Focusing to infinity?

Andyvalver

Andyvalver

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Andy
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Im quite new to photography and wondered if anyone of you guys could explain focusing to infinity to me?

What situation will i need it and why id use it and how to use it or focus to it?

Cheers in advance :P
 
Normally used for landscape photography where much of the subject is very far away.

Most lenses have an infinity symbol on the focussing ring (It' like an 8, but lying on it's side). The focussing ring will turn no further beyond infinity.
 
I actually asked the same question a few months back, from the answers I got back a little research on the tinternet and a little trial and error I realised that the infinite focus position gives a reasonable, probably defined in a lab, amount of focus across the entire field of view.

In reality no lens can be expected to be tack sharp across the entire field of view, some of the best lenses do come very close though, this is not the best option if you want to achieve the most impact in a landscape for instance.

Generally the best option is to decide what, in the scene, you want to be the focal point of the image and focus on that. :thumbs:

Another general rule;) is to view the scene and imagine it separated into 3 and focus one 3rd into the scene. This will again give you an even spread of focus across the image.

Have a look here for a good resource explaining depth of field and hyper focal distances. Online depth of field calculator
 
If your lens has an aperture scale on it (a lot of lenses don't have these days) - then set the infinity mark against the aperture that you are using to get maximum depth of field. So, for example, in a landscape shot you are shooting at f/16 - set the infinity mark against f/16 on the aperture scale - and it will tell you your DoF reading from one side of the aperture scale to the other. Does that make sense? It would be easier to show you than write about it!
 
awp said:
If your lens has an aperture scale on it (a lot of lenses don't have these days) - then set the infinity mark against the aperture that you are using to get maximum depth of field. So, for example, in a landscape shot you are shooting at f/16 - set the infinity mark against f/16 on the aperture scale - and it will tell you your DoF reading from one side of the aperture scale to the other. Does that make sense? It would be easier to show you than write about it!
Click to expand...

awp is talking about the Hyper-focal Distance ...that being, the effective distance of any subject against focal length and F number which can result in the back focus range of the dof being at infinity. This is called the hyper-focal distance.

This shot is taken with a lens of a focal length of 28mm @ F2.8. With the rear dof (edit. dof = depth of field) focus range being at infinity.

dung901.jpg


The focus is on the boat at 40-50 feet, the near dof range is just in front of the lit grass about (guess) 25 feet from me, the foreground in front of that being blurred by the lack of focus because its outside the dof range ..In the background however, the focus is at infinity, because my settings have exceeded the rear dof range and I have entered the hyper-focal distances of that focal length/F stop combination.

How confusing that is I've no idea... :thumbs:
 
lovely shot which nicely illustrates the point.
 
first lesson...dont focus on infinity
you throw away miles of depth of field
infinity is a philosophical point where the lens is at furthest focus...and sharp...this could be around 200 metres
if you do a landscape at say f5.6 with a 28mm wide angle...you can really focus on a point around 50 metres away and get...'infinity' in 'focus'...
really anything so far away cant really be seen...on earth..due to the effects of atmospheric conditions
i say on earth because we all know how sharp a moon shot etc can be...
landscapes in superb atmospheric conditions..in mountainous clear day conditions...i just click back a notch from infinity setting..or if autofocus...focus on something in the 2/3 distance area
chances are your furthest objects wont excite the focus sensitivity anyhow



infinity is that bit way far away...the most intersting parts are nearer...focus on them..in this case the lake...and the dof will generate an acceptable focus for ...far away...

**dont focus on infinity..get used to using the dof parameters and let infinity be at the far end of the dof...



"whats dof?"...new thread
 
Thanks alot, i understand it mor now, also been reading on the net. Nice pictures :)
 
If you have an iPhone there is a nifty little app that will work out focal distances etc if you put in your aperture, focal length and distance to subject, it's helped me get to grips with DOF and is useful when out and about. Its free as well, think its called Depth Of Field Guide (field tools) and has different settings for different types of camera (Nikon DX, Canon APS-C etc etc). As others mentioned I used to think that focusing to infinity meant focusing on the sky to get everything in focus, obviously very wrong!
 
Infinity is the position on a lens where light from distant objects is focused in parallel lines. All subjects at that distance or greater will be rendered with maximum sharpness.

Ininity distance varies with focal length a lot. With ultra wides it can be only a dozen or so metres away, with super telephotos it can hundreds of metres.

Modern lenses have a variable infinity focus position because they are affected by heat expansion, and it shifts a bit. But the distance remains the same, just that the lens needs to move a little further to get it spot on.
 
arclight said:
Most lenses have an infinity symbol on the focussing ring (It' like an 8, but lying on it's side). The focussing ring will turn no further beyond infinity.
Click to expand...

Sorry.... not correct. Early manual focus lenses - yes. But modern autofocus lenses will turn beyond infinity, and the image de-focuses. The autofocus function needs to be able to sample focus either side of the correct focus point, to ensure that it is focused. So the lens needs to be able to move past infinity to make sure that it pinpoints infinity precisely.

Also, temperature variations will change the length of the lens. Admittedly, not a lot, and varies with lens size and temperature obviously. The infinity point will change, hence the need to sample beyond the marked point.

So, if you're manually setting infinity, don't just crank the lens all the way round to the stop.
 
MisterE said:
Sorry.... not correct. Early manual focus lenses - yes. But modern autofocus lenses will turn beyond infinity, and the image de-focuses. The autofocus function needs to be able to sample focus either side of the correct focus point, to ensure that it is focused. So the lens needs to be able to move past infinity to make sure that it pinpoints infinity precisely.

Also, temperature variations will change the length of the lens. Admittedly, not a lot, and varies with lens size and temperature obviously. The infinity point will change, hence the need to sample beyond the marked point.

So, if you're manually setting infinity, don't just crank the lens all the way round to the stop.
Click to expand...

Yes, you are correct.
 
i think andyvalver went home...:(
 
Cheers for the advice guys. I forgot about this thread lol :P
 
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