Infinity, and beyond

[Josh66]

OK, so I decided not to derail this thread any further...


I was going to reply to this post, but thought a new thread would be more appropriate.

And all these years I thought is was because infrared has a different focus point than visible light.

I don't think it has anything at all to do with temperature. And it's not IR either. IR focus is on the near side of infinity, not the far side (look at your lenses if you don't believe me).

My theory is that the over-travel allows the AF mechanism to overshoot it a little, then come back. (To avoid damage from the lens slamming into infinity.)


Just a couple observations - every AF lens I have used will go past infinity. Every MF lens I have used has a hard stop at infinity.


edit
In case anybody is wondering about the different focus point for IR - that's what the red dot is for. When you're shooting IR, use the red dot instead of the line.
And if that still doesn't make any sense to you, you probably don't have any lenses that have distance scales on them.

I recall my 35mm film camera lenses saying in the instructions that's the reason the lens goes past infinity. And none of them were AF.

Now, on all of my lenses, the red dot for IR focus reaches infinity before the 'normal' focus mark. That means that you would never have to go past infinity to focus IR at infinity.

pgriz says that it's to allow for temperature variances (and cites the manual to a lens). I said that it's to allow the AF mechanism to overshoot it and come back to prevent damage (just seems right - I don't have any references).

So, we have 3 theories - (1) IR focus; (2) Temperature; (3) Built in 'slop' to prevent damage.



Since IR reaches focus before visible light, there is no reason to go past infinity in IR. The temperature differences seem kind of small to me (relatively) for it to affect a lens to that degree. The 'slop' theory sounds good to me, but I can't back it up with anything.



So, the question is - why will some lenses allow you to focus past infinity? Also, is there any situation (other than those mentioned) where it would be desirable to focus past infinity?

 

[Garbz]

How about correct for variance in manufacturing? It's much easier to manufacture a lens with tolerances that allow it to focus past infinity then use calibration tools to adjust the focusing system.

 

[EDL]

The auto-focus theory seems to be one of the most prevalent if you google it and I guess that makes sense, allowing the mechanism to roll past infinity.

Another point I saw was that zoom lenses might have a slightly different infinity focus at different focal lengths.

 

[tirediron]

I can't say definitively, but: Physics tells us that it is definitely NOT #1, and as for #2, all of my old glass (the good stuff anyway) stopped dead on infinity, and I'm sure that with the all-metal construction, thermal expansion would have been even more of a factor (and by that token, wouldn't there need to be slop in the actual element mounts so that when the lens go very cold they weren't compressed to the point of damage?). Therefore, it seems reasonable and logical based on the 'overshoot and return' design of most AF systems that choice #3 is the correct one.

 

[Buckster]

All I can say for sure is that the title of this thread makes me think of Buzz Lightyear.

 

[KmH]

Different wavelenghts of light focus to different distances - that's what chromatic aberration (CA) is all about.

IR has a lower frequency than visible light, so in a simple lens IR will focus the furthest, distance wise, from the lens.


However, CA can be corrected by making a compund lens:


Consequently, the issue of 'Infinity, and beyond' usually boils down to who made the lens, and how much design time and manufacturing effort is put into making a lens.

 

[Josh66]

All I can say for sure is that the title of this thread makes me think of Buzz Lightyear.

The kids have been watching Toy Story 3 a lot lately, lol.

 

[Dao]

All I can say for sure is that the title of this thread makes me think of Buzz Lightyear.

Thanks to you ... now the words "Buzz Lightyear" stuck in my mind.

 

[Josh66]

Nobody really touched on this:

Also, is there any situation (other than those mentioned) where it would be desirable to focus past infinity?

"Those mentioned" being the possible reasons that it goes past infinity in the first place.

I can't think of any time where you would want to focus past infinity... UV photography maybe. That's the only thing I can think of. In order to focus UV light at infinity, you would have to 'go past' infinity in visible light, correct?

Edit
UV is sort of in a special place though. When you start talking about quartz elements, I would just sort of expect them to account for the different focus point in some way. A lens like that might not even have focusing marks on it for visible light...