What is the mm product of APS-C cameras?

[Anonymous]

I have a rebel t6i.  The kit lens it came with was the 18-55MM STM.  It is my understanding that 18MM is ultra wide angle, correct?  So if my lens, with these specs, were compatible with a full frame camera it would be an ultra wide angle lens, correct?  It is my understanding, however, that it is not compatible with full frame cameras.  It is also my understanding that the ultimate product of the pictures I take with my T6i are not ultra wide angle because of the crop factor of the APS-C sensor, is this correct?  If so, when I have the lens set to 18MM what am I actually seeing?  Is there MM you can put to the final product with the crop factor?  Finally, if I understand all this correctly, why don't they just name the lens with the MM it puts out with an APS-C sensor if it's not compatible with full frame anyway?

[ebiggs1]

Or......

ebiggs1 wrote:

...

A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on.

The AOV for a 18mm lens on a Rebel full frame is approx. 100 degrees but the very same lens used on a FF Rebel will be only 74 degrees. ... 

The take away here is this.

A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on. The accidental transposing of a number or name not withstanding. There is no cropping or enlarging or alteration of the frame at all.  Simply the AOV is different.

[RobertTheFat]

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@ebiggs1 wrote:

Or......

@ebiggs1 wrote:

...

A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on.

The AOV for a 18mm lens on a Rebel full frame is approx. 100 degrees but the very same lens used on a FF Rebel will be only 74 degrees. ... 

The take away here is this.

A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on. The accidental transposing of a number or name not withstanding. There is no cropping or enlarging or alteration of the frame at all.  Simply the AOV is different.

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Yeah, I noticed that the two values appeared to be reversed. But that's beside the point. Either way, it contradicts the assertion that the AOV "is constant and never changes no matter what camera body the lens is on".

[ebiggs1]

B from B

Yeah I guess I see that too. OK here is another stab at it.

"ebiggs1 wrote:

...

A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  

The AOV for a 18mm lens on a full frame is approx. 100 degrees but the very same lens used on a  Rebel will be only 74 degrees. ... "

If you want to match scenes you need to find a lens that offers 100 degrees of AOV.  In this case a 10mm. The AOV which is the constant here does not change. If you want to know what lens is a telephoto or WA or UWA or even normal all you need check is the AOV.  Forget the millimeters part.  Forget the crop factor which doesn't actually exist anyway.

I do hope this makes it all very much more clear?  I knew what I was saying all along.  I am just not good at typing it.