What is purpose of allowing focus ring to travel past infinity?

[scott squire nonfiction]

Okay, I just got a new (used) 16-35 2.8L (and I'm quite chuffed about it, as our

English friends say). I note that the focus ring allows travel (and 'focus') past the

infinnity mark. Real infinity (or at least a hillside five miles away) is in focus just when

the indicator says infinity, but the ring will travel further, and the focus changes too.

 

Looks clear that it was designed to do this, but Why?

 

Thanks,

 

Scott

[ashtonsmith]

I'm pretty sure that lets you focus on infinity with infra-red film. That or maybe heat/cold

tolerances. I'm almost positive on the infra-red thing though.

[PuppyDigs]

To allow focus with various sensitivities of infrared films. That is, infrared achieves focus differently than visible light.

[johnlund]

The user guide notes that it's to account for temerature. Did you get a guide with your lens? At any rate, the <a href="http://photonotes.org/articles/beginner-faq/lenses.html#infinitymark">photonotes FAQ</a> contains a lot of useful information like this.<br/>

<br/>

Compensating for IR works goes in the other direction, doesn't it? That's what the manual makes it look like anyway.

[jerry thirsty]

I always figured it was so the autofocus motor could overshoot without hitting a "hard stop" and possibly damaging itself.

[sumo_kun]

I have a 28-135 IS and that lets you twist the focus ring past either focus limit, but the focus doesn't change. Isn't that to allow for the full time manual feature? It feels like theres some sort of clutch system to decouple the motor from the ring...

[robert_roaldi1]

I have an old Takumar 400mm screw-mount lens that does the same. It overshoots at both distance extremes.

[zax]

The lense seems to past the infinity mark for heating tolerances. The red dot below the metering scale, which is a little bit right of the right focus sign is for infra-red fotography... Just look at the manual and everything is there....

[scott squire nonfiction]

Manual Schmanual! Who needs manufacturers' literature when you got photo.net?

Thanks!

[johnlund]

<i>"Who needs manufacturers' literature when you got photo.net?"</i><br/>

<br/>

Well, some of the answers you got were well intentioned, but wrong. PN is nice, but you can save yourself a lot of lurking time by skimming the manuals. It's worth at least flipping though the manual if only to know what's there for future reference.

[david_gonzalez]

I'd go with the thermal expansion explanation. Reason - My modern EOS plastic construction lenses all will go past marked infinity. My old manual Minolta lenses, all made with solid metal barrels, stop rock solid at their marked infinities. A lot more thermal expansion with polycarbonates than brass.

[byron_w]

Polycarbonates will simply flex more as they warm up, so thermal expansion is hard to control.

 

However there is not "a lot more thermal expansion is polycarbonates than in brass"

 

It is difficult and costly to make a lens. The tolerances are very fine. In your manufacturing plant, you want to eliminate as many of these fine tolerances as possible, in order to allow cheaper "high tolerance" machining. Stopping your lens element exactly at the infinity mark counts as a high tolerance machining, and because its not strictly nessisary, the manufacturer adds a buffer(your past infinity focusing distance), and uses cheaper machining.

[mormegil]

Well a guy I know who has a giant Russian made reflex lens, that focuses passed infinity told me it's used for astrophotography. He was into that, so maybe he's right.

[akajohndoe]

Would it not also allow one to approach infinty focus more closely with an extension tube?

[digitmstr]

to reach the "Twilight Zone"? ;-P

[jarl_rise1]

On all my old manual focus Pentax lenses the physical stop corresponded to infinity, on my newer AF Pentax and EOS lenses you can focus "past infinity".

No matter the reason for this, it's a pain in the neck when doing night photos, it's to dark to see if the focus is correct, and you need a small flashlight to see the focus ring...