• Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.

    Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
ZEISS DTI thermal imaging cameras. For more discoveries at night, and during the day.

Image moves when focusing? (1 Viewer)

looksharp65

Well-known member
Sweden
Now and then you read about slight image shifts when focusing. Yesterday the very same happened to me with my beloved and mucho-babied Nikon ED82A.
Since this never happened before and I'm reasonably careful with my gear, I started to investigate what was going on.

I found that the lock screw for the rotatable collar wasn't properly tightened, so when I focused, the scope moved a little before the focus actually changed.
The optical axis is not perfectly aligned with the collar's geometrical centre.

I'm not saying this always is the problem when the image shifts when focusing, but try tightening everything up before assuming there's something wrong with your scope.

//L
 
Hi,
This image movement happens regularly with Maksutovs, SCTs, some Dall Kirkhams and other scopes that use main mirror focus.
High quality ones minimise this, but it is often detectable.
The main mirror moves slightly sideways.
They also change focal length as you focus. In larger scopes this is minimal.
 
Hi,
This image movement happens regularly with Maksutovs, SCTs, some Dall Kirkhams and other scopes that use main mirror focus.
High quality ones minimise this, but it is often detectable.
The main mirror moves slightly sideways.
They also change focal length as you focus. In larger scopes this is minimal.

I recall something about moving prism for focusing and image displacement when focusing. This was with spotters, not astro scopes.

//L
 
Warning! This thread is more than 7 years ago old.
It's likely that no further discussion is required, in which case we recommend starting a new thread. If however you feel your response is required you can still do so.

Users who are viewing this thread

Back
Top