Can someone help me better understand zoom and lens "mm", etc?

snapsnap1973 · Apr 12, 2013

Hi. Right now I have a little crappy Sony Cybershot DSCH20 and it has a 10x zoom. When I look at the EXIF info after taking a photo at "10x" zoom it says "63 mm". What I'm wondering is how this "63 mm" relates to say a DSLR's lens. For instance, if I have an 18-55mm kit lens and zoom out to 55mm on my Cybershot, does this mean the 55 mm on the cybershot is the same as the 55 mm on the kit lens?

Also, I heard that the 63mm on my Cybershot (10x zoom) is comparable to 200mm (?) on an SLR lens?

ph0enix · Apr 12, 2013

Let's start with the definition of "focal length" which is the distance between the center of a lens or curved mirror and its focus. Those mm values you're talking about are all focal lengths.
The next concept is zoom ratio. That's the 10x. It's nothing other than the ratio of the lens's longest focal length to its shortest focal length. The zoom ratio of a 18-55mm lens is 55/18 = 3.05X. If a lens has a focal length range of 10-100mm, for example, its zoom ratio is 10X (100/10). Zoom ratios are a pretty meaningless marketing spec. Some newer cameras have zoom ratios of 50X. It all sounds great in theory but realistically the high ratios are achieved by compromising IQ (image quality) so they're not desirable if you want quality photos. That's why people buy expensive DSLRs and bags of lenses to go with them.

How do focal lengths compare between P&S and DSLR cameras? Well, the "field of view" (what you see in the camera's viewfinder or on its LCD screen) is different. When set to the same focal length the P&S camera will be a lot more "zoomed-in" when compared to a DSLR. This is due to the fact that the P&S has a lot smaller digital sensor. The sensor is the part that records light (the image/photo). DSLRs sensors usually come in two flavors - crop and full frame. Full frame sensor is roughly the same size a 35mm film frame. Crop sensor is smaller by the crop factor which varies between manufactures. Canon APS-C crop factor is 1.6, Nikon is 1.5 (sensor size x the crop factor = full frame size). Full frame DSLRs are more expensive than crop sensor ones.

I bet KmH and TimCambell will respond with a lot more great info. In the meantime, take a look here:

https://en.wikipedia.org/wiki/Focal_length
Zoom lens - Wikipedia, the free encyclopedia
Crop factor - Wikipedia, the free encyclopedia

Big Mike · Apr 12, 2013

When a camera like that, lists the zoom as a factor...it is comparing the shortest (widest) to the longest focal length. So I'd guess that you lens can zoom from 6.3mm to 63mm.

That doesn't mean much to most people, so they tend to compare focal length & zoom to the 'standard' of 35mm film cameras. It's often called the 'focal length equivalent' Basically, what they are comparing is the field of view. It has to do with the size of the sensor in the camera. Since, the sensor is usually much smaller than a piece of 35mm film, shorter focal lengths are used.

Your camera, for example, has a sensor size of 1/2.3" (6.17 x 4.55 mm) and a focal length equivalent to 38–380 mm.

When it comes to DSLR cameras, you need to realize that most of them also have sensor that are smaller than 35mm film. Not tiny like the one in your (and most other) digital cameras.
Canon for example, in most of it's DSLR cameras has a sensor APS-C (22.3 x 14.9 mm). When compared to 35mm film, you get a factor of 1.6. (Nikon has a factor of 1.5).

So a typical 'kit' lens, 18-55m would likely have an focal length equivalent to something like 23mm -88mm.

Some DSLR cameras have a 'full frame' sensor, and thus don't need to use focal length equivalent, as they give the same field of view as a 35mm camera.

So no, you can't compare your camera at an actual focal length of 55mm, to a DSLR lens at 55mm. You have to compare them both back to the standard of 35mm film, and compare those numbers.