How do you focus to infinity?

IanC

IanC

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I really want to have go at Star Trails and the one of the things I need to do it focus to infinity.

Could someone give me an explanation as to what this means exactly please? I'm struggling to get my head round it and how to do it on my Sigma 17-70. The focal length I'm using is 17mm.

Thanks for any help :)



photo (1) by I Carvell, on Flickr
 
Infinity is marked on the lens isn't it? Or you could turn the focus ring until the stars look to be in focus.
 
Yup, the symbol for infinity is like a number 8 on its side. Personally I'd use live view and zoom in to set the focus if there's enough light from the stars :D
 
What confused me, was when photographing the moon I thought because of the distance I would be using the infinity setting.

I presumed this was right to the end of the barrels turning position so turned the lens barrel until it stopped - but now I find I have to back off of the end of the barrel by a small amount to attain focus.

So is infinity the end of the barrels turning, or is it slightly before ?
 
I'm not sure of the technical reasons but its slightly before.
 
Most lenses have a "range" for infinity to allow for changes to the lens caused by extreme temperatures. As another poster has said, zoom in in live view if you have sufficient light, otherwise just focus to the end of the range as with a 17mm lens the depth of field is huge even wide open.
 
Modern lenses usually don't have a fixed infinity focus stop. Lenses can focus a bit 'beyond' infinity so that the AF system knows when it's passed it. Also accommodates fluctuations due to heat expansion.

Infinity symbol is the horizontal 8, and alongside that is a horizontal L mark. The infinity position is around the short/vertical foot of the L. The effective infinity distance varies with focal length and with wide-angles it's quite close, like the end of the garden; with a super-tele maybe a hundred yards or more.

With the OP's 17mm lens, probably focusing on something like a chimney stack over the road, or a TV aerial, or a street light a few houses away will effectively be infinity, then using a mid-range f/number will cover any slight errors.
 
HoppyUK said:
Modern lenses usually don't have a fixed infinity focus stop. Lenses can focus a bit 'beyond' infinity so that the AF system knows when it's passed it. Also accommodates fluctuations due to heat expansion.

Infinity symbol is the horizontal 8, and alongside that is a horizontal L mark. The infinity position is around the short/vertical foot of the L. The effective infinity distance varies with focal length and with wide-angles it's quite close, like the end of the garden; with a super-tele maybe a hundred yards or more.

With the OP's 17mm lens, probably focusing on something like a chimney stack over the road, or a TV aerial, or a street light a few houses away will effectively be infinity, then using a mid-range f/number will cover any slight errors.
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Thanks Hoppy, I think I know what I need to be aiming for now. The only problem I have is I will be finding a location away from light pollution so won't necessarily have a street light to focus on.
 
Don't get too hung up about focussing on a nearby object. Even at F2.8, a 17mm lens on your camera will render everything acceptably sharp when focussed anywhere from 20 feet to infinity. Just turn your focus ring as far as it will go to infinity, then turn it back a tiny amount or use the scale to focus anywhere from 20 feet to infinity.
 
Yellowbelly said:
Don't get too hung up about focussing on a nearby object. Even at F2.8, a 17mm lens on your camera will render everything acceptably sharp when focussed anywhere from 20 feet to infinity. Just turn your focus ring as far as it will go to infinity, then turn it back a tiny amount or use the scale to focus anywhere from 20 feet to infinity.
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Thanks Bob :)
 
Oh just to clarify, that doesn't mean you must shoot at F2.8. In fact assuming you use a tripod, I suggest you use F8 to F11.
 
Yellowbelly said:
Oh just to clarify, that doesn't mean you must shoot at F2.8. In fact assuming you use a tripod, I suggest you use F8 to F11.
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I think the idea with star trails is to stack lots of 30 sec exposures isn't it? In which case I would need a wide aperture to let as much light in as possible?
 
Ah, sorry. With all the posts I had forgotten your original purpose.

