Full Frame Versus DX Cameras
With some full frame cameras now on the market, most notably the Canon 5D and Nikon D3, there is quite a bit of chatter on the internet about full frame versus DX (cropped) cameras. People keep lining up in their corners to watch a new fight posted by yet another pro photographer touting the virtues of full frame. About the only thing that hasn't been done is a high dollar late night event on pay-per-view. ;-) Setting other camera features aside, what does full frame really do for you? Is it time to dump your "old" DX camera with its 1.6x crop and buy into the full frame hype? Let's take a quick look at this topic.
"Full Frame" refers to digital cameras with sensors roughly the same size as 35mm film (36x24mm). Most digital SLR cameras now commonly referred to as "DX" cameras use APS-C size sensors which are smaller at about 22x15mm on a 1.6x camera. In comparison, most consumer point-and-shoot cameras use smaller sensors still, many coming in somewhere around 7x5mm. The following figure will give you an idea of the relative sizes.
So what difference does sensor size make if the camera takes good photos? Of course, if you are happy with your photos, that's all that matters, but having a larger sensor does give you benefits that you may not realize you are "missing" with a smaller sensor. First and foremost is image quality. Due to the fact that larger sensors can hold larger pixels (when comparing cameras with the same resolution), a larger sensor usually is capable of greater dynamic range, less noise, and better high ISO performance. Generally speaking, cramming more pixels into a smaller area will reduce overall image quality so having a larger sensor can alleviate some of the issues related to "pixel cramming". In addition, smaller sensors with the same resolution (say 12 megapixels) cram more pixels into a smaller area which often results in the need to use the highest quality lenses. In contrast, using a 12 megapixel full frame sensor, the pixels are larger and more spread out, making the lens a bit less of a factor for sharpness.
Image quality isn't the only thing that changes when you put a smaller DX sensor in an SLR camera. Because other aspects of the camera remain the same, putting a smaller DX sensor in the camera equates to simply cropping the center out of the full frame image. As a result, you end up with tighter framing of objects and a 35mm lens on a DX camera starts to look more like a 55mm lens on a full frame camera. This may force you to back up from the subject and/or change your zoom. In turn, depth of field will also be affected and you may notice that it is more difficult to get blurry backgrounds with a DX camera. On the plus side (for DX), your 200mm telephoto lens will give you roughly the same framing of the subject as a 300mm lens, albeit with different depth of field (than a 300mm lens on a full frame camera).
If you are not used to shooting film or full frame, you may never notice these differences. Those who have been shooting with DX cameras for years won't notice the difference in being able to get really soft, blurry backgrounds under some situations. In addition, it is now very easy to find good quality lenses in the 17mm range, even in a super zoom, making your ability to get wide angle shots with your DX not as problematic as it used to be!
One down side to using a full frame camera is that you may run into situations where light falloff (sometimes incorrectly called "vignetting") is an issue at short focal lengths. Having shot DX cameras for nearly a decade, I was surprised at how much light falloff was present on some of Canon's best zoom lenses at the wide angle end of the range when using the full frame Canon 5D camera. Usually appearing as darkening in the four corners of the frame when shooting bright or uniform subjects, this light falloff issue with full frame cameras is shown at the very bottom of my 20D versus 5D review. Note that light falloff doesn't indicate something "wrong" with full frame cameras, only that I had been spoiled by DX cameras almost never showing this issue and I was a bit surprised at how easy it was to see this problem in my photos when using the full frame 5D at the wide angle end with almost any lens, even when stopping down the lens.
About image quality
I've seen some posts on other web sites that show full frame cameras like the 5D coming out way ahead as far as image quality. Personally, I find very little difference in image quality when comparing the 5D with some of the latest DX cameras like the Nikon D300. A bit of an unfair comparison with the 5D being more than two years old and rumored to be replaced soon, but I don't find the exaggerated quality differences that I've seen on some other sites when comparing the 5D to the D300. Instead, I find the D300 to be a good match for the 5D when it comes to image quality, at least at lower ISO's (below ISO 800). At higher ISO's of around 800 and up, the 5D pulls ahead as expected, due to its larger sensor and greater sensitivity. In controlled side-by-side testing of the 5D and D300, I've found little difference between the two and in fact, might give the sharpness edge to the D300 up to about ISO 400. Here's a link to a comparison shot. Both shots were developed from raw and only some exposure and a hint of fill light added to adjust for differences in the way the two cameras metered the subject. Both shots were taken at ISO 200.
I believe some of the web sites showing better detail from the 5D were running into issues with the lens or even some issues with the noise filtering on the cameras where too much filtering was used on one camera versus the other. The only significant difference I can see with respect to image quality with full frame sensors is the ability to get better detail and less noise at higher ISO settings. Even evaluating noise at high ISO is becoming difficult these days, however, due to the adaptive noise reduction being used in the latest models. Click here for information on that subject if you haven't read last month's article.
Hopefully this article has provided some information on what to look for when considering a full frame versus DX digital SLR camera. To be honest, I do a lot of wildlife shooting and the 1.6x crop factor equates to more "zoom" which can come in handy when shooting subjects that are far away. I also feel that with many new (and good) lenses available in the 17-85 and even 17-200 zoom range, being able to get good wide angle shots is no longer a problem with DX cameras. DX lenses also tend to be a bit lighter and cheaper due to their size, which can also be a plus. For me, someone who has tried both and someone who didn't come from shooting film, I feel that full frame is more hype than hero. Someone who does a lot of studio work or who shoots differently may disagree. Thankfully (for me) this article is more about what to look for when considering whether or not to buy into full frame than an argument as to which is better for you! Different people obviously have different needs. All I can say at this point is that in my opinion, I don't think the existence of a few full frame cameras is going to push the DX models aside for a while.... if ever.
-- Mike Chaney