 Mike Calderwood |
Flash In a lot of books I have read I am seeing charts showing the distance that your flash is effective for a given ISO and MM. Example: At ISO 100 and at 18mm my built in flash is good for a range of 2-12 feet but at ISO 100 and 55mm only 2-7 feet. From a math stand pont does that mean at ISO 100 and 110mm My range is only 2-3.5 feet? ANd so on...
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Jon Close |
The lens focal length does not directly affect the usable flash-to-subject distance. It is lens aperture that affects the usable range. The smaller the aperture, the less light allowed in, so the flash range is reduced. The maximum flash power is given in terms of a Guide Number. At ISO 100, the built-in flash has a guide number of about 42.4 feet. The maximum range is obtained by dividing the guide number by the aperture number. At 18mm, the EF-S 18-55 has a maximum aperture of f/3.5. 42.4/3.5 = 12.1 ft. At 55mm the maximum aperture is only f/5.6. 42.4/5.6 = 7.5 ft. If you were to use instead the EF 100 f/2 lens, the maximum flash range would be 21.2 ft.
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Alan N. Marcus |
Hi mike, All artificial light is generally weaker than people realize and it falls off with distance at greater rate than most understand. Light from a point source like a flash or flood lamp follows a law of physics known as the law of the inverse square. That’s a bit complicated but generally it means that each time you double the distance from light to subject, the light energy is reduced 4 fold. As an example if a light delivers 400 units (watts) at 5 feet distance, only 100 units (watts) will fall on a subject 10 feet distant. Stated another way, double the distance and you must open your lens 2 f/stops. Each f/stop is a 2x change in light. Because this gets complicated we use a simple system call a guide number. You can check your manual and find these numbers. Each ISO will have a guide number. Let’s say its 80 for 100 ISO. We measure the distance subject to lamp and divide this distance into the guide number. Say the subject is 10 feet away then 80÷10=8 meaning we shoot at f/8. Now if the subject is 20 feet away then 80÷20=4 meaning we shoot at f/4. Now that I have told you all that jazz, your question actually revolves around the mounted lenses maximum aperture. As your subject’s distance gets greater your lens aperture must open up wider to compensate. Your 18mm has a max aperture that only allows correct exposure with your flash unit at a max distance of 12 feet. After 12 feet under-exposure is the result. Now your 55mm has a smaller maximum aperture than the 18mm. When mounted it can’t open as wide as the 18mm thus your effective flash taking range is reduced. When at some later time you choose to purchase longer lenses like 110mm or maybe 200mm, you can consider buying one with a max aperture that’s similar to your 18mm. The problem is, long lenses with large max apertures are expensive. Also, your camera has chip logic that does the guide number math for you and sets the aperture too.
Alan Marcus
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Mike Calderwood |
Thank you for your answers. You both must have gone to MIT or something (HAHAHA). Actually you both helped clarify alot. Thanks
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