Convert EF-S lens on an APS-C Camera (60D)?

[Justin]

Do you have to convert an EF-S lens on a cropped sensor camera (60D)? I have searched the forum and not gotten a very clear answer.

I am looking at 2 lenses: EF 28mm f/1.8 USM and EF-S 24mm f2.8 STM

At first glance they look very similar: only 4mm difference between them. However, since I am shooting with a Canon 60D and it is a crop sensor, then I assume that I must multiply the lens by 1.6 to get the correct size. So 28mm would be 44.8mm on my camera. Since the 24mm lens is an EF-S lens then would it still be 24mm on my camera with no conversion necessary? Or do I still have to convert it to 35.2mm?

Justin

[Waddizzle]

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@Justin wrote:

Do you have to convert an EF-S lens on a cropped sensor camera (60D)? I have searched the forum and not gotten a very clear answer.

I am looking at 2 lenses: EF 28mm f/1.8 USM and EF-S 24mm f2.8 STM

At first glance they look very similar: only 4mm difference between them. However, since I am shooting with a Canon 60D and it is a crop sensor, then I assume that I must multiply the lens by 1.6 to get the correct size. So 28mm would be 44.8mm on my camera. Since the 24mm lens is an EF-S lens then would it still be 24mm on my camera with no conversion necessary? Or do I still have to convert it to 35.2mm?

Justin

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The focal length of a lens describes a physical ratio of the size and distances between the focusing elements inside of the body of the lens.  The focal length of a lens does not change, but the characteristics of the size of the image circle projected by the lens can change.

An EF lens is made to project and image circle into the camera large enough for a full frame image sensor.  An APS-C sensor is smaller than a full frame sensor.  The image circle is much larger than the APS-C sensor, which only sees, just the center portion of the image circle projected into the camera.

Because an APS-C sensor only sees just the center of the image, this is what creates the “cropping” that you hear people talk about.  It is as if you cropped an image in post processing, except now the cropping is a result of the smaller sensor.

An EF-S lens is designed to project a smaller image circle, which is just the right size for an APS-C sensor, but is also far too small for a full frame sensor.  A full frame sensor would capture a circular image, because the image circle projected by an EF-S lens is not large enough to fully cover a full frame sensor.  

An EF-S lens is not a good match for a large full frame image sensor.  This is why EF-S lens mounts are made slightly differently so that they cannot properly mount on a full frame, EF mount camera body.

[kvbarkley]

Besides the image circle, for an EF-S lens the focus plane can be closer to the lens which also makes it easier on the lens designer.

[ebiggs1]

"People are what confused to matter."

See what I mean?  Everybody thinks a 'lens' is a black tube with some glass inside.  However, any convex piece of glass has a FL. That is a lens! The FL is determined when parallel light rays come to a focus point.  In other words it is the distance between the lens, a convex piece of glass, and the sensor of the camera when the subject is in focus,  It also has an f-ratio.

The only differences between EF and EF-S lenses is superficial. The EF-S lenses sit deeper or farther inside the camera body when connected.  EF-S, the S stands for short back focus, because there is less space between the lens and the sensor.