« on: December 03, 2012, 08:47:43 PM »
You are confusing it like other people do. You saying what the focal range would be if the 18-135mm lens were put on a full frame body. But here is your error, you can't put APS-C-only lenses on a full frame body. The 18-135mm CANNOT be attached to a full frame body like the Canon 5D. It can only be put on a APS-C body. Therefore, calculating the crop factor of what it "would" be on a full frame body is irrelevant. The only thing that matters on an APS-C-only lens is the stated focal range, in this instance, the 18-135mm. The focal range for this lens will never need to be recalculated for a full frame camera, because you cant even mount it on a full frame camera. Thats why so many people get confused and automatically multiply it by 1.6. Its 18-135mm, thats it.
Consider this scenario. The view and angle through the viewfinders for the following two scenarios will be identical:
Canon 7D with the 18-135mm zoomed to 18mm.
Canon 5D with an 18mm Full Frame only lens.
If you had no idea what camera I handed you, and gave you these two configurations, the image through the viewfinder would be identical and you wouldn't know which configuration I gave you. The 7D with the 18-135mm at 18mm will not be more "zoomed" in at "29mm". If the lens is at 18mm, then its at 18mm. If its at 135mm, its at 135mm.
The only time this changes is when a lens that is compatible with full frame cameras is put on an APS-C body. Then you multiply the focal length(s) by 1.6.