I continually get emails, read articles, and hear photographers talk about the "advantage" of the "magnification" that many digital SLRs provide. With this much misunderstanding and misinformation out there we thought it was time for an article explaining some basics about digital capture - in plain English.

Plain English however means that we won't go into minute technical detail on some of these points. So we ask forbearance among our more technical readers. For the sake of this article a chip is a chip is a chip - whether it be a CMOS, CCD, or Foveon chip we'll refer to it as an 'image chip'. While these various chips have advantages and disadvantages for the engineers designing the circuit boards we really don't care which style is used to capture the image (Foveon's claims notwithstanding).

Size Matters 
Size matters in two ways. It matters in the size of the chip and it matters in the size of the pixels on that chip. Up to a point smaller pixels are bad and larger pixels are good. Why is it that a 3 megapixel digital SLR can provide better images than a 4 megapixel point and shoot? Part of the equation is optics. But just as important is the fact that the digital SLR has an image chip that is quite a bit larger than the digital point and shoot in this example. That allows it to capture an image with more detail and tonal quality because the larger chip has larger pixels. That image will in-turn enlarge into better-looking 8x10s and 11x14s.

Size also matters when it comes to a full size vs. a smaller image capture chip in a digital SLR. In both general use and in print I've seen people refer to the "advantage" of smaller digital chips in that they give you a free "extension" at the telephoto end. The tradeoff being that your wide-angle lenses aren't as wide. While this generally describes what you see it's not really accurate and has led to a misunderstanding of the real advantage of full-frame chips.

This misunderstanding really originated from the marketing departments of the camera makers. Longtime readers of Vivid Light know that I have a special contempt for copywriters in marketing departments. I'm not sure if marketing departments are the training grounds for the guys who write congressional press releases or if it's where they go to retire but the result is the same - the truth can be a slippery thing.

The image on the left was shot using a 300mm lens on a standard SLR. The image on the right shows the effect of mounting the same 300mm lens on a digital camera with a 1.5x chip. The result is the equivalent of a 450mm lens. This is a good thing right?

Early marketing literature, and the articles in the press based on them, described the chips in digital SLRs as 1.5x and 1.6x chips. The formula, said the marketing folks, was that you multiply the focal length of your lens by 1.5x or 1.6x (depending on the brand) to get the length of your lens when mounted onto one of their digital SLRs. The implication being that these chips provided some kind of magnification.

 This was touted as a great advantage for telephoto shooters and a "minor" disadvantage for wide-angle shooters. After all wide-angle shooters could always pick-up a new wide-angle lens! Every lens manufacturer has indeed made "digital" lenses available and sales have been brisk.

Image chips in digital SLRs crop the image provided by the lens. They don't offer any magnification. Is this really an advantage?

In the above example the image on the left was shot using a 300mm lens on a standard SLR. The image on the right shows the effect of mounting the same 300mm lens on a digital camera with a 1.5x chip. The result is the equivalent of a 450mm lens. This is a good thing right? This was a good thing said the marketing folks. What they didn't say was that it is more accurate to describe that 1.5x chip as a 2/3rds chip !

These chips are smaller than the area covered by a frame of 35mm film. There is no extra magnification, as is implied, and has been erroneously reported by journalists in any number of articles. These smaller chips crop the image rather than magnify it. So the "gain" is in fact no gain at all, it is a loss, and that loss occurs equally throughout the entire range of focal lengths from wide-angle to telephoto.

So is this really an advantage? 

By going to a full frame image chip you are able to capture the entire image provided by your lenses. Those photographers who complain they're losing "magnification" if they switch to a camera with a full frame image chip don't understand what's going on. If they switch to a full frame chip, shoot with the same lens, and crop the final image they'll still have the same quality as shooting with a 2/3rds chip because they're essentially doing the same thing - cropping the image.

The difference is that their wide-angle lenses will still provide their full angle of view with a full frame image chip - meaning they can forego the purchase of expensive super-wide lenses.

