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10-20-2008, 11:50 AM   #1
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Why would you ever want to focus past infinity?

So why would they put it on lenses. It just confuses everything..

Also, why dont more lenses have focus limiters?

10-20-2008, 12:02 PM   #2
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-account for thermal expansion in lenses?
-not have AF smash into the stop every time?

That said, my FA50 seems to have the infinity stop set right at infinity focus and doesn't have any problems that I can tell...
10-20-2008, 12:02 PM   #3
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I think an autofocus lens needs a little margin for error beyond infinity so that the lens doesn't slam into the stop when doing a long focus seek from near focus to an object at infinity.
10-20-2008, 12:08 PM   #4
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QuoteOriginally posted by sewebster Quote
-account for thermal expansion in lenses?
-not have AF smash into the stop every time?

That said, my FA50 seems to have the infinity stop set right at infinity focus and doesn't have any problems that I can tell...
Yup, heard about this one too

10-20-2008, 12:18 PM   #5
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I know what you mean about this, but...

Beyond inifnity. This is surely Buzz Lightyear territory.

If your lens focus into alternate dimensions, then is it for sale, and how much?
I don't care if it's got fungus or scratches!
10-20-2008, 12:26 PM   #6
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QuoteOriginally posted by IsaacEastgate Quote
So why would they put it on lenses. It just confuses everything..
Back in the day of manual everything, the infinity mark was more of a reference mark than a place to go. With the DOF lines on the lens, you could set infinity on the line corresponding to the stop, look at the minimum "in focus" distance, and you knew that everything beyond, say 3 meters, would be in focus.

QuoteQuote:
Also, why dont more lenses have focus limiters?
why would you need a limiter on anything other than a Macro lens?
10-20-2008, 12:37 PM   #7
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QuoteOriginally posted by Spongefingers Quote
Beyond inifnity. This is surely Buzz Lightyear territory.

If your lens focus into alternate dimensions, then is it for sale, and how much?
I don't care if it's got fungus or scratches!
when you focus to infinity you are bringing the glass elements closer to the focusing plane

if your lens is mechanicaly set up, there is a point where if you bring the lgass any closer, you will start to go out of focus again, i'm sure there is a mathematical explanation for that i'm just too lazy to research

but i have a lens that can do that.
10-21-2008, 09:42 AM   #8
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My question would be...why focus at infinity at all? You're throwing away half of your DoF by doing so. Hyperfocal is the way forward.

But seeing into infinity and beyond can be useful if you're fighting HAL, the rogue computer in 2001. "My God...it's full of (SMC) stars (lenses)..."

10-21-2008, 11:29 AM   #9
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QuoteOriginally posted by Miserere Quote
My question would be...why focus at infinity at all? You're throwing away half of your DoF by doing so. Hyperfocal is the way forward.

But seeing into infinity and beyond can be useful if you're fighting HAL, the rogue computer in 2001. "My God...it's full of (SMC) stars (lenses)..."
Not really throwing away half. Maybe focusing at infinity gives me say 10m to infinity and hyperfocal gives me 5m to infinity. Total DoF in both cases is infinity

Rationale would be to avoid infinity out of focus problems, especially if using the "traditional" hyperfocal point, which will most likely have infinity somewhat out of focus if you have decent eyes. Clearly there is some room to focus in front of infinity without problems, but sometimes you don't care about the foreground (or there isn't one) and you don't want to risk it.
10-21-2008, 12:01 PM   #10
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On longer lenses it is to account for thermal expansion, which can change the focal length slightly, but enough to throw it out of focus when the lens is hotter than it was meant to be. This is slightly mitigated by using polycarbonate for lens construction, rather than metal.

I'd also imagine there are (or were) AF systems out there that spin past the point of sharpest focus and back to help nail it.

As for hyperfocal only...this is one of those questions like "Why do they even bother with letting DSLRs capture JPEGS? Why bother with optical viewfinders? Why is it a Green Button and not a Beige Button?"

I'd like it if camera manufacturers would still let me focus exactly on what I want to focus.
It's just as easy to let a lens spin to infinity as to just hyperfocal. And, say, on the FA f1.4 50 wide open, hyperfocal may as well be infinity.

As for "You're throwing away half of your DoF by [focusing to infinity rather than to hyperfocal]" this works on the theory that the there's a sudden, definite change between in-focus areas to out-of-focus areas. Which there isn't. Even at f22.

But I can't help you if you're deathly afraid of that prone 8.
10-21-2008, 12:23 PM   #11
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QuoteOriginally posted by Ed in GA Quote
Back in the day of manual everything, the infinity mark was more of a reference mark than a place to go. With the DOF lines on the lens, you could set infinity on the line corresponding to the stop, look at the minimum "in focus" distance, and you knew that everything beyond, say 3 meters, would be in focus.



why would you need a limiter on anything other than a Macro lens?
A good tele would benefit too, if you know you will only focus from 4m to infinity, why let it hunt between 1-4m? or the other way around.
10-21-2008, 12:24 PM   #12
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I remeber reading somewhere that it is somehow related to lens autofocus calibration during manufacturing.
If i recall correctly fine tuning of lens position is done electronically (not by setting the lens at infinity mark and adjusting the system to be in focus, but by "telling" the lens how far it should turn when it receives "infinity" from camera) , so the actual infinity sometimes may happen to be mechanically the infinity mark.

If i find the link ill uptade this post with it.

I also doubt its for af not to slam into it, because My sigma 30/1.4 on k100ds goes past infinity mark and does that anyway.
10-21-2008, 12:31 PM   #13
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QuoteOriginally posted by ytterbium Quote
I remeber reading somewhere that it is somehow related to lens autofocus calibration during manufacturing.
If i recall correctly fine tuning of lens position is done electronically (not by setting the lens at infinity mark and adjusting the system to be in focus, but by "telling" the lens how far it should turn when it receives "infinity" from camera) , so the actual infinity sometimes may happen to be mechanically the infinity mark.

If i find the link ill uptade this post with it.

I also doubt its for af not to slam into it, because My sigma 30/1.4 on k100ds goes past infinity mark and does that anyway.
To have wiggle room for AF adjustments makes for cheaper cameras/lenses and more profits, instead of holding cameras/lenses to tighter tolerances.
10-21-2008, 01:31 PM   #14
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QuoteOriginally posted by lithos Quote
Why is it a Green Button and not a Beige Button?
Oh come one, beige is so 1970s
10-21-2008, 06:03 PM   #15
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QuoteQuote:

why would you need a limiter on anything other than a Macro lens?
To pre-set perhaps 2 different focus points that you could get to (manually) easily such as two points on a race track, or the goal line/basket, etc.
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