(Not mine) Bird shot through inexpensive ED telescope.

On Monday, March 16, 2020 at 8:59:51 AM UTC-4, Whisky-dave wrote:
On Sunday, 15 March 2020 00:29:42 UTC, RichA wrote:
The bird shot from Dpreview was shot with a Celestron 80mm
f/7.5 600mm telescope. I'm impressed what these scopes can
do terrestrially. Comes from having diffraction-limited
optics, only 2-3 elements and long reach. By "inexpensive"
I mean something in the $600-$800 range.

https://2.img-dpreview.com/files/p/E~forums/63743338/1e3f03dfe2734e52bd90500fd65dda20

I guess the only downside is that they are relatively bulky
and that the images from telescopes are usualy inverted for
an astronomical telescope making the subject harder to keep in frame.

They generally tend to have a higher f/stop, but in this
digital era of high ISO's, that's become a bit less important.



-hh

Whisky-dave wrote:
On Monday, 16 March 2020 21:09:20 UTC, -hh wrote:
On Monday, March 16, 2020 at 8:59:51 AM UTC-4, Whisky-dave wrote:
On Sunday, 15 March 2020 00:29:42 UTC, RichA wrote:
The bird shot from Dpreview was shot with a Celestron 80mm
f/7.5 600mm telescope. I'm impressed what these scopes can
do terrestrially. Comes from having diffraction-limited
optics, only 2-3 elements and long reach. By "inexpensive"
I mean something in the $600-$800 range.

https://2.img-dpreview.com/files/p/E~forums/63743338/1e3f03dfe2734e52bd90500fd65dda20

I guess the only downside is that they are relatively bulky
and that the images from telescopes are usualy inverted for
an astronomical telescope making the subject harder to keep in frame.

They generally tend to have a higher f/stop,

higher or lower terminology is confusing is f11 higher or lower than f16 ?


F/11 is wider open, or more open than f/16. Just as f/4 will be wider, or
more open than f/11.
This will effect the depth of DoF and increase the possibility of
diffraction with narrower apertures say from f/16-f/22.
As far as the telescope's effective f/stop is concerned, that will depend
on several factors including type of glass, and objective diameter.

but in this
digital era of high ISO's, that's become a bit less important.

Yes and most astronomical telescopes invert the image as in the north
becomes south and east west, this doesn't really matter to those viewing
but it makes framing far less intuative, I know I 've tried.
Adding the optics to correct this makes a lens more expensive and heavy
and can lower image quality.


....but there are terrestrial field-scopes which do not invert the subject,
and do very well for photography with the appropriate adaptor.


-hh





--
Regards,
Savageduck

On Tuesday, March 17, 2020 at 7:53:13 AM UTC-4, Savageduck wrote:
Whisky-dave wrote:
On Monday, 16 March 2020 21:09:20 UTC, -hh wrote:
[snip]

They generally tend to have a higher f/stop,

higher or lower terminology is confusing is f11 higher
or lower than f16 ?

F/11 is wider open, or more open than f/16. Just as f/4
will be wider, or more open than f/11.

Yes, sorry for the terminology confusion. In this case,
"higher f/stop" was referring to the number, with the
reported f/7.5 being "higher" than the more typical
camera telephoto lens max apertures of f/5.6 or f/4,
which indicates a physically smaller aperture and thus
what we would call a "slower" lens.

This will effect the depth of DoF and increase the possibility
of diffraction with narrower apertures say from f/16-f/22.

A 'higher' f/stop will increase the DoF which can be desirable,
but of course when one desires a shallow DoF (and a nice bokeh),
a lens not capable of opening up wide enough is a handicap.
All part of the overall trade-off (features/cost/weight/etc).

but in this digital era of high ISO's,
that's become a bit less important.

Yes and most astronomical telescopes invert the image...

...but there are terrestrial field-scopes which do not invert
the subject, and do very well for photography with the
appropriate adaptor.

Also known as "Spotting scopes"...not uncommon in long range rifle shooting.


-hh

-hh wrote:
On Tuesday, March 17, 2020 at 7:53:13 AM UTC-4, Savageduck wrote:
Whisky-dave wrote:
On Monday, 16 March 2020 21:09:20 UTC, -hh wrote:
[snip]

They generally tend to have a higher f/stop,

higher or lower terminology is confusing is f11 higher
or lower than f16 ?

F/11 is wider open, or more open than f/16. Just as f/4
will be wider, or more open than f/11.

Yes, sorry for the terminology confusion. In this case,
"higher f/stop" was referring to the number, with the
reported f/7.5 being "higher" than the more typical
camera telephoto lens max apertures of f/5.6 or f/4,
which indicates a physically smaller aperture and thus
what we would call a "slower" lens.

This will effect the depth of DoF and increase the possibility
of diffraction with narrower apertures say from f/16-f/22.

A 'higher' f/stop will increase the DoF which can be desirable,
but of course when one desires a shallow DoF (and a nice bokeh),
a lens not capable of opening up wide enough is a handicap.
All part of the overall trade-off (features/cost/weight/etc).

but in this digital era of high ISO's,
that's become a bit less important.

Yes and most astronomical telescopes invert the image...

...but there are terrestrial field-scopes which do not invert
the subject, and do very well for photography with the
appropriate adaptor.

Also known as "Spotting scopes"...not uncommon in long range rifle shooting.


Yup! I have two from my target shooting days a great Bausch & Lomb, and a
Kowa. What I don't have are the appropriate "T" adaptors, but those can be
bought.

--
Regards,
Savageduck

On Tuesday, March 17, 2020 at 10:20:47 AM UTC-4, Whisky-dave wrote:
On Tuesday, 17 March 2020 11:53:13 UTC, Savageduck wrote:

F/11 is wider open, or more open than f/16. Just as f/4
will be wider, or more open than f/11.

Yes, lets in twice as much light is another and better way
of looking at it. So f11 gives you twice the light as f16

But which generally has a higher f/stop, I'd say a
photographic lens in most cases. Where higher is defined
as a number rather than the amount of light let through the
diaphram. So higher number means less light.

IMO, it depends on what you mean when you ask 'which is higher'
because the camera lens typically contains an adjustable iris
within, so it has the potential for both higher and lower f/stops.


Do atronomical telescopes have adjustable aperatures ?

No, not that I've ever seen in the hobbyist class stuff.


As such, the comparison is along the lines of:

Scope:
Celestron 80mm f/7.5 600mm telescope
Maximum Apertu f/7.5
Minimum Apertu f/7.5

Camera:
Canon EF 600mm f/4L IS
Maximum Apertu f/4
Minimum Apertu f/32


What I've generally seen is that the astronomical lenses
tend to have a higher Maximum … for which we would call
it a "slower" lens. And because they're also typically
not adjustable, the variables for exposure control drop
from three to just two: shutter speed & ISO settings.


-hh

On 17/03/2020 13:36, Savageduck wrote:

Yup! I have two from my target shooting days a great Bausch & Lomb, and a
Kowa. What I don't have are the appropriate "T" adaptors, but those can be
bought.

B&L were my preferred binocular microscope optics in my "lab" days.

In article , -hh
wrote:

A 'higher' f/stop will increase the DoF which can be desirable,
but of course when one desires a shallow DoF (and a nice bokeh),
a lens not capable of opening up wide enough is a handicap.
All part of the overall trade-off (features/cost/weight/etc).


For a normal camera lens that has a choice of apertures, yes. But when
using a telescope as a lens, the aperture is set by the exit pupil and
cannot be changed. On these lenses, DoF is extremely narrow, often
measured in inches or fractions of an inch depending on distance.

--
m-m
www.mhmyers.com