Techie question

I have a Meade DSI set up with a DSI wide adapter and a Tamron lens. I have
taken several pleasing photo's with this using Meade's Autostar DSI program
ranging from short multiple exposures to long timed exposures both guided
and star trail effect.
My problem is that I can only find focus at one position on the lens and
would like to know why.
The lens and setup are as follows:
Tamron 35-70mm 1:3.5 64'-34' ( ' means degrees!!)
It will only focus at approximately the 55mm setting and focused to 0.4
meters.
This lens works perfectly at all ranges on my SLR.

Any ideas please.

Not enough backfocus?

Andrea T.

Sorry I am not that technical! What is back focus?
wrote in message
oups.com...
Not enough backfocus?

Andrea T.

"Tony Pagett" wrote in message
o.uk...
I have a Meade DSI set up with a DSI wide adapter and a Tamron lens. I
have
taken several pleasing photo's with this using Meade's Autostar DSI
program
ranging from short multiple exposures to long timed exposures both guided
and star trail effect.
My problem is that I can only find focus at one position on the lens and
would like to know why.
The lens and setup are as follows:
Tamron 35-70mm 1:3.5 64'-34' ( ' means degrees!!)
It will only focus at approximately the 55mm setting and focused to 0.4
meters.
This lens works perfectly at all ranges on my SLR.

Any ideas please.


It sounds like the distance from the rear flange of the lens to the CCD chip
is not correct (it needs to be the same as it would be with the lens mounted
on your SLR camera. From your description, it sounds like it could be too
close) Is this adjustable with the adapter you have? Are you using the
correct couplings that are recommended with the adapter (a T mount ring for
example?)

Robin

Yes I purchased the DSI wide and adapter as a package. It has an Olympus
mount and the Tamron lens has the Adaptall 2 Olympus adapter.
"Robin Leadbeater" wrote in message
...

"Tony Pagett" wrote in message
o.uk...
I have a Meade DSI set up with a DSI wide adapter and a Tamron lens. I
have
taken several pleasing photo's with this using Meade's Autostar DSI
program
ranging from short multiple exposures to long timed exposures both guided
and star trail effect.
My problem is that I can only find focus at one position on the lens and
would like to know why.
The lens and setup are as follows:
Tamron 35-70mm 1:3.5 64'-34' ( ' means degrees!!)
It will only focus at approximately the 55mm setting and focused to 0.4
meters.
This lens works perfectly at all ranges on my SLR.

Any ideas please.


It sounds like the distance from the rear flange of the lens to the CCD
chip
is not correct (it needs to be the same as it would be with the lens
mounted
on your SLR camera. From your description, it sounds like it could be too
close) Is this adjustable with the adapter you have? Are you using the
correct couplings that are recommended with the adapter (a T mount ring
for
example?)

Robin

"Tony Pagett" wrote in message
.uk...
Yes I purchased the DSI wide and adapter as a package. It has an Olympus
mount and the Tamron lens has the Adaptall 2 Olympus adapter.

Hi Tony,

You can see a list of correct flange to film spacings (register) for various
cameras here.
http://www.a1.nl/phomepag/markerink/mounts.htm
How do the measurements on your set up compare? eg according to this, if it
is an Olympus OM mount the CCD should be 46mm from the lens mounting flange.

Robin

Hello Robin.

Thank you for your help.

I have just measured the register as you say and if I am measuring to the
correct place then it is only 41mm.

"Robin Leadbeater" wrote in message
...

"Tony Pagett" wrote in message
.uk...
Yes I purchased the DSI wide and adapter as a package. It has an Olympus
mount and the Tamron lens has the Adaptall 2 Olympus adapter.

Hi Tony,

You can see a list of correct flange to film spacings (register) for
various
cameras here.
http://www.a1.nl/phomepag/markerink/mounts.htm
How do the measurements on your set up compare? eg according to this, if
it
is an Olympus OM mount the CCD should be 46mm from the lens mounting
flange.

Robin

"Tony Pagett" wrote in message
.uk...
Hello Robin.

Thank you for your help.

I have just measured the register as you say and if I am measuring to the
correct place then it is only 41mm.

I just did some experimenting with my Canon EOS 35-80mm lens. It looks like
the correct spacing could be very critical. Small errors throw the focus out
significantly across the zoom range. While you would get away with being a
couple of mm too close with a fixed focus lens, this may not be the case
with a zoom.

Robin

What I don't understand is that if I can focus at one setting of the lens
then why not at all settings. Surely the focus point should be at the same
place regardless of zoom otherwise I would have the same problem with my
SLR.
"Robin Leadbeater" wrote in message
...

"Tony Pagett" wrote in message
.uk...
Hello Robin.

Thank you for your help.

I have just measured the register as you say and if I am measuring to the
correct place then it is only 41mm.

I just did some experimenting with my Canon EOS 35-80mm lens. It looks
like
the correct spacing could be very critical. Small errors throw the focus
out
significantly across the zoom range. While you would get away with being a
couple of mm too close with a fixed focus lens, this may not be the case
with a zoom.

Robin

"Tony Pagett" wrote in message
o.uk...
What I don't understand is that if I can focus at one setting of the lens
then why not at all settings. Surely the focus point should be at the same
place regardless of zoom otherwise I would have the same problem with my
SLR.

I suspect there is only one sweet spot for the register which allows the
focus to be maintained without adjustment across the zoom range. I must
confess, I don't know anything about the design of zoom lenses, but to
maintain focus across the range the elements must move to produce a
combination of focal length change and refocussing action as the zoom is
altered (I recall the first zoom lens I owned many years ago required
manual refocussing as the zoom was altered.) On my more modern 35-80mm zoom
at least, the front of the lens is racked in furthest at 55mm, the focal
length at which you are able to achieve focus, albeit by setting the focus
very close. At the extremes of the zoom it is racked out further. Is
there any way you could make a temporary lash up with a spacer of a few mm
somewhere between the DSI and the lens to test it out?

Robin