Saturday, September 15, 2007

APS-C versus Full-Frame. What's the deal?

Are you confused with the conversion factor of 1.4 to 1.6 (and probably other conversion factors I don't know about) applied to most lenses designed for the 35mm format but used on DSLR using an APS-C sized sensor? Well, maybe this will help. Using AutoCAD and PhotoShop, I was able to make the sketches herein.

All lenses designed for the 35mm film format project an image circle on the film plane of roughly 43mm in diameter, regardless of the focal length of the lens. I am showing a 300mm lens in the figures herein, but all lenses designed for the 35mm film format project the same size image circle on the 35mm film plane. A 35mm film frame measures 36mm x 24mm and therefore the image recorded is not round, but measures 36mm x 24mm. The portion of the round image outside the 35mm frame is wasted.
See figure above for an overlay of the image circle, the 36 x 24mm film or full frame , the APS-C sensor and a 4 x 5 inch film sheet. See figure below for the actual 35mm film full frame. It is full frame 35mm film format because it captures the rectangular image of 36 x 24mm within the image circle.

What seems to confuse a lot of people is the perception that a lens designed for a 35mm film format, used on a APS-C sensor, becomes 1.5 times more powerful. I have heard, as example, that a 300mm lens designed for 35mm film format becomes a 450mm on a DSLR with an APS-C sensor (as 300mm x 1.5 = 450mm). That would be magic. A 300mm lens is a 300mm lens is a 300mm lens. Got it? What changes, is that because the APS-C sensor is smaller than the full frame 35mm format, the image is cropped. It appears to be a longer telephoto, but it is not. The angle of view changes, because of the smaller size sensor of 23.5 x 15.7mm. It's no different that cropping an image in PhotoShop. See figure below for the APS-C crop.

Now, if you were to use a lens designed for a 35mm formal and use it on a 4 x 5 inch view camera, the image circle would not be big enough to cover the entire 4 x 5 frame. What you would get is a round image in the middle. See figure below.

Using a wide angle lens would have the same effect. As example, a 20mm lens designed for the 35mm film format would give a angle of view equivalent to a 30mm lens. Again, you are just cropping the image.

Owners of APS-C sized DSLRs claim that they get more telephoto for the money. Similarly, the owners of full frame DSLRs claim that they get more wide angle for the money. Nobody can win at that game, it all depends what you shoot more often. Wildlife and sport photographer will have an edge with longer telephoto, while keeping the same widest aperture, for a lot less money. On the other hand, Landscape and architectural photographers will benefit from the true wide angle of their lenses. If you take all kind of pictures, you decide what's more important to you.

The sensors have evolved a lot in the past ten years or so. Most cameras were using CCD sensors a while back. Now, with technology advances and production costs going down, CMOS sensors are being used more and more. The bigger the sensor, the less noise is introduced and the higher ISO settings can be used. At this time, CMOS full frame sensors seem to be the fashionable sensors because they produce less noise. What about all those lenses I bought for my Pentax K10D? If Pentax joins the current trend and use full frame sensors, all my DA lenses won't work with full frame. The image circle will be too small. I am confident that the evolution of sensors will be such that APS-C sensors will produce less and less noise. I can use my lens designed for the 35mm format on my APS-C sized sensor DSLR, but cannot use the DA lenses on full frame DSLRs. Beside, it costs less to manufacture APS-C lenses and we stand to gain from that.

Just my two cents worth.

Thank you for reading my post. Let me know what you think.

Yvon Bourque
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