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Right now we have multiple interchangeable lenses. So why don't we have a single lens, say a 18-200 f/1.2 lens? Are camera manufacturers deliberately avoiding that or it is that it is physically impossible. To my knowledge creating a f/1.2 lens only means a wider aperture which is independent of lens elements. What are the fastest zoom lenses you know?

There are some fast lenses but they are expensive. e.g. Sigma 137101 120-300mm F2.8 DG OS HSM Lens for Canon is ~3500USD. What makes F/2.8 lens more expensive than a F/6.0 lens?

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It is pretty much impossible to build a lens with that wide of an aperture that is also so long. As mentioned, at 200mm the lens opening is 166mm at f1.2. This would be a physically immense lens, even more so a fi it's a zoom.

If we restrict ourselves to an aperture that's realistic (say f2.8), the only issue is that such a lens will need to be very, very big and expensive. These lenses exists, and are mostly bought by broadcasting companies for covering sports.

An example is the Fujinon 25-300mm f2.8, which weighs 10kg (23 lbs) and will cost you $44,000.

Holding the 25-300mm f2.8, from lens rentals blog

Or even more extreme why not a 9-900mm f1.7-4.7, for a cool $100,000. These lenses are however NOT sharp. They are made for video, which is why they can have such a large range, the latter lens for example only resolves 2MP.

If you also want the sharpness of say, a Canon 200mm f2 prime on your zoom lens, it would need to be even larger. The Sigma 200-500mm f2.8 is an example of a lens made for still photography with both zoom & large aperture. Note that it needs to be even larger than the Fujinon while having a much smaller range of zoom to retain acceptable sharpness for stills.

Handholding the Sigma 200-500mm

You can read more about the 25-300 at lensrentals.

  • Are you confusing entry and exit pupils? You can buy a 600mm f/4 today and that has a 150mm opening, so 166mm is not a stretch. Lens mounts have nothing to do with it. There's nothing "physically impossible" about an 18-200 f/1.2 at all, it would simply by very, very heavy and expensive. – Matt Grum Jul 30 '15 at 13:56
  • You are correct, I'm confused. I corrected my answer. – Hampus Nilsson Jul 30 '15 at 14:00
  • @MattGrum It may be impossible to design an f/1.2 lens depending on the focal length. Eventually the marginal ray angle will exceed that which is possible with the available materials in order to stuff the exit pupil through the lens mount's throat. – Brandon Dube Aug 2 '15 at 1:33
  • @BrandonDube Do you know how the Zeiss 1700mm f/4 is able to cope with the huge exit pupil and H-mount throat? Also do you have any idea what would happen if you took Sigma's 500mm f/2.8 and mounted a 2x focal reducer? Seems like that would get you to 250mm f/1.4 albeit with a much smaller image circle. – Matt Grum Aug 2 '15 at 10:11
  • @MattGrum It is made for the Hasselblad V mount with nearly double the mount diameter available. It is also only f/4, the marginal ray angle is quite mild. – Brandon Dube Aug 2 '15 at 18:06
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The "only" is one of the problems. A 200mm f/1.2 lens needs an aperture of 166,67mm. That would be a huge, heavy lens! Look at this picture of a 200mm/2.0 prime(!):

canon 200mm/2.0

A large aperture means you need larger glass, too, and grinding large lenses precisely isn't easy.

Super zoom lenses are difficult to build and are generally of inferior quality to prime lenses. It's simply an engineering challenge to build a lens which is sharp, without any distortions, etc. at all lengths.

The lens you're imagining would be, if of good quality, extremely expensive and huge and heavy.

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The fastest zoom lens right now is the Sigma 18-35mm f/1.8

Building a 18-200mm f/1.2 might be possible but no one would buy it because it would probably weigh something like 35lbs and cost $35,000 or more.

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