Focal Lengths

The focal length is the distance, from the front of the lense, where incident parallel light rays are focused.

By adjusting the focus of the lense we change the focal point behind it.  If the focal point is different from the distance between the rear lense element and the imaging sensor, we get an out of focus image.

If the rear focal point is in front of the sensor, we get what is called front focus - the focus of the image appears "in front" of where we aimed.  Conversely, back focus is when the focal point is actually behind the imaging sensor.

Field Of View (FOV)
The focal length has a direct affect on the FOV.  Our vision has roughly the same FOV as a 50mm lense on a 35mm camera.  Focal lengths smaller than 50mm result in a greater FOV, allowing us to see a wider view than we can see with our unaided eyes.  Focal lengths longer than 50mm result in a narrower FOV.

Above, on the left, we have a shot taken at 200mm, f/22.  Here, the boat is partly visible, and only the feet of Mr. & Mrs. Noah are visible.  On the right, the same setup was photographed using 18mm, f/22.  In this shot we see more of the boat, and Mr. & Mrs. Noah are both visible in their entirety.

Also, note although both shots were done at the same aperture, in the 18mm shot everything is clearly defined, whereas in the 200mm shot the hippos are slightly defocused, and the lions even more so.  At longer telephoto lengths DOF becomes shallow.  This is because as the distance between the lense and the subject decreases, the effective distance between the subject and background effectively increases and becomes more of a factor.

This trend continues as we get closer to the subject.  In macro shots, where the subject is extremely close to the lense, the DOF is extremely shallow.

Focal Length Multiplier
The FOV differs between camera formats.  More precisely, the larger the imaging area, the smaller the FOV for a given focal length.  For example, many DSLRs have a "crop factor."  Nikon's crop factor is 1.5x - a lense of a given focal length will produce a FOV 1.5x less than if it were covering the entire 35mm frame.  For example, a 60mm lense on a D70 will produce the same FOV as a 90mm lense on a 35mm film body.

This shot was taken at 100mm f/2,8 using the entire area of the 35mm frame.  As we can see here, it gives us a fairly wide FOV and a shallow DOF.  Close examination reveals in addition to the foreground post, the next three posts appear sharply defined.  In this example, the DOF consists of four (4) posts.

This shot was taken at 100mm f/2,8 using a camera that employs an APS-sized sensor.  Shot with the same lense, same zoom setting and aperture, we have only changed the imaging sensor.  There is, however, quite a difference between the first shot and this one.  It appears we have zoomed in.  However, closer examination will reveal the DOF has not changed - it is still four (4) posts deep.  The APS-sized sensor is said to have a 1.5x focal length multiplier; it produces a FOV equivalent to a 150mm lense mounted on a 35mm camera.

This shot was taken at 150mm f/2,8 using the entire area of the 35mm frame.  Here, the FOV is essentially the same as the previous shot, but we notice the posts furthest back appear defocused to a greater degree than the previous shot.  Closer examination reveals our DOF has decreased to three (3) posts.

When we refer to a 1.5x crop, we say a 100mm lense is equivalent to a 150mm lense.  However, we are actually referring only to the field-of-view.  The actual image characteristics of a 100mm lense on an APS sized sensor is identical to that of a 100mm lense on a 35mm sensor - the edges have simply been cropped off.

Keep in mind the DOF is a characteristic of the lense and not the camera as defined by the circle of confusion.  The DOF produced by a lense at a given aperture will be the same on a smaller sensor as it is on a larger sensor.  Remember, DOF will deepen as focal lengths decrease.

Many digicams employ minute sized sensors and equally small lenses. These lenses often have focal lengths such as 8mm and consequently have deep DOF.  Manufacturers often convert these smaller lense values into "35mm equivalent" values.  If a camera/lense combination produces the same FOV as a 50mm lense, this 8mm lense is said to be a "50mm equivalent."  Yet, it will still produce the same DOF as an 8mm lense.

Hyperfocal Distance
Cameras are set up so 1/3 of the focal plane is in front of the actual subject and the remaining 2/3 of the focal plane is behind the subject.  By setting the infinity objects at the end of the 2/3 focused area, everything from the subject back will be sharply defined.

To know where the hyperfocal distance is for a particular lense, set the focus at infinity and look for the leading edge of the focal plane.  Place your subject at this point, focus on it, and everything from that point back will be sharply defined.

Another technique is to focus about one-third of the way into the scene and use a small aperture for a deep DOF.