Focal Length and Field of View

Focal length is a primary characteristic of any lens.  Scientifically speaking it is the distance between the optical center of your lens and your sensor, measured in millimeters.  Using lenses with different focal lengths can have dramatic effects on your final shot.  It alters the angle of view, space compression, and depth of field.

 

 

In fact, lenses are generally classified into different categories based on their focal length.  The three main categories are wide angle, normal, and telephoto.

 

 

Depending on who you ask, wide angle lenses fall between 24-35mm with those falling below 24mm being considered superwide. Lenses with focal lengths between 35 and 70mm fall into the normal category, with 50mm being the most common. Telephoto lenses fall between 70 and 200mm, with longer lenses being considered extreme telephoto.

 

 

Lenses with different focal lengths will allow you to capture a wider or narrower snapshot of a scene. The circular image that your lens projects is know as the angle of coverage. However, your sensor doesn’t utilize this entire circle because it is a rectangle.  The angle of view is the portion of the lens projection, that your sensor actually captures. This can also be referred to as the field of view.

 

 

Focal length has an inverse relationship to your angle of view and the magnification of the image.  As focal length gets longer, the angle of view gets more narrow and individual elements in the scene become more magnified.  Conversely, as focal length gets shorter, the angle of view gets wider and the magnification becomes lower.  

 

 

Put in simple terms, shorter focal lengths have wider angles of view and less magnification, and longer lenses have a narrower field of view with more magnification.

 

 

Lets start with the characteristics of wide angle lenses. They have a broad field of view, things appear smaller. Objects nearest to the camera will appear to be closer and the far away places will seem to be further than they are. However, this can work in your favor, if needing to shoot in a small space, with a wide angle lens you have the capacity to get more in frame in that small space.  

 

 

Another characteristic of wide angle lenses is  the distortion created by the curve of the lens. It is especially easy to see when panning or tilting. Horizon lines flex when tilting and when panning, vertical lines bend. 

 

 

Some wide angle lenses, more typically ones that are more expensive, are built with rectilinear correction. Rectilinear correction is designed to counteract distortion. 

 

 

Normal lenses, or lenses with focal lengths between 35mm and 70mm fall into an area of view that mimics the human eye’s field of view. Because of this, zooms that cover the range of 35-70 are great walking around lenses. They will capture the world as you see it. 

 

 

With telephoto lenses, the longer the throw or zoom, the narrower the field of view and have a greater magnification. In essence they capture a very narrow crop of the world. They require a much shorter shutter speed to avoid motion blur from the shake of the camera. Its best to use handheld shutter speed rules. Those rules state that the shutter speed when shooting handheld should never drop below 1 over your focal length. Here is an example: if you are shooting a 70mm lens on a full frame camera, then your shutter speed should not be less than 1/70th of a second.  

 

 

You might have heard of the term “crop factor” before. The crop factor is referring to the size of your camera's image sensor in its relation to a full frame image sensor. All lens focal lengths are based on the camera having a full frame sensor. A full frame sensor is designed to be the same size as 35mm film. 

 

 

For a better understanding, here is an example:  if you have a full frame camera, and you put a 35mm lens on it, it will capture the expected amount of the angle of coverage that your lens projects, giving you a standard 35mm angle of view. However, if you put the same lens on a camera with a smaller sensor, you will capture a smaller portion of the angle of coverage, and this effectively creates a more narrow angle of view.  This is referred to as a focal length multiplier or crop factor.  The smaller sensor effectively crops the image that your lens is capable of projecting.

 

 

While there is no change in actual focal length or angle of coverage of the lens, the effective focal length and angle of view changes.  The smaller the sensor, the more the projected image is cropped, the more narrow the effective focal length and field of view becomes.

 

 

If you have a sensor that isn’t full frame, you need to take this into account when you’re choosing or using a lens.

 

 

A common sensor size is aps-c, which has a 1.6X crop factor.

 

 

A 50mm lens designed for a full frame camera will behave like a 80mm lens when used on an aps-c sensor.  This works to your advantage if you’re looking to get a more telephoto image out of your lens.  However it works against you on the wide angle end.  A 24mm lens will behave like a 38mm lens.

 

 

 

Now some lenses such as a canon ef-s lens are designed to be used with smaller sensors.  But even these lenses usually have the actual focal length in their name, and you’ll need to look at the tech specs to figure out the 35mm equivalent. Many times these lenses will list the 35mm equivalent.

 

 

Knowing what field of view you require to get the shot you need is key. Its of the utmost importance what focal length of lens you choose. That choice will affect everything from what is in your frame, to how those things feel in relationship to each other. Knowing if you have a cropped sensor, what it’s crop factor is and what that means to the effective focal length of your lens will allow you to make the right choice when choosing a lens for any job. Thanks for watching.