The Quest for Infinity - Excercises in focal frustration

[FlyingPete]

So I am the lucky owner of a certain 70-200 f/2.8L, nice lens.

However recently I have begun to shoot images of the night sky, which requires me to manually focus the lens at infinity, surely that is easy enough, but not so.

Normally you just spin the focus ring as far as it goes to the long end of the focus but you see this particular lens has the interesting (and somewhat baffling) ability to focus past infinity, what use this is I am unsure, two possibilities is more focusing range if there is an extension tube fitted, or the ability to see past infinity into another dimension, if the later is the case then the other dimension is quite blurry. Any other theories on this behaviour please let me know!

Manual focusing is easy in good light, in fact when the auto focus works, but bad light its a different story, in fact when you are often forced to as the auto focus has given up.

I have pondered a solution which I am going to attempt when I get home. Since the focusing distance window is of limited value when manually focusing (it looks cool though), I thought I would try this. The focus ring will continue to move beyond the extremes of the focusing positions, which means that the ring has no position that corresponds to a particular focus distance. That is where my idea comes in.

What I am going to try is to spin the focus ring until the focus reaches on of the extremes, at that point I am going to take a marker pen (yes I am going to draw on my lens!) and mark the position of the ring relative to the barrel, I will then manually focus the lens on a suitable object that is at infinity (we have a currently friendly local volcano that is good for that) then mark the position again, that is my infinity position.

Now to repeat finding the position, if you then turn the focus ring towards and then beyond the extreme end at which it was marked, when you move back the other way the marks will match back up in the correct start position, you should then be able to keep turning it to the marked infinite position.

Will give it a go and post some pics of the process if its not clear.

Am I mad or does anyone else have any better ideas?

[Mark_Cohran]

Some lenses have the ability to focus past the infinity mark to allow for variations in temperature on the internal optics. You might get a better answer for your question in Talk About Astronomy - I'll move this thread there for you.

[XterraJohn]

I guess it depends on where you're photographing. When I was trying to take pictures of stars, I just found a very distant street-light or building with a light on it and used AF to focus on that. Then point the lens up at the stars without trying to refocus.

[Nighthound]

You could mark focus but I prefer this method with lenses, especially under 200mm.

Focus first on a very bright star like Sirius, Arcturus, Vega or Rigel by doing this:

Set camera on Av and lens on Auto Focus.

Point camera at bright star and tap to auto focus just as you would in daylight by placing the center focus sensor box on the bright star. You will hear the lens snap to focus and see it as well(be sure you have adjusted the camera dioptric dial for your eyes before starting, it'll vary from terrestrial shooting).

Next switch the lens to Manual focus and camera to Manual as well.

Carefully point your camera to the region of the sky you want to shoot and don't touch focus on the lens while composing your shot. Shoot a test shot using a remote switch and zoom to check focus. This method works well but you may have to refocus if temps drop considerably through the night.

This method makes refocus go quick with lenses, when shooting with my f/4 Newtonian I also have to make collimation checks/adjustments mid way through the night when temps fall to insure critical focus at 800mm.

[FlyingPete]

You could mark focus but I prefer this method with lenses, especially under 200mm.

Focus first on a very bright star like Sirius, Arcturus, Vega or Rigel by doing this:

Set camera on Av and lens on Auto Focus.

Point camera at bright star and tap to auto focus just as you would in daylight by placing the center focus sensor box on the bright star. You will hear the lens snap to focus and see it as well(be sure you have adjusted the camera dioptric dial for your eyes before starting, it'll vary from terrestrial shooting).

Next switch the lens to Manual focus and camera to Manual as well.

Carefully point your camera to the region of the sky you want to shoot and don't touch focus on the lens while composing your shot. Shoot a test shot using a remote switch and zoom to check focus. This method works well but you may have to refocus if temps drop considerably through the night.

This method makes refocus go quick with lenses, when shooting with my f/4 Newtonian I also have to make collimation checks/adjustments mid way through the night when temps fall to insure critical focus at 800mm.

Once again Steve, thanks for your help! As you can probably tell keeping focused is my main issue at the moment (in life as well as photography!).

[renderwerks]

I find it very handy to use LiveView with the 40D with magnification. Works especially well with a laptop and EOS Utility, you can bump the focus right from the laptop.

[paddycook]

Isn't this ability to focus past infinity something to do with infrared photography?

[Nighthound]

Isn't this ability to focus past infinity something to do with infrared photography?

I'm not aware of focusing "past" infinity. Once focused at infinity astronomical objects will be focused. You can use a distant mountain or horizon as a terrestrial target but celestial objects are a sure thing. Some lenses designed for very close up work may not have the capability to focus at infinity.

[FlyingPete]

I find it very handy to use LiveView with the 40D with magnification. Works especially well with a laptop and EOS Utility, you can bump the focus right from the laptop.

Can you use Live View with a laptop? That would really help with the bigger screen, and especially if you can focus the camera without touching it. :)

[Nighthound]

Pete, here's another option that slipped my mind, sorry.

http://www.dslrfocus.com/

[renderwerks]

Can you use Live View with a laptop? That would really help with the bigger screen, and especially if you can focus the camera without touching it. :)

In a word, yes. That's what I do. EOS Utility can activate LiveView and also activate the magnification (just one magnification) not sure if it's 5x or 10x. It also has fast and slow focus buttons in it for use with motorized (AF) camera lenses. I've used it with everything from a 17-70 through my 70-200 2.8. You do need a rather bright star to focus on, but for me, it works quite well.

EOS Utility also has an interval timer in it. It appears that it doesn't work with LiveView though. It's handy to automate exposure sets of 30sec or longer.

I hooked it up the first couple of times and "played" with it in daylight before I tried using it under the stars. Made remembering and finding the controls much easier in the dark!

[FlyingPete]

So I am all ready to try all this out and wouldn't you know its clouded over tonight :rolleyes:

[hollis_f]

I've always thought of using my green laser pointer. Shine it at something a fair distance away (100m should do in most cases) and focus on that. Must admit I've not got round to trying it.

For those who haven't tried them, green laser pointers can be very useful for pointing out things in the sky.

[Adrena1in]

For those who haven't tried them, green laser pointers can be very useful for pointing out things in the sky.

As long as you don't shine them at a passing aeroplane or helicopter!! ;)

[hollis_f]

As long as you don't shine them at a passing aeroplane or helicopter!! ;)Yes, indeed! Somebody ended up in nick for doing that. Some of the newer ones are quite high-powered as well, so it's important to keep the beam away from people, especially their eyes.