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Top lenses versus top telescopes for telephoto work

 
 
Rich
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      12-29-2006

Roger N. Clark (change username to rnclark) wrote:
> Rich wrote:
>
> > Roger N. Clark (change username to rnclark) wrote:
> >
> >>Rich wrote:
> >> > IMO, a good apochromatic telescope while not possessing a fast f-ratio
> >> > will beat any camera lens when it comes to long tele work. For
> >> > instance, compare a Takahashi FSQ106 telescope against a 500mm Nikon
> >> > both operating around f5, the Takahashi scope shooting wide open, it
> >> > will easily beat the Nikon using a high resolution digital camera body.
> >>
> >>While I agree that telescopes can beat out telephoto lenses,
> >>they do so only at near infinity focus. Camera lenses are
> >>a compromise in giving good image quality over a larger range
> >>of subject distances. A apochromatic telescope will not do as well
> >>on a closer frame filling subject, like birds.

> >
> > In a situation like that, what you can do is stop down the scope
> > slightly to eliminate an
> > additional spherical aberration that might be encountered due to the
> > less than infinity focus.

>
> But stopping down is a problem because you lose light and then
> can not capture action. The point of the super telephotos
> is to provide great image quality wide open over a large
> range of distances.


True, in terms of speed, there are no telescopes to match a fast
telephoto (f2.8, etc).
>
> > But also, this issue might not be a major one for a small (80-100mm
> > aperture scope) and then only at very short distances. Questar
> > Corporation used to supply a 1/2" wide diaphram to stop down their 90mm
> > wide mirror-lens scopes when using them under object distances of 25
> > feet or less.

>
> But Questars were optically too slow for wildlife action photography.


And yet people did use them for that. Questar's Field Model, they even
had one with
electric high speed focusing. But, were talking 1350mm at a minimum
for the 90mm model
so that is a long tele. However, Questar's 700mm mirror camera lens
was the best mirror lens of all time, in terms of definitiion and its
speed was f8 and it wasn't a telescope.

> >
> > Then, in real
> >
> >>world situations, the manual focus, fixed aperture telescopes
> >>would give poor overall performance "to get the image" than
> >>modern autofocus lenses. And even so, the super telephoto
> >>lenses are VERY close in quality to apochromatic telescopes, and
> >>I bet it would be hard to tell the difference of say a FSQ106
> >>versus a Canon 500 mm f/4 L at f/5.6. For example, look at the star
> >>images on this photo of M31 taken with a Canon 500 mm f/4 at f/4:
> >>http://www.clarkvision.com/galleries...-v1.6-700.html
> >>At f/5.6, star images of most of the field are barely over on pixel,
> >>being limited by the blur filter of the camera.

> >
> > Good image, because these lenses are used only at prime focus, and are
> > close enough to diffraction limited to avoid any visible issues at that
> > kind of magnification. But, unlike the camera lens, the telescope's
> > focal length can be increased significantly without any noticeable loss
> > of correction, contrast, etc. In-case the target needed this.
> > Downside being the increase in focal ratio (speed) of course.

>
> Super telephotos are designed to still give good image quality
> with teleconverters. Here is a star field with a 500 f/4
> and 1.4x TC:
> http://www.clarkvision.com/galleries...27-v3-800.html
> The stars a still very good.


At 700mm that is pretty decent.

>
> > Yes, I can't see someone hauling a Newtonian scope into the field to
> > shoot birds, but it would make for an interesting conversation amongst
> > the others there!
> > But you can buy a refractor from TeleVue or TMB with an f5 focal ratio
> > and 500mm focal length that would make an interesing comparison.
> > http://www.televue.com/engine/page.asp?ID=297

>
> I have compared image quality of some of the apochromatic telescopes
> with my 500 f/4. The ones that have great performance, comparable
> to the 500 f/4 L telephoto, are, guess what, as expensive as the
> 500 mm f/4 lens.


Apochromatic telescopes are generally considered on a "price per inch
of aperture" basis.
So, a typical high-end one costs over $1000 per inch. A TeleVue 500mm
f5 is about $3800.00. Some cost more, some less. TMB now has a 130mm
aperture "signature" series that costs about $3800 for a 780mm f6 scope
with superb optics, around $750/inch.
A bargain today. Pentax's SDUF costs $3000 for a 400mm f4 lens and it
will support a medium format film size. Inexpensive apos (from China)
run about $100-$400 inch depending on the scope size.

You get what you pay for. The reason I looked
> into this is that I have one undesirable problem with the
> 500 f/4: the aluminum tube changes length during long night
> exposures (e.g. over 20 minutes). A PLASTIC TUBE WOULD BE BETTER!


Not really, the plastic would actually contract more as it got cold, it
would just take alot longer to do it. In other words, it would
probably not even stabilize during the night.

> But for wildlife photography in rough environments, the
> metal tube is great.


Alot of photogs seem to tape or wrap their lenses. PBS had a guy on
shooting Yellowstone in winter. He took his Nikon 500mm and stuck it
face down in the snow(!) while he used another lens on his camera!
But, the lens cap was on it.

 
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