Are Zoom Factor and Focal Length Related?

[UWannaB]

What is the relationship between Zoom factor (i.e. 2x, 4x) and focal length?

One person says that it means "a 2x zoom means that a subject 100 feet away would appear 50 ft away" and another says it's a multiplier of the minimum focal length of the lens; thus a 15-30mm and a 100-200mm would both be 2X lenses.

If you know this, can you explain magnification also? Does 1X mean that a subject 1cm across would take 1cm of space on the film/sensor? If so, what does the 1.6 multiplier do to that?

Sorry if these are really newb questions, but that's me.

[HoodedOne]

Hi

zoom factor:
as far as I know, the second awnser is the correct one.

1.6 multiplier:
This number has nothing to do with magnification. Because of the smaller cmos in the 10D, the image recorded looks as pictures taken with a "longer" lens
example: on the 10D, pictures taken with a 50 mm lens look like pictures taken with a 80 mm on film.

Normally a magnification tells how large the recorded image appears on the negative. With macro lenses you probably want the 1:1 magnification (the negative shows the recorded image lifesize). If the same lens is used on the 10D the image is still life size on the censor, but you will see a smaller part of the image, because the censor is smaller than a negative.

cheers

[UWannaB]

Thanks.

I guess comparing p&s zoom spec and focal lengths is pointless then.

On the macro side, doesn't that mean that you take the magnification and mult. by 1.6 also? Isn't this the same effect that we see for telephotos? A 1 cm subject would be 1 cm on the CMOS, but then we'd print it, say 4x6. The same subject on film would be smaller on the same 4x6 print, wouldn't it?

Maybe I'm just thinking about it wrong :)

[Tom W]

UWannaB wrote:
What is the relationship between Zoom factor (i.e. 2x, 4x) and focal length?

One person says that it means "a 2x zoom means that a subject 100 feet away would appear 50 ft away" and another says it's a multiplier of the minimum focal length of the lens; thus a 15-30mm and a 100-200mm would both be 2X lenses.

If you know this, can you explain magnification also? Does 1X mean that a subject 1cm across would take 1cm of space on the film/sensor? If so, what does the 1.6 multiplier do to that?

Sorry if these are really newb questions, but that's me.

In relation to the zoom question, the second answer is correct when dealing with cameras. If one were dealing with binoculars, the first answer would generally be correct - if you bought a pair of binoculars and they were sold as "8X", you could be pretty sure that the image would be approximately 8 times the size (or appear 8 times closer) of that seen by the naked eye. But the binoculars are related to a reference point of the naked eye's view. The camera lens' reference point when they talk about its zoom range is the minimum focal length of the lens.

So, when you see a lens that says 3X zoom, all you really know is that the lens' longest focal length is 3 times that of its shortest focal length. Now if a lens has a 35-105 focal length, it is indeed a 3x zoom lens. And, if 35 mm film or an equal-sized digital sensor is used, its focal length will range from a slightly wide angle at 35 mm to a telephoto of approximately 2X magnification of what the normal eye sees (which with 35 mm film is approximately a 50 mm lens).

Naturally, with a smaller sensor, the approximation of what the unaided eye sees will be smaller. For example, my small digital point-and-shoot camera sports a very small CCD sensor. It also has a 3X zoom with a focal length of only 7.4 - 22.2 mm. That would be an incredibly wide angle lens if it were designed to use with 35 mm film, but since it is used with a small sensor, Canon thought wisely enough to relate it to the more familiar equivalent 35 mm focal length. In the case of my camera, Canon equates the focal length as being "35 mm equivalent 36-108 mm". So it is a 3X zoom, but its range again is from slightly wide angle to an approximately 2X telephoto.

As for the 1.6 multiplier, that number represents the somewhat smaller sensor that most digital SLR cameras use instead of a full-sized 35 mm sensor. The ratio of the 35 mm film (for which many of these lense are designed) to the smaller sensor happens to be 1.6 (not all are 1.6). What the multiplier means is that when you shoot with, say, a 100 mm lens, the equivalent image area as taken by the smaller sensor will be as though the picture were shot with a 160 mm lens.

I'm not always very clear in what I say. Feel free to do a search of the site - there's countless others that have a better handle on this.