Irix 15mm on K-1 infinity focus issues

Higaide · 11-03-2019, 08:39 AM

Hi, I have a copy of Irix 15mm lens, but I am not really satisfied with its performnce - I tried adjusting the focus ring and taking some night shots but the best I could get is shown below - corner/centre crops, shot at f2.8/15s, last one at f2.4/20s (all major stars have blue-ish glow). Could someone with experience using this lens give me some feedback if that kind of performance is the best I can get from it or is my copy faulty and should I return it?

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pschlute · 11-03-2019, 09:24 AM

I dont do astro shots but could it be perhaps the lens focusses past infinity ?

**whammerhead** · 11-03-2019, 10:03 AM

Adjust the white balance?

Unregistered User · 11-03-2019, 10:30 AM

see if this helps:
Adjusting infinity focus on the Irix 15mm f/2.4 -- need tips - PentaxForums.com

**Bob 256** · 11-03-2019, 11:40 AM

Fully opened, you can expect some issues with the corners and even the center field. WIth the K-1, use liveview with magnification to obtain the sharpest focus. It's possible your lens won't focus at infinity and in that case, you may consider asking for service, but it's also possible that it focuses beyond infinity, in which case you just can't dial it in to infinity and expect the best focus. That's where liveview and magnification come into play and they should allow you to get a much better focus than by using the optical viewfinder.

Stopping the lens down a bit should improve center and particularly edge sharpness but of course at the cost of less light. Also, there can be the issue of star movement if the exposure is too long. 15 seconds shouldn't pose an issue with a 15mm lens but keep star motion in mind. From your center crops, it looks like there's really no problem with star motion in this case, and the edge problems are lens issues (to wide an aperture and or focus).

**MossyRocks** · 11-03-2019, 12:01 PM

I don't know that lens not owning one but it looks like it has some coma in the first shot. Isn't the front element on that lens fairly large and isn't it fairly heafty as well? If so that may be like my 400/2.8 where it has issues in the punishing case of astrophotography until it reaches thermal equilibrium. My big lens takes close to 2 hours because it is so heavy but I would think letting your 15mm sit for a half an hour and it would probably be good since it has a lot less mass. With my 400 the coma goes away completely however I'm not sure if that will work with an ultra wide since there seems to always be distortion in the corners and I would assume that this image is the corner one. Does the coma look the same in all the corners or is different as that will tell you if the lens is decentered. If it is decentered then it should be sent back to get one that isn't.

As far as the purple fringing it could be from a missed focus but is just as likely to be the lens design. With a lens like this you cant use a bahtinov mask to get that perfect focus so you will have to do star minimization. Put your camera into magnified live view with a star in the center of the frame. Be sure to have focus peaking OFF as it will bloat the star so it becomes hard to tell when you have the smallest possible star. Then get the star as small as you can. You should be really close to a perfect focus. I never trust the infinity mark on my lenses. If that doesn't resolve the purple fringing then you will have to stop the lens down. Some otherwise great lenses do this with astrophotography which seems to show every issue a lens has. The DFA 100/2.8 macro is a purple fringing monster with astro shots, as is the 135/2.3 Vivitar Series 1 even though they are great lenses in normal circumstances.

Higaide · 11-03-2019, 01:46 PM

Originally posted by MossyRocks

I don't know that lens not owning one but it looks like it has some coma in the first shot. Isn't the front element on that lens fairly large and isn't it fairly heafty as well? If so that may be like my 400/2.8 where it has issues in the punishing case of astrophotography until it reaches thermal equilibrium. My big lens takes close to 2 hours because it is so heavy but I would think letting your 15mm sit for a half an hour and it would probably be good since it has a lot less mass. With my 400 the coma goes away completely however I'm not sure if that will work with an ultra wide since there seems to always be distortion in the corners and I would assume that this image is the corner one. Does the coma look the same in all the corners or is different as that will tell you if the lens is decentered. If it is decentered then it should be sent back to get one that isn't.

As far as the purple fringing it could be from a missed focus but is just as likely to be the lens design. With a lens like this you cant use a bahtinov mask to get that perfect focus so you will have to do star minimization. Put your camera into magnified live view with a star in the center of the frame. Be sure to have focus peaking OFF as it will bloat the star so it becomes hard to tell when you have the smallest possible star. Then get the star as small as you can. You should be really close to a perfect focus. I never trust the infinity mark on my lenses. If that doesn't resolve the purple fringing then you will have to stop the lens down. Some otherwise great lenses do this with astrophotography which seems to show every issue a lens has. The DFA 100/2.8 macro is a purple fringing monster with astro shots, as is the 135/2.3 Vivitar Series 1 even though they are great lenses in normal circumstances.