In fact many photographers end up replacing their entire compliment of lenses when switching to digital because of the cropping factor. 

No Free Lunch 
So if full frame chips are so great why aren't they exclusively in digital SLRs and why weren't they used from the beginning?

The answer is that the technology wasn't ready. When you capture larger images you need more memory in the camera, larger image buffers to hold images while they're waiting to be written to memory cards, and faster and larger memory cards to process and hold those large images. While these technologies existed three years ago they were larger, more power hungry, and significantly more expensive - and they remain so today.

Advances in component design have allowed camera designers to package all these features into a camera that is a reasonable size, works reasonably fast (2 or more frames per second), will take a reasonable number of images before it kills the batteries, and can be sold at a (ah hem) reasonable price.

But another serious problem faces the designers of cameras with full size chips. Image chips are made from silicon and silicon is much more fussy than film about the angle at which light strikes it. 

Canon's solution to controlling the angle of light striking their CMOS  chip is to place a grid of micro lenses in front of the chip to bend the  incoming light to a 90 degree angle

When the angle of the incoming light is relatively close to 90 degrees the chip is able to accurately capture the color and tone of the incoming light and render a sharp image. The edges of the 2/3rds image chips are close enough to the center axis of the lens that the angle of the incoming light is a non-issue. 

But when the rear element of a lens extends deeply into the body of the camera the angle of light striking the edges of a full size chip becomes acute. Depending on the chip the results can include color shifts, distortion, loss of sharpness or light fall off.

Canon's solution to the issue of the angle of incoming light is to place a grid of micro lenses over the image capture chip. These micro lenses change the angle of incoming light so that it strikes the chip at an angle closer to 90 degrees. The rep I spoke with from Kodak said this wasn't really an issue with their DCS cameras, but recommends that some lenses not be used with their cameras. One Canon rep indicated that controlling the light angle of incoming light is now a design criterion in new lenses now in development.

Keeping Up 
Digital cameras are going to continue to evolve at a fever pace for at least the next three to five years. The challenge for manufacturers is to maintain some kind of consistency among new models. 

Then there is the problem of maintaining customer satisfaction. If a customer spends $5,000, $1,500 or even $900 on a new camera they expect their new camera to have a reasonable lifespan. If customers perceive that their new baby is obsolete in 12 or 18 months (or less) they won't be happy customers for long.

In part this is a perception problem. The digital camera that you buy today will still be providing the same image quality a year or even five years from now. But as newer technologies are introduced the concept of what is "acceptable" is constantly revised upward.

One way to address this problem is to build on a standardized chassis. When changes are incorporated into newer designs there is not a significant cosmetic difference from older designs. This helps existing customers from an ego standpoint. 

A standard chassis may allow manufacturers the option to sell upgrades when new technologies are introduced such as replacing circuit boards. These upgrades could be offered at a fraction of the cost of a new camera.

Nikon has upgraded the size and write speed of the image buffers on the D1X - a current model. So where does that leave existing D1X owners? Nikon is offering to upgrade the cameras of current owners for $246.50 including shipping. They are also offering firmware upgrades for the D1H/X and D100 and a software upgrade to Nikon Capture that will double the image size for images captured in raw mode with D1H/X cameras (click here for details).

Conclusion
This column is in no way a criticism of the capabilities of SLRs with 2/3rds chips. Cameras like the Canon 10D and Nikon D100 continue to amaze me with their capabilities. Rather it is an attempt to clear up some misunderstandings about what these cameras are actually providing us.

The coming Christmas season will see the release of a new batch of digital SLRs in the 8.0 megapixel range. They'll represent an incremental improvement over the existing 6.0 megapixel cameras. Eventually the market will stabilize as improvements to new models reach a point of diminishing returns. A 200 megapixel camera won't offer a significant improvement over a 100 megapixel camera for most photographers. At that point digital cameras will be judged on innovations that further their image making capabilities - just as film SLRs are judged today. 

Until then things will continue to be interesting, confusing and sometimes frustrating for consumers and manufacturers alike.

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