One of the wonders of exposure is that dozens of aperture, shutter speed, and ISO combinations can provide exactly the same exposure (the overall lightness or darkness of an image), but very different artistic “looks”.Â Experienced photographers know which exposure combination to choose to get the image they want.Â Inexperienced photographers who leave the camera on program mode are turning all of the artistic decisions over to a computer chip.
One of the most important artistic decisions is the depth of field in an image (the amount of near to far sharpness).Â The choice of lens aperture is one of the ways depth of field is controlled.Â So long as the focal length and focused distance don’t change, wider apertures like f/4 or f/5.6 provide more depth of field and smaller apertures like f/16 of f/22 provide more depth if field.
In these two photos, the lens is the same, the focal length is the same (155 mm),Â and the point of focus is the same (the end of the mouthpiece).Â The camera and lens were not moved between the two photos. The only change was the aperture and shutter speed combination.
In the top photo, the aperture was f/4, which minimized depth if field. Only the mouthpiece is sharp.Â In the bottom photo, the aperture was set to f/32. The tiny aperture provides a lot more depth of field.
To keep the exposure the same, the change from a large aperture to a tiny aperture (6 stops less light) had to be accompanied by a 6 stop increase in the shutter speed.
No computer chip knows how light or dark you want your photos to be, or how much depth of field you want.Â By taking the camera off of autopilot (program mode) and taking control of the exposure, YOU get to decide what kind of photos you want.
More information on exposure, including two full chapters on controlling depth of field (more than most other books on exposure), can be found in Digital Photography Exposure for Dummies.Â If you use this link to buy the book, you help support this site (thanks in advance if you decide to do this).
Top photo:Â Canon 5D. 70-200mm lens at 155mm. Aperture at f/4. Shutter speed at 1/25 second. ISO 100.
Second photo:Â Canon 5D. 70-200mm lens at 155mm. Aperture at f/32. Shutter speed at 2.6 seconds. ISO 100.