How do I work out the magnification of a zoom lens for a DSLR?

Question

I recently got a Canon EOS 400D digital SLR camera and I am looking to upgrade the lens for one with a more powerful zoom. However separate lenses are new to me so I am struggling to figure out which lenses are more powerful than others.

I was hoping someone would be able to tell me how you can either work out the strength of the optical zoom on Canon lenses, or give me suggestions of lenses to look at.

Any help is greatly appreciated.
Many thanks,
Martin.

Answer 1

You already have two excellent answers, but I have this one in the can at all times, so here you go. My answer adds a little visual aid.

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 35-80 mm lens is 2.3X zoom. Divide 80 by 35 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.

Here is a mini-tutorial I made myself to compare focal lengths. This is NOT a lens test or a camera test! It is merely intended to show the difference between various focal lengths. The lens was the Nikon 18-200 VR lens, which is (by definition) an 11X lens, but that 11X does not tell you what the final image will look like. I added one more frame taken with a 300 mm lens. The camera was a Nikon D200 so there is a 1.5X "crop factor," "lens factor," or "focal length multiplier." There is further explanation on the image itself. It would help if you click on "All Sizes" above the image.

http://www.flickr.com/photos/7189769@N04/476181737/

This was done with two different lenses: the Nikon 17-55 and 70-300:

http://www.flickr.com/photos/samfeinstein/1245831147/

Answer 2

It's relatively simple.

Let's take the “kit” lens included in the... uh, kit package with the EOS 400D. It's a “18 - 55 mm.”, right? Well, then, just divide the second number (the “tele” focal distance), 55, by the first one (the “wide” focal distance), 18, and you'll get 55/18 = 3,05. Well, then, the 18 - 55 kit lens' zoom magnification is 3.05x

Another example: the Sigma 18 - 200 mm. Divide the 200 mm. tele by the 18 mm. wide, and you'll get 200/18 = 11,11. And, indeed, the Sigma 18 - 200 mm.'s zoom magnification is 11x

Of course, there are a few additional complications. As you've probably deduced already, not all lens have the same visual magnification related to the scene, even if they have the same zoom magnification. A 18-200 mm. and a 27-300 mm. will both have the same 11x magnification, but the first one will have a wider minimum focal distance (18 mm., instead of 27 mm.), which means that it will capture a wider scene, taking the photo at the same distance. On the other hand, the second lens will be able to magnify more distant objects in your frame/photo, because it has a more powerful tele focal distance, 300 mm. instead of 200 mm.

To add a bit more complication, the Canon EOS 400D's factor crop, which is 1.6x, means that, for example, the Sigma 18-200 mm. lens will give you (with that particular camera) a 35 mm. equivalent range of 28.8 - 320 mm. In other words, it won't give you the same wide and tele framing than it would if used in a full-frame camera, such as the Canon EOS 5D.

But that's minutiae. What you need to know, is that simply by dividing those numbers, you'll get the zoom magnification level, and that zoom magnification notwithstanding, some lenses will have either more wide angle, or more tele -or both.

Also, keep in mind that the lens with the most zoom magnification will not necessarily be the best one, at least not in terms of optical quality. Some compromises need to be made when constructing a high-magnification lens, unless you ignore the size, weight and (particularly) price factors.

Answer 3

Hello Martin.

Suppose you are using a 50mm lens. You focus on a subject 30' away. Now you switch to a 300mm lens and focus on the same subject 30' away. The image will appear as though it is 6 times closer - or 6 times bigger - as though you had used the 50mm lens at 5'. A zoom of 50-300mm would have a 6x zoom ratio. A zoom of 18-200mm would have an 11.11x zoom ratio.

Simple, isn't it? Just divide the large number by the small number.

A note of caution: Pay particular attention to the f-stops on zoom lenses. If its a 50-300mm and says f3.5/f6.5 that means it is a slow lens at 300mm (f6.5) - and its not all that fast at 50mm (f3.5). Unfortunately, lens speed costs $$$ - usually lots of it.