Cropped lens on FF body - Sigma AF 8-16mm f/4.5-5.6 DC HSM on a 6D - Effective focal length?

[CgRay]

I sold my 600D and am the very proud owner of a new 6D. I still have my Sigma AF 8-16mm f/4.5-5.6 DC HSM which is for cropped sensors. After some investigation as to why not to use lenses made for cropped sensors on full frame, I decided to give it a go. The reason I read was that often EF-S lenses had more protrusion into the camera body that could be hit by the mirror when flipping up. I compared the back of the lens at focal length 8 and 16 with full frame lenses and it didnt protrude any further than any I compared it to. The lens works great when set to 16mm, but at wider settings it gets severe vignetting obviously 'cause the larger sensor allows it to see out past where it's meant to and the hood fins get in the way. So after all that, the main question I'm asking here is how to calculate the focal length of a cropped lens on my 6D. Is it just x/1.6? So at 16mm, it's effectively a 10mm?

Cheers,

Cg.

[cicopo]

It's my understanding that even though it's designed for crop bodies the industry uses the specs they did for 35 mm film cameras. That would mean it's still an 6-16 mm zoom but it doesn't light up the full frame sensor throughout that zoom range. .

[MikeSowsun]

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CgRay wrote:

I sold my 600D and am the very proud owner of a new 6D. I still have my Sigma AF 8-16mm f/4.5-5.6 DC HSM which is for cropped sensors. ...................................

...............................................................So after all that, the main question I'm asking here is how to calculate the focal length of a cropped lens on my 6D. Is it just x/1.6? So at 16mm, it's effectively a 10mm?

Cheers,

Cg.

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No it is still a 16mm focal length on your 6D.    Your thinking is partially correct because the view you have now at 16mm on your 6D would be equivalent to 10mm on your 600D.  

[ebiggs1]

Just like the leopard and the changing spot thing, a lens can not change it's focal length from what it is.  No matter it is and will always be a 8-16mm even if you put it on a medium format camera.

One way to check a third party lens is to run the zoom from short to long and back with the focus at infinity. Repeat this at Minimum Focus Distance. Set the lens at it's widest focal length, run the focus from MFD to infinity and back. Repeat with the lens it's longest focal length. If no lens elements protrude past the flange on the rear of the lens at any time you are probably safe that the lens will not contact the mirror. When mounted and in use, however, it may not focus properly at all distances.

The real question is, why?

[CgRay]

Wow, all this time I thought that an EF-S lens at 50mm on a cropped sensor would create the same image as an EF 50mm on a full frame sensor.

The reason I want to do this, is that I like this lens and i don't have any other cameras to use it on since I sold my 600D

[ebiggs1]

Yes, this is due to the "angle of acceptence" and not the lens changing it's focal length.