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Explain xkcd: It's 'cause you're dumb.
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Telephoto
I was banned from the airliners.net photography forum by concerned moderators after the end of my lens started brushing against planes as they flew by.
Title text: I was banned from the airliners.net photography forum by concerned moderators after the end of my lens started brushing against planes as they flew by.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Do NOT delete this tag too soon.

Telephoto lenses are special lenses for cameras that have a longer focal length than standard lenses, which allows the photographer to zoom in on an object. Alternatively one could add "converters" and "extenders" to an existing lens to get a greater focal length for the cost of reduced brightness. The joke is that Cueball did not want to spend the money on buying a new telephoto lens or real converters, and instead achieved the same effect by moving his cheap camera (a standard webcam, in this case) close enough to the subject to obviate the need for zoom. This 'solution' only works, if the extended lense with the camera gets relevantly close to the subject. In this comic it is first realized with a bird on a tree. In the title text it is further exaggerated to the case of taking pictures of passing planes, which would mean a camera of about 10km length.

There are many problems with this. First, the end result is completely impractical to carry around; as shown in the comic, Cueball has to set up two tripods just to support the weight of his hulking behemoth of a camera. Second, if you're an animal photographer like Cueball, you need to be able to see the animal as close up as possible in order to get a good picture; a lens with lots of magnification power accomplishes just that without alerting the animal to the photographer's presence, but Cueball's camera would surely scare off any birds he tried to photograph (except in fanciful proof-of-concept diagrams like this comic).

Perhaps most damning of all, though, is the fact that Cueball's idea involves installing a webcam at the far end to be able to photograph anything. Webcams are not designed to capture high-resolution images, so the resulting image will be of considerably lower quality compared to professional photographers' works, although it could be better than a standard camera setup taking account of the huge achievable zoom levels. But more importantly, the presence of the webcam renders the functionality of the extenders (and the base camera itself!) completely redundant, cementing this idea as a total waste of time and effort. The same could be achieved by mounting the webcam on a long stick.

The title text continues this by saying he was banned from the Airliners.net photography forum because his new modified lens got so long from all of the attachments that it started brushing against planes as they flew by. It would seem that having a greater than 100 foot lens set up on or near the runway, along with the potential to damage planes in-flight, being banned from an online forum would be the least of his worries.

Transcript

[Cueball stands behind a huge telephoto lens which rests on two tripods, one at the left in front of Cueball, and an other larger one in the middle. The lens is more than five times longer than Cueball is high. In front of the lens is a tree with a bird on top close to the lens. The bird is labeled "Subject". Inside the telephoto lens at the location of the objective lens a small device is shown and labeled "Webcam". From that device a small cable runs through the entire telephoto lens to the eyepiece, where an other device labeled "Camera" is shown.]
[Caption below the panel:]
Telephoto tip: If you add enough converters and extenders, you don't actually need a fancy lens.


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