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?

[ScottyP]

This is pure Ernie Biggs territory. 

The main idea is the field of view. 

[RobertTheFat]

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@Anonymous wrote:

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?

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The people who can help you tend to scan all the topics in the forum. You really didn't have to post this in two places.

[ebiggs1]

"The main idea is the field of view. "

Great answer.  This is spot on.