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I'm looking at an ad for a used Canon 1200mm f/5.6L EF USM Autofocus Lens where they are asking $180,000. Yes, that's not a typo.

I don't get it.

I'm an amateur astronomer and I know I can buy a killer 1200m refractor for well less than $180k. What makes this so expensive? I know that a good refractor only has a triplet (or a doublet) as the objective and a small, but complex, eyepiece. The Canon lens has 13 elements, 2 of which are flourite.

This isn't about this particular lens, and this question may well be better for an astronomy forum

But why are camera lens so much more expensive than refractor telescopes?

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  • Rarity. There were only approximately 20 of these now out of production lenses ever made. When they were in production they sold for about $90,000 (US). Due to the time needed to grow the large fluorite crystal used in the 3rd element of the lens, once ordered they took about 18 months to produce.

  • Autofocus Capability. These lenses include auto focus capability. Moving focus elements as large and heavy as this requires mechanisms that are both robust and extremely precise. Moving them fast enough to be used to photograph sporting events means they must also be very powerful.

  • Maximum Aperture The EF 1200mm f/5.6 L requires an entrance pupil of 214mm (8.4 inches). A 1200mm f/8 telescope needs only a 150mm wide objective. When you compare the areas of a 214mm circle to that of a 150mm circle, you see that it takes at least twice as much material to create a lens 214mm wide compared to 150mm wide. And that is before you consider that the larger lens element must also be thicker in the center to maintain the same amount of curvature on the surfaces.

  • Optical Image Quality While things like coma and chromatic aberration are expected at the edges of the field of view of a large refracting telescope, they are not as acceptable in a camera lens. And the larger the diameter of a lens is, the more correction must be applied to obtain the same image quality. This requires more elements in the lens, and these additional elements are almost always made of exotic materials with a higher refractive index and lower mass density than normal optical glass. They must also be precisely shaped to almost insane tolerances to perform as designed.

  • Image Circle Diameter A telescope need only to cast an image circle the size of a human eye's pupil: approximately 8mm in diameter. A camera lens intended for use with a Full frame camera must cast an image circle approximately 44mm in diameter.

  • Minimum Focus Distance Many telescopes are designed to focus only at longer distances, some even only at infinity. Camera lenses such as the EF 1200mm f/5.6 L can focus at infinity but are expected to focus at shorter distances as well.

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    \$\begingroup\$ Why then not use a reflecting telescope instead of such a camera lens? \$\endgroup\$
    – Count Iblis
    Apr 28, 2015 at 2:27
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    \$\begingroup\$ @CountIblis Those exist too. They're known as reflex lenses. There's everything from a not uncommon 500mm f/8 which is under $100, to the Nikkor 2000mm f/11 which makes you wonder if you should call it a telescope or a camera lens. \$\endgroup\$
    – user13451
    Apr 28, 2015 at 2:45
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    \$\begingroup\$ An important difference that you can get into the optics bit - the block diagram for the 1200mm f/5.6 shows 13 elements in 10 groups, two of which are fluorite. A 1200mm f/6 telescope has 2 elements. That is a significant difference in the amount of glass. \$\endgroup\$
    – user13451
    Apr 28, 2015 at 2:55
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    \$\begingroup\$ The reason for the difference in number of elements is that a telescope is designed only to focus at infinity, while a camera lens has to be able to focus at a range of distances. \$\endgroup\$
    – 2012rcampion
    Apr 28, 2015 at 3:26
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    \$\begingroup\$ Don't forget things like coatings to increase transmissivity etc. and preventing to get light into the wrong paths. Telescopes are used in the dark where light from the wrong direction is no problem, and you can easily compensate the bad light performance of beginner telescopes by longer exposures. \$\endgroup\$
    – PlasmaHH
    Apr 28, 2015 at 8:56
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Michael Clark missed one important point in his otherwise correct answer:

  • Physical vs. Optical length Think about what the 1200mm really mean. This is the focal length. This normally means that the distance from the center of the lenses to the place where the image forms is 1200mm. (Yes 1.2 meters!) On your telescope I guess that you pretty much see these 1200 millimeters as the telescope is around 1.2 meters in length. On your Canon you're probably not carrying a lens that is 1.2 meters long. The lens in question is 83.6 centimeters. This significant shortening of the physical dimensions can be achieved by optical elements, most probably the ones with fluorite.
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  • \$\begingroup\$ I think that the Zeiss 1700mm f/4 for medium format (?!?! nice image circle though) may be nearly 2 meters long. Its also a bit more expensive than the 1200mm f/5.6. \$\endgroup\$
    – user13451
    Apr 28, 2015 at 20:15
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    \$\begingroup\$ You don't need fluorite to make a lens shorter than it's focal length (telephoto), there are many telephotos at much lower price points. Fluorite reduces chromatic aberrations. \$\endgroup\$
    – Matt Grum
    Apr 29, 2015 at 8:36
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    \$\begingroup\$ @MattGrum, I half-way agree on your first point, you don't need fluorite to shorten your lenses. It's not primarily the price and fluorite will not "magically" reduce chromatic aberration. For both, the length of your lens and chromatic correction (and a bunch of other properties) you need a very careful and balanced selection of different glass materials. Fluorite just adds another option in that equation. "Photography is always the search for the best tradeoff." \$\endgroup\$
    – user23573
    Apr 29, 2015 at 9:09
  • \$\begingroup\$ @MichaelT, Unfortunately there are no physical dimensions for this monster available. But interpolating from the images I've found and looking at the images from exhibitions, I would assume that the whole thing is just about 1.3 meters long. So, there is some optical shortening done on that one. Additionally this lens is for medium format which is usually quite a different caliber when it comes to the price. \$\endgroup\$
    – user23573
    Apr 29, 2015 at 9:19
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    \$\begingroup\$ @BogdanWilli "fluorite will not "magically" reduce chromatic aberration" actually it will, but it's not magic - it's physics, fluorite is a low dispersion material which means different frequencies of light spread out less when passing through, so it will reduce chromatic aberrations compared to a similar element of standard optical glass. \$\endgroup\$
    – Matt Grum
    Apr 29, 2015 at 9:35
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Astronomers use reflectors mostly but are indeed interested in accuracy.

