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04-21-2010, 08:22 PM   #1
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K7 and infinity focus (AF) with the DA*300/4

I am having a little problem which happens only once in a while:

The DA*300/4 is mounted on the K7 for quite some time. Actually, I use this gear almost 100% of the time.
I find that I need to "fine focus" (with the Quick shift feature of the lens) when focusing at infinity.

Has anyone encountered a similar issue?

JP

04-21-2010, 08:57 PM   #2
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I don't have the K7 but my da*300 focuses past infinity on a K20D
04-21-2010, 10:08 PM   #3
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QuoteOriginally posted by imtheguy Quote
I don't have the K7 but my da*300 focuses past infinity on a K20D
It is normal for a lens to focus past infinity.
04-21-2010, 10:45 PM   #4
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QuoteOriginally posted by Lance B Quote
It is normal for a lens to focus past infinity.
Is it even possible to focus past infinity? Theoretically/scientifcally? Or does it mean, the lens barrel goes beyond the infinity mark?

04-22-2010, 06:38 AM   #5
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So I suppose I have to live with this.
Not a big deal but rather annoying when you want to quickly focus at "infinity" for whatever reason, and realize later that the pics come out soft.
Lee, Lance:
That's probably why Pentax has come up with QuickShift on those DA*lenses. At least it does solve such a focusing issue. I had no idea that some lenses would "normally" focus past infinity. To me, that seems rather strange.
Thanks for the replies.

JP
04-22-2010, 06:44 AM   #6
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Have you shot some focus charts to see if the lens isn't front-focusing a bit? I had a bit of focus issues with two of my lenses and broke down to shoot focus charts - one was slightly backfocusing and the other front focusing, both correctable. However, the one that back-focused on the K-7 (DA*50-135) used to occasionally not focus to infinity on the K10, so I wonder if something like that might not be going on with your DA*300.
04-22-2010, 07:00 AM   #7
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If you are referring to the fact that if you rack the focus all the way out it will not be infinity focussed, this is a fact on just about every long lens. In fact, it is the same on the M300/4 I bought in 1983 and it was advertised as a feature at the time of promotion. Longer lenses have the big barrels that are affected by temperature variations which can minutely affect intralens element distance. Thus the need to allow adjustment by providing focus "past" infinity. The physics haven't changed over the years.

Jack
04-22-2010, 07:13 AM   #8
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QuoteOriginally posted by mtngal Quote
Have you shot some focus charts to see if the lens isn't front-focusing a bit? I had a bit of focus issues with two of my lenses and broke down to shoot focus charts - one was slightly backfocusing and the other front focusing, both correctable. However, the one that back-focused on the K-7 (DA*50-135) used to occasionally not focus to infinity on the K10, so I wonder if something like that might not be going on with your DA*300.
I thought of doing that but you see, this lens is absolutely perfect until I "push" it to infinity. Other than that, I have no focusing issue whatsoever.
I tend to agree with the others that this is an inherent feature of longer glass.
Thanks for the reply.
JP

04-22-2010, 07:20 AM   #9
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QuoteOriginally posted by jbinpg Quote
If you are referring to the fact that if you rack the focus all the way out it will not be infinity focussed, this is a fact on just about every long lens. In fact, it is the same on the M300/4 I bought in 1983 and it was advertised as a feature at the time of promotion. Longer lenses have the big barrels that are affected by temperature variations which can minutely affect intralens element distance. Thus the need to allow adjustment by providing focus "past" infinity. The physics haven't changed over the years.

Jack
That is exactly that which I have been doing.
Another thing just popped in my ol' brain as I am writing this: trying to focus at "infinity" on a very far subject when the distance between me and the subject is a wetland of several hundred meters calls for trouble; there will be haze caused by the wet area, especially near ground zero, i.e.: on the surface. How in the world didn't I "click" faster in my head?
Plus the fact that, according to concensus, long glass behaves according to the law off physics which "haven't changed over the years" [quoting you].

Thanks for the reply!
JP
04-22-2010, 07:50 AM   #10
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The entire subject is odd really.

Long glass like 300mm on a fine 5µm pitch sensor like K-7 requires extremely exact focussing.

If you want less than a pixel blur due to misfocus at f/4, you have to focus between 4.5km and infinity. The difference between the two on the focus scale is less than the width of a hair! So, it is almost always impossible to focus precisely with long glass. Even contrast AF may not be able to achieve the 50µm focus accuracy which is required, for mechanical reasons alone.

A well calibrated lens with a fixed infinity setting made of carbonized or sandwhich material with less thermal dependencies may be a desirable thing. Or at least an infinity-button in the firmware which is calibrated once in a while.
04-22-2010, 05:03 PM   #11
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QuoteOriginally posted by falconeye Quote
The entire subject is odd really.

Long glass like 300mm on a fine 5µm pitch sensor like K-7 requires extremely exact focussing.

