To Infinity and Beyond?

[The Stig]

Why do all my three lenses allow me to focus beyond infinity? From what I've understood(?) so far, infinity represents the distance where a point is so far away from you that the light from it enters the lens almost as parallell lines. Why do I need to go past this? Doesn't that mean that beyond infinity focus light rays would actually converge coming towards my lens? So far it's just another version of "out of focus" frem where I'm standing. What am I missing here?

Thanks,

-S

[arith]

Is it a zoom lens? What about temperature. Possible reasons but favourite is zoom.

[The Stig]

Er... Sorry - I'm not sure I understood what you meant there. I have three lenses - two zooms and one prime - they all focus beyond infinity.

[Steaphany]

Yes, it is simply out of focus and just how the lens designers chose to implement the design. Instead of making the mechanical stop of the lens focus adjust ring correspond to the exact position of ∞, they just made it go a little beyond it.

If you want something even more wonky, do you realize that the focal scale on your lenses isn't necessarily accurate ? For example, Where do you physically set the ∞ symbol in relation to the "pointer" to actually have the lens set to ∞ ?

[arith]

Er... Sorry - I'm not sure I understood what you meant there. I have three lenses - two zooms and one prime - they all focus beyond infinity.

I only mean't is it a zoom; but anyway I have found an answer. Did you know the numbers for dof cannot be right for any camera; since a 1.6 crop has different dof to a 1.3 ect.

[The Stig]

Thanks for the replies. I think your lenses are more expensive than mine - I just have a very basic (and not very detailed) distance scale on my zoom lenses. It really stems from me observing that no matter what I point my camera at (including stars) I could always get the focus to overshoot infinity. I was just wondering if there was any good reason for it, but from all of your replies I guess there isn't. I actually have a Minolta SLR from 1969 that has a hard stop at infinity on its 55mm f/1.7 lens, so it obviously can be done

Cheers,

Stig

[FrankMi]

I think your lenses are more expensive than mine - I just have a very basic (and not very detailed) distance scale on my zoom lenses.

LOL! Consider yourself lucky, Stig! My Nikon 55-300mm zoom doesn't have any distance scale!

[krispix]

Hi Stig,

Unfortunately this is pretty common these days. Most of my lenses do it. A bit annoying really, you would hope that you could manually wang it round to the stop and have infinity in focus, but you can't. The IR issue is a bit of a red herring because IR wave length is shorter than the visible spectrum and therefore focus is in front of the indicated range.
Unless they're anticipating an Ultra Violet sensitive sensor (LOL) which would need to focus beyond. I think the real reason is modern manufacturing tolerances are sloppy and they won't spend the time and money necessary to set the lenses up properly, so they err on the side of caution and place the stop beyond infinity.

[nickjohnson]

Folks,
One possible reason why so many of our lenses don't have a hard infinity end stop is because they have auto focus. Most autofocus systems work by tuning to a null (highest contrast point – for example) – using an iterative convergence cycle – on either side of the null. Where the null happens to be at infinity it is necessary for the mechanism to be able to focus past infinity so that the auto focus can converge on the correct point of focus.
HTH

Regards,

Nick.

[Dave Humphries]

Folks,
One possible reason why so many of our lenses don't have a hard infinity end stop is because they have auto focus. Most autofocus systems work by tuning to a null (highest contrast point – for example) – using an iterative convergence cycle – on either side of the null. Where the null happens to be at infinity it is necessary for the mechanism to be able to focus past infinity so that the auto focus can converge on the correct point of focus.
HTH

Regards,

Nick.

Give that man a prize for the most plausible (to me) explanation

[arith]

I'm a bit shocked to find Canon must therefore be telling lies.

[Dave Humphries]

I'm a bit shocked to find Canon must therefore be telling lies.

I have often heard it quoted (no doubt because it is in the manual), but as an engineer, I've never really bought into the idea that temperture could make that much difference.

Nick's explanation makes far more sense, but is perhaps harder for the lay person to understand without gaining the false impression it is a weakness*, hence (IMHO), the 'Canon party line', which makes them look good

* "To allow the Auto-Focus to home in on objects at infinity without crashing into a hard endstop we allow the lens to move past the infinity focus point" - see what I mean?

[krispix]

I'll buy Nick's explanation, although it does make life harder for people who still remember how to focus manually.

[AlanP]

I believe that they were used when shooting infrared film. Different colors will refract at different angles and infrared film required a focus correction (focus at a greater distance than non ir film) to focus properly.

[arith]

I have often heard it quoted (no doubt because it is in the manual), but as an engineer, I've never really bought into the idea that temperture could make that much difference.

Nick's explanation makes far more sense, but is perhaps harder for the lay person to understand without gaining the false impression it is a weakness*, hence (IMHO), the 'Canon party line', which makes them look good

* "To allow the Auto-Focus to home in on objects at infinity without crashing into a hard endstop we allow the lens to move past the infinity focus point" - see what I mean?

I like that as well actually; I'm not an engineer but it is easy to see that the mechanism would survive longer if it didn't keep banging up at the end.

But why put infinity at the end of an L, why not just put a line in? It is all a bit odd.