Is the Canon EF-S mount lens doomed?

I recently purchased a Canon 20D camera. One of the interesting things about this camera is that it uses a new lens mount. The seemingly cool thing about this new mount is that it is compatible with all the standard Canon EF lenses and also with the new EF-S lenses. For those not familiar with Canon lenses, the EF lenses were introduce in 1987 and featured a fully electronic (non-mechanical) interface to the camera. There are about 50 Canon EF lenses available and even more EF lenses made by other companies. Many people consider the "L" series Canon EF lenses to be the best in the world. Needless to say there is a strong momentum behind these lenses.

 

The EF-S lenses take advantage of the fact that the 20D has a smaller "APS-C" size sensor. This sensor is smaller than a "full frame" sensor which is the same size as 35mm Film.

 

 

Now because the sensor is smaller the lens only needs to focus an image on the smaller area. This is kind of interesting. One thing you get is a 1.6x crop factor. The crop factor makes your lenses feel like they are zoomed in closer. This isn't as good as it first appears, but that's a subject for a different blog.

 

Since EF-S lenses make images that are too small for a full frame, they can't be used with full frame cameras. So why would somebody make a camera with smaller sensor? Simply because of economics. Smaller sensors are cheaper to manufacture and the sensor is a key component in the price of the camera. Sensors are very much like computer chips. They are made on silicon wafers with several sensors on each wafer. Large sensors cost exponential more. There are several reasons for this:

• You can put more small sensors on a wafer than large ones.
• Wafers are round which causes more area on the edges to be wasted with large rectangular sensors.
• When there is a flaw in the wafer, it usually ruins the one sensor under it. Large sensors are more likely to be flawed because of their bigger area.

You can make a sensor of any size but with a given process the number of pixel/mm stays the same. For any given process a large sensor will have more pixels. Therefore the trade off you are making with a small sensor is not just size, but also the number of pixels.

 

I have a theory about these sensors that nobody seems to have written about on the web. I predict that Canon's next high-end camera will have a full frame sensor but it will also accept the EF-S lenses. When a EF-S lens is put on the camera it will switch to a mode that only records image data in the APS-C area of its sensor and throws out all the pixels around the edge. There's no reason for them to disallow these lenses and it is in Canon's best interest to make an upgrade path for somebody that has an investment in these lenses. If the price of sensors comes down Canon can also use this scheme to put a full frame sensor in their low to medium price cameras.

 

I'm actually unsure about some of the technical details. I think the EF-S format allows the glass to be closer to the sensor. There is room because the mirror is smaller when the sensor is smaller. So a full frame sensor with a larger mirror might not actually have a enough room without hitting the EF-S rear lens element.

 

It would also be possible to make a converter that magnified the image. A EF-S lens would work on a full frame camera but it requires the light to travel though another element which would cost a stop or so. I suspect that optically correcting the frame size would produce a better quality image than my idea of simply cropping it.

 

Since Canon is a reasonable company I'm quite confident that my investment in APS-C lenses. They will not be obsolete any time in the near future. As long as cameras support EF lenses they will also support the newer EF-S lenses.