Help understanding lenses

dutch27

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So the used 50D I picked up had a few lenses, none being EF-S. One is a 35-80 from the film bodies. My understanding is the sensor in the 50D is a 1.6x crop factor, so the 35-80 is equivalent in field of view to 56-128mm on a full sensor body.

I'd like a wider lens to toy with, I mountain bike and recently am starting to explore hiking spots, and want to get wider landscape shots. I'm not going to invest in an expensive wide angle since I'm just starting to exploring this as a hobby, but I'd like to pick up a used 18-55 EF-S kit lens.

So, my question is, is the 18-55 EF-S still subject to the 1.6 crop factor? From what I understand the EF-S are built specifically for DSLR crop bodies since the lens can be placed closer to the mirror. So, is the EF-S 18-55 equivalent to a 18-55 EF on a full frame, x1.6, or somewhere in between? Is the difference in field of view going to be drastic between the 18 EF-S and 35 EF?

If I can find something a bit wider or with a bigger range on the used market for a reasonable price I may consider it, but don't want to spend a ton in the starting stages of exploring a hobby.
 

Gavjenks

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Focal length as advertised in millimeters is always the same. A 100mm lens has the same optical focal length whether it is a crop frame lens, a full frame lens, medium format, giant view camera with bellows, a piece of glass you ground yourself out of silica in the forges of Mordor... doesn't matter.

The reason they sell them as different lenses is that the crop frame lenses have smaller pieces of glass (as in diameter), since they don't need to cover as big of an image sensor. But the curvature and refractive power (assuming an idealized simple lens) is identical to a lens advertised as the same focal length in full frame.

In other words, a 100mm crop frame lens has at no point been divided by 1.6 to arrive at that number. In fact, in most (but not all!) cases, you can physically fit crop frame lenses onto full frame bodies without damaging your mirror, and they will act like lenses of their advertised focal length on full frame. But you'll just have big black corners in your shot since it isn't covering the whole area.
 

Juga

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dutch27

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Thanks for the info guys. Probably going for the basic kit lens since I'm starting out, but am going to hunt and see it I can't happen on a great deal on a 17-85. I'm guessing I'll end up with the kit lens, and I'm sure I'll be ecstatic with it for awhile. Learning composition is more important than a higher level lens for now.
 

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