Clarifiacation and help

safehaven wrote in post #2240019
Alrighty...lets try this with some other lingo...

If you mount an EF 70-200mm f/4L to a body with an APS-C sensor, it "behaves" as though it is a 112-320mm lense, correct?

If you mount an EF-S 18-55mm to a body with an APS-C sensor, will it "behave" like an 18-55mm, or will it "behave" as though it is a 28-88mm?

Just to be direct - YES the 18-55 wil behave like a 28-88.

The focal length of a lens DOES NOT CHANGE when you move it from camera to camera. EF-S DOES NOT CHANGE the focal length of a lens. It just changes the size of the light circle that the lens produces.

The last statement above "It (changing cameras) just changes the size of the light circle that the lens produces" is not absolutely correct. As I understand it, the EF-S designation indicates that the image size is optimized for the size of the smaller APS-C sensor and is, therefore, smaller than the image size produced from the non EF-S lens. This is why they cannot be used on a FF camera. The camera itself has nothing to do with the size of the image produced by a particular lens. Am i correct?

KCMO Al wrote in post #2242641
The last statement above "It (changing cameras) just changes the size of the light circle that the lens produces" is not absolutely correct. As I understand it, the EF-S designation indicates that the image size is optimized for the size of the smaller APS-C sensor and is, therefore, smaller than the image size produced from the non EF-S lens. This is why they cannot be used on a FF camera. The camera itself has nothing to do with the size of the image produced by a particular lens. Am i correct?

I think you would be more correct to think of it not as a smaller image, but less of the image being captured. If you have a given sized subject, at a given distance, at a given focal length then it will be the same size on the sensor with EF or EF-S lens on an APS-C or full frame camera. The difference is, because the APS-C sensor is smaller, more of the image projected through the lens will be cropped.

Dazecoop wrote in post #2238226
It was superb build quality and a lovely lens to use, but focusing was slow, it was missing and had front/back focus problems. I know that I got a bad copy, but its put me off of 3rd party lenses for good.

Seems like a trifling reason to give up on third party lenses. A friend of mine bought a Canon 10-22mm, which always produced images that were blurred on the right hand side. He sent it back and got a replacement. Should that have put him off Canon lenses for life?

dontblink wrote in post #2241486
Just to be direct - YES the 18-55 wil behave like a 28-88.

The focal length of a lens DOES NOT CHANGE when you move it from camera to camera. EF-S DOES NOT CHANGE the focal length of a lens. It just changes the size of the light circle that the lens produces.

THANK YOU!!! That is all I wanted to know. I have been mistakingly believing that crop factor did not apply to the EF-S lenses.

Thanks to everyone else as well for the Lense Engineering 101 lesson. Quite informative.

So, now that I am less confused, I have another questions. Do the EF-S, Di-II, and DC lense have a significant advantage on an APS-C sensor over that of a FF mount.

After reading more reviews from this site, reviews from other sites that people here have suggested reading, and from other threads, it appears that the Tamron's are slightly more reliable than the Sigma's in these focal ranges. So, I am kind of leaning toward either the Tamron 17-50mm or the 28-75. I think I would really use the 17mm end, but I also think that I would really use the 75mm end. But between the two Tamron's, I think I am leaning toward the 17-50mm. I already have a 70-200mm, so I have 70mm covered. But, 50-70mm seems like a pretty big gap.

Choosing a lense is extremely difficult! LOL. What does everyone else think? Am I overlooking something? Should I still be considering the Sigma's (I am very happy with my 70-200)? I don't know, I don't know

Thanks again everyone!