Focal length

A simple and clear explanation of focal length

Understanding focal length, brightness, and other key terms and concepts that define camera lenses

Camera lenses are an essential part of taking photos. There are a good number of important attributes that describe them: focal length, brightness, quality… and it’s best if you are on friendly terms with all of them!

This knowledge will come in handy when shooting photos, buying your next lens, or talking to fellow photographers. The lesson below summarizes the most crucial aspects you should be familiar with.

1. Focal length

The first and most important characteristic of a camera lens is its focal length. This is a strange measurement, because in fact it describes the lens’ angle of view. The two values are inversely proportional. In other words, the longer the focal length, the narrower the angle of view. See the infographic below:

Gyújtótávolság

“…uh, sure, but what IS focal length?” you might ask.

The official explanation goes like this:

A camera lens always consists of multiple lenses (elements). Some optics are simple, while others are more complex lenses and lens systems.

 However, from an optical point of view, it’s possible to…

substitute even a complex system with a single glass element/lens (see illustration below). 

Then, if we measure the distance (in millimeters) between the lens and the point where the parallel rays of light coming from the left are assumed to cross, we get the focal length.

Please don’t ask why we don’t just say/use angle of view when measuring this value… Focal length is the widely accepted term, it’s what we all say, it’s all the rage now, like six shooters and broncos used to be in the Old West.

One would hardly claim they bought “a lens with a 46-degree angle of view”, yet “I’ve got a new 50mm standard lens” is a perfectly common thing to hear (because buying a standard lens is just everyday procedure! 😉).

Degrees are put to better use on the label of a good French brandy, not on our lenses 🙂 >> Even so, it’s good to have a general understanding of how this measurement works. (I mean when it comes to brandy… if you drink “too many degrees”, you’ll be sure to get an extraordinary angle of view for the night! 😉)

Focal length

Check out the interactive block below to follow how focal length changes on various lenses.

Just move the slider up and down! 🙂

 

Uh-oh! Interactive sliders may not work perfectly on all touch screen devices. If you are having problems, please try again with a mouse! 🙂

2. Different types of camera lenses

You probably know that there are different types of camera lenses. These are usually categorized by focal length, but sometimes they are also classified or ranked according to other factors.

Here is a list of various lens types >>

There are:

  • Prime lenses
  • Zoom lenses
  • Macro lenses
  • Wide angle lenses
  • Fish eye lenses
  • Telephoto lenses
  • Standard/normal lenses
  • Mirror lenses

There is a wide array of lens types (enlarge the image by clicking on it)

Let’s check out a few typical focal lengths and when to use them. Mind you, the only rule in aesthetics is that there is no rule! You are okay to try anything and everything. The guidelines below are not exact science!

Wide angle lens

Ideal for capturing outdoor landscapes, buildings or interiors. It is characterized by a wider angle of view, great depth of field, and will convey a nice sense of space.

Typical focal length: 16-24-35mm

Check out the photos below! The forefront, the middle and the background are all in sharp focus, and you can feel the space between them.

Standard lens

The standard lens has a focal length of 50mm. This is where portrait lenses start. They range up to the 100mm short telephoto lens.
Comparing different focal lenghts for portrait photography:
On the left photo the model’s face appears distorted, and the frame contains several unnecessary elements around her (the images can be enlarged).

Telephoto lens

Lenses over 50mm are called telephoto lenses. They are great when you can’t get too close to your subject; they are often used for wildlife photography. A telephoto lens “compresses” the sense of depth into a flat plane. It has a narrow angle of view and a shallow depth of field.

Photo by Bence Máté

3. Lens brightness

Brightness is the second most important trait of a camera lens. The brighter the lens, the more light it can collect (…and the more money it costs). Brightness is measured by the lens’ maximum aperture value. The lower this number, the brighter (faster) the lens is. E.g., a lens with f/1.4 maximum aperture is better/brighter than one that only opens to f/4.0.

Brightness is in fact the ratio of focal length to lens diameter. E.g., if a lens with a 50mm focal length has an f/1 aperture (brightness), its effective diameter needs to be 50mm. Brightness = focal length divided by lens diameter. Another example: a 200mm telephoto lens has an f/2 lens brightness if its diameter is 100mm >> 200/100=2. See the image below:

Key advantages of a bright lens:

  • It’s possible to shoot with lower ISO and/or faster shutter speed even in low-light conditions. The resulting photo will contain less noise and/or won’t blur.

  • Aperture impacts the depth of field (click here to view our lesson on depth of field), and a brighter lens has a wider aperture >> which makes for a shallower depth of field, allowing the photographer to emphasize the subject.
  • When using flash or continuous lighting, a smaller burst/less powerful light will do the job.

Raw materials for manufacturing high-quality optical glass are rather expensive. A bright lens requires a large diameter >> which needs lots of glass. That is why top-notch lenses allowing for high brightness come with a hefty price tag. If you are interested in finding out more about why lenses cost so much, click here to read our article on this topic.

You have reached the end of this lesson. Enjoy exploring our other content too! You can always pick the next lesson from the  top menu!

About the author:

Bence Gyulai is a photographer and professional photography teacher. Grand Prize-winning photographer of the Hungarian Academy of Sciences, regular invited speaker at photography events, founder and host of visualgangster.com.  More info & images >>