Why doesn’t moving the whole lens farther from the sensor create zoom?
Asked 10/4/2016
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If changing the distance between a lens and the sensor affects magnification, why aren’t cameras designed to zoom by moving the entire lens assembly forward and backward instead of moving internal lens groups? Wouldn’t that be a simpler way to make almost any lens into a zoom lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Moving all of the glass elements away from the imaging area is what extension tubes do, and you will note that they do not increase the lens' focal length. An extension tube just moves the focusing points, which allows the lens to focus closer, at the cost of loss of infinity focus plus some light loss. That puts paid to your idea right there: it doesn't do what you want, and even if it did, there are unwanted consequences to doing it, so that's reason enough not to do it.
You can fix some of this by adding more optical elements, at which point you have essentially reinvented the teleconverter. You still lose light when doing this, but you have at least achieved the desired end of "zooming" by extending the length of the optical system.
Originally by user4141. Source · Licensed CC BY-SA 4.0
user4141
9y ago
0
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Moving the entire lens away from the sensor does not really change the lens’s focal length, so it doesn’t create true zoom in the usual sense. What it does is act like an extension tube: it changes the focus range so the lens can focus closer, but you lose infinity focus and also lose some light.
A zoom lens works by changing the optical relationships between internal lens groups, which changes effective focal length while keeping the image usable across a range of distances. Simply shifting the whole lens would also tend to throw focus off as you “zoom,” and optical performance would usually suffer because the lens was designed to project an image at a particular distance from the sensor.
In other words, moving the whole lens mainly changes image distance and focusing behavior, not focal length. If you add optics to increase magnification while preserving function, you’re no longer just moving the lens—you’re getting into teleconverter or true zoom-lens design territory.
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