Infinity focus of the canon 350D 18mm - 55mm lens

[proflight2000]

Hi all

I have now set up a piggyback system for my Canon 350D on top of my telescope, what I am after are just wide shots of the sky, so basically i am shooting through the lens of the camera not through the telescope. I have read some info on ensuring the focus of the lens is set to infinity, that the lock is on the lens etc, however, how do I go about placing the lens into infinity. Sorry for the dippy question.

[studdedsole]

The object is to have the light rays enter the lens parallel - best way i've found is to focus on an object at great distance - say a distant hill during the day or the moon or any other suitable bright object you can find at night (obviously this can be a bit tricky). When the object is in focus this will be the closest you get to infinity.

[Albireo380]

Yeah, no easy way - pick a bright street light a couple of miles away (they do have uses, after all) and focus on that, then zoom up the image on the LCD - if in focus - okay, if not, change the focus slightly and repeat, again and again until you are happy with a magnified/zoomed in image.

Then mount piggy back on your 'scope - and away you go.

Tom

[robindurant]

Hi

Infinity used to be when the focus ring was fully in or clockwise. On my canon it is slightly out or anti clockwise. I set it up on the scope, wind the ring in and take shots of small stars, moving the ring out a bit at a time and check the lcd enlarged until I get the stars as small as possible, then dont touch it at all during your shots.

[Ant]

Hi,

What I did was spend some time one clear evening taking images to get the best focus I could once I was happy that I had the best focus I could get was to mark the lens body and focus ring with white tipex (very thin line).

I did that for 18mm and for 55mm (both ends of the zoom).

Now when I go out for wide field shots I just adjust the lens so that the two lines match up and bobs yopur uncle.

These images were taken using that method.

Ant

[moondog]

Ant's suggestion is a good one.

You can use the landscape setting during the day and focus on a distant object using auto focus. Then switch the focus to manual on the lens and don't touch the focus ring.

MD

[Ant]

Yep sorry forgot to mention that it's vital to set the lens to Manual Focus.

Ant

[studdedsole]

Nice shots Ant - just be aware that although you have a mark on the lens to indicate infinity - it will actually vary with temperature which is why an infinity focus mark in the lens window on short focus lenses isn't conincident with the lens at full extension - it is an indication but requires a tickle to achieve absolute focus due to the temperature of the environment you are operating in.

[Robd]

What can also help is the use of a right angle viewfinder - available off eBay for about £20. Plenty of references on this site. I have also made myself hartman masks for the higher focal length lenses but no reason why you couldn't make one for wide angle. Easy to do with a piece of cardboard and cellotape.

Rob

[daz]

Is it possible (and I suspect not otherwise it would've been done) to control the focus ring from software at all?

If not, the hartman mask is the best one I think...

[daz]

I seem to remember seeing a website, or even a program, which calculated the hartmann mask template for you, and produced a PDF document..

Anyone else remember this??

[proflight2000]

http://billyard.servehttp.com/Hartmann.html

[daz]

Thats the one!!