The f/stop has a dual function complicating its understanding by the beginner. The f/stop is half the exposure changing mechanism along with the shutter speed.Another function of the f/stop is to regulate depth of field. The smaller the iris, the more in-focus depth is seen.

f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16

Focal LengthThe measurement from where the image is the smallest size to the image plane when the lens is focused at infinity.

Focal Length

Focal Length is often used to describe the angle of view of a lens. They are not necessarily the same thing.

90 Degrees Angle of View

Angle of View more accurately describes the view that a lens offers.

21mm Wide Angle Lens

A couple of examples where Focal Length is not as marked.

• Telephoto Lens DesignIn a telephoto lens design the actual lens length is shorter than the focal length. However, the lens is marked as though it were as long as its focal length.

• Single Lens Reflex “Wide Angle” Lenses

Wide angles for SLR cameras are not true wide angles, they are actually reversed telephoto lens designs.

Canon EF 800mm f/5.6 IS USM

Nikkor 28mm f/2.8 AI-S

(31.5 inches long!)

If a “wide angle” lens is almost completely mounted exterior to the camera body, and the camera body is 46.5mm from lens flange to image plane, how can a focal length of 28mm work?

46.5mm

Somewhat longer than 28mm

Answer: Design a telephoto lens and turn it around backwards so that the lens has a longer than 28mm focal distance to the image plane. It works, but not as well as if it were a true wide angle design. This is one of the main advantages of a rangefinder camera over an SLR.

This is the Contax G1. It has no mirror so a wide angle lens can sit as close to the image plane as it needs to for highest performance. That boost in optical performance is around 20-25% depending upon the lens comparison.A 16mm true wide angle lens can almost fit inside a rangefinder camera body.

Inside the camera body.

Pictured here is a 16mm f/8 Carl Zeiss Hologon. It has zero distortion due to its true wide angle design.

Resolution

In the bad old film, days resolution had a completely different meaning than it does in the digital world.With film, resolution was described as a measurement of potential quality. What limited resolution was a host of unrelated issues. The build quality of the lens, the steadiness of the photographer, the fineness of the grain of the film, etc. All these disparate things impinged on resolution.

USAF Resolution Chart 1951

USAF Resolution Chart 1951

The measurement unit is Line Pairs per Millimeter or LPPM.The question is how many pairs of black and white lines could be distinguished within 1 mm on the film before the image simply turned to a blur?The highest performing color film ever released could capture 200 line pairs per millimeter. The best camera lens could capture a bit over 225 line pairs per millimeter (35mm lenses).

Resolution Continued

Today in digital photography, resolution is no longer an attainment goal. Resolution is simply the number of light gathering pixels found on a sensor.

Bayer Mosaic Sensor

Resolution Continued

The resolution chart on the left preserves perfect imagery while the chart on the right is a more “real-world” presentation of lens performance at the point where we can no longer distinguish between lines and spaces.

USAF Resolution Chart 1951

Resolution Continued

ContrastContrast is the most important aspect of any lens. Without high contrast, resolution is virtually invisible. Contrast can be described in two ways, one, how the image distributes tonal gradation across the scene as seen in these two images.When discussing lenses we are not concerned with that description, but we are concerned with micro-contrast, the ability of the lens to differentiate between smaller and smaller details of more and more nearly similar tonal value.