how compare camera lens in mm with x's?

Answer 1

Clavestone is correct, but here's more useless information that I already have written. Just divide the smaller number into the bigger number and you get the "X" power of the lens. For instance, a 35-80 mm lens is 2.3X zoom. Divide 80 by 35 and you'll get the result.

It is usually better to know what the focal length of a lens in "35 mm equivalent" is and judge by that, rather than relying on the "X" power of the lens. For instance, most point and shoot cameras start at about 35 mm and have either a 3X or 4X zoom. This would make it a 35-105 or a 35-140. I've seen some that start at 28 mm, though. A 3X starting at 28 mm is 28-84 and a 4X is 28-112. Neither one is a particularly strong telephoto lens and the 4X is just about the same as the 3X that starts out at 35 mm.

It's also important to realize that tradition dictates that lens focal lengths are usually expressed in terms of "35 mm equivalent," where "35 mm" refers to a 35 mm film camera. This is because of the relation between the sensor size and the actual focal length of the lens and the resultant angle of view of the lens.

I have one point & shoot that is actually a 5.8-24 mm zoom. This is a 4X zoom. The 35 mm equivalent is 28-116 mm. The sensor is 7.2x5.3 mm. (1/1.8") (And I wish I knew someone who could explain how the heck they came up with sensor size terminology!)

I have another point & shoot that is actually a 5.7-17.1 mm zoom. This is a 3X zoom. The 35 mm equivalent is 34-102 mm. "How could a shorter focal length give a longer 35 mm equivalent?" you might ask. It's because the sensor is only about 5x4 mm. (1/2.5")

I have a few Nikon DSLR's and - thankfully - they all have the same size sensor. They all have a "lens factor" of 1.5. This means that you just multiply the actual focal length of the lens to get the 35 mm equivalent and then you can make comparisons accurately from camera-to-camera. Most Canon's, for instance, have a lens factor of 1.6. On a Nikon DSLR, a 28 mm lens is the "35 mm equivalent" of a 42 mm lens. On most Canon DSLR's, the same 28 mm lens is the equivalent of a 45 mm lens.

These example are just to show you how freaking confusing it can all become if you try to make sense of the "X" power of a zoom lens.

Bottom line...

Check the 35 mm equivalent specifications for the lens. This way, you will be leveling the field and comparing apples to apples. More or less.

Answer 2

Just to add to the useless information, here's an explanation of why some point and shoots have large zoom whilst expensive SLRs 'appear' to have little zoom, this also covers what wide angle, normal and telephoto lenses are, as well as the difference between zoom and telephoto.

For reference the zoom of the lens is simply the big number divided by the little number. so for an 18-55 lens it is 55 / 18 which is simply 3X zoom. Why the apparently low numbers? Because with SLR cameras you have a choice. If the current lens doesn't do it for you, then you change it! By making lenses with a smaller zoom, it's actually possible to make them better quality, because there is not so much compromise to work at a wide range of focal lengths. Having said that, you can buy 18-200mm lenses, so that is quite a large 11.1X zoom.

Moving into SLR photography you need to forget about this magic zoom factor and concentrate on focal lengths.

A focal length of 50mm is regarded as "Normal" and all other references are drawn from here. This is something supposedly similar to what the human eye sees. Normal focal lengths are used for portraits, although up to 85mm is still considered a portrait lens.

Any number lower than 50mm is considered a wide angle lens, and any number over 85mm is considered telephoto. SLR lenses can have fixed focal lengths, or they may zoom. An 18-55mm lens is wide angle to normal zoom lens. A 70-200mm lens is a zoom telephoto lens. Notice how a lens can be a zoom lens but not necessarily a telephoto lens, this is a popular misconception.

Answer 3

take the larger number and divide by the smaller number to get the x's.... ie 28-300 (300/28=10.7x).

However remember that the human eye see's about 55-60mm (film) or 35-40mm (DSLR). so anything smaller than those numbers mak it wider and anything larger makes it more telephoto....