Focusing Beyond Infinity.

**GUB** · 05-08-2021, 02:07 AM

Yes I agree that "focusing past infinity" appears impossible.
But the point where "infinity" is in focus is a very specific point for a lens and it is easy to bring the elements closer to the sensor than that.
So the term is merely a simplification of the fact that the elements are closer to the sensor than the infinity point.
What would the alternative be. Perhaps the lens has been "adjusted to less than its focal length"? Rather clumsy and un-explanatory and probably technically incorrect for a lot of lens designs.

**GUB** · 05-08-2021, 02:17 AM

Originally posted by AggieDad

Perhaps the better phrase would be “past the ∞ mark.

No this is not what is meant. The infinity mark is at the stop of the focusing ring - so if a manual focus lens is focusing beyond infinity the infinity mark will be also beyond the point of infinity. (anyone know the linux keyboard code for "infinity"?)

bertwert · 05-08-2021, 07:34 AM - **1 Like**

Originally posted by GUB

(anyone know the linux keyboard code for "infinity"?)

If you have a Compose key set up it's just Compose 8 8 - as an aside I set up my Caps Lock key to be a Compose key as it is far more useful to me as that than a Caps Lock key.

Alternatively, if you know the Unicode designation of any character, you can type it using Control+Shift+u {code} - for ∞, which is U+221e, you'd type Control+Shift+u 2 2 1 e and then hit space.

dlhawes · 05-09-2021, 02:21 AM

The purpose of focusing beyond "infinity" is that the infinity mark might not be located exactly and precisely correctly on the lens body, so what you're really doing is making up for the fact that the lens is a manufactured item made to conform to "tolerances"; in other words, you're actually focusing AT infinity as to the capabilities of the lens' construction, which happens to put the indicator beyond the mark.

**EssJayEff** · 05-11-2021, 06:38 AM

Sorry if someone already wrote this and I missed it. I've always used the ∞ symbol on a lens as the guide to the outer limit of my depth of field for a given f/stop. For example, if I want to shoot at f/8 and want everything "up to infinity" in focus, I find the pair of lines for f/8 (on many old lenses the f/stop pairs are the same color, but many modern lenses don't even have them), place the ∞ on the distant line and then everything in between that and the distance for the closer line would be my approximate depth of field range. I found it not to be highly accurate, but close enough for jazz.

Okay, tell me what I've been doing for decades is all wrong . . . I can handle it!

bertwert · 05-11-2021, 06:40 AM

Originally posted by EssJayEff

Sorry if someone already wrote this and I missed it. I've always used the ∞ symbol on a lens as the guide to the outer limit of my depth of field for a given f/stop. For example, if I want to shoot at f/8 and want everything "up to infinity" in focus, I find the pair of lines for f/8 (on many old lenses the f/stop pairs are the same color, but many modern lenses don't even have them), place the ∞ on the distant line and then everything in between that and the distance for the closer line would be my approximate depth of field range. I found it not to be highly accurate, but close enough for jazz.

Okay, tell me what I've been doing for decades is all wrong . . . I can handle it!

Well, that's sort of what the marks are there for

**EssJayEff** · 05-11-2021, 06:51 AM

Originally posted by bertwert

Well, that's sort of what the marks are there for

Well that's what I thought, but to me that means that focusing at infinity means you are throwing away a goodly chunk of your depth of field unless you needed critical focus.

Wheatfield · 05-11-2021, 07:56 AM

Lenses, especially long ones will allow the focus ring to move past the infinity mark when cool to allow the lens to focus to infinity when heat causes the lens to expand and limit focusing on the long end.
The term focusing past infinity is just shorthand for explaining this.

**MossyRocks** · 05-11-2021, 08:42 AM

Originally posted by mee

Aww no one enjoyed my space balls joke. Ah well.

I completely missed that. I thought it was funny when seeing it.

**GUB** · 05-11-2021, 02:36 PM - **1 Like**

Originally posted by EssJayEff

Well that's what I thought, but to me that means that focusing at infinity means you are throwing away a goodly chunk of your depth of field unless you needed critical focus.

Yes but the catch to that is that dof guide on your lens is based on a circle of confusion (coc) of .03mm. This is the old standard for full frame and this converts to .02mm for apsc. But with the resolving power of modern digitals and changed expections of the size the image is going to be presented at most people now believe this standard is too wide. For instance if you look at a 28mm if you set it to infinity it suggests that @ f2.8 everything back to just over 3 metres will be fine and this is confirmed with the dof calculator online as per the screenshot which says if set to 50 metres (near enough to infinity) dof down to 7 metres. But take a look at this screenshot from my recent Testing Pentax 28mm infinity focus. - PentaxForums.com . As you can see this A28 2.8 @f2.8 is fairly well focused at infinity and the 6metre target (right at the bottom of the clip) is well out of focus. Even the 12metre target has obvious focus drop off. How much lee way you want to use is up to you but the scale on the lens is probably too blunt a tool now days.

stevebrot · 05-11-2021, 08:26 PM

Originally posted by GUB

But the point where "infinity" is in focus is a very specific point for a lens and it is easy to bring the elements closer to the sensor than that.

