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This is a photograph from a wedding I recently shot. What kind of exposure problems are present in this photo. AN all white dress, head covered by a veil, and window back light just for good measure. How do you expose for this setting? Might there be flash involved? If so, why and how is it used. Stay tuned for future columns that discuss use of flash.

Q9: Provide a brief explanation as to why an aperture of f/5.6 allows twice as much light as an aperture of f/8 to strike the film plane. Answer: Aperture controls the area of the opening in the lens. The aperture value represents a relationship between that opening and the focal length of the lens. Dividing the focal length by the aperture yields the mathematical measure of that area. Therefore dividing a focal length by f/5.6 yields a larger area then dividing by f/8.

Q10: Provide a brief explanation as to why a shutter speed of 1/500s allows half as much light to hit the film plane as a shutter speed of 1/250s. Answer: Shutter speed controls the amount of time that an amount of light strikes the film plane. A value of 1/500s is twice as fast as 1/250s, therefore, light enters the film plane for half the time.

Q11: Explain why a 300/2.8 lens is physically larger than a 300/5.6 lens. Answer: As in Q9 above, aperture is a mathematical relationship between the focal length of a lens and the diameter of the opening of the aperture. The relationship is derived by dividing the focal length by the aperture. 300 divided by 2.8 yields a larger aperture opening then 300/5.6.

Q12: Explain why exposure settings of f/5.6 at 1/500s and f/4.0 at 1/1000s yields an equivalent exposure. Answer: f/4 allows twice as much light as f/5.6 BUT 1/1000s shutter cuts in half the amount of time that the light strikes the film plane. There the opening up of the aperture and the faster shutter speed cancel each other, yielding the exact same exposure value.

Q13: Determine the amount of contrast (in stops of light) between the darkest object in a scene that meters at f/5.6 at 1/30s and the brightest object in a scene that meters at f/11 at 1/125s. Answer: You are trying to determine the number of stops of light between the two exposures. Start with aperture. From f/5.6 to f/11 is 2 stops of light (f/56 to f/8 to f/11). 1/30s to 1/125s is another 32 stops of light (1/30 to 1/60 to 1/125). Therefore, there is a difference of 4 stops of light.

Q14: Metering an object in a scene yields a reading of f/16 at 1/125s. In order to place the object on Zone III what exposure would you use. Answer: Zone III is 2 stops less light than Zone V, which is what f/16 at 1/125 is. Therefore, close down two stops in any combination that works. F/16 at 1/500s is one such value.

Q15: Why is there only 9 stops of light represented on the Zone System. Answer: For practical purposes, 9 stops of contrast is the limit of film and digital sensors ability to record detail. Anything below Zone I is pure black, anything above Zone IX is pure white.

OK! We can now put the discussion about exposure to rest for awhile. How did you do? Do you know more now than when you first started reading this column? Or, do you now know why you don't know? The mechanics of photography is not easy, but an overall understanding of why things work the way they do is important to make meaningful exposure decisions. So, get out there and try stuff. What's the harm. See what effect changing aperture or shutter speed has on an image, record the values you use and compare them to the readings suggested by your camera. Who is right more often? Are you ready, willing and able to take pictures with the camera on manual? On aperture priority? On shutter priority? In any case, keep shooting and see ya next time with a brand new topic.