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January/February 2009: Helpline

Digital Differences

This Article Features Photo Zoom

While you can use lenses designed for “full-frame” 35mm-sized sensors on smaller-format (APS-C) sensors (subject to the telephoto, or magnification effect), the converse isn’t true. For example, the Canon EFS 18-55mm IS, included in the EOS Rebel XS kit, is designed specifically for use with APS-C-sized image sensors, like those used in the Rebel series. It isn’t designed to work with Canon’s full-frame sensor cameras like the new EOS 5D Mark II. No matter what camera brand you shoot, if you move up to a full-frame model, don’t expect to use the lenses designed for your sub-full-frame camera. The issue at hand is the size of the image circle that the lens produces. Lenses designed for smaller sensors produce smaller image circles that aren’t large enough to cover a 35mm-sized sensor.

Does 12=12?
Q) Are my “kit” lenses going to work if someday I buy a full-frame digital camera? Or how about my wide-angle zoom, which is for digital cameras with APS-C-sized sensors? I realize the new cameras say that the camera knows when you use a lens like this, and that it only uses part of the sensor. If these lenses are for that size only (APS-C sensor), then how can they legally say it’s 12-24mm, when in actuality, on that size sensor, it isn’t? My hope is you’ll say that on a full-sized digital-camera sensor, these lenses will actually be the size written on them (just like they would be if it were a lens for a 35mm camera), and with my 12-24mm, I’ll actually have that wide of a lens, instead of an 18mm at the widest. And if you say, no, my 12mm (digital lens) will never be a 12mm, then I think people should know this.
Connie McAyeal
Via the Internet

A) Most likely, your “kit” lens (“kit” meaning it came with your digital SLR body) is one designed for use on the digital SLR it was packaged with. So, if that SLR has an image sensor that’s not a full-frame sensor (the same size as a 35mm film frame), then the lens won’t work on a full-frame sensor camera. Even if you could physically mount your (non-full-frame) lens on a full-frame body, the light passing through the lens would not create an image circle that would cover the entire sensor surface.

Why? The lens is designed to create an image of a certain size at the image sensor plane. If that image sensor is smaller than a 35mm film frame, it doesn’t make sense to create a lens with optical elements larger than needed. Lenses can be less expensive to produce this way. As an analogy, if you make a bedspread for a bed and your bed is a double, it doesn’t make sense to make a king-sized bedspread. The extra fabric would be wasted.

There’s a lot of confusion about the issue of using lenses on APS-sized digital cameras. All sorts of terms have sprung up: focal-length conversion factor, magnification factor, cropping factor, field-of-view factor or focal-length multiplication factor. And the list seems to grow every time I get asked about this topic.

All of these terms attempt to explain how lenses designed for 35mm film (and full-frame sensors) will work on a sub-full-frame camera. Unfortunately, while the terms try to help clear up the confusion, often they just add to it. (I probably get more feedback letters on this topic than any other. And let me just say that this can be a very passionate subject for some readers.)

To put it another way, if the 35mm film format had never been created, and APS-C was the only format around, there wouldn’t be all this confusion. A 50mm lens would be considered a 50mm lens, and there would be no conversion ‘factors.’

The issue all boils down to the focal length of the lens. Focal length is an optical term not specific to photography. It’s essentially the measurement of the optical power of a lens—some might call it the magnification power of the lens.

Notice that nowhere in the previous paragraph is there any mention of a camera. And this is the key to answering your question. Focal length is a “lens and lens only” measurement.

In the case of your 12-24mm lens, it is a 12-24mm lens. That’s it. At its widest setting, it’s a 12mm lens, and at the other range, it’s 24mm. The focal length doesn’t change when you attach the lens to your camera.

The confusion creeps into the equation when people say the lens “changes to a 19-38mm lens when mounted on an APS-C-sized sensor.” While it may seem that the lens “acts” differently, the fact is that the lens is still a 12-24mm lens. In reality, it’s the camera that changes things.

This situation isn’t new to digital. Photographers who use both medium-format cameras and 35mm cameras have always had to deal with the difference between a 50mm lens on a medium-format body and a 50mm lens on a 35mm body. Once again, it’s still a 50mm lens; the field of view changes—not the focal length.

To put it another way, if the 35mm film format had never been created, and APS-C was the only format around, there wouldn’t be all this confusion.

A 50mm lens would be considered a 50mm lens, and there would be no conversion “factors. Now back to your request that “lenses actually be the size written on them.” They are. If manufacturers started labeling their lenses based on an “apparent” focal length, they’d be mislabeling the product. I know that doesn’t make sense or help the situation, but it’s the reality.

It gets even more confusing when some companies talk about “digital lenses,” and people think this means that the focal length printed on the lens has been adjusted for their smaller image sensor. It hasn’t.
More importantly, if a lens is specifically designed for a sensor smaller than a full-frame sensor, the lens won’t work with a full-frame camera. First, it may not mount on the camera. Second, even if it did, the light presented to the sensor won’t cover the entire sensor area.

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