What is the equation for the conversion of mm in a lense to actual zoom distance?

Question

Ok so for example, what does it actually mean when the lense on a camera is an 18-55mm lense. What do those numbers mean, and is there a way to figure out the converstion to the same scale as compact digital cameras that give the optical zoom in the scale of like 5 times zoom?

Answer 1

There is not a lens-to-distance equation. If we are talking about a "35 mm equivalent," a 50 mm focal length is approximately "normal" view without any magnification. For most DSLR's, the lens factor of 1.5-1.6 means a "normal" focal length would be about 31-33 mm. If we go back to a 50 mm "normal" lens then, a 100 mm lens would be a "2 power" lens, 200 mm would be 4X, and so on.

Just divide the smaller number into the bigger number and you get the "X" power of the lens. For instance, a 18-55 mm lens is 3X zoom. Divide 55 by 18 and you'll get the result. 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.