A compositional feature of focal length is the way it alters perception of depth. Short focal lengths make objects look farther apart whilst longer lengths compress distances and make objects seem much closer together. Therefore, a wide-angle lens makes backgrounds seem far away whilst a telephoto lens brings them much closer.

A lens suited to most subjects, from portrait to landscape. There is minimal distortion and can use large apertures down to f/1.8, etc. Good for portrait, nature, low light, and general use.

Very wide angle of view, up to 180 degrees. This causes considerable distortion but captures a huge amount of the scene.

Short focal length lenses have a wide angle of view, hence the name “wide angle”. Longer focal lengths have a narrower angle of view, covering a smaller area of the scene. That’s why long focal length lenses seem to magnify the image - they capture a small part of the scene but still fill the sensor with it. A focal length of round 50mm is generally accepted as being about what our eyes see, which is why a 50mm lens is so often the standard lens that comes with the camera.

Deprecated by LightingOrigin keyvalue and SetLightingOrigin input for point entities (these however are not in Valve's FGD by default), use those instead, as they can accept any entity as a lighting origin.

Depth of field, how much of the shot is in acceptable focus, changes with focal length. Shorter focal lengths not only capture a wider view, but they also have a deeper depth of field. They can capture almost everything from right in front of the camera to infinity in acceptable focus. As focal length increases the depth of field decreases, so a longer lens captures a narrower band in acceptable focus.

Focal length is a technical characteristic of a lens, related to the way it works, rather than its actual size. You can experiment with focal length in your photography to change perspective, play with depth of field and zoom in / out on your subject.

info_lighting_relative is a point entity available in all Source games before Left 4 Dead. An entity that allows a model to be lit from a different light origin without affecting the environment around the model. Great for lighting up trees with many lights or doing manual light correction. It is used in d1_trainstation_01, where inside the train car Gordon encounters G-Man.

Focal length influences how your camera records a scene. It determines the angle of view, or how much of the scene ends up projected on the camera sensor. It controls how magnified an image looks, it influences the depth of field and can alter perspective, making objects look closer together or farther apart.

Picks out distant objects and fills the frame with a small part of the scene. The shallow depth of field is popular with portrait photographers because it throws the background out of focus.  Also used for general nature and wildlife work.

Technically, focal length is the distance between the point of convergence and the camera sensor, but that’s not a particularly easy way to understand it. Instead, imagine a simple lens with one piece of glass focusing the light onto the sensor. The distance between the lens and the sensor is the focal distance. So, in a 100mm lens the distance between the glass and the sensor is 100mm. It’s as simple as that.

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Distortion is small – vertical lines may curve but can be corrected later. It’s good for journalistic and real estate work because the angle of view is huge and so much is in focus.