BetterPhoto Member |
Amount of Light In a Telephoto Shot I know the rule of thumb when using a telephoto lense is to shoot with a shutter speed roughly equal to the mm of the lense. This is to reduce blur from shake right? But when zooming in on a subject, does the fact that you're zooming in also reduce the amount of light reaching the sensor? Does the shutter speed/lense length rule hold up there as well?
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Alan N. Marcus |
Hi Matthew, As far as the lens is concerned, the exposing light energy projected on film or chip is a function of the lens aperture (working lens diameter) and the operating focal length. The two are intertwined in a complex way. The complexities are taken away by the f/number system. These values are ratios that take into account these two factors. As you zoom you are changing focal lengths. Not to worry! The f/number remains constant because the modern zoom lens automatically changes lens diameter as you zoom. Makes no difference if you zoom in or out the lens compensates nicely, both ways. A little too much detail follows: Now when we zoom the light energy at the image plane (film or chip) changes drastically. As you zoom in 2x, this new magnification results in a 4x drop in light level. Yes that’s right, 4x light fall off because as you zoom 2x more magnification, you have changed the subject’s image area by 4x (height x width). Stated another way, the same light must now be distributed over 4x more surface area. Spread out thinner, the light is weaker. Zooming out is exactly the reverse. 50mm to 25mm zoom and the image becomes 4x brighter. Gearing to the rescue: For the purest who might find some fault: Exposure is easier now with digital because of all the modes available to you. Your camera is smarter than us old fxxt's ever were. Go and shoot and don’t worry about all this. Alan Marcus
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BetterPhoto Member |
Wow. That was a fantastic explanation! Thanks Alan!
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BetterPhoto Member |
He Alan, are saying we're all old fxxts?
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