Thank you for all the replies, as for the focusing, I am using max magnification and live view, with no peaking, the images above are the best results I could obtain, turning focus ring even slightly either ways makes the distant objects more blurry/fringing intensifies. The image does become acceptable around 3.5, it just kind of defeats the purpose of having f2.4 hence why I was hoping for input from other users of this lens. I used to have a Samyang 10mm 2.8 on APSC body and the stars were perfectly in focus with no color fringing even at f2.8. About what "MossyRocks" said, there might be something about the size of the front element, it is indeed much bigger compared to the mentioned samyang. About thermal equilibrum or such, I have a Blackstone version (full metal), so I imagine it should reach it pretty fast.

**Bob 256** · 11-03-2019, 04:25 PM

It sound's like you're doing everything ok by using liveview and mag, so that most likely defaults to the lens itself. You might just have a bad copy with a decentering issue or other problems. Sorry about that. As others have noted, you might have to wade through several returns to find a good one or resort to a Pentax issued lens in that focal range. Still, even with a good lens, don't be surprised with less than excellent performance wide open. Few lenses will give their best performance until they are stopped down somewhat. Star images are very unforgiving where you're trying to image a point source and get it to be a point source, so closing the lens from full aperture may be needed even with a better example of a lens as well. That said, shooting stars is a good test for helping to find a good copy.

**MossyRocks** · 11-03-2019, 06:43 PM - **2 Likes**

Originally posted by Higaide

Thank you for all the replies, as for the focusing, I am using max magnification and live view, with no peaking, the images above are the best results I could obtain, turning focus ring even slightly either ways makes the distant objects more blurry/fringing intensifies. The image does become acceptable around 3.5, it just kind of defeats the purpose of having f2.4 hence why I was hoping for input from other users of this lens. I used to have a Samyang 10mm 2.8 on APSC body and the stars were perfectly in focus with no color fringing even at f2.8. About what "MossyRocks" said, there might be something about the size of the front element, it is indeed much bigger compared to the mentioned samyang. About thermal equilibrum or such, I have a Blackstone version (full metal), so I imagine it should reach it pretty fast.

Metal cools down quickly however glass is much slower and with a big hung of glass it will take it a while to reach thermal equilibrium, that is why I said to wait like 30 minutes and see if that clears things up some. The first 2 times I borught out my 400/2.8 I only waited 30 minutes which wasn't enough. The 3rd time I thought ahead and let it cool down for 2 hours before going and getting things setup and the coma I had was gone and the CA was almost entirely gone. I still have to stop that lens down to f/3.5 but it is perfect at that point. Astro shooting is really punishing and you will find out about every flaw your lens has when you do it.

beachboy2 · 11-03-2019, 06:49 PM - **1 Like**

My Irix 15 wide open. No astrotracer. Some coma. Star is Canopus, second brightest after Sirius.

Higaide · 11-04-2019, 12:58 AM

Originally posted by beachboy2

My Irix 15 wide open. No astrotracer. Some coma. Star is Canopus, second brightest after Sirius.

Thanks for the sample, I will try shooting with similar settings and see what I get, might also try MossyRocks idea and take samples before and after cooling down the lens.

beachboy2 · 11-04-2019, 01:12 AM

MossyRocks makes a very good point.
PS I used factory infinity focus point; locked focus; and used 12 sec delay after pressing shutter. Was first attempt at astro with the K1m2 and I would hope to do better.

**Digitalis** · 11-04-2019, 01:15 AM

FYI the stars near the corners are always going to look like that. The corner blurring is caused by astigmatism, a property of the lens design itself. Nothing can be done to mitigate astigmtism other than stopping down...until diffraction becomes a bigger problem.

Unregistered User · 11-04-2019, 03:46 AM

Originally posted by beachboy2

MossyRocks makes a very good point.
PS I used factory infinity focus point; locked focus; and used 12 sec delay after pressing shutter. Was first attempt at astro with the K1m2 and I would hope to do better.

I'd suggest using a cable-release with a two-second delay. The reason is, as I understand it, the two second delay causes the mirror to lock up immediately, to stay up for the full two seconds, while the twelve-second delay keeps it down right up until the shutter opens, with concomitant vibration. My practice using the cable release is to wait until all apparent movement in the viewfinder stops, then hit the button and allow the two second delay to kick in.

