To infinity and beyond

[rastis95]

Hi all,
A quick question with focus on a dslr. Ive read that taking night shots with a lens that people set the focus to manual and wind it out to infinity. On mine when i do this it is out of focus and I need to wind the focus back about 5mm then take a test shot. Is this normal.

Cheers
Scott

[Hagar]

Absolutely normal. Every lens I have ever used for astro imaging has been like this.

[jjjnettie]

This is normal.
I'm sure someone will pipe in explaining why it is so.;)

[rastis95]

Thanks for the replies, i was starting to think that it might be the lenses. Is there any tricks of the trade to achieving sharp focus with a dslr, at present i use live view and expand 10x on a bright star. Looks good on the camera but once its on the computer it just doesn't seem spot on.

[bojan]

It is because of tolerances :D
It is better that focus in inside the moving limits of the focusing ring than out ;)

[bojan]

Try to use FocusAssist and Magnifier (both are freeware).
http://www.xmission.com/~jstanley/focusassist/ (http://www.xmission.com/%7Ejstanley/focusassist/)
http://www.iconico.com/magnifier/

Sharpness is also affected by the lens quality and f number.. try to close the lens 1-2 stops - this will require 2-4x longer exposure time for the same effect, but it will be worth it in terms of sharpness.

[rastis95]

Thanks Bojan, have downloaded, now all i need is a bit more practice :thumbsup:
Cheers

[bojan]

Also. try Bakhtinov's mask
http://www.iceinspace.com.au/forum/showthread.php?t=35676

I am using it with my 1000mm lens, it works very well. Some others claim it works with shorter lenses too, but I haven't tried, my 200mm lens (and 50mm as well) can be focussed perfectly well without B-mask.
(and focus assist and Magnifier are still useful here)

[tlgerdes]

This is the method I use, but i do it through the EOS utility on my computer. This means I am working on a 12in screen rather than 2.6in.

[[1ponders]]

Bakhinov Mask for sure! As well as laptop onscreen assist. If you have the laptop there, use it to the best of its ability. Get that star as sharp as possible. While I don't have, nor have i used live view in camera, I can't see how a 2.6in screen (with dubious resolution) at 10X zoom can compete with a 15 or 16" laptop screen at 6x zoom. You just know you've nailed it on the laptop screen.

Plus you can adjust all your camera settings there and not muck around with the back of your camera and risk bumping and misaligning your shot

[Octane]

You still see the same star size image, regardless, Paul.

LiveView works wonderfully well, even when zoomed in. The tiniest movement of the focuser shows the star image change.

Couple the Bahtinov Mask along with the Bahnitov Grabber software (interactively shows you how far out of focus you are and suggests racking in or out until star is in focus), and it is wonderful for DSLR work.

H

[[1ponders]]

Even the same size diffraction pattern?

[Octane]

Sure. And, I wear glasses to boot.

It /is/ a lot easier when you're looking at a netbook/notebook screen, that's for sure, not having to contort yourself.

H

[Wavytone]

The old solution from film SLR days was a 5X magnifying finder (with right-angle prism) that attached to the camera eye lens. I used to have one but tossed it along with my Pentax SLR years ago.

Focussing with these is easy, if you have a bright object - anything - in the field.

They are still around on eBay; for example this:

http://cgi.ebay.com/1-25-2-5x-Angle-Finder-A1C-for-Pentax-K20D-K200D-K100D_W0QQitemZ280470349459QQcmdZVi ewItemQQptZLH_DefaultDomain_0?hash= item414d55e693#ht_3223wt_1167


The other trick is being able to lock the lens focus so it wont shift; most lenses don't. One solution is a piece of adhesive tape between the lens body and the focus ring, use this to nail it in place once you have focussed.