Lens Basics Part 3 (The MMs)

This week we are going to talk about the focal length of lenses. At it’s simplest, the focal length is the effective length of a lens as is measured from the front element of the lens to the film plane (the digital sensor). The focal length is measured in millimeters (mm). The longer the lens (greater focal length), there greater the number of mm’s. Make sense?

OK, so what does that mean? The longer the lens (more mms), the greater the magnification. As we briefly talked about in Part 1 of this series, more magnification comes at a price. The greater the focal length, the less light makes it to the sensor.

Lenses can be divided into two basic groups. The first are lenses that have a fixed focal length (called prime lenses). For instance a 300mm prime lens will always be a 300mm lens. If you want to get more magnification, you have to move closer to your subject. The second type of lens is a variable focal length lens or zoom lens. For instance, I frequently use a 24-70mm lens in the studio. This is a lens that adjusts between 24mm and 70mm. So if I have it set at 50mm and want to tighten up a shot, I don’t have to walk any closer. I can just zoom in to 70mm.

So all this begs the questions, why doesn’t everyone just use zoom lenses to save the effort of walking back and forth? There’s a couple of reasons why prime lenses are popular. First of all, a good prime is almost always sharper than a zoom lens. The reason is that there are fewer elements inside the lens barrel. When you introduce more elements for the light to pass through it can hurt image quality. The other advantage is that prime lenses transmit more light to the sensor make them faster than zoom lenses.

Some zoom lenses have a constant maximum aperture, while others have a maximum aperture that is determined by it’s focal length. For instance my 24-70mm lens has a constant max aperture of 2.8 throughout it’s focal length. However my 10-22mm lens has a max aperture that changes from f/3.5 to f/4.5 as I zoom from 10mm to 22mm.

So is the focal length all about magnification? Well sorta, but not always. It’s also about perspective. For example, consider the two photos below. The one on the left was shot with a 200mm lens from about 20 feet. The one on the right was shot with a 10mm lens from less than 2 feet away. The size of the figurine in each is the same, but because of the different proximity the perspective changes. At 10mm much more of the background is visible.

This next photo shows how proximity can cause a distorted view of reality. In the photo below both markers are in the same position however they appear very different due to the camera’s distance to the subject. The photo on the left was shot with at 200mm, whereas the photo on the right was shot at 10mm. Notice that at 10mm the red marker appears to be twice the size of the blue one, even though they are the same size. At 200mm the markers appear to be next to each other even though there is a about a foot between them.

This distortion is particularly important when photographing people. When you are too close, body parts can extremely disproportionate, which can be very unflattering. For portrait work a focal length of about 100mm is ideal because it provides enough distance between the camera and the subject so that everything is proportional.

To see more of my portrait work click here.

Well that’s it for this time. Thanks for reading and if you have any questions let me know.

-Ken