a 35mm sensor, though, and that has to do with lenses. Because these cameras are designed to work with lens systems that were developed for 35mm film sizes, using the same lenses with a smaller sensor means that your image gets cropped in a way that effectively changes the field of view of the lens. So, if you stick a 50mm lens on a Canon 20D, it gives you the same field of view as an 80mm lens on a 35mm camera, or on a full-frame digital camera.
Though this focal length multiplier is a boon to people who want telephoto lenses, for wide-angle shooters it presents a serious drawback. Stick a 28mm lens on your 20D and you’re stuck with an almost “normal” 44mm lens.
Obviously, high-end pro cameras – which can cost upwards of $6000 more than their smaller-sensored brethren – pack other “pro” features such as faster shooting rate, weather proofing, finer control of image processing, better light meters, and so on.
In the end, though, it’s the sensor size – and its effect on image quality and focal length – that separates the high-end digital pros from those who can’t justify $10K on a digital camera system.
Same sensor, different lens
Some vendors argue that there are other drawbacks to the current “let’s stick a smaller sensor in a body engineered for 35mm film” strategy. Olympus has completely eschewed this approach in favor of developing a completely new body/lens system. According to Olympus, compatibility with 35mm lens systems has an inherent disadvantage because, basically, it’s a hack. They claim that a more sensible approach is to design a camera/lens combo with a particular sensor in mind. To that end, they support the 4/3 Consortium that specifies sensor sizes and shapes. Though Olympus’ theory is interesting, they have yet to produce a digital SLR with interchangeable lenses that proves their advantage.
Canon has taken a different approach. With the release of the Digital Rebel, a camera that packs an APS-sized sensor, Canon introduced a new lens mount, the EF-S.
Canon recognized that, with the smaller sensor size, you can get away with a smaller mirror and pentaprism system, which means you can free up some space inside the camera’s body. With this extra space to exploit, it’s possible to build lenses with rear elements that protrude farther into the camera. The EF-S lens mount, while fully supporting Canon’s entire line of EF and L lenses, also supports the mounting of special S lenses, with these deeper rear elements. (The “S” actually stands for “short back focus.”)
This new mount accomplishes two things. First, it allowed Canon to design a mount that’s more appropriate for the APS-sized sensor used in the Rebel, 20D, and any other “mid-range” SLRs they choose to produce. In theory, this should answer the claims of those who find cramming a small sensor into an existing 35mm lens system to be somehow nauseating. More importantly, it allows for the creation of smaller, less expensive lenses without sacrificing quality.
When engineering a lens, it’s optically advantageous to have the lens as close to the focal plane as possible. As you position the lens closer to the image sensor, the lens itself can become physically smaller. What’s more, it’s much easier to engineer a lens that can focus onto a small area than a large area. Since EF-S lenses only have to cover an APS-sized area, eliminating aberrations and flares is much easier than with a lens that has to cover a 35mm-sized area.
This is one reason that tiny point-and-shoot cameras deliver such good images. Their sensors are tiny, so it’s easy to engineer a good lens for them. The EF-S lens system allows Canon to exploit the small sensor advantage of an APS-sized sensor. With easier lens engineering, extremely wide-angle lenses (a type of lens that is usually difficult to engineer and therefore very expensive) are suddenly easier, and therefore cheaper, to build.
Such is the case with Canon’s new 10-22mm EF-S lens, a $799 (list) lens for the Canon Digital Rebel and EOS 20D. With a 35mm equivalency of 16-35mm, this lens effectively answers the “but how can I shoot wide angle with an APS-sized image sensor” problem that has been frustrating many digital photographers.
The lens itself is excellent, and you can read a full review of it here, but the lens is also interesting for what it implies about the digital SLR market.
How much would you pay? Well don’t answer!
Many pro and semi-pro digital photographers have been waiting for full-frame digital SLRs to come down in price. The attitude is that the current generation of cameras is a somewhat “stop-gap” measure, while we wait for full-frame sensors to become cheap enough for everyone to afford. So, the idea goes, it shouldn’t be too long before we have full-frame digital SLRs at the $1500 price point.
Given the rapid progress of digital SLR development, in terms of resolution, performance, and improvements in image quality, this isn’t an entirely unreasonable assumption. After all, we’re all used to Moore’s law cropping up in all digital devices, so it should only be another year or so before we have the equivalent of a Canon 1DS Mark II 14-megapixel full-frame camera for under $2000, right?
Canon’s 10-22mm lens makes this assumption less certain. When Canon released the Digital Rebel, they also released two EF-S lenses. Then the 20D came along, the Rebel’s slightly bigger brother with a $1500 price tag. One of the core improvements that the 20D had over its predecessor, the 10D, was support for EF-S lenses. With the 20D came the announcement of 2 new EF-S lenses, the 17-85 IS and the 10-22mm.
In other words, with the 20D came the assertion that the S lens system is something Canon is taking seriously. In addition to expanding the system across their mid-range market, they’ve proven that they’re taking active steps to shore up the lens selection for their new mount, offering an image stabilized “walk-around” lens, and a lens that provides an excellent solution to the problem of shooting wide-angle on small-sensor digital SLRs.
So, those photographers who’ve been holding out for a full-frame sensor camera to appear at the current price point of something like the 20D, might have a very long wait ahead of them.
With the EF-S mount and current selection of lenses, Canon has answered the problem of how to shoot wide angle with an affordable digital SLR. They’ve also addressed the claims of those who believe lenses should be engineered for a particular sensor size. Since Canon’s excellent L-series lenses work fine on the Rebel and 20D, the S lenses serve to shore up the more difficult wide end. In other words, Canon has done everything they need to do to assure a full range of high-quality lenses for their mid-range cameras.
That’s not the attitude of a company that’s aiming to shift everyone toward full-frame digital cameras.
With the D30, Canon invented the “affordable” digital SLR market. (Granted, that camera was $3000, but that price was significantly lower than the $5000 SLR that Nikon was offering.) From there, they moved very quickly to push the SLR entry price down to $2000, then $1500, and finally with the Rebel, $899.
At the same time, they did an excellent job of building a high-end line-up. It seems obvious that their goal has not been to push everyone toward full-frame pro cameras, but to develop and exploit the obviously large mid-range market.
Selling cameras is great, but the real money is in lenses, and now Canon has three separate lines of lenses that they can market to very specific, vertical niches. That’s a great position to be in.
What’s more, there’s no reason for them to eliminate this market. Just because we always shot film of a particular size doesn’t mean we need to use digital sensors of that size. Canon and Nikon have proven that you can get excellent, perfectly usable images from an APS-sized sensor, and with Canon smartly addressing the lens selection question, the drive toward full-frame seems far less pressing.
From this perspective, it’s obvious that the sensor-size line that separates high-end pros from everyone else is here to stay. Yes, there will be improvements in both areas, but the market will remain thusly delineated.
So, if you’ve been putting off going digital, or moving to a digital SLR, because you’re hoping for an affordable full frame camera, look again. You may not need full frame, and the big camera companies may not want to sell you one at the price you’re hoping to pay.