Crop Factor and 17-55 Lens

While I was researching the recent purchase of the 17-55mm lens, I read the sticky about the influence of the smaller sensor on focal length and beginning to understand the importance of understanding this. I do have a question.

I have the 30D, smaller sensor than a full-framed camera. Mine would have a 1.6 crop factor.

Therefore, the Canon 24-105mm L lens would actually perform as a 38 - 168mm attached to the 30D? From what I understand this lens was made for a FF camera.

From what I have read, the 17-55 was made specifically for the cropped sensor cameras (I don't know what to call them, sorry) and not a FF camera.

Due to the fact that the lens was made specifically for a cropped sensor in this case, does that mean it will be a 17-55mm truely, or does the crop factoring still apply and the focal range is actually 27-88 ?

Hope this makes sense, it's hard to articulate the question I have.

Another question I have from all this:

The front glass of the 17-85mm IS lens I presently have is 67mm. The new 17-55 f2.8 is 77mm - this is the same size as my 100-400mm L lens, which shocks me.

What determines the diameter of lenses? Both of the above lenses are IS USM, one is f/2.8 and one f/4, but the 1-400mm is only f/4?

I'm really curious about things here that I've not been interested in before this...can someone enlighten me (if my questions make sense that is.)

Thanks / Pat

EF-S Lenses are specifically made for smaller sensors found in 40D, XTi, XT, etc. Imagine theEF lens projecting a round image diameter onto a full-size sensor and, when used on a smaller one, there is much wasted image since the sensor is smaller. The EF-S lenses help the lens perform as the EF lenses were originally designed, so that the diameter of the image cast onto the sensor is not overly large.

As for what determines the diameters of lenses, its all part of optical science and what speed and focal length you are designing for the lens. I am sure the manufacturers like to keep the glass smallest possible for the relative lens speed as larger glass adds more weight and expense to the lens.

The format of your camera does nothing to the focal length. The 24-105L is still a 24-105mm lens. It just has a different field of view on your camera than it does on a full frame. Likewise, the 17-55 IS has the same FOV at 17mm as the 17-40L does at 17mm. 17mm is 17mm is 17mm. The focal lengths NEVER change.

Here is a good explanation, made easier by photos showing the differences between cropped sensors and full sensors (35mm film):
http://www.kenrockwell​.com/tech/crop-factor.htm

Thanks everybody... it's about the field of view not the focal length of the lens - got it finally! Thank you for the site, it is very helpful and it looks like there is a lot more there of value. As it suggests, I think I will pull out my old pentax film camera tomorrow and have a look for myself.

Doug, your image circle analogy was really helpful.

I guess I'm trying to get a clear understanding of it so I know when I need to consider these facts. Great, thanks again!

This thread from the "Stickies" also explains the "crop factor" pretty well.

To answer your question in much simpler terms, yes the crop factor applies.

plbb wrote in post #5398911
Therefore, the Canon 24-105mm L lens would actually perform as a 38 - 168mm attached to the 30D? From what I understand this lens was made for a FF camera.

From what I have read, the 17-55 was made specifically for the cropped sensor cameras (I don't know what to call them, sorry) and not a FF camera.

Due to the fact that the lens was made specifically for a cropped sensor in this case, does that mean it will be a 17-55mm truely, or does the crop factoring still apply and the focal range is actually 27-88 ?

Pat, focal length is focal length is focal length. A 24-105 is never anything but a 24-105 and performs just like a 24-105 should.

The "crop factor" is simply for comparing the field of view with the same focal length on two different camera formats. "Format" refers to the size of the film frame (or digital sensor) in a camera.

A 24-105 mounted on a 30D will provide the same field (angle) of view range that a 38.4 - 168 lens mounted on a 35mm film camera (or "full-frame" DSLR) would.

The image of a subject produced by any particular focal length will be the same - at the "film" plane - in a 35mm film camera or your 30D. There will just be less of the scene recorded in the 30D because the sensor is smaller than the 35mm film plane.

Regardless of lens design - EF, EF-S, or whatever - any lens of the same focal length attached to your 30D will provide precisely the same size image as any other lens of the same focal length.

If you don't have experience with 35mm film cameras (or "full-frame" DSLR's, for that matter), you can totally forget the "crop factor" in your daily photography. Just consider the focal lengths printed on your lenses. Remember that approximately 30mm is the "normal" focal length for your 30D. Any lens that has a shorter focal length than 30mm would be termed a wide-angle and lenses with focal lengths longer than 30mm would generally be termed telephoto (although there's a technical reason why telephoto is not necessarily a perfect term to use).

As far as the lens size, alot has to do with how much light they let in, thus a 2.8 will be larger for the same focal lengths as a f4, or f3.5/5.6 lens.

BTW,

Alot of Canon's lenses use a 77mm filter, which makes it much easier to use filters, only having to buy one for that size.