digital photography jargon

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Digital photography jargon M is for....

  • Macro
  • Magnification
  • Memory card
  • Metering

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Digital photgraphy exposure

Angle of view, magnification and Crop factors

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Digital photography jargon

M is for........

Macro

True macro digital photography is when a subject is reproduced at (or greater than) life size on the image sensor.

The size of the image compared to the real life subject is called the reproduction ratio.

True macro lenses have reproduction ratios of at least 1:1

(Reproduction ratios greater than 10:1 are known as MICRO – vis a vis MICROscope)

Example

A caterpillar that is 18mm long when photographed with a 1:1 macro lens mounted on a full frame camera (image sensor size 36mm x 24mm) will form an 18mm image on the sensor – you can visualise this as the caterpillar actually crawling inside the camera and physically laying down on the sensor!)

If a 2:1 macro lens is used, the caterpillar's image would occupy the full 36mm width of the sensor.

Macro lenses are specially modified with longer barrels to enable close up focusing.

(The closer a subject is to a lens – the greater the distance behind the lens to bring the image to focus. To achieve this extra distance between image sensor and focusing element, the focusing element is moved forward in the barrel of the lens. In a standard lens there simply isn’t sufficient space in lens barrel to enable close up focusing – lenses normally list “minimum focus distance” in their specifications.)

Accessories are available that give standard lenses macro capability.

Extension tubes are effectively extra sections of barrel that can be mounted between camera body and lens to increase lens to sensor distance (and therefore allow closer focusing) Extension tubes contain no glass elements and so do not affect optical quality of the image, although the extra distance does result in a fall off of light intensity so exposure needs to be increased to compensate.

Close up lens attachment – these are fixed to the front of the lens via the filter thread. Think of these as photographing through a pair of reading glasses! This is the lower cost option but generally considered to compromise image quality.

Other relevant macro photography facts

The closer the focusing distance, the shallower the depth of field. In macro photography with high reproduction ratios this can reduce to just a few millimetres.

High f numbers (small apertures), tripod, and manual focusing are therefore recommended.

As smaller image sensors have a deeper depth of field for the same angle of view – macro is one area of photography where smaller sensors can offer an advantage.

The term macro has become synonymous with any form of close up photography.

Many compact cameras and lenses have a “macro” setting (often indicated by a flower icon) which allow closer focusing than with the standard setting – but rarely a true 1:1 reproduction ratio.

Macro effect by cropping

If you don’t have a macro lens – but do have camera with a high mega pixel sensor – then it is possible to create a macro effect by cropping the captured image so that the subject fills the frame. This does of course reduce the maximum size of print that can be produced – but if the the image is for “on screen” display , then even if you crop to only a 1000 pixel wide you will have a large enough image to display with reasonable quality.


Magnification

This term can be somewhat confusing in digital photography, as it is used to describe a variety of effects. Listed below are the main examples.

  • Narrowing the angle of view. This is the result of increasing the focal length and results in the subject occupying a greater proportion of the image and therefore appears closer or magnified. Lengthening focal length is typically called “zooming”.
  • Crop factor/ focal length multiplier. When a lens of any given focal length is mounted on a camera with a smaller sensor the image is “cropped”. This effectively has the same affect as narrowing the angle of view and thus the image appears closer or magnified.
  • Subject distance. Just as with your own eyes, the size of objects appears to increase or diminish with a change in distance. If you move twice as close to an object it will appear twice as big.
  • True magnification. Some Macro lenses (see digital photography term “Macro” above) really do magnify the subject. A Macro lens with a 2:1 reproduction ratio is capable of doubling the real life size of the subject image.
  • Post Cropping and “digital zoom”. Most modern image sensors are several thousand pixels wide and high. This means that even at high resolution the original image may be printed in relatively large format. However many images are only intended for display on lower resolution computer monitors. For example if an image is recorded on a sensor that is 5000 pixels wide x 3200pixels high (16 mega-pixel) then cropped to a typical screen size of 1000 x 800 pixel – then this portion of the image will appear to be magnified 5x (as though the focal length were 5 x longer) and can be displayed without loss of quality. This is similar, but superior to using the “in camera digital zoom”. It is generally recommended to avoid using this and to crop in post processing – as this gives greater control over composition.

