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04-26-2012, 07:20 PM   #1
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Hard Stop at infinity

Just curious, why don't most modern lenses have a hard stop at infinity? I think its a handy feature. And this isn't pentax specific, I noticed it with my newer Nikon lenses as well.

04-26-2012, 07:31 PM   #2
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Do you mean why do they focus past infinity? From what I have been led to believe, plastic auto focus lenses are much more susceptible to temperature. If you calibrate it to stop at infinity at a particular temperature, you might loose infinity at another temperature. I'm guessing many do not have the precision tolerances that some of those good old all metal lenses had. If it's a little out of spec in an opposite direction of what your camera is you might also loose infinity focus too. Another reason is to allow infinity focus of infra red light. You have to focus a bit past infinity for visible light to get infinity focus when shooting infra red.
04-26-2012, 07:32 PM   #3
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I'm no expert but how about adjustment of AF?

I just picked up F28/2.8 that would not lock focus at infinity. I had to fine adjust +6 to get it to focus confirm.
04-26-2012, 08:01 PM   #4
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AF lenses focus past infinity simply because "They Have To" in order to get a focus [for multiple points] [and also for the "Trap" that is required for focusing] - without this there could be no multi-point focusing and thus the only auto focusing you could ever have would be single point focusing. This really does not have much to do with infinity but in order to focus the AF system needs to key it at contrast then past contrast to know to bump it back. Of course this isn't the correct way to explain it but should give you a little more insight into the AF engineering...

04-26-2012, 10:17 PM   #5
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From the manual for the FA*600/4

"The distance scale on the lens stops slightly past the infinity mark. This allows the lens to focus on a point past the infinity scale in the case where a temperature change causes the focusing point to shift. Even when shooting at infinity confirm that proper focus is achieved through the viewfinder before releasing the shutter."

From the Pentax 67 Interchangeable Lenses Operating Manual

"Note on Infinity Indication: With a powerful telephoto lens, there may be a focus shift between the distance indicated on the lens and the actual camera to subject distance, influenced by temperature conditions. For this reason, the focusing ring is specially designed to rotate slightly past the infinity indication. So, even when you take pictures at infinity, be sure to confirm good focus through the viewfinder."
04-27-2012, 06:47 AM - 1 Like   #6
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I don't know but it sounds like a great title for a book.
04-27-2012, 02:53 PM   #7
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Good catch...definitely a best seller!
04-27-2012, 03:44 PM   #8
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Ah...that's why I like this place, I learn something new everyday


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