Back to Image Editing

Simple explanations of some of the words used in photography.

RAW – a file containing the raw sensor data from a camera plus other information. It usually includes an embedded JPEG file that has the camera settings applied to the RAW data. It also includes metadata about the camera (so that the application knows how to handle the raw data) and the camera settings covering such things as the picture style, white balance etc. that are applied to the raw data to make the JPEG, other static data related to the camera – owner, serial number, etc. lens fitted and data related to the camera settings for the image – shutter speed, aperture, ISO Speed, focal length, exposure compensation, metering mode, auto focus settings, GPS co-ordinates. Much of this information is stored as EXIF data and can be in “name” – “value” pairs so that the information stored can be unique to the camera used.

JPEG – a widely used image file format for the lossy compression of digital images as defined by the Joint Photographic Experts Group. A compression rate of 10:1 is not uncommon with JPEG files – producing a much smaller file than other loss-less file formats. Because of the lossy compression this file format should really only be used to produce the final file that is to be published – though there are a limited number of operations that can be done on a JPEG file that do not reduce the quality. Intermediate stages of processing the image should ideally be done using loss-less file formats such as TIFF.

TIFF – Tagged Image File Format – a standard format for raster image files. This file format is often used to preserve the quality of images that are being edited. Any compression used in saving the file is loss-less. Therefore areas of the image that are not being edited maintain their quality. With lossy compression areas of colour or edges can start to show visible artifacts that were not in the original image.

Non-destructive edits – various applications allow you to “edit” an image but the original image data is not lost. The edits done to the image are stored in the file – as in Canon DPP with the RAW file – or as a separate file that is read by the application when the file is loaded again. These edits are often only read by the application and so when publishing the image it still needs to be exported in a “standard” file format such as JPEG. With this method you can often go back several stages of edits or throw away all edits and be back with the original image with no loss of quality.

ISO – a standard managed by the International Organisation for Standardisation. Many people will know of ISO9001 Quality Management. In photography the standard used for the speed of sensors in digital cameras is ISO 12232:2006. The numbers normally used to represent speed are from the arithmetic series where a doubling the number represents a doubling of the responsiveness of the sensor. The numbers used in this standard represent similar sensitivities to those previously used in ASA and ANSI standards that were commonly used to represent film speed. The halving/doubling of the ISO in this arithmetic way follows the principle of the “stop” used in exposure control.

Aperture – the variable hole in the lens through which the light travels to get to the sensor. It’s size is quoted in f:numbers which are the focal length of the lens divided by the diameter of the hole. Therefore a 25mm diameter aperture on a 50mm lens is quoted as f:2. Because this is based on diameter but the amount of light allowed through is proportional to the area of the “hole” we get a range of f:stop numbers based on the square root of 2.

Starting with f:1 each subsequent number in this range is 1 stop different (see below) to the 1 before it – allowing for a bit of rounding.

f:1, f:1.4, f:2, f:2.8, f:4, f:5.6, f:8, f:11, f:16, f:22, f:32.

Stop – a stop represents the doubling or halving of an exposure. The term can be used when referring to adjustments of shutter speed, aperture and/or ISO film speed. For a given exposure – say one that gives you the brightness of image that you want – if you change the shutter speed by 1 stop you can maintain the same exposure by changing the aperture by 1 stop in the opposite direction. Changing from 1/125s to 1/250s can be balanced by changing aperture from f:11 to f:8. The brightness of the image will still be the same but the changes in shutter speed will influence how frozen any motion is and changes in aperture will influence depth of field and possibly general clarity.

Depth of Field – a measure of the distance in front of and behind the focus point that is of an acceptable sharpness. Changing to a larger aperture will reduce the depth of field. The maximum depth of field for a lens is achieved at its smallest aperture however care should be taken that the lens does not show diffraction issues at small apertures where the quality of the image can suffer.

Portrait photographers often want to get a model’s eyes in pin-sharp focus but for the image to quickly start to blur as you get closer or further away from the model – hence they want lenses with large apertures, f:1.4 or f:1.2 or larger.

When the subject distance from the camera is getting close to the focal length of the lens the depth of field is noticeably small and can cause difficulties for close-up work.

Hyper focal distance – the closest distance at which to focus a lens that still leaves objects at infinity in focus. The distance behind the focus point for which the image is sharp is approximately twice the distance in front of the focus point. Today’s lenses seldom have a depth of field indication on them, mainly because many lenses are zoom lenses and such markings would be very difficult to make sense of. A lens focused on infinity and with at a small aperture the image may well be sharp as close as 10m. However by focusing on a point closer than infinity the image at infinity can still be in focus and the close sharp area brought closer. In a landscape it is therefore possible to focus on a tree that is perhaps 1/3 to 1/2 way into the image. The far distance will be within the depth of field and the depth of field to wards the camera will be measured from the tree and not the distance. The image below shows this on a 50mm prime lens with f:16. By setting infinity on the f:16 mark the image will be in focus from about 2.5m to infinity. The red line on the lens shows that the hyper-focal distance for this lens at f:16 is 5m.

This image is showing that the aperture of the lens is set to f:16 and infinity aligned with the 16 on the depth of field guide. The depth of field is shown as stretching from less than 3m to infinity. If the lens was focused on infinity then the depth of field would be from more than 5m to infinity.

It is possible to calculate/estimate hyper-focal distances – I expect there is an App For That.

GIMP – is an Open Source GNU Image Manipulation Program that can be found here.

UFRAW – is an Open Source program for handling RAW files. It can be used in batch mode to extract the embedded JPEG file.

Darktable – is an Open Source program for developing RAW files that can be found here. It is very powerful for those that understand image processing but does not have some of the single button “make it good” options of other applications.

Enfuse – is an Open Source program that can be used for blending images for HDR or (I believe) focus stacking. It can be found here.

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