Yes, that is true. Infinity focus is when the rear principal plane is focal length distance forward of the focus plane. Precisely determine the focal length* and one can approximate infinity focus to the limits of manufacturing and measurement tolerance.

Steve

* Requires infinite precision...

stevebrot · 05-11-2021, 08:29 PM

Originally posted by EssJayEff

Okay, tell me what I've been doing for decades is all wrong . . . I can handle it!

Works for me and has for years...

Steve

stevebrot · 05-11-2021, 08:37 PM

Originally posted by Wheatfield

The term focusing past infinity is just shorthand for explaining this.

Unless they mean using an EVF in magnified mode to zero in (past and back) on a distant object. We get a lot of requests on this site from mirrorless users wanting howto instructions for defeating the infinity stops on Pentax lenses.

Steve

**pentaxus** · 05-11-2021, 09:27 PM

You can't say, however, to infinity and beyond without paying.

**GUB** · 05-12-2021, 04:58 AM

Originally posted by stevebrot

Yes, that is true. Infinity focus is when the rear principal plane is focal length distance forward of the focus plane. Precisely determine the focal length* and one can approximate infinity focus to the limits of manufacturing and measurement tolerance.

Steve

* Requires infinite precision...

That leads me to something I was thinking about when I said;

Originally posted by GUB

What would the alternative be. Perhaps the lens has been "adjusted to less than its focal length"? Rather clumsy and un-explanatory and probably technically incorrect for a lot of lens designs.

In the case of a modern lens that internally focuses does the focal length of the lens change when you focus? Otherwise how else can it work?

05-11-2021, 02:36 PM - 1 Like	#25
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	Originally posted by EssJayEff Well that's what I thought, but to me that means that focusing at infinity means you are throwing away a goodly chunk of your depth of field unless you needed critical focus. Yes but the catch to that is that dof guide on your lens is based on a circle of confusion (coc) of .03mm. This is the old standard for full frame and this converts to .02mm for apsc. But with the resolving power of modern digitals and changed expections of the size the image is going to be presented at most people now believe this standard is too wide. For instance if you look at a 28mm if you set it to infinity it suggests that @ f2.8 everything back to just over 3 metres will be fine and this is confirmed with the dof calculator online as per the screenshot which says if set to 50 metres (near enough to infinity) dof down to 7 metres. But take a look at this screenshot from my recent Testing Pentax 28mm infinity focus. - PentaxForums.com . As you can see this A28 2.8 @f2.8 is fairly well focused at infinity and the 6metre target (right at the bottom of the clip) is well out of focus. Even the 12metre target has obvious focus drop off. How much lee way you want to use is up to you but the scale on the lens is probably too blunt a tool now days. Attached Images

05-11-2021, 08:37 PM	#28
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Wheatfield The term focusing past infinity is just shorthand for explaining this. Unless they mean using an EVF in magnified mode to zero in (past and back) on a distant object. We get a lot of requests on this site from mirrorless users wanting howto instructions for defeating the infinity stops on Pentax lenses. Steve Last edited by stevebrot; 05-11-2021 at 10:20 PM.

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05-08-2021, 02:07 AM	#16
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	Yes I agree that "focusing past infinity" appears impossible. But the point where "infinity" is in focus is a very specific point for a lens and it is easy to bring the elements closer to the sensor than that. So the term is merely a simplification of the fact that the elements are closer to the sensor than the infinity point. What would the alternative be. Perhaps the lens has been "adjusted to less than its focal length"? Rather clumsy and un-explanatory and probably technically incorrect for a lot of lens designs.

05-08-2021, 02:17 AM	#17
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	Originally posted by AggieDad Perhaps the better phrase would be “past the ∞ mark. No this is not what is meant. The infinity mark is at the stop of the focusing ring - so if a manual focus lens is focusing beyond infinity the infinity mark will be also beyond the point of infinity. (anyone know the linux keyboard code for "infinity"?)

