Top lenses versus top telescopes for telephoto work

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.
The difference would be even more apparent using a Canon 1DsMkII with
its 16 megapixels. The Tak also supports 6x7 medium format. These
telescopes range in price from entry-level "semi-apos" costing $300 for
a 360mm f5.5 to over $4000 or more for a colour free 500mm-800mm f4-6
that can be "sped-up" via dedicated compressor lenses.

The reason these scopes have to be so good optically is that shooting
celestial objects with stars in the field is a severe test of optics.
Any residual off-axis aberrations show up in the star (point source)
images. Most of us have seen the flary, blurred out images at the edge
of a field of lights seen when night shots are taken with camera lenses
used wide open. These scopes can't allow that.

The reason these instruments haven't been made use of by more
photographers is that they don't operate (no IS, focus radically
different) like camera lenses. However, for wildlife photogs who use
blinds and wait for the opportunity to get good shots, they would be
ideal.

Interesting thing; In 1977, Modern Photography tested a 550mm f5.5
telescope put out by an American company called, "TeleVue." They put
it up against other long teles from Nikon, Canon, Leica, etc. It beat
them all. In fact, its MTF figures where so high they looked like those
from a macro lens. That telescope wouldn't compare to what is avalable
today from TeleVue or other makers of high end scopes.

The closest these instruments have come to integrating with the general
photographic community is though "Birders" who often use high-end
spotting scopes that they can attach their cameras. However,
"digiscoping" using P&S cameras and terrestial spotting scopes is not
the same as using a DSLR on a non-terrestrial telescope operating as a
"prime" lens.

The apochromatic telescope (otherwise known as an apochromatic
refractor) made its debut around the late 1970s with the introduction
of brands like the American Astro-Physics, Takahashi, Nikon and Pentax.
Nikon left the business some time ago and it's telescopes are highly
sought-after collector's items. Pentax is about to enter the American
market with its apos.
They make a 400mm f4 called the 100 SDUF II.

You will never see f2.8 or f2 apo telescopes, the need for diffraction
limited performance negates that possibility. But at f4-8, they
represent the pinnacle of imaging systems.

"Rich" writes:
IMO, a good apochromatic telescope while not possessing a fast f-ratio
will beat any camera lens when it comes to long tele work.

The reason these scopes have to be so good optically is that shooting
celestial objects with stars in the field is a severe test of optics.
Any residual off-axis aberrations show up in the star (point source)
images. Most of us have seen the flary, blurred out images at the edge
of a field of lights seen when night shots are taken with camera lenses
used wide open. These scopes can't allow that.

That's part of it. Another factor is that telescope objectives are
normally used as part of a system for visual observing, and the
magnification is changed by changing eyepieces. A typical good
refractor has a 2 inch focuser, and might be used with a very low-power
eyepiece (e.g. 55 mm FL) at about 10X magnification. At that power, it
is expected to produce an image nearly 2 inches in diameter with good
quality.

Or it may be used with a series of successively shorter focal length
eyepieces to give higher magnification. A 4 inch diameter objective is
capable of 200X useful magnification if the objective is diffraction
limited, and refractors are *expected* to be diffraction limited, at
least in the central portion of the image that would be visible at such
high power. This is somewhat comparable to finding a zoom lens that
performs well over a 20X zoom range.

The reason these instruments haven't been made use of by more
photographers is that they don't operate (no IS, focus radically
different) like camera lenses. However, for wildlife photogs who use
blinds and wait for the opportunity to get good shots, they would be
ideal.

I've used a TeleVue Oracle on a film camera. This is a 3 inch diameter
"semi apo" - not as good as the 4" TeleVue apo refractor referred to,
but also a lot cheaper. It was sharp, but it's also quite a pain to use
compared to a camera lens: (These apply to most astro telescopes used
as camera lenses)

1. Focusing is by rack and pinion, which is really too quick to get
precise focus with a camera. (It's fine for telescope use, where
your eye itself provides some degree of final fine focus).

2. There's no auto diaphragm. In fact, there's no diaphragm at all,
so you're always shooting wide open.

3. The optical tube is longer and more awkward than a camera lens
of comparable focal length.

Interesting thing; In 1977, Modern Photography tested a 550mm f5.5
telescope put out by an American company called, "TeleVue." They put
it up against other long teles from Nikon, Canon, Leica, etc. It beat
them all. In fact, its MTF figures where so high they looked like those
from a macro lens. That telescope wouldn't compare to what is avalable
today from TeleVue or other makers of high end scopes.

Dave

Dave Martindale wrote:
"Rich" writes:
IMO, a good apochromatic telescope while not possessing a fast f-ratio
will beat any camera lens when it comes to long tele work.

