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02-11-2012, 06:52 PM   #1
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K7 AF Fault?

When I first bought my K7, it seemed to take much sharper images than it does now. It seems to have the most trouble picking infinity focus. I took a couple of photos today using the same Pentax 16-45 f4 lens - one with the K200 and one with the K7. The image from the K200 is distinctly sharper. I didn't struggle with getting the f-stop and shutter speed the same but here's the crazy thing - the K200 image was f6.7 and the K7 image was f14 but the K7 image is still a lot sharper! It's difficult to compare the images side by side because there's a difference in the resolution of the cameras, but I think the issue is reasonably obvious:





I had a look at the settings of the K7 and the AF adjustment was enabled even though no adjustment was set. I've turned the AF adjustment off for the moment. Does anyone have any idea why the K7 AF accuracy might deteriorate? Should I just send the camera for servicing? I thought the issue may have been the lens, but I noticed that the sharpness of images with the 55-300 had also deteriorated. Does anyone have any thoughts on this issue please? Maybe it's an excuse to buy a K5, but I'm resisting that - I bought the K200 just before the K7 was released, and bought the K7 just before the K5 was released. It's possible a successor to the K7 could be released soon and I don't want to make that mistake again!

02-11-2012, 07:09 PM   #2
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The two shots look fine to me- the level of detail is about the same. The K-7's shot just needs a bit more processing.

At infinity, though, sometimes wide lenses like the 16-45 do have trouble focusing.

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02-11-2012, 08:07 PM   #3
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QuoteOriginally posted by Adam Quote
The two shots look fine to me- the level of detail is about the same. The K-7's shot just needs a bit more processing.
Both images are cropped from jpegs out of the cameras at 1:1 on the pixels. I avoided any processing - other than obviously there will be some introduction of artifacts because the source and destination images are jpeg. Maybe the K200 firmware sharpens the image by default? I notice the colour balance is also very different; the K200 image seems to have a higher colour saturation and the K7 image tends toward magenta.

QuoteQuote:
At infinity, though, sometimes wide lenses like the 16-45 do have trouble focusing.
OK, thanks. What set me chasing this was actually looking at photos I took with the K200 and 16-45 at an airshow of the static displays, where the images looked distinctly sharper and cleaner than the K7 photos taken the same day on the 55-300 (and yes I realise that the 16-45 is better quality glass). Perhaps part of the difference I'm seeing is a generally higher level of digital noise in the K7 images compared to the K200.
02-12-2012, 07:14 AM   #4
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You've gone from 10 to 16 megapixels, which is harder on any lens, and will generally need a bit more sharpening. Also, try adjusting your AF to see if you can get things a little better.

02-12-2012, 08:17 PM   #5
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If the pictures were taken at different f-stops, I don't see why you'd blame any difference on the camera. but in any case, it's easy enough to correct for the difference in resolution - just downsize the K-7 image to match the K200D. I think you'll find they are virtually identical. The K-7 image just appears softer because it's blown up larger.
02-12-2012, 09:30 PM   #6
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QuoteOriginally posted by RobG Quote
I took a couple of photos today
Are you in Hughes or something? That view looks familiar.
02-13-2012, 01:21 AM   #7
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QuoteOriginally posted by Marc Sabatella Quote
If the pictures were taken at different f-stops, I don't see why you'd blame any difference on the camera.
Because the K200 image is sharper at f6.7 than the K7 at f14?

QuoteQuote:
but in any case, it's easy enough to correct for the difference in resolution - just downsize the K-7 image to match the K200D. I think you'll find they are virtually identical. The K-7 image just appears softer because it's blown up larger.
OK, maybe I chose a poor example - for curiosity's sake I'll try doing exactly what you suggest although I'd be surprised if the detail in the trees turned out to be the same. It's hard to post photos of exactly the same thing over a space of a couple of years to demonstrate that the K7 AF used to be more consistently accurate than it is now, and impossible to demonstrate in photos that the camera hunts for focus more often than it used to.
02-13-2012, 01:23 AM   #8
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QuoteOriginally posted by rawr Quote
Are you in Hughes or something? That view looks familiar.
The photo is taken from Isaacs Ridge. Hughes is on the wrong side of Mount Taylor.

02-13-2012, 01:26 AM   #9
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QuoteOriginally posted by Wheatfield Quote
You've gone from 10 to 16 megapixels, which is harder on any lens, and will generally need a bit more sharpening. Also, try adjusting your AF to see if you can get things a little better.
I'll have to find a guide to setting up a focussing chart. I'd have thought that the resolution of a DA 16-45 at mid apertures was well over a 16MP sensor's needs. It's certainly a heck of a lot sharper than the Sigma zoom which came with the K200.

Last edited by RobG; 02-13-2012 at 01:30 AM. Reason: more info
02-13-2012, 01:52 AM   #10
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Maybe try a series of shots with the ISO 12233 resolution test chart:
http://www.graphics.cornell.edu/~westin/misc/ISO_12233-reschart.pdf

QuoteOriginally posted by RobG Quote
The photo is taken from Isaacs Ridge.
Ah. My bad. Looked a bit like that low hill behind Curtin.
02-13-2012, 03:45 AM   #11
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QuoteOriginally posted by rawr Quote
Maybe try a series of shots with the ISO 12233 resolution test chart:
Ah. My bad. Looked a bit like that low hill behind Curtin.
No worries. Thanks for the chart!
02-13-2012, 09:20 AM   #12
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QuoteOriginally posted by RobG Quote
Because the K200 image is sharper at f6.7 than the K7 at f14?
That's as expected. You're already diffraction-limited by f/8 or so; sharpness goes steadily down from there, even though DOF gets larger. f/6.7 is usually around the peak sharpness for most lenses; f/14 is virtually always going to be noticeably softer when you start pixel peeping.

QuoteQuote:
for curiosity's sake I'll try doing exactly what you suggest although I'd be surprised if the detail in the trees turned out to be the same
I can see from the crops you posted it will be close, but again, you gave the K200D an unfair advantage by shooting at a much sharper aperture, so I would expect the K200D image to still have a slight edge in this comparison. Unless the loss of sharpness from diffraction is small enough that downsizing the K-7 image actually hides it, but actually, I suspect it won't - you're comparing what is probably the lens, best aperture (f/6.7) with one of that is probably only around a stop from its worst as far as diffraction effects are concerned.

Anyhow, the sort of focus issues that people test obsessively for have nothing to do with infinity focus. They have to do with focus at closer distances and larger apertures where DOF is shallow enough that a difference of a couple of millimeters in exactly where focus is achieved will affect the image. That isn't going to be relevant in examples like this.
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