Focal length is a number that is vital to photogrammetry but often misunderstood.
What is it?
A strict technical definition of focal length is difficult without providing a lot of background in lens theory, so we will use a simplification. You can think of focal length as the distance between the imaging surface plane (piece of film in a 35mm camera or CCD chip in a digital camera) and a point where all light rays intersect inside the lens. So a focal length of 50mm means that the distance from the point where all the light rays focus to the imaging plane is 50mm long (about 2 inches). More strictly, focal length is this distance when the lens is focused at infinity.
Zoom lens / Fixed lens
A zoom lens is a lens that can change focal length. Most modern consumer digital cameras have zoom lenses. Some inexpensive cameras have a fixed focal length lens and most high-end professional cameras have interchangeable lenses (either zoom lenses or fixed focal length lenses).
What does it mean?
The focal length number tells us how much of the scene is captured in the picture. The lower the number, the more we can see. A wide angle setting (zoomed out), where you can see a lot of the scene, has a small number or shorter focal length. A narrow angle setting (zoomed in), where you can see only a small part of the scene, has a large number or longer focal length. On a 35mm film camera a normal focal length is 50mm, a wide angle is less than 50mm, and a narrow angle (often called telephoto) is larger than 50mm.
As an example, if the zoom lens of your 35mm film camera is set to 28mm you will see a lot in your picture and objects that are far away will look very small. This is a wide angle setting (because we can see a wide scene). If your zoom lens is set to 200mm you will not see much in your picture and objects that are far away will look big. This is a narrow or telephoto setting and makes objects appear large, as you would see through binoculars or a telescope.
Terms
There are a number of names and terms that you may have heard that relate to focal length; zoom setting, wide angle, telephoto, and principal distance (a technical photogrammetry term). The zoom setting is the current focal length of a zoom lens. A zoom lens labeled 28-200mm means you can manually change the focal length from 28mm (wide angle) to 200mm (telephoto). Principal Distance is a more precise photogrammetric term. While many photogrammetric packages use the Principal Distance term, we use Focal Length in PhotoModeler as it is a more recognizable term.
Focus and Macro
Focal Length is the Principal Distance when the lens is focused at infinity. When the lens is focused at a closer distance, the Principal Distance changes. When PhotoModeler lists focal length for a camera it is actually the principal distance that is shown.
All lenses have a focal length value (or range of values for a zoom lens). A 28mm wide angle lens has one focal length of 28mm long. Or does it? The number printed on the lens as its focal length is actually its nominal length or the principal distance when the lens is focused at infinity. As you focus on objects that are closer to the camera the principal distance (or as PhotoModeler shows as "focal length") increases. So for example, a 50mm lens focused on an object a few feet away might actually be a 55mm lens at that time. The most extreme example of this is with the macro setting (a lens setting that allows you to focus on very close, very small objects (under 5" in size for example). A 50mm lens that has a 50mm focal length (so a 50mm principal distance when focused at infinity) might in fact have a 100mm principal distance when focused at a few inches! This is why we calibrate a camera at the best setting for the distance and size of objects we are working with.
Format Size
When you buy a digital camera you will often see the specification "equivalent 35mm focal length". What does this mean? Most digital cameras have imaging chips that cover less area than a standard 35mm film camera. A 35mm film camera makes a negative that is about 36mm wide by 24mm high. Consumer digital cameras can have imaging chips that are as small as 6mm by 4mm. This affects what is considered to be normal, wide angle or telephoto.
Let's say you take a picture of an automobile. You have two cameras, a 35mm film camera and a digital camera. You stand in the same spot and take two photos, one with each camera. In both cases you want to take a photo of the automobile that fills the frame. If the 35mm film camera lens has a 50mm focal length, the digital camera's focal length might be 8.3mm. So even though they are very different numbers they produce the same result because of the size of the imaging surface. So the "equivalent 35mm focal length" for this camera at 8.3mm is 50mm. Camera manufacturers sometimes list these equivalents because generally we are more familiar with 35mm cameras and they want to make it easier for us to understand.
Photogrammetry
If all of this seems rather confusing, not to worry. There are only two important items we need to remember when using PhotoModeler:
For the most consistent and accurate project results, calibrate your camera and then always use it at the same focal length that you calibrated it at. Please click here for details on our pre-calibrated cameras.