1. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Essential principles chapter 1 Aperture, Shutter timing and ISO (sensitivity) setting

2. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Aperture, Shutter timing and ISO (sensitivity) setting Your camera records light. –Too much light and the image is too light –Not enough light and it is too dark. To let more light from your subject into the camera you need either : –the hole must be open for longer (shutter timing) –or a larger hole (aperture)

3. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Automatic exposure control Your camera's automatic exposure control works this out for you and adjusts the shutter timing and aperture to give the correct exposure –BUT…. there is more than one way to achieve the correct exposure and the results have some important differences.

4. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Look at this pair of images They were taken at the same time, same camera, same light. Why are they different ?

5. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes In this image the movement of the water is 'frozen' by a fast shutter timing

6. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Here the water is allowed to blur, much as your eye sees it

7. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Both are correct exposures You can choose, it is part of the creative process –but you have to tell the camera what to do –if you use fully automatic exposure control, the camera decides for you.

8. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Another pair of images taken at the same time, same camera, same light - why are these different ?

9. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Here the background is sharp the impression is of a room with a bottle in the foreground

10. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Here the background is out of focus the main impression is of the bottle in the foreground

11. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Both are correct exposures. You can choose, but once again –only if you tell the camera what to do, otherwise it decides for you. –To do this, you need to take control of the settings for aperture, shutter timing and ISO, the three elements of a correct exposure :

12. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Aperture Aperture (the size of the hole) is specified by a series of values written as f/4, f/11 etc. There is a complicated technical reason for this, but you can ignore it what you need to know is : –Bigger ‘f ’ number = smaller aperture = less light

13. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Aperture Bigger ‘f ’ number = smaller aperture = less light –Each change of 'f' setting, say from f/4 to f/5.6, or from f/8 to f/11 reduces the light by half

14. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Shutter Timing This is often called shutter speed –very confusing because speed is distance per unit time, eg km/hr. –The settings are actually times eg 1/100s or 1/1000s for which the shutter remains open. Most modern cameras have a huge range of setting from say 10s to 1/2000s

15. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Shutter Timing What you need to know is : Longer time = more light –(obvious really ! )

16. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes ISO (or sensitivity) The final leg of the triangle is ISO, the measure of sensitivity of the camera's sensor. –Higher ISO number = higher sensitivity = less light needed for a correct exposure

17. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes A choice Now you can already make a choice of how to achieve the correct exposure : –Bigger ‘f ’ number (smaller hole, less light) + longer shutter time (more light) Is the same as : –Smaller ‘f ’ number (bigger hole, more light) + shorter shutter time (less light)

18. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Try it. Set the camera to Av (Aperture priority), set f/5.6, see what shutter timing is proposed by the automatic metering for, say, the wall opposite. Now set to f/16 and try again – write down the values, notice the difference.

19. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes What can I do with this ? You can control the style of picture your camera takes, not just the subject. 2 important rules –A fast shutter (short timing) will freeze the action of moving subjects –Larger apertures have smaller depth of field (backgrounds will be more likely to be out of focus)

20. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Using ISO Higher sensitivity settings produce noise that tends to blur the image, use the lowest ISO that will work in the shot you are taking –lower sensitivity needs more light - that will mean longer exposures or bigger apertures –Slow shutter speeds tend to produce blurred images due to camera shake (use a tripod).

21. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes How do I achieve the correct exposure with the auto turned off ? Mode semi auto ! Most cameras have two semi-automatic modes –aperture priority (called Av on Canon cameras and A on Nikon) –shutter priority (called Tv on Canon cameras and S on Nikon).

22. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Av aperture priority mode you select the aperture you want to use –the automatic exposure control measures the light and sets the shutter timing required for a correct exposure.

23. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Tv shutter priority mode you select the shutter timing you want to use –the automatic exposure control measures the light and sets the aperture required for a correct exposure.

24. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Possible problems It is, of couse possible to select a value that cannot be achieved with the light available, eg. a fast shutter timing in poor light. –Each make and model of camera reacts differently in these situations. Some override the settings, some refuse to take the shot, some flash a warning sign - you will have to read the handbook and learn how your own camera behaves.

25. Photo Club 8 décembre, 2011Principes Fondamentaux, Ch1 - Ouverture/temps d’obturation/ISO Copyright David Keast/Association Fils et Cordes Possible problems Sometimes (quite often actually), the automatic exposure control is not clever enough to work out the correct settings. How to predict when this will happen and how to do better is covered in a future session (ch3).