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I'm getting very poor focus particularly at the long end of an 80-320mm tele lens that I am using with my Pentax K10D.

The lens was originally bought for my 35mm Pentax SLR (maybe an MZ5 or something, I don't remember, and it's at teh back of a cupboard these days), and worked very well with the film camera.

Theoretically the lens is compatible with the K10D, though obviously the different sensor sizes mean that it's not a direct 80-320mm equivalent.

The problem has been present since I first bought the camera about 15(?) years ago, but I encountered it a couple of nights ago again and the frustration led me to post this question in case there is a solution.

The photo below shows the issue.

Out of focus image

The image was shot at the '320mm' end of the tele lens and the settings are shown below.

Image information

The image is absolutely pin sharp in the veiw finder, but as you can see the resulting photo is blurred. Not only that, but the 'in-focus' beep and dot are on when the image appears to be in focus.

A while after gettng the camera I swapped the focus plate for a 'Katz Eye Optics' split prism partly to see if this led to any improvement and partly because I always loved this type of focus on my dad's old Pentax film cameras. Sadly it made no difference so it seems that the problem was not that the image was out of focus in the VF.

I've read about the AF issues on the K10D, but this is when manually focussing and also when ignoring the 'in-focus' beep (I tried ignoring the beep years ago when I first saw the issue - long before I read about the K10D AF issue).

I haven't seen the problem with the 28-80mm lens supplied with the K10D, but I have assumed that that is because it has a max focal length of 80mm and the problem only seems to manifest at the long end of the tele lens.

Is this problem likely to be lens incompatability, an issue that is only noticeable when shooting subjects like this at a distance, or something inherantly wrong in the body (like the AF / back focus issue)?

Edit: I forgot to mention that I also had a Circular Polarizer fitted for this shot as the moon was very very bright that night, but the same blur occurs without when shooting other subjects.

Fat Monk
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  • Are you using a tripod? And a remote shutter release or a self timer in the camera? The most likely problem is user error (nothing personal). Misalignment in the camera's optical system is also possible, but it is less likely. Also remove the polarizer so it can be eliminated from the equation. – Bob Macaroni McStevens Mar 03 '21 at 23:19
  • Yes, sorry, forgot to mention that as well - tripod mounted and remote shutter release. The shot posted is with the CP filter, but I shot others without and they are also blurred. Ki posted the CP shot as the blur is more obvious as there is less glare). – Fat Monk Mar 03 '21 at 23:22
  • Also possible that the camera is just worn out. The popular forum for Pentax cameras suggests mirror failure in the K10d can be an issue. That would explain the focus problems in both manual and auto modes. It's an old camera. – Bob Macaroni McStevens Mar 03 '21 at 23:38
  • Related: How can I achieve more clarity in my photos of the moon? and Do cheap filters have an effect on image quality? and What factors affect the detail and clarity of a photo taken by a digital camera? and How do I diagnose the source of focus problem in a camera? – Michael C Mar 04 '21 at 01:43
  • @BobMacaroniMcStevens As I said in the question, the issue has been present since I bought the camera (from new) so I doubt it is a wear and tear issue. – Fat Monk Mar 04 '21 at 08:48
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    @MichaelC thanks for the links. I haven't read them all, but that first one seems spot on with my fears - limits of the lens is, at least, better than a faulty camera body. I still have the same problem without filters, so that's not the issue, but seeing the problem at the long end of an 80-320mm film camera lens fits the description of the most likely cause in that first linked post. I'll keep experimenting to try to confirm. – Fat Monk Mar 04 '21 at 09:00
  • Is the diopter of the viewfinder adjustable? Is it properly adjusted? – Bob Macaroni McStevens Mar 04 '21 at 14:03
  • Dioptr is correctly adjusted - internal displays are pin sharp. I wouldn't have thought the dioptre adjustment could correct for an image that was blurred through the main lens system, though, anyway.... Could it? – Fat Monk Mar 04 '21 at 17:43
  • Yes the diopter won’t fix a misadjusted mirror. But a misadjusted diopter is another way an image can be sharp in the viewfinder and soft on the sensor. – Bob Macaroni McStevens Mar 05 '21 at 01:27
  • Have you tried “focusing through” the object? One picture focused a little short of sharp in the viewfinder; one sharp in the viewfinder; and one a little long of sharp in the viewfinder. It will help if the plane of focus is all supposed to be sharp. – Bob Macaroni McStevens Mar 05 '21 at 01:34
  • Based on your comment to an answer below, rather than being a duplicate of How can I achieve more clarity in my photos of the moon? (which still answers your question with the first possibility suggested in the accepted answer), it's pretty much a duplicate of How do I diagnose the source of focus problem in a camera?. In your case it appears to be the last major point in the accepted answer to that one. I've reopened the question so I can't vote to close it again. – Michael C Mar 06 '21 at 06:23
  • @BobMacaroniStevens I'll try focussing each side of sharp to see if there is any improvement, and I'll also do these tests on daylight subjects to avoid confusion such as MichaelC's that this is a moon specific question. (I just need to find some time to experiment, and a distant object with enough detail to demonstrate the issue sufficiently). – Fat Monk Mar 06 '21 at 11:19
  • @FatMonk I'm not confused at all. You presented a question with an example of the moon and your question, as written, indicated no other use cases. We're set up to deal with to specific questions regarding specific use cases here. Once you made it clear (in a comment, not in the rather nebulous hint you later edited into the end of the question) that you were asking regarding other use cases, I adjusted my response based on the information you finally provided. Apologies for not being able to read your mind in advance of that explanation, if that's what you mean by "confused". – Michael C Mar 10 '21 at 12:20
  • @M8chaelC Check the dates. The comment added about the issue being present with other subjects was added a day before you commented on the question. If I had posted an example of a picture of a cat would you have assumed that I was only having problems photographing cats? You didn't read the question fully and made an incorrect assumption and answered according to that assumption. – Fat Monk Mar 10 '21 at 12:28