05-08-2021, 07:34 AM - 1 Like	#18
bertwert Veteran Member Join Date: Jun 2015 Location: Golden, BC Posts: 15,173	Originally posted by GUB (anyone know the linux keyboard code for "infinity"?) If you have a Compose key set up it's just Compose 8 8 - as an aside I set up my Caps Lock key to be a Compose key as it is far more useful to me as that than a Caps Lock key. Alternatively, if you know the Unicode designation of any character, you can type it using Control+Shift+u {code} - for ∞, which is U+221e, you'd type Control+Shift+u 2 2 1 e and then hit space.

05-09-2021, 02:21 AM	#19
dlhawes Guest	The purpose of focusing beyond "infinity" is that the infinity mark might not be located exactly and precisely correctly on the lens body, so what you're really doing is making up for the fact that the lens is a manufactured item made to conform to "tolerances"; in other words, you're actually focusing AT infinity as to the capabilities of the lens' construction, which happens to put the indicator beyond the mark.

05-11-2021, 06:38 AM	#20
EssJayEff Site Supporter Join Date: Jul 2011 Location: near Saxapahaw, NC Photos: Gallery \| Albums Posts: 969	Sorry if someone already wrote this and I missed it. I've always used the ∞ symbol on a lens as the guide to the outer limit of my depth of field for a given f/stop. For example, if I want to shoot at f/8 and want everything "up to infinity" in focus, I find the pair of lines for f/8 (on many old lenses the f/stop pairs are the same color, but many modern lenses don't even have them), place the ∞ on the distant line and then everything in between that and the distance for the closer line would be my approximate depth of field range. I found it not to be highly accurate, but close enough for jazz. Okay, tell me what I've been doing for decades is all wrong . . . I can handle it!

05-11-2021, 06:40 AM	#21
bertwert Veteran Member Join Date: Jun 2015 Location: Golden, BC Posts: 15,173	Originally posted by EssJayEff Sorry if someone already wrote this and I missed it. I've always used the ∞ symbol on a lens as the guide to the outer limit of my depth of field for a given f/stop. For example, if I want to shoot at f/8 and want everything "up to infinity" in focus, I find the pair of lines for f/8 (on many old lenses the f/stop pairs are the same color, but many modern lenses don't even have them), place the ∞ on the distant line and then everything in between that and the distance for the closer line would be my approximate depth of field range. I found it not to be highly accurate, but close enough for jazz. Okay, tell me what I've been doing for decades is all wrong . . . I can handle it! Well, that's sort of what the marks are there for

05-11-2021, 06:51 AM	#22
EssJayEff Site Supporter Join Date: Jul 2011 Location: near Saxapahaw, NC Photos: Gallery \| Albums Posts: 969	Originally posted by bertwert Well, that's sort of what the marks are there for Well that's what I thought, but to me that means that focusing at infinity means you are throwing away a goodly chunk of your depth of field unless you needed critical focus.

05-11-2021, 07:56 AM	#23
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,981	Lenses, especially long ones will allow the focus ring to move past the infinity mark when cool to allow the lens to focus to infinity when heat causes the lens to expand and limit focusing on the long end. The term focusing past infinity is just shorthand for explaining this.

05-11-2021, 08:42 AM	#24
MossyRocks Loyal Site Supporter Join Date: Nov 2017 Location: Minnesota Photos: Gallery \| Albums Posts: 2,982	Originally posted by mee Aww no one enjoyed my space balls joke. Ah well. I completely missed that. I thought it was funny when seeing it.

05-11-2021, 08:26 PM	#26
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by GUB But the point where "infinity" is in focus is a very specific point for a lens and it is easy to bring the elements closer to the sensor than that. Yes, that is true. Infinity focus is when the rear principal plane is focal length distance forward of the focus plane. Precisely determine the focal length* and one can approximate infinity focus to the limits of manufacturing and measurement tolerance. Steve * Requires infinite precision...

05-11-2021, 08:29 PM	#27
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by EssJayEff Okay, tell me what I've been doing for decades is all wrong . . . I can handle it! Works for me and has for years... Steve

05-11-2021, 09:27 PM	#29
pentaxus Site Supporter Join Date: Jun 2010 Location: Central Coast, CA Posts: 1,313	You can't say, however, to infinity and beyond without paying.

05-12-2021, 04:58 AM	#30
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,760	Originally posted by stevebrot Yes, that is true. Infinity focus is when the rear principal plane is focal length distance forward of the focus plane. Precisely determine the focal length* and one can approximate infinity focus to the limits of manufacturing and measurement tolerance. Steve * Requires infinite precision... That leads me to something I was thinking about when I said; Originally posted by GUB What would the alternative be. Perhaps the lens has been "adjusted to less than its focal length"? Rather clumsy and un-explanatory and probably technically incorrect for a lot of lens designs. In the case of a modern lens that internally focuses does the focal length of the lens change when you focus? Otherwise how else can it work?