Focus length due to cropped sensor

[blaz7]

Hi,

Some question i have been wanting to answer but could not.

Is the 17mm of the EF 17-40mm f/4L USM on a FF body the same as the 17mm of the EF-S 17-55mm f/2.8 IS USM on a cropped body?

Since EFS cannot be used on a FF body.
If we use a EF lens on a cropped body, it gets multiplied by the crop factor.

Many thanks.

 

[J-Chan]

17mm is still 17mm, its the angle of view that is different..

 

[blaz7]

Then is the wideness(angle) the same or different.

 

[wdEvA]

EF / EF-S lens when used on cropped body, u still have to multiple the focal length by the cropped factor.

17mm on FF will be 17mm
but 17mm on 1D series will be 17x1.3 = 22.1mm on FF
and 17mm on xxxxD / xxxD / xxD series will be 17x1.6 = 27.2mm on FF

 

[frankchn]

I think the OP is trying to ask if the focal length is still specified on the same scale even it is an EF-S lens that can only be mounted on 1.6x bodies. The answer is yes. The EF-S 17-55 when mounted on a compatible body will have a 35mm equivalent focal length of 27.2-88mm.

 

[blaz7]

I see, so no matter if it is EF or EFS, on a cropped body, still need to multiply by the cropped size.

Then in the first place, why did they come out with EFS lens, and not just make all lens to be EF.

 

[frankchn]

Cheaper to design/build as the glass doesn't need to project a 43mm diameter image circle onto the sensor across all focal lengths, but can make do with a 27mm circle. Reduces the size of glass needed at least.

 

[pl4sMa]

Cheaper to manufacture

You don't have to manufacture glass that provide the whole coverage of a full-framed sensor.
That's why EF-S lenses that are used on full-framed sensor have vignetting issue

 

[frankchn]

Here is what happens when a lens designed for cropped sensors (APS-C) is mounted on a FF body. Lens in question Sigma 10-20 zoomed out to 10mm, body is 5D2.

 

[estel]

and due to the smaller size of the mirror hence less clearence needed for mirror flap, it can have short "back focus" i.e. the rear element of the lens can protrude into camera body. This makes the design cheaper in some cases. They can't be used on a full frame camera without damaging the mirror.