The reason these scopes have to be so good optically is that shooting
celestial objects with stars in the field is a severe test of optics.
Any residual off-axis aberrations show up in the star (point source)
images. Most of us have seen the flary, blurred out images at the edge
of a field of lights seen when night shots are taken with camera lenses
used wide open. These scopes can't allow that.

That's part of it. Another factor is that telescope objectives are
normally used as part of a system for visual observing, and the
magnification is changed by changing eyepieces. A typical good
refractor has a 2 inch focuser, and might be used with a very low-power
eyepiece (e.g. 55 mm FL) at about 10X magnification. At that power, it
is expected to produce an image nearly 2 inches in diameter with good
quality.

Or it may be used with a series of successively shorter focal length
eyepieces to give higher magnification. A 4 inch diameter objective is
capable of 200X useful magnification if the objective is diffraction
limited, and refractors are *expected* to be diffraction limited, at
least in the central portion of the image that would be visible at such
high power. This is somewhat comparable to finding a zoom lens that
performs well over a 20X zoom range.

The reason these instruments haven't been made use of by more
photographers is that they don't operate (no IS, focus radically
different) like camera lenses. However, for wildlife photogs who use
blinds and wait for the opportunity to get good shots, they would be
ideal.

I've used a TeleVue Oracle on a film camera. This is a 3 inch diameter
"semi apo" - not as good as the 4" TeleVue apo refractor referred to,
but also a lot cheaper. It was sharp, but it's also quite a pain to use
compared to a camera lens: (These apply to most astro telescopes used
as camera lenses)

1. Focusing is by rack and pinion, which is really too quick to get
precise focus with a camera. (It's fine for telescope use, where
your eye itself provides some degree of final fine focus).

The Oracle is a 16 year old telescope. Most apos now use two-speed
focusers for course and fine focus so that issue is resolved. But they
can't focus fast like a collar focusing lens or an AF lens.


2. There's no auto diaphragm. In fact, there's no diaphragm at all,
so you're always shooting wide open.

That is something they could implement, but most observers want full
aperture.

3. The optical tube is longer and more awkward than a camera lens
of comparable focal length.

Yes, these kinds of scopes don't incorporate (generally) elements
further down the tube to increase focal lengths, but generally rely on
the main lens (objective) to provide the focal length. Additionally,
few are below f5 (Pentax makes a 400mm f4 photographic model
and Takahashi makes a Sky 90 which is a 450mm f5). Both are fairly
compact.

There was a craze about 25 years ago for using telescopes with camera
adapters in place of a long telephoto lens. Two problems limited the
popularity and the craze died out.

First, as you say, relative aperture was limited, so exposures were
slower in low light. This was not too big a deal, as these things HAD
to be used with tripods anyway. Secondly, they did not have adjustable
apertures- exposures HAD to be set by varying shutter speed only.

This later problem meant that the lenses could not be used with
automatic exposure, which was getting popular at the time. While auto
focus was not around yet, AE was, and many folks had already come to
depend on it by then.



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.
The difference would be even more apparent using a Canon 1DsMkII with
its 16 megapixels. The Tak also supports 6x7 medium format. These
telescopes range in price from entry-level "semi-apos" costing $300 for
a 360mm f5.5 to over $4000 or more for a colour free 500mm-800mm f4-6

"Don Stauffer in Minnesota" writes:

Secondly, they did not have adjustable
apertures- exposures HAD to be set by varying shutter speed only.

This later problem meant that the lenses could not be used with
automatic exposure, which was getting popular at the time. While auto
focus was not around yet, AE was, and many folks had already come to
depend on it by then.

That depends on the camera. In aperture priority mode, you set the
aperture and the camera determines shutter speed from the light meter
reading and how much it thinks the lens will stop down (if the meter
reading is taken before exposure). To work with a lens that has no
diaphragm, you just have to convince the metering system that the
shooting aperture is the same as the viewing aperture.

I know my old Minolta X-570 could do it. I'm sure the X-700 would too,
since it was a superset of the X-570. These cameras actually metered
off the film after the lens had stopped down to taking aperture.

Dave

PS: the "meter at shooting aperture" behaviour once caused me some
puzzlement for a while. I once shot a bunch of photos at a world's
fair using a lens whose diaphragm was stuck open. I didn't figure this
out until well after the fair. The camera noticed the bright "stopped
down" image and used an actual shutter speed that was much higher than
what it indicated it was going to use, based on the aperture ring
position. In the end, most of the exposures were pretty good, not
grossly overexposed like you'd expect from a stuck diaphragm. But I was
shooting with far less DOF than I expected to have, and some subjects
ended up out of focus because of that. I didn't understand why until I
noticed the stuck aperture; the camera doesn't record the actual shutter
speed it used anywhere.

Dave