Focussing Aid

[William W]

In another conversation there's a specific discussion about using a Star Crossed Filter as a focussing aid for Astro-photography.

For those interested in other methods I have found a Bahtinov Mask is a useful tool, in some situations.

Making a Bahtinov Mask can be a DIY project.

WW

[TonyW]

I looked up a few references about the Bahtinov mask and it didn't look too difficult to make.

However, I have only tried astro-photography a couple of times with partial success and it is not something I would expect to spend a lot of time on. This is probably not a good time to do astro-photography on the east side of Australia and perhaps not the west either.

I still haven't worked through the physics/mathematics to discover why they produce the pattern they do.

[rpcrowe]

I don't have any interest in star photography but, my curiosity has been piqued. I am wondering how well manual focus with a combination of Focus Assist (enlarging the viewed image) and Focus Peaking (colored areas indicating the in-focus portions of an image) on cameras so equipped would work.

I think that I might try this combination during one of the new few nights if there is a night with a clear sky...

[Manfred M]

I don't have any interest in star photography but, my curiosity has been piqued. I am wondering how well manual focus with a combination of Focus Assist (enlarging the viewed image) and Focus Peaking (colored areas indicating the in-focus portions of an image) on cameras so equipped would work.

I think that I might try this combination during one of the new few nights if there is a night with a clear sky...

I'm going to have to say no.

Contrast detect focusing, whether you use full autofocus or focus peaking relies on the same mechanism; contrasty edges. Unfortunately, a star-lit sky has very little contrast, so neither mechanism will work.

I had tried to take some sky shots with my mirrorless camera when we were trekking in Nepal a few years back and had zero success.

[xpatUSA]

Originally Posted by rpcrowe I don't have any interest in star photography but, my curiosity has been piqued. I am wondering how well manual focus with a combination of Focus Assist (enlarging the viewed image) and Focus Peaking (colored areas indicating the in-focus portions of an image) on cameras so equipped would work.

I'm going to have to say no.

Contrast detect focusing, whether you use full autofocus or focus peaking relies on the same mechanism; contrasty edges. Unfortunately, a star-lit sky has very little contrast, so neither mechanism will work. <>

Does this comment apply to manual focusing - which is what Richard is asking about?

Do bright stars not show up on a good, high-res, magnified, auto-brightening LCD? (never tried it myself ...).

[xpatUSA]

I think that I might try this combination during one of the new few nights if there is a night with a clear sky...

Perhaps no need to wait, Richard. A mini-mag light at about 300 yards ** would probably give you a good idea as to the efficacy of manual magnified Focus Assist in the dark.

** based on a half-pixel CoC ...

[Manfred M]

Does this comment apply to manual focusing - which is what Richard is asking about?

Do bright stars not show up on a good, high-res, magnified, auto-brightening LCD? (never tried it myself ...).

Contrast detect requires sufficient contrast for the camera to be able to use it. A dark sky with stars is low contrast, at least it is when I have tried contrast detect focus.

With manual focus and magnification, the noise overwhelms the stars, so its challenging to figure out what the noise is and if there are any stars buried on the noise. Focus peaking, which is used to help in manual focus is little more than a visual representation of what the contrast detect mechanism "sees".

The only technique, using a fairly wide angle lens, that has worked for me is to focus on a brightly lit object in the distance (i.e. close to infinity) and then use that pre-focus to capture the sky (plus the subject).

[Stagecoach]

With manual focus and magnification, the noise overwhelms the stars, so its challenging to figure out what the noise is and if there are any stars buried on the noise.

That has certainly not been my experience when focusing manually with contrast detect on a bright star using my D800. The trick and fiddly bit is to spend time finding and aligning the camera to that 'brightest' star as you 'up' the magnification.

[Manfred M]

That has certainly not been my experience when focusing manually with contrast detect on a bright star using my D800. The trick and fiddly bit is to spend time finding and aligning the camera to that 'brightest' star as you 'up' the magnification.

I suspect there is less "light pollution" in Fiji than in Ottawa and that might be the difference.