Focal length of the Canon 350 D

[+Graham]

Hi all.

I want to start using a barlow with my canon 350 D.

To do this I need to know what EP will have an equal focal length to the camera.

What I want to do is put an EP into the barlow so I can set the focus up visually.

Then when I swap the EP for the camera I am going to be roughly in focus.

Trying to set the camera up without this method is such a long winded affair it is nigh on impossible at night.

Many thanks

Graham

[Guest Kheldar]

Very roughly it's the same as the diagonal size of the sensor (I think!!!)

That would make it about 30mm : however all 30mm focal lengths do not focus equally!

What you are describing is called a parfocal EP, and it doesn't have to be the same focal length : any EP will do.

Step 1 : Focus camera

Step 2 : Place any EP in the focuser

Step 3 : Move the EP out of the focuser or use barlows until you get it into focus WITHOUT moving the focuser

Step 4 : Make or buy a parfocal ring that attaches round the barrel of the EP

Next night out, focus the scope with the EP/barlow combination, and your camera will be about in focus

Edited July 30, 2012 by Kheldar

[+Graham]

Thanks Stephen makes perfect sense to me.

[Brantuk]

Focus can be obtained in just a few mins in the dark using only a bhatinov mask - and you don't even need to take the camera off the scope. Once practised it's dead easy and quick - and probably more accurate cos the eye won't see what the camera does - unless you have parfocal eyes lol

[Guest Kheldar]

Focus can be obtained in just a few mins in the dark using only a bhatinov mask - and you don't even need to take the camera off the scope. Once practised it's dead easy and quick - and probably more accurate cos the eye won't see what the camera does - unless you have parfocal eyes lol

Have to agree with you there

[+Graham]

Agreed here to but as I have never used a barlow with this set up I have absolutely no idea where the focus point will be so doing it this way the first time will save me hours of adjusting the focus doing a 60 second sub moving the focus 60 second sub move the focus ect ect ect.

Once I have found the focal point I can mark up my focus tube so I have a starting point for subsequent times.

[Brantuk]

IIRC all you have to do is locate a star and rough visual focus - pop the camera on - hook it up to a pc and run up the live preview in the capture software - slip the mask on and adjust the focus till you get the pattern - it was all done in 60 secs lol.

(we did use a motor focuser though)

[+Graham]

Problem is Kim I don't have live view on the 350

[glider]

If you pick a bright star though it shouldn't be needing 60s test exposures though, I use 5 to 10s with a nudge of the focuser between each.

[Brantuk]

Sorry Graham I shoulda been clearer - I didn't mean "camera live view" - I was refering to "live preview" in the pc capture software. You'll see whatever the camera is looking at well within a couple of secs - you can do that with the 350D.

Try it in daytime - you can pan around the room and you'll see whatever the camera is looking at and focus just like when you take a normal photo looking in the view finder.

[+Graham]

Ahh I have understanding, thanks folks I will give that a go if it ever stops raining down here.