BetterPhoto Q&A
Category: New Answers

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? To love this question, log in above September 05, 2006

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: Each time you zoom, you are changing the lens’s focal length and thus the image magnification. Shoot a picture of a person, 20 feet distant. You get a specific head size. Zoom 2x, say change from 50mm to 100mm and the apparent head size gets bigger, in fact doubling. You could have kept the lens set at 50mm and stepped forward the subject to the 10 foot mark. Head size will be the same for both circumstances however, prospective is different. 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: The zoom lens aperture is geared to the zoom mechanism. As you zoom the aperture changes in diameter, this action compensate for any focal length change keeping the f/number the same. Same f/number - same light energy at the image plane. For the purest who might find some fault: As you zoom you change the angle of view. Telephoto means a narrow angle of view and Wide angle is the inverse. Now as you change the angle you probably will seen changes with regard to exposure. Your camera meters by through-the-lens sensors. As the magnification changes, the average values metered will change. More or less white shirt, more or less gleaming chrome, more or less blue sky etc. 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 ammarcus@earthlink.net To love this comment, log in above September 05, 2006

BetterPhoto Member		Wow. That was a fantastic explanation! Thanks Alan! To love this comment, log in above September 05, 2006

BetterPhoto Member		He Alan, are saying we're all old fxxts? To love this comment, log in above September 08, 2006

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