The Basics Part 5: Focal Length

02/20/20 • 29 min

The Basics: Focal Length

The focal length is one of the fundamental aspects of photography. It is how we describe lenses, and it's how we change perspective without moving our feet. Changing your focal length will have a dramatic change in your final image.

What is the focal length?

The focal length is a calculation of an optimal distance from the point where the light rays converge to form a sharp image to the digital sensor or film at the focal plane of the camera.

The focal length of a lens is determined with the lens focused at infinity. The millimeters that we use to describe a lens describes the focal length, not the physical dimensions of the lens itself.

What does focal length tell us?

First of all, it tells us the field of view and angle of view or put simply, how much of the scene is captured on the sensor. We generally refer to this is a wide-angle (zoomed out), or telephoto (zoomed in).

Secondly, it tells us the magnification or how large the individual elements will be captured on the sensor. So, the longer the focal length, the narrower the field of view, and the higher the magnification. The inverse is true for shorter focal lengths. Short focal lengths will have a wider field of view and lower magnification.

Check out the cheat sheet to see examples.

Types of Lenses

There are two types of lenses, zooms, and primes. Zoom lenses have a variable focal length. You'll see this noted on a lens like this: 18mm-55mm. This means that the focal length of this lens can be adjusted "zoomed" between 18mm and 55mm, as well as all focal lengths in between. Prime lenses, however, have a fixed focal length. This means they cannot be adjusted. They will only have one focal length listed, such as 50mm.

Zoom Benefits

Zoom lenses are more versatile. They offer a range of focal lengths, so you don't have to move. This means you can get a variety of shots without changing lenses or moving around. When shooting horses, this means you'll get less dust on your sensor because you do not have to change lenses so often. If you shoot subjects that are moving, but you cannot move, then zoom lenses will let you get more images that are framed how you want. Zoom lenses are what most camera companies use as kit lenses.

Prime Benefits

Prime lenses are typically lighter weight and more compact because they only cover one focal length. They also tend to have a larger maximum aperture (smaller number). Example, 85mm f/1.2 or the 200mm f/2 compared to the 70mm-200mm f/2.8. Although this seems like a small difference on paper, it is a massive change in the final image. Because of this larger aperture, you can shoot pictures with a faster shutter speed, shoot in even lower light, have a blurrier background (shallow depth of field), and prime lenses typically have the better quality glass. Since there are fewer moving parts, the manufacturers make fewer compromises in image quality.
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The Basics: Focal Length

What is the focal length?

The focal length is a calculation of an optimal distance from the point where the light rays converge to form a sharp image to the digital sensor or film at the focal plane of the camera.

The focal length of a lens is determined with the lens focused at infinity. The millimeters that we use to describe a lens describes the focal length, not the physical dimensions of the lens itself.

What does focal length tell us?

Check out the cheat sheet to see examples.

Types of Lenses

Zoom Benefits

Prime Benefits

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Previous Episode

The Basics Part 4: ISO

ISO adjusts the brightness of your photo.

Define ISO:

ISO adjusts the brightness of your photo. It increases the sensitivity of your sensor. In a non-geek talk, this means that as you raise the ISO, you increase how sensitive the sensor is to light. This can brighten your image by raising the ISO and darken it by lowering the ISO if all other settings are the same.

Raising the ISO has consequences, though. When you increase the ISO, you add grain/noise to your images. The photos produced at higher ISO’s are less sharp and less detailed. Generally, we want to use the lowest ISO we can.

I typically adjust my ISO after setting the aperture and shutter speed because I use it to get the exposure right. If the ISO is too high, then I’ll see what I can do about adjusting the other settings (aperture and shutter speed). Later we will talk about adding light, but for now, let’s assume we can’t do that. If you can’t open the aperture more and if you need a fast shutter speed, then you’ll have to deal with having grainy/noisy pictures.

There are, of course, options for using a lower ISO with less noise while keeping the brightness (exposure) of your photo where you want it. For one, you can upgrade to a better camera with cleaner high ISO photos. You can find better light or shoot from an angle that sheds more light on your subject. You can add light via strobe or constant light source.

Let’s try at home!

Set your camera to manual mode, grab your owner’s manual.

Start on ISO 100 (or 200 for some cameras). If you’re inside, then try 1/500th of a second shutter speed and f/4 for your aperture. That should make for a pretty dark picture. Later, try increasing your ISO and taking more photos. Raise it until your image is bright enough. Then, raise it until it is too bright.

Try other settings too! Take your camera outside. Pick out some random settings and then adjust your ISO, so your images are too dark, too bright, and just right. ISO adjusts the brightness of your photo.

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