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The following is all guesswork, and should be a comment on the question rather than an answer, if it weren't so long.

First off, motion blur could be the reason, but the uniform dizziness around the moon's border suggests to me that it's not the case here.

I do see a little bit of chromatic aberration on the right side of the moon. Are we looking at the full picture here? If the moon was in a corner section of the image, it could suggest that this is normal for an old, cheap lens of this type (although, this is a full-format lens on an APS-C sensor, so it's not that far outside of the center either way).

It could be that the mirror or sensor system is somehow out of alignment, as that would cause the image to be sharp in the view finder but out of focus on the sensor plane. I've had this happen to my K3.

To verify if that's the case, use the camera's Live View and have it focus. Use the widest aperture possible in order to get a small depth of focus. Note the position of the focus ring. Then switch back out of Live View and have it focus again. Compare the results. If you consistently get a different focus positions, or if view finder is out of focus if Live View was in focus, and vice versa, then there's an alignment issue (which is probably not fixable other than using the Camera menu to adjust the AF offset, although that won't fix the issue that the view finder will show the image slightly out of focus then).

If the focus positions are the same, then it's rather likely that your lens is bad, such as having gotten the dreaded lens fungus.

As to the question why it's only visible with the tele lens: I guess it's the smaller depth of focus you get with the long focal length. It could, however, also simply be that the lens is not sharp at the end ranges. I remember having had cheap lenses in the 80s that would be like that, and even today's lenses still suffer from that in the corners, especially the zoom ones. Although that effect is more likely to happen in shorter focal ranges.

