How to focus a DSLR

[Earl]

I tried last night to take some wide field using my 24-105 on my 5dmk2 mounted on a regular tripod but could not get a sweet focus.

So whats the trick?

Live view dos not show anything and I cant see anything through the view finder.

I did move the focus ring to infinity then back it off a touch and kept backing of more but still did not get pinpoint stars.

Any tips will be appreciated.

Oh i was using 30 seconds, which i think was giving slightly oval blobs LOL, so ill back off to 20 seconds tonight.

[jimmyjamjoejoe]

I either focus to infinity during the day, or use liveview with absolute max settings - highest iso, longest exposure etc and it can usually pick up some brighter stars like vega. Then just zoom in 10x on liveview and focus.

[Steve Ward]

I use the focus control in EOSUtilities , when tethered you can see the Liveview on the big screen with the settings pushed way up so you can use a bright star to focus on ,

The s/w lets you see at 10x in Liveview and the fine focus is much more accurate than using the focus ring.

If there's no bright star in vicinity you can take short 4-5 second test shots too , enabling focus to be checked.

[steppenwolf]

I played this game last night and used the free 'EOS Camera Movie Record' software at 5X Zoom to focus on Vega in Lyra. This worked very well indeed and I simply re-pointed at my target (which was invisible to Liveview and, of course, the viewfinder) and started imaging. Because of atmospheric refraction you need to choose a bright star at a similar latitude in the vicinity of the object you want to image but you don't need to bust a gut over being too choosy.

Follow some excellent advice from Peter (Psychobilly here on SGL) and position the test star at one of the 'thirds points' in the frame to get the best average focus as camera lenses do tend to suffer from field curvature and this technique certainly helps.

[Psychobilly]

Also resist the temptation to adjust the lenses zoom setting after you have focused.. the 24-105/f4L is supposed to be parafocal across the full zoom range - it might be for terestrial targets but it's needs focusing at the imaging fl for astro use...

Peter...

Sent from my GT-P7300 using Tapatalk HD

[Steve Ward]

Agreed , another good reason for using Liveview on the laptop , you don't go anywhere near the lens ...

[Earl]

Thanks for all the good advice Gents, Laptop is charging eos stuffs installed, just waiting for darkness.

[Ags]

1. Set selected F ratio. Manually focus to infity using a distant daytime object.

2. Set ISO to maximum, point camera at bright star.

3. Bright star will be visible on LCD.

4. Position star midway to edge of LCD.

5. Zoom in x10, and very carefully focus! Helps if the camera is bolted to something fairly stable. My 4SE mount and AZ3 are both good enough.

The reason for (4) is that you lose a bit of center sharpness by focussing midframe, but you gain a lot of sharpness at the edges.