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Author Topic: Infinity focus  (Read 7100 times)
Ben Rubinstein
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« on: December 31, 2005, 07:23:02 PM »
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I've owned and shot many a photo using manual and AF camera, 35mm, digital and Med Format but although I use hyperfocal focusing when I can, I can't say that I've ever used infinity focus.

I know I used to know this but years of not bothering with it has made me forget. On the distance scale of my lenses I will see a number of distances and then a longer throw and the infinity symbol. Does that mean that anything from the last distance marked is included in infinity? Evidently not as when I focus on an object past that marking the lens is not focused at infinity. Infact often when focusing far beyond the markings with an AF lens, such as 100m away or further, the lens still doesn't show infinity on the distance scale. Then it seems that it is possible to rotate the focus ring past the infinity mark on the scale.

As I thought I had understood it, infinity focus is a certain distance from which anything further will be in focus, at any aperture. Is this the case that if I set the dial to the infinity mark? How do I know what distance the inifinity mark is from, why can I focus further than the infinity mark?

I'm rather embarressed asking this, I'm a pro wedding photographer and I dabble in landscape photography for recreation. I know hyperfocal focusing, I know zone focusing and use both in day to day shooting but I realised that although most of my photos would not warrant or make use of it, I should remind myself of what Infinity focus is useful for.
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Jack Flesher
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« Reply #1 on: December 31, 2005, 08:46:19 PM »
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How do I know what distance the inifinity mark is from, why can I focus further than the infinity mark?


Pom, I'm sure you know this and have probably forgot  

All lenses are designed to focus beyond infinity.  Temperature extremes can expand or contract lens barrels and shorten or lengthen them which in turn can alter the actual focus points on the lens.  The reason they focus beyond infinity is to allow for infinity focus if those changes occur.

As for the infinity point, it is standardized and I believe it is something like 100x the focal length of the lens.  

Happy New Year,
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Jonathan Wienke
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« Reply #2 on: December 31, 2005, 10:49:53 PM »
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Pom, at least you can take solace in the fact that you're not arguing in favor of the idea of actually focusing beyond infinity...

And Jack's explanation is exactly right. The focus mechanism allows movement past the infinity focus point to allow for thermal expansion & contraction, as well as manufacturing tolerances.
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Ben Rubinstein
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« Reply #3 on: January 01, 2006, 05:50:44 AM »
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So how the heck would I know when to use the infinity mark or further than the infinity mark because my lens is warm today?
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Jonathan Wienke
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« Reply #4 on: January 01, 2006, 08:31:03 AM »
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Don't use the infinity mark to set infinity focus; aim the camera at something sufficiently distant and focus on it. When the moon or a distant mountain is in the sharpest possible focus, you're at infinity.
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Ben Rubinstein
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« Reply #5 on: January 01, 2006, 09:32:51 AM »
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In other words more trouble than it is worth... back to hyperfocal focusing.
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Jonathan Wienke
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« Reply #6 on: January 01, 2006, 09:44:34 AM »
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In other words more trouble than it is worth... back to hyperfocal focusing.
Unless you're shooting celectial bodies and everything is at infinity, you're better off with hyperfocal anyway. Just make sure you're using .00888mm as your CoC when calculating the hyperfocal distance or you may be disappointed to discover your DOF isn't quite as hyperfocal as you intended.

BTW, the 1Ds AF is sufficiently accurate that it will find correct infinity focus just fine in all but the darkest conditions. It's not much trouble IMO.
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Ben Rubinstein
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« Reply #7 on: January 01, 2006, 10:48:33 AM »
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Is that what was finally decided on as the real CoC for FF digital?
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Jack Flesher
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« Reply #8 on: January 01, 2006, 10:56:03 AM »
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So how the heck would I know when to use the infinity mark or further than the infinity mark because my lens is warm today?
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Actually, it would depend on how warm it was      

Fact is, those marks are only 100% accurate at standard temperatures; I'm guessing ~~ 20*C/68*F  After that you are flying blind.    

In the "old" days when CoC's for our DoF calculations were .03" it wouldn't have mattered.  Now that we view at "actual pixels" our CoC's are so small it does matter.  Maybe we need to add accurate digital distance readout to that Canon 1-series wish-list.

HNY,
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Jonathan Wienke
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« Reply #9 on: January 01, 2006, 01:41:40 PM »
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Is that what was finally decided on as the real CoC for FF digital?
That is the pixel pitch spacing for the 1Ds. If you were using a 1Ds-MkII, it would be .00721mm, and the correct 5D CoC value is .00824mm. As you can see, higher resolution cameras require a more stringent CoC value.
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Ben Rubinstein
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« Reply #10 on: January 01, 2006, 02:38:25 PM »
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At least you can use f22 then with a FF camera without diffraction being too noticeable as it would be with a crop camera, that CoC really does change everything.
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Jack Flesher
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« Reply #11 on: January 01, 2006, 03:09:37 PM »
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At least you can use f22 then with a FF camera without diffraction being too noticeable as it would be with a crop camera, that CoC really does change everything.
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Not in my experience -- f11 is about max now on a FF camera.  Any smaller and you can see the detremental effects of diffraction.  Same goes for f8 on crop cameras.
« Last Edit: January 01, 2006, 03:10:11 PM by Jack Flesher » Logged

Ben Rubinstein
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« Reply #12 on: January 01, 2006, 05:12:35 PM »
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Maybe I should have said 'acceptable' level of diffraction. Then again I'm saying that to the man with the exotic OM WA's and a whole bunch of T/S lenses where stopping down is not the issue that it is for me shooting near/far landscapes with a 70-200!
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Jack Flesher
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« Reply #13 on: January 01, 2006, 05:44:25 PM »
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Maybe I should have said 'acceptable' level of diffraction. Then again I'm saying that to the man with the exotic OM WA's and a whole bunch of T/S lenses where stopping down is not the issue that it is for me shooting near/far landscapes with a 70-200!
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Well, you could always buy a wider lens...

But then again, the good news is that you can actually get by with smaller apertures as you go up in focal, so can probably get by with f16 on your 70-200 towards the long end
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crspe
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« Reply #14 on: January 03, 2006, 03:57:06 AM »
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Actually you cannot go to smaller apertures in longer lenses - the diffraction is only dependant on the f-number, not on the focal length ... On a longer lens, (with identical f-stop), the aperture is larger (less diffraction), but the focal plane is further away because of the longer lens (more diffraction).  These 2 cancel out exactly and so diffraction is only dependant on f-number.

This website has a great description & pictures:
http://www.cambridgeincolour.com/tutorials...hotography.htm#
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