New guy with dumb question

[jc.dempsey]

I think so far my best bet is cranking up the exposure on live view to get it in focus. I have yet to try it.

 

[TCampbell]

On another note, just cranking the focus as far is it will go on the distance scale is not necessarily infinity. Most of my lenses will focus "past" infinity, whether it's a mechanical or electronic focus ring. Looking at the moon, for example, it's not focused when I crank the lens over against the stop; I have to back it off just a smidge. I suppose it's to cover a manufacturing tolerance to make sure the lens CAN reach infinity, but that's just ignorant me making a guess., but anyway, it makes finding the max distance focus position more difficult than you'd think it ought to be, especially at night with nothing out there but the stars you want to shoot.

Different wavelengths of light focus at a different distance. These same lenses can be used on film cameras and if you load infra-red film into the camera then the focus point for "infinity" is beyond the focus point for infinity in the visible light spectrum. (It's also possible to convert a DSLR to do IR photography but the conversation is more-or-less permanent since the amount of work involved is non-trivial. Some photographers will convert an older body that they no longer use.)

In any case, many lenses have a focus distance indicator on the lens. On Canon lenses, this mark is an "L" shape (but the bottom of the "L" is longer). The vertical part of the "L" indicates the position of "infinity" when shooting visible light. The end of the horizontal part of the "L" indicates the "infinity" focus position when shooting IR.

But another reason for the extended focus mark is due to the thermal properties of the lens. As metals heat up, they expand, and they contract as they get cold. This is the original reason that Canon started painting their longest lenses "white". It's not much of a problem on short lenses since the expansion/contraction due to temperature differences won't amount to much of a difference, but the longer the lens, the more this becomes a problem. Astro-imagers typically start shooting as soon as it gets dark enough... but the night time temps tend to drop the later you continue to shoot. As the temperatures drop, they have to keep re-focusing their telescopes due to the thermal contraction of the cooling optical tube body. High end scopes will sometimes use carbon fiber optical tubes not because they care about the weight... but because carbon fiber doesn't expand and shrink as much as metal tubes.

I've noticed Nikon actually uses the infinity symbol ∞ and I believe the center of that mark is the intended focus position for "infinity". I'm not sure where they place the focus position for "IR" or how that works on Nikon.

 

[jc.dempsey]

That was very informative

 

[Solarflare]

Lifeview, 100% mag, move focus manually around until the stars are tack sharp.

I would strongly suggest getting a 24mm f1.4 or something of that sort, though. This way you can keep the exposure short and avoid star trails.

The alternative is to get hardware that compensates for the movement of the stars.

 

[careysb]

When we had the lunar eclipse I was shooting with live view and manual mode. When the eclipse got to the "blood red" stage I couldn't get the shutter speed to be slower than 1/30 of a second. (Didn't matter because the moon went behind the clouds at that point.) I was scratching my head about this though and came to the conclusion it had something to do with live view.

I just found this and it may help you.

This only really becomes a problem when you decide to switch into shooting stills in Live View, especially if you’re shooting landscapes and want a slower shutter speed – it’s a frustrating couple of minutes trying to figure out why the camera won’t allow you below 1/30sec when you really want 2sec.

The solution? Pop into the menu and switch the Live View mode from Movies to Stills. Voila, you’ll now have access to the full shutter speed range!

 

[jc.dempsey]

Hey thanks for the heads up! I haven't tried it yet so it's good to know that before I get frustrated.

 

[480sparky]

.............The solution? Pop into the menu and switch the Live View mode from Movies to Stills. Voila, you’ll now have access to the full shutter speed range!

At a different aspect ratio and resolution that you were shooting at.