[Kavan]

Hello Forum,

 

I am new to photography but willing to learn. I have several questions/discussions I would like to start.

 

For now, let me pose this question:

 

What is the difference between a telescope and a telephoto optical system? Both are used to get enlarged images of a scene.

 

I know a telescope is an afocal system when it is used to view distant object but is not afocal when viewing nearby objects.

 

Telephoto lenses provide an enlarged image and a narrow field of view.

 

Thanks,

Kavan

[popo]

As a history lesson you can look up elsewhere for more detail, telephoto started off as a lens configuration which gave a physical size less than that of the focal length. This gives a size benefit. In modern common usage it generally refers to any lens of longer focal lengths, even if the size criteria might not be met.

 

Telescopes come in several classes.

1, Refractors - These use refracting optical elements just like most camera lenses. Cheaper telescopes may be as simple as 2 elements (not including eyepiece) to provide a degree of colour correction. And just like lenses, the more elements you use, the better the correction can be, if the design and construction are up to it. Astronomical ones can go as fast as about f/4 ball park, although most are slower.

2, Reflectors - these use mirrors which have a benefit of not introducing any colour problems. In typical designs there will be some obstruction in the light path from needing a secondary mirror, which can reduce contrast a bit. For bigger aperture sizes, this is the cheapest way to "go large". Again, astronomical ones can go as fast as about f/4 ball park, although most are slower.

3, catadioptric  - these use a combination of mirrors and refracting elements. The more affordable astronomical scopes in this class tend to be Maksutov–Cassegrains for smaller apertures, and Schmidt–Cassegrain for bigger ones, but there are a lot of other types too. These two have a common characteristic of having relatively long focal lengths in a short body, but slower focal ratios. For an example of each, I have a Celestron NexStar 4SE (4" aperture) which is 1325mm f/13, and I have a Meade SCT 200mm aperture, or 2000mm f/10. But again because of the central obstruction, the contrast isn't as good as a refractor.

 

Putting aside the construction differences, if you can work out the focal length and focal ratio, it will do pretty much the same as a telephoto lens of the same specification. There are some "mirror lenses" in the photography world too, of which only Sony made one with AF. These also show another feature of reflectors and catadioptric systems, which is the "donut bokeh" caused from the centre obstruction.

[davidmanze]

  Just to add a couple of points to the user side of "refractor" type telescope lenses; a lot of of nature photographers use telescope lenses basically because of their lower price point, eg. a 560mm F7.1 lens can cost in the region of $500-600. Usually they come with a "T" mount for which you buy the appropriate adapter for your make of camera. 

   Normally they are "all manual fixed aperture" and use a simple form of "rack and pinion" manual focus system, as such they're mostly suitable for photography in "hides" on a tripod where the shooter can wait unseen and has the time to focus and catch his shot. In spite of their simple construction (often only three elements) they are very sharp lenses and light, but their size (length) is about equivalent to their focal length.  Modern telephoto lenses have a built in tele-converter to keep the lens compact which is why telescope lenses can get away a much more simple construction.

 The "bokeh" is fine compared with mirror/reflector counterparts, which often result in awful and very distracting "donut highlights" as well as softer images and low contrast which pretty much counts them out for nature photography.  

  I have a semi-pro friend who uses one and he gets the most remarkable results out out of it, he actually prefers the fact that he does all manually!

 However, with the recent lenses from Tamron/Sigma 150-600mm producing very good results for around $1,000 with variable aperture, image stabilization and fast auto-focus,  unless you find a very cheap S/H telescope example suitable for DSLR photography, it is difficult to find a good reason to try one!

[Brightcolours]

Of the DSLR camera manufacturers, only Pentax offers a telescope lens, a 560mm f5.6. It does AF, is expensive, and not the greatest performing lens one can imagine. And pretty expensive.

[davidmanze]

Of the DSLR camera manufacturers, only Pentax offers a telescope lens, a 560mm f5.6. It does AF, is expensive, and not the greatest performing lens one can imagine. And pretty expensive.

The lens was only tested by PentaxForums in their early days, surprisingly user results are filtering through showing in fact that it "is" a very sharp lens indeed, some bemoaning it's unfortunate review, even it's price is now down to a reasonable level sometimes at  $4,000, not out of the way considering.

   It's ludicrously unwieldy size unfortunately, remains unchanged!

[Brightcolours]

It has strong CA like old fashioned long focal length lenses used to have. It is an oddity, this lens Pentax. There has been a lot of progress since the days that long focal length lenses were telescope types, for photography...

[davidmanze]

I've seen nothing from forums or the test from Pentax Forum to indicate "strong CAs",  Pentax Forums did say however say it had "a little more than we would expect from FF lens", which does nothing to indicate "strong", ?................where are you getting your information about this little sold lens?.....even then moderate CAs are hardly the end of the world theses days.

  At it's original price point there was little to defend it's performance but at it's current price some of it's smaller sins can be pardoned a little!

[popo]

The "bokeh" is fine compared with mirror/reflector counterparts, which often result in awful and very distracting "donut highlights" as well as softer images and low contrast which pretty much counts them out for nature photography.  

 However, with the recent lenses from Tamron/Sigma 150-600mm producing very good results for around $1,000 with variable aperture, image stabilization and fast auto-focus,  unless you find a very cheap S/H telescope example suitable for DSLR photography, it is difficult to find a good reason to try one!

The contrast reduction for obstructed designs probably isn't significant for normal photography. It is only of concern if you're trying to pick out faint details for astronomical uses.

Also, the value of picking a telescope over a photographic lens really depends on the application. Up to 600mm for so, if you don't need a fast aperture, then photographic lenses are hard to beat. It is once you reach longer focal lengths that telescope designs really take the value crown, but of course their use and handling will be different. I've tried imaging at 6m focal length before, and that's tough at my technology level. There isn't really any gain from 4m, which does give a big difference from 2m.
 

Of the DSLR camera manufacturers, only Pentax offers a telescope lens, a 560mm f5.6. It does AF, is expensive, and not the greatest performing lens one can imagine. And pretty expensive.

What makes that a "telescope lens"? Ok, in a quick look, the length + flange distance would suggest it isn't telephoto design, but regardless I'd still consider it a lens.

[Kavan]

Thanks to everyone. 

 

 

So, in essence, if the subject to be imaged is 100 meters away, portability of the system is not an issue and all we want is an enlarged image, we could use either a telephoto lens system or a telescope. But there are many other factors involved (aberrations, f-ratio, etc.)

 

Kavan

 

 

 

 

[Brightcolours]

 What makes that a "telescope lens"? Ok, in a quick look, the length + flange distance would suggest it isn't telephoto design, but regardless I'd still consider it a lens.

It not being a telephoto design makes it a long-focus lens, but not a telelens. I guess it is not done to call these telescope lenses