Memory card

When you take a digital photograph, the image captured by the image sensor is converted into digital data by the camera's processor and then stored on a memory card.

Memory cards exist in a variety of formats of different physical sizes. Some cameras will accept more than one type – check your cameras specifications for details of which format yours uses.

Memory cards use a type of memory known as “flash” . This system is “solid state” i.e. it has no moving parts nor does it require power to maintain the stored digital data.

Memory card specifications

Memory cards come in a variety of storage capacities – typically from 1GB to 32GB and are capable of storing hundreds or even thousands of images.

They also come in different classes of “speed” - this refers to the rate at which digital data can be written to the card and subsequently transferred from the card when downloading to a computer.

Higher write speeds are important to facilitate shooting full resolution shots continuously at maximum frames per second rate in “burst” drive mode – or when shooting data intensive RAW files.

High read speeds permit faster transfer rates when you download your images.

Price generally increases with storage capacity and read / write speeds.

IMPORTANT NOTE the IMAGE QUALITY is consistant across all the types and prices – i.e. an expensive high capacity, high speed card WILL NOT give any improvement in image quality.

Some cameras now support “wifi” cards which have built in wi fi capability which enables wireless transfer of the data to a computer directly from the camera.

Memory cards are robust, durable and don't “wear out” with use - they can however become corrupted. It is recommended to use the in-camera function to reformat them from time to time and to download and make copies of your images regularly.

It is also a good idea to carry spare memory cards with you.. just in case!!


Metering

Is the process by which the camera measures the intensity of light emanating from a scene in order to determine optimum exposure settings.

Sometimes the range of brightness levels within a scene are greater than the dynamic range that the camera can deal with in a single exposure. In these instances if the camera calculates exposure by averaging the all the brightness levels from the scene then it is possible that the primary subject of the image will be either under or over exposed.

Common examples are

  1. A portrait photo when the subject is backlit against a bright background – in trying to correctly expose for the background, the subjects face will end up underexposed.
  2. Photographing the bright moon in a dark night sky. In trying to expose for the dark sky, the moon will end up being overexposed.

Modern Dslr cameras offer a variety of metering modes designed to help the photographer prioritise which part of the scene's luminance is given greater weighting when calculating exposure.

These modes are as follows:-

  • Evaluative/Matrix metering. This divides the frame into numerous zones. The weighting for each zone takes account of various factors including which contains the active focus point and the orientation of the camera. For the majority of scenes this is often the best default mode.
  • Centre weighted. As the name suggests gives greater weighting to the central area but still takes account of the peripheral areas.
  • Partial metering. This takes account of the central area but ignores the peripheral areas.
  • Spot metering. This only takes account of a very specific small area – normally the centre – useful if the primary subject is considerably darker or lighter than the rest of the scene.

Metering systems are normally calibrated to expose for the mid tones of a scene. When mid-tones are difficult to distinguish – e.g. when the entire scene is bright (think polar bear in snow) or dark (think black cat in shadow) then the metering system can be “fooled” and either under or overexpose the photo.

Most dslrs therefore offer exposure compensation, which enables the photographer to instruct the camera to deliberately use exposure settings up to 3 stops different from what the camera's meter determines.

Metering is computed instantaneously, normally a fraction of second before exposure commences. Exposure lock (AE lock – normally a button with a * icon) enables the photographer to meter the scene and then lock the exposure settings so that the shot may be recomposed without adversely affecting the exposure of the primary subject.

NOTE: The different exposure modes DO NOT extend the dynamic range of the the camera. They merely enable the photographer to ensure that the most important elements of the scene are correctly exposed. Other areas of the scene will still be under or over exposed if the dynamic range is high. The only solution to this is to physically alter the lighting of the scene – e.g. using flash or reflectors. Or restrict the light in certain parts of the scene e.g. Using a graduated filter to reduce brightness from the sky. Or finally to take “bracketed” exposures at a series of different exposures and then combine these in post processing.


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