11-04-2019, 01:12 AM	#12
beachboy2 Senior Member Join Date: Dec 2012 Location: Perth Western Australia Photos: Gallery Posts: 222	MossyRocks makes a very good point. PS I used factory infinity focus point; locked focus; and used 12 sec delay after pressing shutter. Was first attempt at astro with the K1m2 and I would hope to do better. Last edited by beachboy2; 11-04-2019 at 01:16 AM. Reason: extra

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11-03-2019, 09:24 AM	#2
pschlute Moderator Loyal Site Supporter Join Date: Mar 2007 Location: Surrey, UK Photos: Gallery Posts: 8,215	I dont do astro shots but could it be perhaps the lens focusses past infinity ?

11-03-2019, 10:03 AM	#3
whammerhead Site Supporter Join Date: Aug 2019 Location: Belfast Posts: 128	Adjust the white balance?

11-03-2019, 10:30 AM	#4
Unregistered User Guest	see if this helps: Adjusting infinity focus on the Irix 15mm f/2.4 -- need tips - PentaxForums.com

11-03-2019, 11:40 AM	#5
Bob 256 Site Supporter Join Date: Jun 2008 Location: Idaho Photos: Gallery Posts: 2,378	Fully opened, you can expect some issues with the corners and even the center field. WIth the K-1, use liveview with magnification to obtain the sharpest focus. It's possible your lens won't focus at infinity and in that case, you may consider asking for service, but it's also possible that it focuses beyond infinity, in which case you just can't dial it in to infinity and expect the best focus. That's where liveview and magnification come into play and they should allow you to get a much better focus than by using the optical viewfinder. Stopping the lens down a bit should improve center and particularly edge sharpness but of course at the cost of less light. Also, there can be the issue of star movement if the exposure is too long. 15 seconds shouldn't pose an issue with a 15mm lens but keep star motion in mind. From your center crops, it looks like there's really no problem with star motion in this case, and the edge problems are lens issues (to wide an aperture and or focus).

11-03-2019, 12:01 PM	#6
MossyRocks Loyal Site Supporter Join Date: Nov 2017 Location: Minnesota Photos: Gallery \| Albums Posts: 2,982	I don't know that lens not owning one but it looks like it has some coma in the first shot. Isn't the front element on that lens fairly large and isn't it fairly heafty as well? If so that may be like my 400/2.8 where it has issues in the punishing case of astrophotography until it reaches thermal equilibrium. My big lens takes close to 2 hours because it is so heavy but I would think letting your 15mm sit for a half an hour and it would probably be good since it has a lot less mass. With my 400 the coma goes away completely however I'm not sure if that will work with an ultra wide since there seems to always be distortion in the corners and I would assume that this image is the corner one. Does the coma look the same in all the corners or is different as that will tell you if the lens is decentered. If it is decentered then it should be sent back to get one that isn't. As far as the purple fringing it could be from a missed focus but is just as likely to be the lens design. With a lens like this you cant use a bahtinov mask to get that perfect focus so you will have to do star minimization. Put your camera into magnified live view with a star in the center of the frame. Be sure to have focus peaking OFF as it will bloat the star so it becomes hard to tell when you have the smallest possible star. Then get the star as small as you can. You should be really close to a perfect focus. I never trust the infinity mark on my lenses. If that doesn't resolve the purple fringing then you will have to stop the lens down. Some otherwise great lenses do this with astrophotography which seems to show every issue a lens has. The DFA 100/2.8 macro is a purple fringing monster with astro shots, as is the 135/2.3 Vivitar Series 1 even though they are great lenses in normal circumstances.

11-03-2019, 01:46 PM	#7
Higaide New Member Join Date: Nov 2018 Posts: 10 Original Poster	Originally posted by MossyRocks I don't know that lens not owning one but it looks like it has some coma in the first shot. Isn't the front element on that lens fairly large and isn't it fairly heafty as well? If so that may be like my 400/2.8 where it has issues in the punishing case of astrophotography until it reaches thermal equilibrium. My big lens takes close to 2 hours because it is so heavy but I would think letting your 15mm sit for a half an hour and it would probably be good since it has a lot less mass. With my 400 the coma goes away completely however I'm not sure if that will work with an ultra wide since there seems to always be distortion in the corners and I would assume that this image is the corner one. Does the coma look the same in all the corners or is different as that will tell you if the lens is decentered. If it is decentered then it should be sent back to get one that isn't. As far as the purple fringing it could be from a missed focus but is just as likely to be the lens design. With a lens like this you cant use a bahtinov mask to get that perfect focus so you will have to do star minimization. Put your camera into magnified live view with a star in the center of the frame. Be sure to have focus peaking OFF as it will bloat the star so it becomes hard to tell when you have the smallest possible star. Then get the star as small as you can. You should be really close to a perfect focus. I never trust the infinity mark on my lenses. If that doesn't resolve the purple fringing then you will have to stop the lens down. Some otherwise great lenses do this with astrophotography which seems to show every issue a lens has. The DFA 100/2.8 macro is a purple fringing monster with astro shots, as is the 135/2.3 Vivitar Series 1 even though they are great lenses in normal circumstances. Thank you for all the replies, as for the focusing, I am using max magnification and live view, with no peaking, the images above are the best results I could obtain, turning focus ring even slightly either ways makes the distant objects more blurry/fringing intensifies. The image does become acceptable around 3.5, it just kind of defeats the purpose of having f2.4 hence why I was hoping for input from other users of this lens. I used to have a Samyang 10mm 2.8 on APSC body and the stars were perfectly in focus with no color fringing even at f2.8. About what "MossyRocks" said, there might be something about the size of the front element, it is indeed much bigger compared to the mentioned samyang. About thermal equilibrum or such, I have a Blackstone version (full metal), so I imagine it should reach it pretty fast.

