Achieving Focus

[NickRose2006]

I was wondering I have two cameras. One is a Canon 300D Rebel and the other is a Canon 1D Mark3. I'm using the 300D for astrophotography since I'm using the other one for only my Freelance work. Out of the two cameras my Mark3 has the live view. My question is, can I use that camera to achieve focus since it has the live view and then change over to my 300D? Would there be any change in focus?

[FraserClarke]

Is the back focal distance (flange->detector; usually ~44mm on Canons??) the same on both cameras? If so, then in theory yes you could, but the tolerances might mean it doesn't quite work.

The focus will also probably change with time/temp, so you need to focus quite regularly -- it's not like you could focus once and then leave it for a month...

[Bizibilder]

It's always a bit dodgy swapping DSLR's (or anything else) once you have set focus. Even with the locks "on" it is just too easy to move something critical or not quite set things "exactly" when "exactly" matters!

[Tim]

Because perfect focus is achieved with tiny tiny tiny tiny movements of the control knob, I'd be surprised, actually shocked, if you could get perfect focus this way.

Have a word with Steve or James at FLO. They should be able to provide a bahtinov mask for your telescope which will tell you absolutely if your focus is perfect with just a few short frames on a bright star.

I can honestly say that using a bahtinov mask made the single biggest difference to my imaging, vastly reducing setup time, and vastly reducing or eliminating wasted hours of slightly out of focus shots.

TJ

[ollypenrice]

I agree with Tim, you need to focus properly with a capital P. With a DSLR the Bahtinov is a 100% no brainer, it is essential. You'd do better to be looking at a PC monitor as well, rather than just the screen. In astro imaging focus needs to go to a level not practicable or necessary in most daytime work, or so it seems to me.

Another golden rule of mine is don't touch the telescope unless you absolutely have to, so mauling it around swapping cameras would be anathema to me.

Olly