Focusing / camera (Kenny)

[MarkH]

With focusing any slr with a prime lens, am I not right in believing that if the focus ring is set to infinity that any celestial object should be well within the depth of field, even at f1.8. If so that would solve Kenny's focusing issues. I do appreciate that the lens would have to be designed for that specific slr.

[Kenny]

The infinity symbol on my lens is wide so that's quite a focus range, plus it goes well beyond infinity as well. Mark Shelley (in my Decisions Decisions thread) has suggested a masking tape calibration mark solution.

http://forum.orpington-astronomy.org.uk/index.php?topic=10030.15

[Mike]

With focusing any slr with a prime lens, am I not right in believing that if the focus ring is set to infinity that any celestial object should be well within the depth of field, even at f1.8. If so that would solve Kenny's focusing issues. I do appreciate that the lens would have to be designed for that specific slr.

No. Sometimes the infinity mark is WAY off. If only it were that easy. *sigh*

[Carole]

I haven't even got an infinity mark on either of my lenses anyway.

Carole

[Canadian Roger]

I have heard of people having great success with a vernier type of system for focussing.

This fellow suggests doing this:
https://sites.google.com/site/boriskrassi/photography/astro/focusing-canon-100-400l

I have an old Pentax SMC 300mm f/4 lens that works very nicely for ordinary photography, but the deep red is not focussed as tightly as I might like.  However, I'm going to try what Boris suggests but using just a 6nm bandpass H-alpha filter.  I'll use a nice bright red star like Betelgeuse so that my exposures are short enough that it won't take all night.

[MarkH]

Then I have a luxury on all my zuiko lenses as they all stop and focus exactly at infinity 

[Kenny]

I'm going to search all your images now for out of focus stars! 

[MarkH]

I'm going to search all your images now for out of focus stars! 

I'll have to get the old om2 out then Kenny

[Fay]

Forget infinity it will be waaaaaay out! mine are usually around 0.3 - 0.6 to focus

[The Thing]

Forget infinity it will be waaaaaay out! mine are usually around 0.3 - 0.6 to focus

Ah but yes but Fay, your lenses are usually on some sort of adaptor to convert one fitting to another so the lens to sensor distances are not as the designers planned for. As are mine. It's another factor.

[MarkH]

Yes Duncan, exactly that's the problem I had in. Lanzarote, the lens wasn't designed for the camera I was using. I recon lens designers in the big slr companies design their lenses to stop at infinity. From my experience at f 1.8 focussed at infinity with a 50 mm lens the depth of field is about 30 m-infinity. My only enquiry is not where the dividing line lies at 30 m but where it lies at infinity. My guess I the designers for all intents and purposes set it at infinity.

[MarkS]

Old fashioned lenses tended to have distance markings and they tended to stop at infinity.  Modern lenses are designed with auto focus in mind which works by "hunting" back and forward - hence they are designed to go beyond infinity - to facilitate auto focus.  Even if infinity is marked then you cannot assume it is marked sufficiently accurately for our purposes.

Mark