The lens in question is much faster than a telescope.

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  • \$\begingroup\$ I can buy a 10" f3.9 reflector for under $1,000. \$\endgroup\$
    – Paul Cezanne
    Apr 29, 2015 at 9:48
  • \$\begingroup\$ The f-number used to describe a telescope doesn't describe the same thing the f-number used to describe a camera lens does. With a telescope it is an indication of maximum field of view. With a camera lens it is an indication of maximum light gathering capability. For the telescope, the light gathering capability is indicated by the diameter of the objective lens. \$\endgroup\$
    – Michael C
    Nov 16, 2015 at 11:11
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Different tools for different jobs. A telescope is for looking at objects with the naked eye, a camera lens is for recording high resolution images.

It is acceptable for a telescope to have large distortions around the edges and nobody much cares about colour correctness. This makes it simple (cheap) to make the lenses which are good enough to do the job. Now add the sheer size of the optics required to get an f-stop of 5.6 from a 1200mm focus length.

---- edit to correct the edits ---

A 1200mm telescope is for looking at Stars, a 1200mm f/5.6 lens is for looking at Celebrities. If you expect to sell a picture of Venus then you really don't want it to look like a picture of Jupiter. If you expect to sell it for over $30,000 then it had better look like Venus.

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  • \$\begingroup\$ @NickM, I know you thought you were trying to help, but you have it utterly backwards. Viewing stars requires the ability to detect the presence or absence of light, nothing more. That is why they can be use much lower quality optics then lens used to view people. The naked eye is the most descerning optical instrument we know of, not the least \$\endgroup\$
    – Paul Smith
    Apr 28, 2015 at 20:42
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    \$\begingroup\$ I disagree completely. Astronomers are very concerned about distortions, and to some extent color correctness. \$\endgroup\$
    – Paul Cezanne
    Apr 28, 2015 at 22:41
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    \$\begingroup\$ The 1200mm refractor telescope is that of a backyard astronomer rather than a professional. The backyard astronomer is much less concerned about these issues than a professional... and a reflector has much less (to no) issue with distortion and chromatic aberration (a mirror reflects all frequencies of light (that it reflects) equally). There really aren't that many refractors in professional use anymore. \$\endgroup\$
    – user13451
    Apr 29, 2015 at 13:34
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    \$\begingroup\$ GAAAAAH! Absolutely not! There are plenty of amateur astronomers out there willing to pay $10,000 for their OTAs alone, plus accessories like application-specific cameras (which can run for nearly as much, if not more), computers, and software for image processing. On top of that, there are at least a dozen pro-am astronomers in the world who sell their photos to fund their hobby (Jack Newton comes immediately to mind, look him up). Your assertion is born of ignorance, and is paramount to saying that photographers never need to use anything more expensive than a cellphone camera. \$\endgroup\$
    – Ernie
    Apr 29, 2015 at 18:11
  • \$\begingroup\$ @Ernie - It is not clear who you are railing against, so the charge of ignorance becomes difficult. Also no one has suggested photographers do not require quality? Perhaps you have misinterpreted what I said to imply that I think astronomers do not require quality, but I didn't say that. I just said that the quality they require, and are happy to pay a lot for, does not compare to that required of celebrity publications. Jack Newtons pictures are pretty, but do not compare to Hubbles, do they? \$\endgroup\$
    – Paul Smith
    May 7, 2015 at 16:46
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Part of the reason is doubtless that professional photographers earn more money than professional astronomers, and so can be milked a bit more for their cash.

Score: +5 Funny

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  • \$\begingroup\$ I very much doubt that. Optics is complicated. It's more to the effect that the price tag of this item reflects the high costs of research and the small number of people who have any need or desire for it. This increases the price because there is no economy of scale for mass production. \$\endgroup\$
    – Ernie
    Apr 29, 2015 at 18:03
  • \$\begingroup\$ I used to be a professional photographer. I can tell from my personal experience that this absolutely not the case. It's the same short-circuited assumption as the following: All footballers are incredibly rich, see what Ronaldo earns! \$\endgroup\$
    – user23573
    Apr 30, 2015 at 6:32
  • \$\begingroup\$ @BogdanWilli It's funny because (a) it's true, professional astronomers don't get paid, and (b) geez, lighten up, it's a joke. \$\endgroup\$
    – Nicholas Shanks
    Apr 30, 2015 at 10:38
  • \$\begingroup\$ @Ernie You have too much serious in your reply. \$\endgroup\$
    – Nicholas Shanks
    Apr 30, 2015 at 10:39
  • \$\begingroup\$ @Nicholas See it this way: It's not so funny for me, it's the reason why I am no more a professional photographer. A profession I really liked. Nevertheless I've at least tried to reply with a joke. \$\endgroup\$
    – user23573
    Apr 30, 2015 at 12:29

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