What is Exif Data?

Exif is short for Exchangeable image file format and is the standard by which meta data tags are defined on most digital images. Every image has data embedded in it which include the date and time the picture was taken and the make and model of the camera used to produce the image. In some cases this can also be used to show the lens used on the camera, the owners information and contact details and even geotagging. It also shows the settings used to take the picture – Aperture, Shutter Speed, ISO, Focal Length etc. Below is, in simple terms, some of the settings that you will have on your typical D-SLR camera and also some compact cameras and what they mean.

What is Aperture?

Put simply Aperture controls the amount of light that is allowed to reach the sensor in your camera thus controlling how bright or dull the image is. That is only the start of the story however. Aperture also controls the depth of field. A smaller depth of field (DoF) or higher F-Stop (e.g f/22) allow objects at varying distances to remain in focus at the same time. A low F-Stop or wider aperture, e.g f/1.4, only allows a very limited amount of your picture to remain in focus but in turn this helps blur the background emphasising the part of the picture you want to focus on. A lens is described as fast if it has a lower F-Stop number as this in turn affects the exposure time. It is calculated that an aperture of f/2 has an exposure time of one quarter the time of f/4. Perfect for low light when it isn’t practical to have a really fast shutter speed/exposure time. This photograph was set up to show the difference in aperture from the range of f2.8 through to f22. Click on the photograph to view the photograph in Flickr so you can zoom in and see the difference more clearly. The focus point was on the left hand Octonaut’s face, the body used was a Canon 60D, the lens was a Sigma 24-70 f2.8 EX DG HSM and to keep the lighting consistant I used a Speedlite 430 EX ll.

What is Shutter Speed?

Shutter speed/exposure time is the length off time that the cameras shutter is held open, e.g 1/4000sec is a very fast speed and 30sec is a relatively long time. When the shutter is held open the light is able to reach the sensor and begin to form the picture. The length of time you want your shutter to remain open for varies depending on what you are trying to photograph and whether or not you have you camera on a stable platform or tripod. If I wanted to photograph motorsport for instance I would use a speed of around 1/160 as this combined with panning at the same speed as the vehicle will blur the background while still giving the appearance of movement with the wheels still rotating. While 1/2000 would still take the shot without the need to pan the vehicle would be ‘frozen’ or look like it was stopped. On other occasions a slow shutter speed can be used to create brilliant effects such as traffic movement while photographing a building at night. This gives the sensor time to ‘collect’ all the information it needs and the traffic is just a blur. As a rule of thumb if you are using a focal length of 100mm you would use a shutter speed of 1/100 or faster if you are holding the camera to reduce the likely hood of having camera shake.

What is ISO?

Have you ever taken a picture that when you looked at it appeared to be grainy? This is caused by ISO. Generally image noise is worse in poor lighting because the amount off light that is allowed to enter the camera is limited. If your camera is set to auto adjust ISO it will in this case boost the ISO number to compensate for this allowing a brighter image. Most cameras now can go as high as 3200 or 6400 (and some higher) but the higher the number the more grainy the image and so the quality suffers. This isn’t a problem in most cameras until after 1600 and some much higher.

This sequence was set up to show the difference in ISO-Noise and probably explains the difference much better than my words ever could. The photographs were all taken with a Canon 6D and Sigma 24-70 f2.8 EX DG HSM lens on a Manfrotto tripod. The shutter speed varied as detailed in the photographs and the aperture remained constant at f5.6. Please click on the photograph below to view]

What is Focal Length?

Almost every camera, from your phone to your professional D-SLR, has the option of zoom (on a camera a zoom lens). In short this brings the image closer to you rather that you walking to it. The standard kit lens on most D-SLR has an effective range of 18-55mm although on more consumer based cameras this has a crop factor, on a Canon this is 1.6x and Nikon 1.5. This is taken from the older SLR cameras and this is were the distance comes in. A super wide angle lens has the ability to be as wide as 10mm or less and at the opposite end of the spectrum it is possible to buy telephoto lenses that have a range off 800mm. The standard range for a telephoto lens is somewhere between 70-300mm. Which lenses you decide to buy depend very much on what sort of photography you want to do. For sport or wildlife photography were you need to be close to the action, without having to actually be physically close, a telephoto is for you but if you want to take portrait photography a fixed focal length but lower F-Stop, e.g 50mm f/1.4 would suit you more. The example given here lets you take pictures without the need for flash or a really high ISO in low light but means you might have to move around to get exactly the right image you want. Focal length is the magnification of the image inside the lens and the greater the magnification the more the angle of view becomes decreased. The simple explanation is that when the lens is focused on infinity the distance between the optical centre and the imaging sensor in mm make up the focal length.

Summary

Everyone has there own opinion on what lenses they consider to be their most important but as was said in the last section it does come down to the user to decide what is right for them. On a full frame camera (professional grade) the focal length are more accurate than on the camera fitted with the crop sensor which means that on a Canon a 200mm lens has an effective focal length of 320mm. This can be very handy for wildlife to get that extra bit of length. On this page we haven’t gone into any great detail on any of the subjects but I hope I have helped explain things for you. We didn’t cover other details that can be contained in the Exif Data such as if the flash was fired, metering, white balance, geotagging etc but the above should hopefully cover the basics and help you progress from that Automatic mode!