Digital Photography Jargon


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Digital photography jargon f is for.....

  • F number
  • F stop
  • Field of view
  • Filter
  • Focal length
  • Focal plane
  • Focus
  • Full frame

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related links

Digital Photography exposure explained

Focal length, aperture and f stop summarized

Shedding the light on f numbers

Angle of view, focal length and magnification

Depth of field


Digital photography jargon f is for......

f number

The f number refers to the focal ratio – which is the focal length divided by the aperture diameter.

F numbers are important because irrespective of the focal length ,any given focal ratio will yield an equivalent exposure.

This enables photographers to more easily set correct exposure when zooming in or out or changing to a lens of different focal length.

For a more detailed explanation see

Focal length aperture and f stops summarized

Shedding the light on f numbers


F stop

The sequence of focal ratios whereby aperture diameter is changed such that the amount of exposure is halved or doubled as the aperture diameter is increased or decreased.

So plus or minus 1 f stop halves or doubles exposure.

As apertures are essentially circular, and halving or doubling exposure requires aperture area to be halved or doubled – this in turn requires the aperture diameter to be changed by the square root of 2 between each number in the focal ratio sequence.

Hence the f stop sequence starting from f:1 is

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 etc

Remember – the higher the f number the small the aperture diameter.

For a more detailed explanation see

Focal length aperture and f stops summarized

Shedding the light on f numbers


field of view

Field of view describes the extent of the scene in front of the camera which can be captured and recorded for a given focal length.

Increasing the focal length  decreases the angle of view, thus restricting the field of view to a narrower part of the scene falling more directly in front of the lens. This in turn has the effect of magnifying the resultant image.

For a more detailed explanation see

Angle of view, magnification and crop factors

Filter

Filters are typically interchangeable glass elements that may be mounted in front of a lens to control the way in which the light passes through.

Typical filter types and uses are

  • UV and Skylight filters- There is a difference of opinion between professional photographers as to whether these essentially clear filters have either beneficial or detrimental effect upon image quality. Many photographers (myself included) choose to use them as added protection for the front glass element of the lens – the theory being that if you do bump into something when shooting, its going to be cheaper to replace a scratched filter than repair a lens. In this case it's important to get a high quality filter otherwise you may well compromise the image quality that your lens is capable of delivering. i.e. sticking a $10 filter in front of a $500 lens is probably counter productive.
  • ND filters – Neutral density filters reduce the intensity of light passing through the lens. This enables the photographer to use either a larger aperture or slower shutter speed than otherwise would be possible and therefore have more options for creative control of the image.
  • Graduated – Reduces the intensity over a smooth gradient. This helps the photographer balance the exposure when part of the scene is much brighter – typically used to help achieve detail in the sky in landscape photography.
  • Polarising Filters – By limiting light rays to those moving on a specific orientation, a polarising filter reduces reflections and glare from surfaces such as water or glass and reduces haze in skies giving darker blues and greater contrast in clouds. Other colours may also be intensified - which can enhance foliage and flowers. Digital cameras with TTL (through the lens metering) and autofocus systems will only function with circular polarisers. Circular polarisers may be rotated so the degree of polarisation effect can be controlled from zero to 100%. At full effect a couple of stops of light may be lost, so exposure has to be increased to compensate.For best effect when photographing landscapes with skies, take the photo approx. 90deg from the sun. Be aware that, because with very wide angle lenses the angle of different parts of the scene to the sun will vary considerably, some parts of the photo will show a greater polarising effect than others.
  • Other filters – Many of the other types of filters previously used in film photography to filter light for creative effect, are no longer really necessary with digital photography as similar effects may be achieved during post processing.

Focal length

Focal length is the distance over which light rays are converged to a single point by a lens.

If the focal plane (see below) is at “infinity” which in photography terms means far enough away that by the time the light waves reach the lens they are parallel (collimated in full photography jargon) then the point at which they come to focus is equal to the focal length.

As the focal length increases, the acuteness of the convergence decreases, narrowing the angle of view and the resultant image is magnified.

Using a given focal length on a camera with a smaller sensor size, further limits the effective angle of view and therefore increases magnification.

For full explanation of this topic please read

Focal length, angle of view and crop factors.


Focal plane

This is the exact distance from in front of the lens from which all light rays will be brought to sharp focus beyond the lens.

Subjects beyond or in front of the focal plane will be progressively registered as unsharp on the image sensor.

see Depth of field


focus and autofocus

When subjects are in focus they are located exactly on the focal plane (see above)

Most lenses have adjustable focus – enabling the focal plane to be repositioned such that subjects closer than “infinity” can be brought to sharp focus. This is normally achieved by moving the focusing elements within the lens. As the focusing distance is decreased (ie the subject to be focused is nearer to the lens) the lens elements have to be moved farther from the image sensor to enable sharp focus to be maintained. The physical limitation of this movement ultimately limits how close to the lens a subject can be and still be brought to focus.

Macro lenses are specially designed to allow lens movement away from the sensor and thus a closer focusing distance and therefore greater magnification.

Auto focus

Modern cameras or lenses have systems detecting when a subject is out of focus and making the required adjustments automatically.

On dslrs using interchangeable lenses this system is typically a phases detection type. This system based on optics has the benefit of being extremely fast – allowing accurate refocusing for moving subjects.

Compact cameras tend to use a contrast system that uses data from the image sensor itself – whilst this system has the potential to be even more accurate it has the downside of being somewhat slower and is therefore better suited to static subjects.


full frame

Full frame describes sensors that approximate the dimensions of traditional 35mm photographic film.

Dimensions are typically around 36mm x 24mm

Sensors at this size are expensive to produce and so "full frame" cameras also tend to be very expensive and are normally the reserve of professional photographers, serious amateurs or the  weatlhy!

Larger sensors are able to deliver significantly higher image quality than smaller sensors and are less susceptable to digital noise - particularly at higher ISO settings.

They also return a shallower depth of field for a given focal length.

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useful links from digital photography jargon f

Digital Photography exposure explained

Focal length, aperture and f stop summarized

Shedding the light on f numbers

Angle of view, focal length and magnification

Depth of field

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