# What is the difference between a super-telephoto lens and a telescope? **URL:** \ **Category:** Factual Questions\ **Created:** [March 25, 2008, 9:52pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726 "2008-03-25T21:52:17Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [March 25, 2008, 9:52pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/1 "2008-03-25T21:52:17Z")
Canon sells a “super-telephoto” lens that costs $90,000, the EF 1200mm. > **[Canon EF 1200mm lens](https://en.wikipedia.org/wiki/Canon_EF_1200mm_lens)** > > The EF 1200 mm f/5.6 L USM is a super-telephoto prime lens that was made by Canon Inc. It uses an EF mount, and is compatible with the Canon EOS camera range. It has a focal length of 1200 mm and so on a digital body with a sensor size of 22.5 mm × 15 mm (called 1.6× crop), such as a Canon EOS 40D or 450D, it provides a 35 mm field of view equivalent to that of a 1920 mm lens. With a body with a sensor size of 28.8 mm × 19.2 mm (called 1.3× crop), such as a Canon EOS-1D Mark IV, the field of view... What is the difference between this, and a $1600 telescope with a camera attachment? ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 25, 2008, 10:12pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/2 "2008-03-25T22:12:25Z")
[QUOTE=Absolute] What is the difference between this, and a $1600 telescope with a camera attachment? [/QUOTE] Just for starters, a $1600 dollar telescope won’t be an 8" refractor. It’ll be a reflector of some sort. 214 mm refractors go for more like $17,000. ---
**Author:** ![squeegee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/squeegee/32/14537_2.png) [@squeegee](https://boards.straightdope.com/u/squeegee)\ **Post date:** [March 25, 2008, 10:39pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/3 "2008-03-25T22:39:35Z")
I don’t know what telescope’s specs are, but a 1200mm **f5.6** lens is a lot of expensive glass. The f/ ratio on the telescope may be something like f/32 or worse. I mean, jeezus, [look at the primary lens](http://k43.pbase.com/o5/33/205833/1/67739895.rANtdvik.Canon12002a.jpg) on that thing, its the size of your head! ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [March 25, 2008, 10:43pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/4 "2008-03-25T22:43:21Z")
More light gathering power for starters. The Canon lens has a aperture of f/5.6, while a [$2000 8 inch reflector](http://www.meade.com/lx90-acf/index.html) has an aperture of f/10. It is a faster lens than the telescope, by a factor of two. ---
**Author:** ![squeegee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/squeegee/32/14537_2.png) [@squeegee](https://boards.straightdope.com/u/squeegee)\ **Post date:** [March 25, 2008, 10:56pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/5 "2008-03-25T22:56:10Z")
[QUOTE=Fear Itself] More light gathering power for starters. The Canon lens has a aperture of f/5.6, while a [$2000 8 inch reflector](http://www.meade.com/lx90-acf/index.html) has an aperture of f/10. It is a faster lens than the telescope, by a factor of two. [/QUOTE] But also, isn’t that telescope you cited a Schmidt-Cassegrain , not a refractor? I assume there’s be more chroma aberration (I’m forgetting the proper term) in that design than a refractor, no? ETA: you did say reflector, sorry. But anyways, what would an apples-apples comparison be with that lens vs a telescope? Is that meade a reasonable one? ---
**Author:** ![garygnu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garygnu/32/2924_2.png) [@garygnu](https://boards.straightdope.com/u/garygnu)\ **Post date:** [March 25, 2008, 11:26pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/6 "2008-03-25T23:26:53Z")
An aperture of f/5.6 for a 1200mm lens is insane. In addition to the sheer speed of that, attaching a normal telescope to a camera and using it to shoot wildlife (for example), _everything_ would end up being in focus, including the background. You couldn’t get pictures like [this](http://www.thaimtb.com/webboard/114/57037-1.jpg)\* or [this](http://www.pbase.com/image/80600804). (Good illustration of the difference [here](http://www.hp.com/united-states/consumer/digital_photography/take_better_photos/tips/depth.html#4).) Still, the law of diminishing returns applies. I think a lot of the cost stems from the autofocus. \*:from a Thai website, I think it was taken with the lens in question, but I’m not sure. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [March 25, 2008, 11:38pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/7 "2008-03-25T23:38:09Z")
[QUOTE=Fear Itself] It is a faster lens than the telescope, by a factor of two. [/QUOTE] The speed of the lens (f 5.6) is a big part of what drives the cost up. Fast lenses are more affected affected by [Spherical aberration](http://www.vanwalree.com/optics/spherical.html) than lenses with a high F number. The aberration worsens as some funky power (3?) of the lens diameter, so an 8 inch apochromat will take either fancy aspheric grinding or clever correction to give a flat focus plane. With 13 lens elements, I suspect Canon went with the latter approach, or probably both. That’s a lot more lenses than you’ll find in a typical refractor, 2-3 for the objective plus maybe 6 for the eyepiece. Of course the telephoto also needs to be optimized for a flat focusing plane down to 50 feet, while a telescope concentrates on infinity. That too requires a more complicated optical setup. The telephoto probably also has a much more sophisticated anti-scatter system than your typical eight inch reflector. Light bouncing off the tube walls of a telescope and through the eyepiece is usually taken care of with black flocking paper, or paint. That’s good enough for night work, but in the daytime a reflection of the sky off out of frame objects can trash the contrast of an optical system. No doubt the Canon has all sorts of exotic black coatings and ingeniously placed light baffles to minimize this problem. ---