I still recommend that you don't shoot totally wide open as most lenses do not perform at their best then. Rather stop down one or two stops.
 
Yellowbelly said:
Ah, sorry. With all the posts I had forgotten your original purpose.

I still recommend that you don't shoot totally wide open as most lenses do not perform at their best then. Rather stop down one or two stops.
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Thanks Bob, I'm guessing experimentation is the key with this type of photography :)
 
With such long exposures there will always be a an amount of guesswork initially. Just review your first shots early and adjust exposure to suit.
 
IanC said:
Thanks Hoppy, I think I know what I need to be aiming for now. The only problem I have is I will be finding a location away from light pollution so won't necessarily have a street light to focus on.
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Do a quick pre-check. Focus on a distant subject by the most accurate means possible, in good light. Probably live view, max magnification. You may find normal AF might fluctuate a bit, though still within tolerance, so live view may be more precise.

Note the position on the scale, and use that as your future manual focus reference point.
 
If your camera has live view... get a bright star in shot, zoom in n live view... focus MANUALLY (switching AF off... not just manually over riding it) until the star is as sharp as possible... then do not move your focus ring again. Shoot. Enjoy.


I'm a big fan of live view focusing for work like this.
 
Right, first off don't focus to infinity for a star trail, focus on the foreground interest that your including as part of the scene has a star trail with perfectly sharp stars bit an OOF foreground looks absolutely awful, where as due to stars being such a small source of light if they are a tiny bit OOF but the foreground is sharp no one is going to notice ;)

For further info on star trails you may find it helpful to check out my guide to star trails in the tutorials section, there's a link to it in my signature :thumbs:
 
Yellowbelly said:
Don't get too hung up about focussing on a nearby object. Even at F2.8, a 17mm lens on your camera will render everything acceptably sharp when focussed anywhere from 20 feet to infinity.
Click to expand...

While this works for most subjects, and wouldn't matter too much for star trails (as pointed out by Matthew), it doesn't work with ordinary star images. When shooting a landscape 'acceptably sharp' has quite a bit of flexibility, something that's a little fuzzy won't be obvious.

But when shooting stars it is only too obvious if the focus is not spot-on. Values that would give you 'acceptably sharp' any other time will give you 'white blobs' when shooting stars.

David's advice on using LiveView is probably the best way of getting the focus spot-on.
 
MWHCVT said:
Right, first off don't focus to infinity for a star trail, focus on the foreground interest that your including as part of the scene has a star trail with perfectly sharp stars bit an OOF foreground looks absolutely awful, where as due to stars being such a small source of light if they are a tiny bit OOF but the foreground is sharp no one is going to notice ;)

For further info on star trails you may find it helpful to check out my guide to star trails in the tutorials section, there's a link to it in my signature :thumbs:
Click to expand...


True for trails... not so for just stars, as they become little blobs.

This is why I always use a very wide lens, as even at f4, a very wide lens will probably keep everything sharp.
 
MWHCVT said:
Right, first off don't focus to infinity for a star trail, focus on the foreground interest that your including as part of the scene has a star trail with perfectly sharp stars bit an OOF foreground looks absolutely awful, where as due to stars being such a small source of light if they are a tiny bit OOF but the foreground is sharp no one is going to notice ;)

For further info on star trails you may find it helpful to check out my guide to star trails in the tutorials section, there's a link to it in my signature :thumbs:
Click to expand...

Got you :thumbs: Thanks Matt, I have been reading and re-reading your tutorial to get it all in my head! :)
 
Yellowbelly said:
Just turn your focus ring as far as it will go to infinity, then turn it back a tiny amount or use the scale to focus anywhere from 20 feet to infinity.
Click to expand...
That may be quite hit and miss with the Sigma 17-70. It goes well beyond infinity. I agree live view is the way to go. If there's not enough light for that, set it to infinity in the light, then tape it up.

I now use the Tokina 11-16 for night photography. Just bang it on the endstop - it's focussed :love:
 
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