Lens Length and Other Factors

Photography Tips

Perhaps one of the more confusing subjects in the world of digital cameras is lens length. We hear terms like “telephoto,” “wide angle,” “field of view,” “5X,” and a host of others to describe our lenses. Then once we get all of that business sorted out we crank in other arcane phrases like “crop factor” and “35mm equivalent.” Is it any wonder that it’s tough to make a decision on a lens?

Photo by Korina Segaya; ISO 1250, f/1.8, 1/60-second exposure.

While it is easy to grasp that a longer lens length offers more magnification and a short lens length provides a wider view, it helps to understand what a standard lens length is before we begin making comparisons.

There is a way to use math to determine what constitutes a standard lens length for any camera, and I’ll add that bit of mathematic fun at the end of this discussion. That kind of mental gymnastics is interesting stuff to us photo geeks but the other 98 percent of the world, I believe, would rather have a root canal.

That said, we are going to make an assumption: On average, digital cameras using an APS-sized image sensor will have a standard (sometimes called normal) lens length of 28mm.

OK, big deal, what does it mean? What this little bit of information tells us is that if we mount a lens on a DSLR and set it to a length of 28mm, the view through the camera shows the world in the same perspective as we might see it through one open eye. In other words, this is a “normal” or “standard” lens length. We have our benchmark length that can be used to compare lenses.

Let’s do some mental housekeeping first. I would like to encourage you to disregard a few terms. We can forget “crop factor” and “35mm equivalent” for now. Since the majority of new DSLR camera buyers never owned a film camera with interchangeable lenses, these terms simply add needless layers of numbers and confusion.

Another term to set aside for the moment is the “X Factor.” Compact camera lenses are described with phrases like “5X zoom lens,” which frankly is kind of meaningless. Expressing lens length this way is handy but ineffective and here’s why. A lens that is 20-100mm is a 5X lens, but then so is a 50-250mm lens. The “X Factor” is ad-speak and only superficially useful in the world of compact cameras.

Photo by Albert Huynh; ISO 100, f/1.4, 1/50-second exposure.

So we are left with wide angle, telephoto, and field of view. These are three good terms we can sink our teeth into, and here’s how: As we have just discussed, a DSLR with a 28mm lens has a field of view similar to that of one open eye. Any lens length shorter than 28mm will have a wider field of view (wide angle), a longer lens a narrower field of view (telephoto).

There is some easy, do-it-in-your-head math that we can now use to compare lenses. If 28mm is a normal view, then a lens that is 14mm will have a view twice as wide. A lens of 50mm will provide a view almost two times closer. Do we really care what the 35mm film camera equivalent or the crop factor is? Nope.

So in the real world we can now make some well informed judgments when we compare camera lenses. If we are offered a lens that is 18-55mm, we now know that we have a slightly wide angle lens and a telephoto length that will make the world look twice closer. Another kit offers a lens of 28-80mm and we understand that we are getting a lens with zero wide angle capability and a telephoto view about three times closer than our eye sees it.

We’ve added just a little bit of new knowledge and our confidence in making a decision between lenses has increased.

In conclusion I’d like to offer a bit of advice. With every manufacturer in the world trumpeting their high magnification telephoto lenses, it’s easy to overlook the wide angle side of the equation. If you ever shoot pictures in your own living room you’ll be better served if your camera lens offers wide angle coverage below 28mm.

The difference between 28mm and 17mm—we know from this discussion—offers a wider view with the shorter lens, but how much wider? At 28mm we can capture three people scrunched together on the couch. At 17mm we are now getting the whole couch and the end tables, too.

Photo Geeks’ Corner: The Math behind Normal Lenses

As we did earlier, there are a few assumptions I’d like to offer up as a way of cutting to the chase:

1. We are discussing digital single lens reflex cameras collectively using APS-sized image sensors.
2. There is a lot of difference between the major manufacturers of image sensors in the actual size of their sensors. However, the real impact of these size differences is minimal.
3. The terms “Normal” and “Standard” refer to the perspective that a lens renders and not necessarily its total angle of view.

The short and easy rule proven over time is that a camera format’s normal lens is equal in length to the diagonal measure of the camera’s film frame or sensor. This lens length will capture the world showing subjects in perspective to one another as the scene would be viewed through our own eyes.

Photo by Piotr Szczepankiewicz; ISO 100, f/2.0, 1/10-second exposure.

An average APS-C sensor measures 22.7 x 15.1mm, which gives us a diagonal of 27.26mm. Nikon uses a sensor that is slightly larger and yields a diagonal measure of 28.4mm. There have been other variations of size in APS digital sensors but they all hover around a 28mm diagonal measurement. This is why I have chosen to state that a 28mm lens length is generally considered “normal” on all APS-sized digital cameras.

The math looks like this: 22.7mm2 + 15.1mm2 =743.3mm. Then we find the square root of 743.3mm, which is 27.26mm.

What other advice would you give to a photographer looking to buy a new lens?

About the Author:
Stu Eddins is blogger, instructor, merchandiser, and is generally in charge of a lot of things for Porter’s Digital Cameras and Imaging (digital camera lenses). Years of experience over the counter and in classrooms have turned Stu into an evangelist for image preservation, capturing and sharing memories, and helping people understand digital cameras.