Question about optical zoom vs. focal length

[Helevitia]

I'm trying to learn as much as I can abut my new camera but I still have some questions.

My previous camera, Canon G3, had an optical zoom for 4x - 35-140mm equiv.

My new Rebel XT has an 18-55mm lens.

The question is, why don't they talk about optical zoom for DSLR camers? And why such a small telephoto number (55mm vs. 140mm) on the XT?

[Jon P. Inghram]

Umm, because the zoom on a DSLR is dependant upon which lens you use?

Part of the confusion with digital camera focal lengths is that a focal length of one size will have a significantly different field of view on a typical digital camera compared to a 35mm film camera due to the image plane being much smaller on many sensor chips. I think they sometimes indicate a "35mm equivilent focal length" so people who are used to 35mm camera lenses will get the results they were expecting.

[Liquid Snake]

4x simply refers to the zoom ratio (e.g. 140/35 is 4). Zoom ratio tells you the range from the widest to the longest on that particular lens. It isn't a particularly useful spec. For example, an 18 - 50 lens and a 70 - 200 lens are both about 2.8x, but the 70 - 200 is telephoto and the 18 - 50 is not. This is why almost all SLR lens manufacturers just tell you the focal length range and not the zoom ratio, because the former tells you a lot more than the latter. Consumer cameras are marketed with zoom ratios because the average consumer probably has no idea what focal length is.

About what JPI said, most DSLR cameras have what's known as a "field of view crop." In the case of the Canon Rebel XT, it's 1.6x. This means that mounting a 20mm lens on it will give you a field of view equivalent to a 32mm lens (20 x 1.6) on a standard 35mm camera.

[Helevitia]

So are you saying my 18-55mm lens that came with my camera is really equivilant to 28.8 - 88mm because the field FOV crop? And what does FOV crop really mean? Where is this "crop"? Is it the viewfinder? Or the actual picture? Sorry for all of the questions, I want to understand

I took 400 pictures over the weekend and learned a ton about ISO settings and exposure settings.

[Topha]

I think it means that where the analog cameras had 35mm sensors (film), the digital cams use smaller sensors. The lenses, however, are the same, so while you still get what you see when taking a picture (nothing is cropped there), the lens just takes in more information than it passes on to the sensor

[Helevitia]

Does that mean the actual picture will have 1.6x more?

[Topha]

I think it will be magnified by the factor 1.6, as opposed to a regular 35mm sensor, but I dont know much about it myself

edit: Thinking about it, my guess would be that the edges are lost, yet because it is not yet transformed into digital data, no quality is lost, so what FOV crop means is that you only get the central part of the information taken in by the lense, magnified in this case by the factor 1.6, yet without any loss in quality

[Liquid Snake]

Here's an explanation with pictures:

http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_Multiplier_01.htm

[Helevitia]

Cool, thanks. That helped out a bunch.

So basically I really have a 28.8 - 80mm lens equivilant and this puts me at a dis-advantadge on the wide angle side but gives me an advantadge on the telephoto side, correct?

[degrub]

https://www.luminous-landscape.com

Cheers!

[Dr Mordrid]

I'd put in my 2cents worth, but my eyes keep drifting left when I read Helevitia's posts

Dr. Mordrid

[Helevitia]

Sorry, didn't mean to divert your attention

I am interested in your 2 cents though, if you are still interested in giving it?