**Author:** ![lobotomyboy63](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobotomyboy63/32/2900_2.png) [@lobotomyboy63](https://boards.straightdope.com/u/lobotomyboy63)\ **Post date:** [March 26, 2008, 12:09am UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/8 "2008-03-26T00:09:10Z")
The Canon would also give you a choice of f/stops (5.6, 8, 11…) whereas the telephoto would be fixed. More automation—autofocus or programmed modes—would probably work with it, which could be critical in low-light. Reflector telescopes tend to actually transmit less light than their theoretical f/stop because of the light loss in mirrors. And being designed for night time use, I’d expect more problems shooting in daylight because of flare etc. But they’re lighter and obviously, cheaper. For an amateur photog who doesn’t need such a lens often, maybe it’s a good solution. For a pro, maybe not. ---
**Author:** ![ticker](https://avatars.discourse-cdn.com/v4/letter/t/d07c76/32.png) [@ticker](https://boards.straightdope.com/u/ticker)\ **Post date:** [March 26, 2008, 9:43am UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/9 "2008-03-26T09:43:27Z")
[QUOTE=garygnu] …attaching a normal telescope to a camera and using it to shoot wildlife (for example), _everything_ would end up being in focus, including the background. [/QUOTE] No it wouldn’t. You would not have any _control_ over depth-of-field with (i presume) a fixed aperture telescope, but at those sorts of focal lengths DoF will be pretty limited even at f32. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 26, 2008, 12:15pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/10 "2008-03-26T12:15:43Z")
The definition of “telephoto lens” used to be that its principle plane was out in front of the first lens element. This would be convenient in a long focal length lens because the lens assembly could be shorter than the focal length. By this definition, the opposite of a “telephoto lens” is a “retrofocus lens”, in which the principle plane was behind the last lens element. This is useful in extremely wide angle lenses intended for SLR cameras, because it lets the mirror move up and down even though the mirror must do so through the principle plane. In practice, “telephoto” nowadays more often means “long focal length”. Astronomical telescopes differ from huge long camera lenses in not having adjustable apertures, not being able to focus very close, and (sometimes) not having as constant an image quality over the entire field of view (because you use higher magnification when looking at the center of the field of view). Also, astronomical telescopes are often designed specifically to be looked through or specifically to take pictures. I once spent a summer photographing variable stars through an 8" astronomical camera. This thing had a Cooke triplet lens with 8" of clear aperture at the front and an 8" by 10" glass plate holder at the rear end. You could not look through it at the sky, unless you count leaving the plateholder off and using a magnifying glass to examine the image. It did, though, have a 5" refractor as its guidescope. The refractor had been made by Alvin Clark and I’ve actually spent many more hours looking through this than any other single telescope. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [March 26, 2008, 2:11pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/11 "2008-03-26T14:11:14Z")
[QUOTE=squeegee] …[look at the primary lens](http://k43.pbase.com/o5/33/205833/1/67739895.rANtdvik.Canon12002a.jpg) on that thing, its the size of your head! [/QUOTE] :: looks at pic :: Nice! Oh yeah, there’s a lens in the picture too. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [March 26, 2008, 3:39pm UTC](https://boards.straightdope.com/t/what-is-the-difference-between-a-super-telephoto-lens-and-a-telescope/442726/12 "2008-03-26T15:39:07Z")
[QUOTE=Fear Itself] More light gathering power for starters. The Canon lens has a aperture of f/5.6, while a [$2000 8 inch reflector](http://www.meade.com/lx90-acf/index.html) has an aperture of f/10. It is a faster lens than the telescope, by a factor of two. [/QUOTE] Actually you can get a 1200mm F/4.7 telescope for [$500](http://www.telescope.com/control/product/~category_id=reflectors/~pcategory=telescopes/~product_id=09959). In practical terms, the difference between this and the 1200mm Canon lens is the size and flatness of the field of view. A camera lens is designed to provide a sharp image for a 35mm film. That means it’s sharp right to the corner of a flat 35x24mm area. The $500 telescope will be just as sharp at the center of the image, but its image would be much worse in the corner. Also, most telescopes in this size (\>200 mm aperture) is a reflector, and that means the secondary mirror is sitting in the middle of the light path. So out-of-focus image of a light source will not be a pretty circle, instead it’s a donut shape. This isn’t an issue for astronomical observations, but can be very distracting for sports or wildlife photography. (Further discussion on the Wiki entry for [bokeh](http://en.wikipedia.org/wiki/Bokeh), including an example.)