Photographic Parameters

In this day and age, the vast majority of cameras sold (digital, especially dSLRs and compacts) tend to mask the optical properties that determine features of your photographs. An algorithm chooses the settings that gets some part of the scene exposed the way it thinks it should be; another chooses the distance at which to focus; a third might decide whether it needs to pop up an internal flash unit. Unfortunately, this brings much scope for error; if the camera is designed to make the entire scene average out to a midtone grey, then snow will be grey, inside a coal-mine will be grey, back-lit portrait scenes will either have the person underexposed, the background overexposed, or both. You might be able to learn the nuances of your camera's behaviour in various situations, but there's still a risk it will choose the wrong parameters for a critical important photo. An alternative approach is to understand what the camera's controls typically achieve optically or physically, in order that one might have the control with which to make artistic choices.

Hail Manual Mode

Most cameras should have the options to disable automatic mode, in favour of varying degrees of semi- or fully manual modes.

Parameters Available

The following parameters affect the final image:
  • ISO sensitivity
  • Aperture diameter
  • Shutter speed
  • Exposure algorithm
  • Focal distance
  • Lens focal length (zoom/telephoto/wide-angle)
  • Sensor format (size)
  • Image Stability (IS) / vibration-reduction (VR)
  • In-camera processing (digital chip or type of film)

Consequences

All photographs require a juggling of settings of the above parameters in order to achieve a desired effect.

Exposure

The scene in front of your camera will have a particular brightness, or range of brightnesses if you crop into certain parts of it. The combination of shutter-speed and aperture diameter, taken together, controls how much of the light coming from the scene will contribute towards the brightness of the result. The ISO sensitivity will also determine how much of the incoming light is required in order to make a pixel as bright as a midtone in the final image. The camera's light-meter can analyse a scene in one of typically three ways:
  • "centre-weighted", meaning the entire scene contributes towards the camera's understanding of its brightness, but the centre of the frame counts for more than the surroundings
  • Matrix or multi-segment: as before, the entire scene counts towards determining the overall brightness, but the camera analyses it split into many segments in order to guess your subject-matter and make a choice accordingly (for example, it might have been programmed to notice a dark blob in the centre of a light blue scene and assume it's a portrait)
  • Spot-metering: here only a tiny point (around 1 degree or less than 2 percent of the frame) counts.
Parameter Effects when low Effects when high
ISO setting At low settings such as ISO 100 or lower, the sensor will take more light to register the same increase in brightness of a pixel. This means the computed values will be more accurate, the thermal noise from the electronics will contribute less - ie the image will be relatively noise-free.
Because more light is required, the shutter-speed must be slower and/or the aperture wider.		 At high ISO settings (ISO800 for digital compacts, ISO3200 for modern dSLRs), less light is required so the shutter-speed can be faster and the aperture narrower, but you'll get more noise in the resultant image.
Shutter speed "Slow" is relative to the subject you're shooting. This end of the spectrum is characterised by motion: either an object in your scene has had time to move (eg rivers/waterfalls becoming blurry like combed cotton-wool) or the action of pushing the shutter-release button has caused camera-shake. Because there is more time for light to enter, the aperture can be narrower or the ISO lower. At higher speeds, motion is frozen. Closeup photographs of flowers will not appear blown around in the wind, but aeroplanes and birds will appear stuck static against the sky.
Aperture Recalling that apertures are given as reciprocal numbers, a small (narrow) aperture, such as f/16, has the effect of reducing the amount of light coming in (so the shutter-speed needs to be slower or the ISO higher), and extends the depth of field so that more of the scene will be in focus front-to-back. A wider aperture (eg f/2.8) lets in more light, so the shutter speed can be faster and/or the ISO sensitivity lower; in the process, the depth of field is reduced, ie less of the scene will be sharp from front to back, and you will achieve differential focussing on a subject versus a blurry background.
Sensor Format A small sensor (eg four-thirds or other compact / dSLR formats) is more susceptible to image-noise but, all other things being equal, will give you greater depth of field at a given aperture A larger sensor (eg medium- or large-format film) has less depth of field, so requires the aperture to be stopped-down narrower. However, there is a greater signal-to-noise ratio: either on "full-frame" 35mm digital sensors where there is more space to each photosite, or on film where the grain is a fixed small size but the image is over a larger area so the grain detracts less from the quality.
Focal length A wide-angle lens has a greater depth of field therefore can be used at wider apertures and/or faster speeds. A long telephoto or zoom lens has a very small depth of field, so helps isolate subjects against a blurry background. A good rule of thumb is that it should be safe to hand-hold at speeds faster than the reciprocal of the (35mm-equivalent) focal length - so for example, a 300mm lens on a DX-format dSLR (such as a Nikon) would need a shutter-speed of 1/450th (call it 1/500th) to be safe; below that shutter-speed, it would be best to use a tripod.
As the above table should make apparent, all these parameters are interwoven. What matters is the ability to identify the type of scene and desired results and choose the other two or three parameters accordingly. For example, in a landscape, you typically want lots of depth of field so everything from nearby to infinity is sharp; this means a narrow aperture and hence slow shutter speed. In low light, or with slow (low-ISO) settings to reduce noise or grain, a tripod quickly becomes essential. Alternatively, shooting macro photographs of small wildlife (e.g. dragon-flies), you benefit from throwing the background out of focus (narrow depth of field) and the resultant faster shutter-speed reduces the risk of camera-shake.

IS/VR

Image-Stabilization (IS) or Vibration-Reduction (VR) are techniques (implemented either in the sensor/body, or on a given lens) that help identify camera-shake at the point of pressing the shutter button, and reduce motion-blur. Typically relying on this might allow you to shoot a shutter-speed 1 to 1.5 stops slower than you might otherwise have used - using a long zoom for wildlife at dusk, for example.

The Buttons On My Camera

Typically, the exposure-related controls on a camera take the form of a mode dial, including options such as
  • M - fully manual where you specify all the above
  • S or Tv - shutter-priority, where you specify the shutter-speed and it computes an aperture for you
  • A - aperture priority, where you specify the aperture and it computes the shutter-speed for you
  • P - programme mode, where it calculates a pair of shutter-speed and aperture values, allowing you to vary the balance from one to the other or compensate the overall exposure brighter or darker
  • Auto - we don't go here!
  • Landscape, portrait, closeup, etc
This reasonably reflects the variety of users' preferences; for sports photographers, often speed is the primary concern and the scene will be a uniform distance away (eg the width of a racing track is small compared to the distance away, so focussing somewhere on a passing Formula 1 car at the time is bound to give a sharp result). For some closeup or music-gig photographers, the aperture is important and the speed can vary within a small range according to available lighting. The other aspect the "canned scenes" menu options control is the tonality or response-curves of the sensor. Specifically, it might be that landscape brings greater saturation especially in blue and green, or that portrait is softer and more delicate (of lower contrast). In film terms, one might choose Fuji Velvia for a landscape scene but Provia or Astia for a portrait, because of their colour-reproduction tendencies.

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