16 August 2009

DSLRs: Less Is More

I've posted before about how more megapixels is a bad thing in compact cameras. I pointed out that the Fujifilm F30/F31fd cameras, with a modest 6.3mp, produced better photos than brand new 12mp compacts. It comes down to a fact of physics that smaller pixel sites packed closer together on any given size of sensor will gather less light and generate more noise than fewer, larger pixels. Past a certain point, cramming more pixels on a sensor is just a cynical marketing gimmick.

I've been disappointed to read that manufacturers have now crossed the megapixel quality line in DSLRs with small-format sensors, a format called DX by Nikon and APS-C by Canon. These sensors are used widely in entry- and mid-level cameras. They are much smaller than the full-frame 35mm sensors in the top-of-the-line DSLRs, though MUCH larger than compact camera sensors.

The blue (1.5x) and red (1.6x) rectangles represent DX/APS-C sensors respectively. Most compact camera sensors are smaller than even the innermost 1/3" sensor size.

DX/APS-C sensors have to this point produced superb, low-noise images even at ISOs as high as 1600. This allows, for example, the effective use of even entry-level DSLRs for night photography. Their main disadvantage as compared to larger full-frame sensors has been in megapixel counts, crop factor, and diffraction.

Crop factor is the reduced field of view that smaller sensors offer, which effectively turns that great 24mm wide-angle lens from your film camera into a 40mm normal lens on your DSLR. This is very problematic, and pushes people up to full-frame cameras in itself.

Diffraction means, in practice, that as you reduce your aperture, usually for increased depth of field, the point at which image quality starts to degrade increases as pixel density increases. So you may start to see diffraction at f7.4 on a 15mp APS-C camera, but only at f11 on a 16mp full-frame camera.

I have not read many reviews on Nikon cameras, since I shoot Canon. But I think Nikon is still putting quality imaging first. On Canon APS-C DSLRs, tests show that the latest 15mp models, the entry-level T1i and mid-range 50D, both have slightly poorer image quality than earlier models with 12mp or less, like the XSi and 40D. Dpreview concludes that the "40D stills beats the newer model [50D] in terms of per pixel detail. Despite a 22% increase in vertical and horizontal resolution the extra detail captured by the 50D is marginal." The 12.3mp sensor in Nikon's D90 and several other models beats it, too. It's looking like 12mp is about as high as you can go on DX/APS-C sensor while retaining optimal image quality.

Also, noise levels have really taken a jump, especially when working with RAW images ("digital negatives"). So, again dpreview: "When shooting in RAW the 500D [T1i] actually shows visibly more noise at higher ISOs than its predecessor [XSi]."

Two other problems are, first, as explained above, diffraction has necessarily increased as megapixels have risen. Perhaps more seriously, at these pixel densities it takes really good lenses to resolve images sharply, so that with the T1i, "At least towards the edges of the frame the kit-lenses struggle to resolve all the detail in a scene."

All together, when viewing T1i and XSi images side by side, the differences in quality are very apparent. Our reviewer concludes, "The 'extra quality' you can usually get out of RAW files compared to shooting in JPEG is relatively limited on the 500D [T1i]. One reason for that is the quality of the camera's JPEG engine. It is doing a pretty good job at 'optimizing' the JPEG output when converting the RAW data. However, the 500D's [T1i's] RAW images are also slightly lagging behind some of the competition and surprisingly even the 450D [XSi] in terms of high ISO noise and to a smaller degree in terms of pixel level detail. It's not going to be an issue when checking images at screen size but it's certainly visible up-close."

Canon should be introducing two or more new DSLRs on Tuesday (or Sept. 1; rumors vary). Almost certainly a new 7D to replace the 50D and a downspeced T1 to slot beneath the T1i. I'm now a little less enthusiastic about the T1i and will be interested to see what these new models bring to the table. Very sensibly, rumor has it that the new 7D will not increase in megapixels. I think Canon realizes that they've crammed all the pixels they can on a APS-C sensor without seriously damaging the image quality. Already they've gone a little too far.

For myself, I'm now looking at a refurbished Canon 40D or perhaps a slightly less expensive Canon XSi. There are pros and cons to both, but I'm leaning towards the 40D. It's 10mp are enough for me and it has a much better viewfinder than the XSi. That's important to me. But we'll also see what the new product launch brings.

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