infinity and Beyond!

[YT2095]

I have a Camera lens (a 300mm) that I use to photograph the moon and other far away things, the "problem" I have is that when I set the lens to Infinity for the Moon for instance it`s a little bit out of focus and I have to turn it back just a little bit to be in perfect focus.

so my question is, when it`s set to infinity (just beyond what is perfect focus for the moon), what IS it focusing on?

 

what sort of distances am I looking at for that setting be perfect?

[CaptainPanic]

Infinity probably means that there is no focus.

A lens has a focal point, and if the object is at the same distance as that object, then it will be a sharp (focussed) picture.

A lens that has the focal point at infinity will never completely focus. I think it means that in the "thin lens formula", the value of S1 is at infinity, meaning that the lines originating from a point on one side of the lens (picture side, S2 side) will never converge, or diverge, but in stead will run perfectly parallel.

[Klaynos]

I suspect, at that point it's assuming the incident light is collimated.

[YT2095]

so perhaps for a 300mm lens, the Moon isn`t point-like enough of a light source to use the Infinity marker?

[insane_alien]

yep. only parallel light beams will be focused perfectly. i think thats the definition of infinity focus.

[YT2095]

ok dokey, cheers for that, next time we have a nice clear night I`ll try focusing it on a star and see that works, it Should be a point-like enough source to work.

[doG]

For parallel rays you could trying photographing the sun through a welding filter....

[insane_alien]

the sun is hardly point like.

 

it is further away but it is also bigger. and to about the same ration which is why we can get a total eclipse.

[swansont]

the sun is hardly point like.

 

it is further away but it is also bigger. and to about the same ration which is why we can get a total eclipse.

 

Right. The divergence is about 9 milliradians.

[doG]

the sun is hardly point like.

 

I wasn't suggesting it as a point source but simply an alternate parallel source to experiment with to see what difference it has on the focus....

[YT2095]

it doesn`t make any difference to be honest, I have tried the sun and it`s also more or less the same size as the moon on film when developed also.

 

I think it needs a star or maybe street lights across a bay when I go on holiday, they`re pretty tiny and might work.

 

either that or an element in the lens has slipped or it needs recalibrating or something???

[John Cuthber]

I think that it's down to manufacturing tolerance.

Imagine if the slight errors in making the lens meant that, at "infinity" it focussed at say 100 M. You wouldn't be able to get a clear pic of anything further away.

It makes sense to have the lens able to move a bit beyond "infinity" in order to ensure that, even if it gets a bit worn, it will still focus at infinity. You can then rely on the autofocus or your own abillity to sort it out.

The answer to the question is that, for a lens set at infinity it will fucus light that starts out parallel. Set nearer than that it will focus divergent rays.

Set slightly beyond infinity it will focus light that is slightly convergent.

[Flashman]

There might be 5mm allowance for using it on an older/newer body with an adaptor ring.

[YT2095]

There might be 5mm allowance for using it on an older/newer body with an adaptor ring.

 

That makes sense, as it`s originally an M42 mount, and has an M42 to Pentax K mount adapter on it.

I`ll have to try it on my Praktica SLR without the adapter and see what that does, Cheers