Lens related concepts and terminology

In order to properly use lenses of different focal lengths to achieve a desirable visual impact, one must clearly comprehend the relationship between f-stops, depth of field, focal lengths and perspective. At the same time, one should also understand what aberrations a lens might exhibit so as to attain the best image quality possible or make image quality compromises intelligently. Let's have a look at some basic definitions and ascertain relationships between them; further, let's examine what aberrations lenses might normally show.


  • Angle of view simply describes how much of a scene a lens covers from a given position.

  • Perspective is an impression of distance between and relative size of objects in a scene.

  • Depth of field is the distance between the closest and the furthest points in a picture between which all subjects appear sharp.

  • Focal length is the distance between a lens' optical centre and the plane of an image-registering element (either film or digital censor) when the lens is focused at infinity. From any given location, the shorter the focal length, the wider the angle of view. 50mm is considered the "normal" focal length in 35mm format; any lens with a shorter focal length is usually referred to as a "wide-angle" lens, whereas a lens with a longer focal length as a "telephoto" lens.

  • Aperture is the variable opening in a lens which controls the amount of light which reaches an image-registering element. It is usually expressed in series of f-stops (e.g. f/2, f/2.8, f/4, f/5.6, f/8, etc.) - it is measured as a function of a lens' focal length. For example, f/4 indicates that the diameter of the opening is one forth of the focal length.

    Why do we express aperture in such a strange way? Because what really interests us is not the size of the opening itself, but rather the amount of light that a lens transfers at a given size of the opening. Expressed this way, any lens transmits the same amount of light at the same f-stop regardless of its focal length. For example, at f/2 the diameter of the opening in a 100mm lens is 50mm, whereas in a 50mm lens - 25mm. In practice we never think of it this way, though - what we think about is that both lenses will transmit the same amount of light to the film or digital censor surface.

    And why such a peculiar sequence of numbers - 2, 2.8, 4, 5.6, etc.? These numbers are derived so that any f-stop transmits one-half or twice the amount of light of the adjoining f-stop. This in turn has a direct relationship with shutter speed sequences (e.g. 1/30, 1/60, 1/125, 1/500, 1/1000sec.) - in these sequences, the amount of light that an image-registering element receives also doubles or halves as you move to the adjacent shutter speed. This way, the same (correct) exposure can be achieved by choosing different combinations of shutter speeds and f-stops. For example, a shutter speed of 1/30sec. combined with an aperture of f/2.8 will result in exactly the same exposure as when choosing a shutter speed of 1/60sec. and an aperture of f/2 (this is the reciprocity principle). And, of course, we need this shutter speed/f-stop choice as aperture directly influences depth of field and shutter speed controls how moving objects are rendered.

    A lens' maximum aperture is often referred to as its speed. A lens with a maximum aperture of f/1.4 is very fast, whereas a maximum aperture of f/5.6 is way too slow.

The relationships

  • For any given lens at a given shooting position, the smaller the aperture, the shallower the depth of field. Depth of field also depends on the lens-to-subject distance - depth of filed is greater when your subject is further away from you.

  • At a given f-stop and from a given shooting position, every lens captures the same perspective regardless of its focal length but has a different angle of view (subject coverage); image size is proportional to focal length.

    If you do not change your shooting location and switch from, say, a 100mm lens to a 50mm lens (or simply zoom out if you use a zoom lens), all you do is double the coverage - apparent perspective remains the same. If you have discovered a very nice looking perspective, shoot from the location you discovered it from using a lens with a focal length that provides adequate subject coverage. Walk away from that spot and that beautiful perspective will disappear!

  • At a given f-stop and given the same coverage of a (foreground) subject, lenses of all focal lengths have the same depth of field but they show different perspective - lenses of shorter focal length capture a wider area of background.

    In other words, if you shoot a scene with a sharp foreground subject and an out-of-focus background, the longer the focal length, the greater the magnification of the background relatively to the foreground subject (that is different perspective). However, background will be blurred to the same degree regardless of focal length. It is because of the different degree of background magnification that telephoto lenses appear to have greater depth of field.


A good lens, ideally, should produce sharp images, show neither light fall-off in the corners nor distortion nor chromatic aberration. It should also handle flair well and produce beautiful bokeh. However, designing a lens involves making quite a few compromises. Due to this, not all lenses are very sharp. Almost all of them show different degree of distortion and light fall-off when wide open. Many of them are prone to flare and exhibit color fringing. And only a few lenses render out-of focus areas beautifully.

  • Intuitively, sharpness is probably the easiest concept to understand, and simply means how well-focused and crisp the final image is. However, speaking more strictly, it consists of two independent concepts, namely resolution and acuteness, with resolution referring to how many lines per mm a lens is capable to reproduce, and acuteness referring to how different in brightness two areas are towards their common edge. These two terms can be combined back together into one term named "contrast". In real life, however, apart from qualities of a lens there are quite a few other factors at play, such as film resolution, film flatness, film grain, stillness of the object and camera, etc., and all of them might influence what we perceive as overall sharpness of the image.

  • Light fall-off is an effect when a lens loses elimination towards the sides of the image. This effect is usually worst and most visible at full aperture and gradually disappears as the lens is stopped down.

  • Distortion is an effect when lines that were perfectly straight in real life bend in or out when they run along the edges of the image. It is called "barrel" distortion if lines bend in towards the center of the image, and "pincushion" distortion if they curve in the opposite direction. Most wide-angel lenses (as well as zoom lenses at their shorter end) tend to have barrel distortion, while normal and telephoto lenses (as well as zoom lenses at their longer end) have a tendency to show pincushion distortion.

  • Flare is a general term that refers to undesirable haloes, colored patches, and/or ghost images in the final photograph. These are caused when undesirable light reflected from lens surfaces, diaphragm blades, and/or the inside of the lens barrel reaches the film. Flare degrades sharpness of a part of or even of the whole image and is most visible in the dark parts of the image. Ghosts, on the other hand, are the kind of flare that appears as sharp, colored, hexagonal images or as an out-of-focus area of light fogging; their shapes have the same shape as the aperture (when caused by the surface reflection in front of the aperture). Ghosts appear in the image in a position symmetrically opposite the light source.

  • Chromatic aberration (colour fringing) arises when a lens is not capable to focus light of different wavelengths (i.e. different colours) onto the same focal plane. This happens because in an uncorrected lens light of different wavelengths undergoes different degrees of refraction as it passes through it. Chromatic aberration results in colour (usually purple) fringes around objects in the image.

  • The term bokeh, which is of Japanese origin, is used to describe how well, or "silky", out-of-focus objects in an image look, as well as how gradually transition from in-focus to out-of-focus objects occurs. In my opinion, this is a relatively subjective criterion that is hard to compare. Nevertheless and generally speaking, most people tend to like when an out-of-focus light blur is round and gradually diminishes in brightness towards its outer boundaries as opposed to hexagonal forms with bright edges and dim center.