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Tuesday, September 20, 2011

Speed Up To Slow Down


Slo-Mo Video • What You Get Isn’t What You Got • False Magnification?

Labels: ColumnTech Tips
False Magnification?
Q I notice in your captions, you state the lens used and the effective focal length of the camera/lens combination, i.e., "100-400mm at 400mm (640mm with 1.6x crop factor)." As I understand crop factor, there's no additional magnification, but a perception based on a field of view equal to 640mm due to the sensor being smaller than a 35mm-sized sensor, although there are many people who think they're getting the equivalent magnification of a larger lens. Is it important to state the crop factor in caption info?
R. Gilbert
Via the Internet

A We've addressed this point on several occasions, but I really believe it's important, so we'll talk about it again.

The discussion begins with a holdover from film days. While our digital cameras are equipped with many different sizes of sensors (which record the images we capture), lenses are still standardized to 35mm film (24x36mm), based on the field of view (or angle of view) that the given lens records on a sensor of that size. Canon, Nikon and other manufacturers make cameras with sensors that are full frame (24x36mm, like 35mm film) and smaller. Given the same lens, a smaller sensor records a lesser field of view than the full-frame sensor, essentially cropping the image so that only the center is recorded. The relationship of the field of view achieved by the smaller sensor compared to the full-frame sensor is called "a crop factor" or a "focal length multiplier," and while each type of sensor has its own specific name, it's commonly expressed in terms that suggest magnification, such as Canon's 1.3x and 1.6x sensors, Olympus' 2x sensors and Nikon's 1.5x sensors. The Canon PowerShot SX30 IS advanced digital compact camera has a true focal length range of 4.3mm to 150.5mm, but the effect (angle of view) as compared to 35mm full frame equals 24mm to 840mm. The extended reach of the lens by a factor of about 5.6x is real, whether it's called magnification, a multiplier, the crop factor or angle of view. I always try to provide this information in my captions, but I certainly don't think it's necessary for all photographers to do so.

Some photographers with highly technical approaches are unhappy with this characterization of the phenomenon, but there's no reason to get emotional about it. We don't really need to continue to define everything about digital photography in terms of film equivalents. Rather, I hope that photographers can begin to recognize the benefits and drawbacks of the various options and choose the ones that are best suited to their photography. For example, cameras with full-frame sensors are excellent choices for landscapes where a wider view is desirable, while smaller sensors that extend the reach of the lens are especially suited to wildlife photography. Smaller sensors generally yield an image of less quality, but even this digital axiom is being challenged as digital technology advances and all sensors are optimized for their particular uses.

For information about upcoming seminars and digital-imaging workshops, visit www.georgelepp.com. If you have any tips or questions, address them to: OUTDOOR PHOTOGRAPHER, Dept. TT, George Lepp, 12121 Wilshire Blvd., Suite 1200, Los Angeles, CA 90025-1176 or online at www.georgelepp.com.

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