If you want less than a pixel blur due to misfocus at f/4, you have to focus between 4.5km and infinity. The difference between the two on the focus scale is less than the width of a hair! So, it is almost always impossible to focus precisely with long glass. Even contrast AF may not be able to achieve the 50µm focus accuracy which is required, for mechanical reasons alone.

A well calibrated lens with a fixed infinity setting made of carbonized or sandwhich material with less thermal dependencies may be a desirable thing. Or at least an infinity-button in the firmware which is calibrated once in a while.
That really got me lost with your scientific jargon here, FalconEye!
Am I right to assume, then, that no matter what excellent lens/camera body one uses, one should not expect perfect infinity focus with a 300mm/4 lens?
What I experienced for "infinity" focusing, using the K7 + DA*300/4 was somewhat disappointing because I was trusting this AF "blindly". Perhaps I shouldn't.
Anyway, the "subjects" were at least 500 to 600 feet away ... !
The expanse of terrain between me and the said subjects was covered with marsh land, or wetland if you prefer. This would most likely cause athmospheric ( best term I found to explain this) disturbances in the form of haze, even if not visible; certainly some humidity which would "unsharpen" the area I was focusing on ... makes sense?
Now, would this have anything to do with having to use the quickshift of that lens?
A dedicated infinity button would be great, wouldn't it?

JP
04-22-2010, 05:52 PM   #12
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QuoteOriginally posted by jpzk Quote
That really got me lost with your scientific jargon here, FalconEye!
JP, I do sincerely apologize.

QuoteOriginally posted by jpzk Quote
Am I right to assume, then, that no matter what excellent lens/camera body one uses, one should not expect perfect infinity focus with a 300mm/4 lens?
Yes, this is what I wanted to illustrate. If you pixelpeep, then one will probably find that infinity is difficult to hit exactly. With a lens 10x as long, one must focus about 10x as precisely for the same infinity objects.

QuoteOriginally posted by jpzk Quote
Anyway, the "subjects" were at least 500 to 600 feet away ... !
The expanse of terrain between me and the said subjects was covered with marsh land, or wetland if you prefer. This would most likely cause athmospheric ( best term I found to explain this) disturbances in the form of haze, even if not visible; certainly some humidity which would "unsharpen" the area I was focusing on ... makes sense?
Makes sense.
I wrote an article in my blog about this subject exactly.

The disturbances are aerosol scattering (humidity) and turbulences. The former destroys contrast, the latter destroys sharpness. A've put up a table which says that at bad conditions, 600 feet can already limit a 300 mm lens.

Nevertheless, I guess your main problem is a loss in contrast which makes the AF focus less accurately. A lack in contrast can easily be healed in post-processing if you shoot low ISO.
04-22-2010, 06:11 PM   #13
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QuoteOriginally posted by falconeye Quote
JP, I do sincerely apologize.


Yes, this is what I wanted to illustrate. If you pixelpeep, then one will probably find that infinity is difficult to hit exactly. With a lens 10x as long, one must focus about 10x as precisely for the same infinity objects.


Makes sense.
I wrote an article in my blog about this subject exactly.

The disturbances are aerosol scattering (humidity) and turbulences. The former destroys contrast, the latter destroys sharpness. A've put up a table which says that at bad conditions, 600 feet can already limit a 300 mm lens.

Nevertheless, I guess your main problem is a loss in contrast which makes the AF focus less accurately. A lack in contrast can easily be healed in post-processing if you shoot low ISO.
Gotcha, Falk!
Not at all disturbed with your explanations; as a matter of fact, it's right to the point!
So, next time I try to focus that far with this lens, I have to remind myself that there are a few things to take into consideration, just as we have been discussing.
Yes, the loss of contrast/sharpness would be the culprit under those circumstances, not the lens itself.
Indeed, I have had some luck at times when the temperature was cold, low humidity and good sunnny conditions. This usually happen in the middle of the winter, in our area, when the temperature drops to minus 10-20C. Of course, we can't expect this kind of condition(s) when the Spring comes.
So, thanks a bunch for your replies ... appreciated!

JP
04-22-2010, 06:30 PM   #14
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QuoteOriginally posted by siva.ss.kumar Quote
Is it even possible to focus past infinity? Theoretically/scientifcally? Or does it mean, the lens barrel goes beyond the infinity mark?
I'm sure that others have explained it well enough, but my inference was that long lenses will focus past the infinity focus mark on the lens, not past infinity itself, which you rightly questioned.

The reason they can focus past the infinity point has to do with the expansion/contraction of the whole lens due to ambient temperatures which will obviously affect the calibration of the various distance marks on the lens. The distance marks on the lens are a guide only and should not be taken as exact representations, especially on these long lenses.
04-23-2010, 12:33 AM   #15
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QuoteOriginally posted by falconeye Quote
A well calibrated lens with a fixed infinity setting made of carbonized or sandwhich material with less thermal dependencies may be a desirable thing. Or at least an infinity-button in the firmware which is calibrated once in a while.
I think focus bracketing would be a desirable thing.
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