11-03-2019, 04:25 PM	#8
Bob 256 Site Supporter Join Date: Jun 2008 Location: Idaho Photos: Gallery Posts: 2,378	It sound's like you're doing everything ok by using liveview and mag, so that most likely defaults to the lens itself. You might just have a bad copy with a decentering issue or other problems. Sorry about that. As others have noted, you might have to wade through several returns to find a good one or resort to a Pentax issued lens in that focal range. Still, even with a good lens, don't be surprised with less than excellent performance wide open. Few lenses will give their best performance until they are stopped down somewhat. Star images are very unforgiving where you're trying to image a point source and get it to be a point source, so closing the lens from full aperture may be needed even with a better example of a lens as well. That said, shooting stars is a good test for helping to find a good copy.

11-03-2019, 06:43 PM - 2 Likes	#9
MossyRocks Loyal Site Supporter Join Date: Nov 2017 Location: Minnesota Photos: Gallery \| Albums Posts: 2,982	Originally posted by Higaide Thank you for all the replies, as for the focusing, I am using max magnification and live view, with no peaking, the images above are the best results I could obtain, turning focus ring even slightly either ways makes the distant objects more blurry/fringing intensifies. The image does become acceptable around 3.5, it just kind of defeats the purpose of having f2.4 hence why I was hoping for input from other users of this lens. I used to have a Samyang 10mm 2.8 on APSC body and the stars were perfectly in focus with no color fringing even at f2.8. About what "MossyRocks" said, there might be something about the size of the front element, it is indeed much bigger compared to the mentioned samyang. About thermal equilibrum or such, I have a Blackstone version (full metal), so I imagine it should reach it pretty fast. Metal cools down quickly however glass is much slower and with a big hung of glass it will take it a while to reach thermal equilibrium, that is why I said to wait like 30 minutes and see if that clears things up some. The first 2 times I borught out my 400/2.8 I only waited 30 minutes which wasn't enough. The 3rd time I thought ahead and let it cool down for 2 hours before going and getting things setup and the coma I had was gone and the CA was almost entirely gone. I still have to stop that lens down to f/3.5 but it is perfect at that point. Astro shooting is really punishing and you will find out about every flaw your lens has when you do it.

11-04-2019, 12:58 AM	#11
Higaide New Member Join Date: Nov 2018 Posts: 10 Original Poster	Originally posted by beachboy2 My Irix 15 wide open. No astrotracer. Some coma. Star is Canopus, second brightest after Sirius. Thanks for the sample, I will try shooting with similar settings and see what I get, might also try MossyRocks idea and take samples before and after cooling down the lens.

11-04-2019, 01:15 AM	#13
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	FYI the stars near the corners are always going to look like that. The corner blurring is caused by astigmatism, a property of the lens design itself. Nothing can be done to mitigate astigmtism other than stopping down...until diffraction becomes a bigger problem.

11-04-2019, 03:46 AM	#14
Unregistered User Guest	Originally posted by beachboy2 MossyRocks makes a very good point. PS I used factory infinity focus point; locked focus; and used 12 sec delay after pressing shutter. Was first attempt at astro with the K1m2 and I would hope to do better. I'd suggest using a cable-release with a two-second delay. The reason is, as I understand it, the two second delay causes the mirror to lock up immediately, to stay up for the full two seconds, while the twelve-second delay keeps it down right up until the shutter opens, with concomitant vibration. My practice using the cable release is to wait until all apparent movement in the viewfinder stops, then hit the button and allow the two second delay to kick in.