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03-19-2017, 03:49 PM   #1
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IRIX 15mm blackstone focus doubts

Hei Pentaxians,
I got a new 15mm IRIX lens, and I am playing a bit with the lens these days. I noticed that I don't really manage to get a focus that satisfies me completely.
I am not sure if I got too spoiled with my Sigma 18-35 art or if there is something wrong with the lens.
I would like to have your opinion on the focus quality of this sample picture: do you think that the quality in that picture is what I can get or do you think that the focus is unsatisfying (as I think). Notice also that the left part seems even worse...
I upload the raw file, so that you can have a look at the picture in its full quality.
ps. I shoot with a Pentax K-3 II.

I couldn't upload the file (too big). Here's a link to the dropbox file:
https://www.dropbox.com/s/1w6aeucacoyxapx/IMGP2748.DNG?dl=0

Thank you all for your feedback!
Mauro


Last edited by candeloromauro; 03-19-2017 at 03:55 PM.
03-19-2017, 04:21 PM   #2
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Ciao Mauro,

The focus (if you call it that) is either completely off (you are focused at the closest focus distance) or the lens is badly de-centered. I wouldn't accept any new wide-angle lens with that quality.

But, before you send it back, two questions:

1) You shot at f/2.4 at 1/8000 of a second - did you intend to shoot the lens aperture wide open? (If not, does the lens have an aperture lever sticking out of the back for Pentax mounts? Meaning you can control aperture from within the camera.) The reason I ask is because most lenses are sharper stopped down a bit, and wide-angle views of landscapes or architecture need even more than that (like f/5.6 or f/8). You can hand-hold it down to a shutter speed of 1/125, so a smaller aperture (higher f number) should be no problem...

2) For that shot, was the focus scale (if there is one) at or near "infinity" (which is the end of the scale)? If the lens focuses beyond infinity (many do), that may be part of your problem. Take some test images at infinity and a bit less than all the way out, to see if there's a difference.

HTH
03-19-2017, 04:33 PM   #3
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QuoteOriginally posted by candeloromauro Quote
Hei Pentaxians,
I got a new 15mm IRIX lens, and I am playing a bit with the lens these days. I noticed that I don't really manage to get a focus that satisfies me completely.
I am not sure if I got too spoiled with my Sigma 18-35 art or if there is something wrong with the lens.
I would like to have your opinion on the focus quality of this sample picture: do you think that the quality in that picture is what I can get or do you think that the focus is unsatisfying (as I think). Notice also that the left part seems even worse...
I upload the raw file, so that you can have a look at the picture in its full quality.
ps. I shoot with a Pentax K-3 II.

I couldn't upload the file (too big). Here's a link to the dropbox file:
Dropbox - IMGP2748.DNG

Thank you all for your feedback!
Mauro
Did you use live view with magnification to verify the focus?

Adam
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03-20-2017, 02:19 AM   #4
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QuoteOriginally posted by panoguy Quote
Ciao Mauro,

The focus (if you call it that) is either completely off (you are focused at the closest focus distance) or the lens is badly de-centered. I wouldn't accept any new wide-angle lens with that quality.

But, before you send it back, two questions:

1) You shot at f/2.4 at 1/8000 of a second - did you intend to shoot the lens aperture wide open? (If not, does the lens have an aperture lever sticking out of the back for Pentax mounts? Meaning you can control aperture from within the camera.) The reason I ask is because most lenses are sharper stopped down a bit, and wide-angle views of landscapes or architecture need even more than that (like f/5.6 or f/8). You can hand-hold it down to a shutter speed of 1/125, so a smaller aperture (higher f number) should be no problem...

2) For that shot, was the focus scale (if there is one) at or near "infinity" (which is the end of the scale)? If the lens focuses beyond infinity (many do), that may be part of your problem. Take some test images at infinity and a bit less than all the way out, to see if there's a difference.

HTH
Thank @panoguy for your answers and your comments!

1) was intentional, this lens was supposed to have good sharpness also wide open, and I thought that by having the highest aperture I could actually evaluate if I was having at least part of the image with good focus... was it a stupid thought?

2) you are correct: this picture was taken at infinity or close to that. But notice that this is the SHARPEST picture I got out of a set of 10 pictures taken with small adjustments in the focus. So my impression is that I don't really get anything better if I focus in the vicinity of that focusing point....

---------- Post added 03-20-17 at 02:21 AM ----------

QuoteOriginally posted by Adam Quote
Did you use live view with magnification to verify the focus?
Thank you @Adam for your answer.
I will make another test where I focus using live view and I will upload the result as soon as I have time. Will be nice if you can look at that too, then!

03-20-2017, 05:45 AM   #5
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Hi Mauro

yes, try again with Liveview though i have a feeling that this is not a good copy. My first copy was decentered and i am waiting for a new copy arriving this week. The first batch of pentax mounts was unfortunately not very good, i read from some forum members that they did have to return their copy aswell.

Cheers
03-20-2017, 06:35 AM   #6
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Irix 15mm has W-shaped focus field, in the center it is sharp, between the center and the edges (fullframe coverage) it dips a bit and towards edges it picks up sharpness again but never reaches center performance. With APS-C format the corners are located in that "dip" area so it will suck if you focus for in the center of the image.
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