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01-11-2017, 02:48 AM   #1
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Strange edge softness issue

Hi everyone,

I own a K-3II with few lenses: 70 Ltd, DA* 300, 12-24mm and Sigma 17-70 and have no problem with them.

I just purchased a 20-40mm lens 3 days ago and encounter a very strange and anoying issue... at 20mm the edges of the frame are ok, but at 40mm they're very very soft at every aperture. When I say edge, I mean everthing outside the 1/3 center of the frame...

Here is the strange thing: on the 500 pics I took with the lens... 2 are perfectly starp across frame, but I don't know why and I cannot reproduce this... The problem is more visible outdoors.

This morning I tried the lens with my old K10D, and the border seems to be far more sharp than with my K-3II... how is this possible???

It cannot be back/front focus as focus has been done in LV/tripod/timer... on most of my pics. So I really don't know what is the problem. Can my K-3II be faulty? even if it work perfectly fine with all my others lenses?

Here are samples pics:

The two firsts are taken with K-3 (focus 1/3 into the frame).



This one is perfectly sharp, I don't know how I do!?



This one is from my K10D: it is sharp edge to edge

Any idea? Should I return the lens?


Last edited by superdave; 01-12-2017 at 01:26 AM.
01-11-2017, 03:03 AM - 1 Like   #2
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On my laptop screen and at these sample sizes, I don't see any obvious problem with those shots. Could you provide some 100% crops of the areas you're concerned with? Also, what aperture did you use for these?

There seems to be some sample variation with the 20-40. Some users here report their copies as very good across the frame even wide open. In the centre of the frame, I'd agree - however, away from the centre, my own copy gives results very similar to combined reviews I've read... it really needs stopping down to f/5.6 for good results, and ideally f/8 (I like it at f/8). Also, I believe I see some degree of field curvature on my copy. You can roughly test this by selecting a focus point to the far left or far right of centre and check your resulting shots to see if the part you focused on is sharp.
01-11-2017, 03:15 AM   #3
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QuoteOriginally posted by BigMackCam Quote
On my laptop screen and at these sample sizes, I don't see any obvious problem with those shots. Could you provide some 100% crops of the areas you're concerned with? Also, what aperture did you use for these?

There seems to be some sample variation with the 20-40. Some users here report their copies as very good across the frame even wide open. In the centre of the frame, I'd agree - however, away from the centre, my own copy gives results very similar to combined reviews I've read... it really needs stopping down to f/5.6 for good results, and ideally f/8 (I like it at f/8). Also, I believe I see some degree of field curvature on my copy. You can roughly test this by selecting a focus point to the far left or far right of centre and check your resulting shots to see if the part you focused on is sharp.
Hi Mike, I agree you cannot see the problem on the thumbnails, but you can right click on the picture and select Display Image in the menu for full-size images.

Aperture is f/8 on all images.

Here a screenshot with a 100% crop of the right side of the frame: Left is one of my 2 sharps images, Right is 99% of the pics I took with this lens.


---------- Post added 01-11-2017 at 11:31 AM ----------

Mike, here is the field curvature test:

Left : The right side of the image, I focused on a text panel in the middle of the frame (center is sharp, borders are more than soft).
Right : The text panel is on the right of the frame, and I focused on it manually in LiveView; border is sharp here and center too... but I cannot use the lens such way.



So do you think it is the field curvature? Is it a problem with this specific lens?

What is strange is that the edges looks fine when I use my K10D... I really don't understand

Last edited by superdave; 01-11-2017 at 03:32 AM.
01-11-2017, 03:55 AM - 1 Like   #4
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The field-curvature test, whilst undoubtedly crude, tends to suggest this might be what you're experiencing. I see it too with my copy, even at smaller apertures such as f/8. On my copy, field curvature isn't an issue for close-up shots (even at wider apertures), but at distance it can be very noticeable.

I don't understand why you're getting different results with different cameras, though. Of course, you're using PDAF on the K10D (as there's no Live View), and it could be that it's front or back-focusing slightly - enough that, for the photos you've taken, the borders are in better focus. Does that make sense?