Thomas Tempelmann
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  • Thanks, some intersting thoughts there. Unfortunately the K10D doesn't have Live View (I wished I'd waited 6-12 months after bying as LV was much more common then). I suspect you may be right about limits of the lens, unfortunately. I also took some shots at 1/4000 so motion blur shouldn't be an issue. I'll have a bit more of a play and see what I can work out. (I've also updated the question as I forgot to mention I had a CP filter on as well as the moon was very very bright that night). – Fat Monk Mar 03 '21 at 12:47
  • What kind of CP filter were you using? Have you tried comparing shots taken with it and without it in less challenging focus scenarios to see if it could be affecting IQ? We've got an old question here somewhere that demonstrates the effect of a less than adequate filter on telephoto shots, but I can't find it. – Michael C Mar 04 '21 at 01:35
  • @MichaelC I have the same issue without the CP (without any filter) but I posted the CP one as the blur is easier to see without the glare. – Fat Monk Mar 04 '21 at 10:40
  • The CP filter has one of two effects: If you don't adjust exposure when you add the CP it reduces exposure, thus reducing any "blur" due to overexposure, if you do adjust exposure, then your exposure time will increase by around one stop, so blur from the motion of the moon would be increased. – Michael C Mar 05 '21 at 02:42
  • @MichaelC The camera was on full manual. I added the CP filter to reduce any over exposure (the moon was too bright) and to remove any scattered light from the atmosphere - is the filter seemed like a good thing to add. As the shutter speed was fixed at 1/1000 for the shot there would be no motion blur. Yes I could have opened the aperture and increased the shutter speed to 1/4000 but as I understand it a wider aperture is more likely to produce a blurred shot (assuming shutter speed is fast enough which 1/1000 should be for the moon at this magnification). – Fat Monk Mar 05 '21 at 08:40
  • @FatMonk I usually begin at around ISO 200, f/8, and 1/125. That's using a 200mm lens + 2X TC + 1.6X APS-C body. Even at 1/125 I don't get any noticeable motion blur when I use a tripod, cable release, and mirror lockup. At 1/1000 and f/5.6, you're letting two stops less light into the camera and compensating by increasing the ISO two stops. You're working with 1/4 as much light and amplifying "shot" noise, along with the signal, by a factor of four. You're trading usable light for shorter Tv than you need. – Michael C Mar 05 '21 at 19:25
  • Having said all of that, some nights the "seeing" is just terrible and no matter what one does, short of stacking a bunch of frames (like the best several hundred out of a few thousand video frames shot over several seconds), you're not going to get clear photos of anything above the atmosphere. That was the case for my locale back during the hyped "Christmas Star" conjunction of Saturn and Jupiter on 21 Dec, 2020. – Michael C Mar 05 '21 at 19:30
  • @MichaelC please note that this is not a question about getting better photos of the moon. I also have photos of birds of prey which have the same issue - you have obviously completely missed the point simply because I chose to use a shot of the moon as the example in the question because it was the most recent shot I had to hand showing the issue. – Fat Monk Mar 06 '21 at 00:22
  • @MichaelC and the seeing was pretty much as clear as it ever gets at around sea level inland in the UK on the night I shot the sample photo - that's why I tried a few shots. Through my reflector the moon looked incredible. You are right about the trade off in useable light, and my mistake was that I missed the iso setting. However ignoring the noise introduced by the higher than optimal ISO, the problem here is blur and I tend to believe that is just a limitation of the extreme ends of the telephoto lens range - it was not an expensive lens when I bought it and it was for 35mm film, not DSLR. – Fat Monk Mar 06 '21 at 00:30
  • I'm wondering if your tele lens is suffering from a back focus issue. – frank Mar 06 '21 at 02:53
  • @FatMonk Based on your comment above, rather than being a duplicate of How can I achieve more clarity in my photos of the moon? (which still answers your question with the first possibility suggested in the accepted answer), it's pretty much a duplicate of How do I diagnose the source of focus problem in a camera?. In your case the issue seems to be the last major point in the accepted answer to that one. I've reopened the question so I can't vote to close it again. – Michael C Mar 06 '21 at 06:26
  • @FatMonk Re: "completely missing the point" - In your question you mentioned no other use cases and provided a photo of the moon. What point, exactly, did we miss? Was there one that you even attempted to communicate in the original question? – Michael C Mar 06 '21 at 06:29
  • @MichaelC the edit to the original question - added before you commented - might give a clue: 'but the same blur occurs without when shooting other subjects." – Fat Monk Mar 06 '21 at 11:15
  • @FatMonk "... when shooting other subjects." That isn't really much of a "point" being made, is it? One general reference to nebulous and unspecified "other subjects"? – Michael C Mar 06 '21 at 19:51
  • @MichaelC any mention of 'other' subjects implies that anything other than the moon could be the subject in this context. I cannot help if if you did not read the question fully and missed that, but you assumed I was only talking about a problem shooting the moon when the edit clearly encompasses the possibility of other subjects being anything other than the moon. You are making an utterly pointless argument which sadlybdetracyabfeom the fact that you have actually pointed to some useful other answers. – Fat Monk Mar 06 '21 at 22:07
  • "sadlybdetracyabfeom"? – Michael C Mar 06 '21 at 22:20
  • @MichaelC sadly detracts from – Fat Monk Mar 10 '21 at 12:29