7D Crop Factoer Question

[carlc]

I realize that an EF lens on a crop sensor camera must be multiplied by 1.6 to arrive at the equivalent focal length on a crop camera, but my question; when using an EF-S lens, has the crop factor already been converted?  So that an EF-S 17-55 on a crop would be equivalent to a EF 17-55 (if one existed) on a Full Frame without having to multiply by the crop factor?

It kind of makes sense that on an EF-S lens Canon might have already factored in the crop equivalent when stating the focal length.  And then again maybe not.  Please advise

[IIIHobbs]

when using an EF-S lens, has the crop factor already been converted?

No

The EF-S lens mount is a derivative of the EF lens mount created for Canon digital single-lens reflex cameras with APS-C sized image sensors. Cameras supporting EF-S, however, have more clearance, allowing lens elements to be closer to the sensor than in the EF mount. Only Canon cameras with the APS-C sized sensor released after 2003 support the EF-S mount.
The "S" in EF-S comes from "Small image circle", meaning the lens is designed to provide a smaller circle than a normal EF lens.

The focal length of the lens is still measured as any other EF lens.

[Mt Spokane Photography]

I realize that an EF lens on a crop sensor camera must be multiplied by 1.6 to arrive at the equivalent focal length on a crop camera, but my question; when using an EF-S lens, has the crop factor already been converted?  So that an EF-S 17-55 on a crop would be equivalent to a EF 17-55 (if one existed) on a Full Frame without having to multiply by the crop factor?

It kind of makes sense that on an EF-S lens Canon might have already factored in the crop equivalent when stating the focal length.  And then again maybe not.  Please advise

NO!!!
EF and EF-s lenses have their actual focal length specified.  There are some P&S cameras that put the equivalent crop factor on their lenses, but the specs normally show the actual focal length.  A EF-s 17-55 and a EF 17-55 would be the same except for the ability of the EF to cover the larger sensor.

[Axilrod]

What they said.  But pretty much the 17-55mm is the full frame equivalent of 27-88mm. 

[dr croubie]

Easy way to remember:
The focal length is printed on the LENS, not the BODY. It is a property of the GLASS.
A 17mm lens is always a 17mm lens no matter what you mount it on, J1, m4/3, APS-C, FF, or MF.
A crop BODY is what the BODY does to the image.

When you mount a 17mm lens on a Crop Body, it is the Body that does the 'crop' or the 'focal length multiplication' or whatever you want to call it. 17mm times the 'crop factor' of 1.6x = 27.2mm, means that the image produced is the same as if you put a 27mm lens on a FF body.

[Daniel Flather]

The crop factor for Canon APS-C is 1.62, not 1.6.

[weixing]

Hi,

The crop factor for Canon APS-C is 1.62, not 1.6.

    Not all Canon APS-C sensor had the same size, so it should be: ~1.6x   

   Have a nice day.

[DB]

Think of a circle with a rectangle inside it (a 36mm x 24mm rectangle) and all 4 corners of this rectangle are just touching the circumference of the circle....this is what 35mm equivalent "full-frame" sensor looks like.

Now picture a smaller rectangle (less than 40% the size of the first rectangle) placed inside the first rectangle, this is a crop sensor.

Now imagine light travelling through the glass of a lens in straight diagonal lines such that the outside image is reflected onto the sensor upside down (visualize very elongated X's or lines of light travelling down the glass cylindrical lens placed in front of this circle with the 2 rectangles in it)

So because the smaller sensor is smaller, the angles of these lines of light are not as steep as they would be on the FF sensor (think small-x on crop and big-X on FF). The difference between the angles on both the big and small rectangles is the "crop" or multiplier factor. The angle of view is different.

There's also an aperture factor to consider as well - as the lines of light (if we use that analogy) are steeper in a camera with a bigger sensor, which is why a larger sensor camera needs a smaller aperture to get the same DoF (as the sensor size affects the angle of view).

For example, using a 100mm lens on an APS-C body at say f2.8 with it's 1.6x crop factor, would give you the same perspective as a 160mm lens @ f4.0 on a FF body.

[Meh]

The crop factor for Canon APS-C is 1.62, not 1.6.

Ummmmm... yes, that is correct.  Are you being serious?

[justsomedude]

The crop factor for Canon APS-C is 1.62, not 1.6.

Is this a serious post?