Focal Lengths...

[jts01]

I'm curious if anybody can explain the idea of focal length. I use a Sony DSC S90 and don't even know where to find the focal lengths it has. What do they mean? I've seen them listed as a range x-mm to xxx-mm and have NO idea what that means or what you do to go from one end of that range to another.

I'm not entirely sure if this is the forum for this question, but this has been killing me and I would LOVE for some kind soul to enlighten me. Thanks in advance and have a great weekend!

[wulf]

On a point and shoot camera, it is often called zoom. Serious photographers prefer to talk about focal length, which sounds much more scientific and technical!

Do you know how to view the EXIF data on your photos? That will tell you what focal length was used although what effect this has also depends on the size of the sensor in the camera - f/2.8 on my D40 will give very different characteristics to f/2.8 on your Sony.

Wulf

[Major_Small]

Here are the lens specs for the DSC-S60 and DSC-S90:

• 39 - 117 mm (35 mm equiv) 3x optical zoom
• F 2.8 - 5.6

from dpreview

Going from one point in the range to another is usually called zooming, as Wulf already stated. Focal length is the length between the center of a lens and (in photography's case) the film plane or digital sensor. If you want to look into it, you could start here or here. Be warned though, any in-depth discussion of focal length quickly turns into a discussion of physics. It's usually enough to just know that focal length equates to 'zoom' though.

[inkista]

Ok, I'll be daring and go tech on this thing.

"Focal length" refers to the distance from a lens's center to its foci (focal points). It's a physical attribute of any lens.

The shorter the focal length is, the wider the angle of view, and the father "back" you feel. The longer the focal length, the narrower the angle of view, and the "closer" you feel. The Canon website has an interactive demonstration of focal length for 35mm film photography. In 35mm film photography, 50mm most equates to "normal" view (i.e., what you would see with the naked eye), and lengths below 50mm are considered "wide" and lengths above 50mm are considered "telephoto".

[This, btw, is why the zoom rocker on a P&S camera is mysteriously labelled "W | T".]

As wulf has stated, with a P&S camera, focal length relates to zoom, but the numbers won't be the same as with SLR lenses because the sensor size is smaller than a frame of 35mm film. Your sensor is 1/2.7", and your crop factor's 6.5x. So, if you look in your EXIF, if you multiply the focal length you find there by 6.5 that will get you the "35mm equivalent" focal length.

If you look at the lens of your DSC S90 it tells you everything, it says "Carl Zeiss / Vario-Tessar" (brand/model), "2,8-5,2" (this is your max. aperture: f/2.8[wide end]-f/5.2[tele end]) and "6-18" (this is the zoom's focal length range: 6mm-18mm, which gives the "35mm equivalent" of 39mm-117mm).

The 3x, 10x zoom thing basically just gives you the tele/wide ratio (e.g., 18/6 = 3, so you have a 3x zoom), so it's a relative scale. Actual focal lengths, however, give you more of an absolute scale by which to judge the amount of "zoom" you get with a lens.

[RittyRats]

To Inkista:

Wow! That was technical, but very well written! Thank you!

[reddevil08]

thats a lot to take. but thanks a lot inkista for your tachnical explanation. Although its not all clear to me yet....