EF-S 18-55mm

[TonyMan]

I am taking an online course on DSLR camera operation and someone posed a question about this lens. At 18 it's extended, at about 34 it's pulled in, but at 55 it's extended again. I see that my lens exhibits the same behavior. Why is this so? I've serach around and cannot find a picture or diagram of the inside of the lens.

Thanks

[MikeSowsun · Accepted solution]

This behaviour is very common with  zoom lenses in the slightly wide to slightly telephoto category. It has to do with the way the internal lenses and groups move inside to change focal length using both retrofocus and telefocus lens design. 

This diagram can give you an idea of the complexities involved. 

[TonyMan]

Thank you Mike!

[kvbarkley]

I believe it is the 15-85 that has a mark on the zoom ring at the lower end of the zoom range. I finally figured out that this was the zoom position for the smallest travel and the mark was to indicate where to "park" the lens before you put it in the bag!

[MikeSowsun]

My 15-85 has no markings on the zoom ring. Are you thinking of another lens maybe? 

Many of my other lenses have a mark on the zoom ring but it doesn't always denote the shortest length. I think it is just there because the spacing on the zoom scale markings don't match the travel on the zoom ring. 

[kvbarkley]

It is the 17-85:

[ebiggs1]

A more in detailed answer is, not all lenses are designed with internal zoom, constant aperture, and internal focus. The most expensive and difficult designs to make.  Different lenses are designed to do different specific tasks and to be produced at specific price points. Your kit lens the consideration was keeping the cost down. Period. In low cost designs the internal zoom elements might bump into other elements of the lens before they could travel far enough to extend, or contract, to the desired focal length. Compromises are done and room is made for the internals without worrying on what is happening to the outside.

[ebiggs1]

BTW, another factor of this class of lenses, as you close focus, the FL and aperture are effected. The f-ratio can be lesser by 1/3 stop or so. Yeah, it is small but it is there.