EDIT: One other thought... the K10D has a 10 megapixel sensor, which is relatively forgiving compared to the K-3II's 24 megapixels. Lack of sharpness, especially in the rendering of small or fine detail, will be more noticeable on the K-3II....


Last edited by BigMackCam; 01-11-2017 at 04:13 AM.
01-11-2017, 04:10 AM   #5
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QuoteOriginally posted by BigMackCam Quote
The field-curvature test, whilst undoubtedly crude, tends to suggest this might be what you're experiencing. I see it too with my copy, even at smaller apertures such as f/8. On my copy, field curvature isn't an issue for close-up shots (even at wider apertures), but at distance it can be very noticeable.

I don't understand why you're getting different results with different cameras, though. Of course, you're using PDAF on the K10D (as there's no Live View), and it could be that it's front or back-focusing slightly - enough that, for the photos you've taken, the borders are in better focus. Does that make sense?
Thank Mike, I am reading an article about field curvature and that explain this strange behavior.

As I plan to use the lens for landscape, my 20-40mm is totally useless as my old Sigma 17-70 is sharp from edge to edge.
Do you think field curvature is a problem with my copy? and another copy will not have the issue, or do you think it is a design problem with the 20-40mm lens?

---------- Post added 01-11-2017 at 12:15 PM ----------

QuoteOriginally posted by superdave Quote
Thank Mike, I am reading an article about field curvature and that explain this strange behavior.

As I plan to use the lens for landscape, my 20-40mm is totally useless as my old Sigma 17-70 is sharp from edge to edge.
Do you think field curvature is a problem with my copy? and another copy will not have the issue, or do you think it is a design problem with the 20-40mm lens?
There is one think I still don't understand.

At f/8, the image should be in focus up to infinity, so why edges are not sharp ???

I try to take several short at f/8 f/11 and f/16... they are sharper to sharper in the corder, even with CA f/16 is sharper than f/8.. except in the center of the frame.
01-11-2017, 04:23 AM - 1 Like   #6
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QuoteOriginally posted by superdave Quote
Thank Mike, I am reading an article about field curvature and that explain this strange behavior.

As I plan to use the lens for landscape, my 20-40mm is totally useless as my old Sigma 17-70 is sharp from edge to edge.
Do you think field curvature is a problem with my copy? and another copy will not have the issue, or do you think it is a design problem with the 20-40mm lens?
My belief is that field curvature with the 20-40 is inherent in the design, David. Many people don't seem to notice it, and that may be because they're either (a) not photographing subjects where it would manifest, (b) taking close-up shots, where the field of focus seems to be flatter (based on my own experience), or (c) not viewing images at the pixel level - or, if they are, not having high expectations at this level of reproduction.

I definitely see this behaviour in my own copy, and I've read too many comments on forums for me to think it's a sample variation thing.

I've never tried the Sigma 17-70, but I have the 17-50 f/2.8 and it has a much flatter field of focus than my 20-40. I would pick my 17-50 every time over the 20-40 for landscape shots... that said, I have some nice landscapes taken at f/11 and smaller with the 20-40.
01-11-2017, 04:33 AM   #7
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QuoteOriginally posted by BigMackCam Quote
My belief is that field curvature with the 20-40 is inherent in the design, David. Many people don't seem to notice it, and that may be because they're either (a) not photographing subjects where it would manifest, (b) taking close-up shots, where the field of focus seems to be flatter (based on my own experience), or (c) not viewing images at the pixel level - or, if they are, not having high expectations at this level of reproduction.

I definitely see this behaviour in my own copy, and I've read too many comments on forums for me to think it's a sample variation thing.

I've never tried the Sigma 17-70, but I have the 17-50 f/2.8 and it has a much flatter field of focus than my 20-40. I would pick my 17-50 every time over the 20-40 for landscape shots... that said, I have some nice landscapes taken at f/11 and smaller with the 20-40.
Sad... I cannot even stich a panorama.

I seen fullsize landscape shots with 20-40mm on flicker that were perfectly sharp, they probably have been took at 20mm.
I have to find sharp photos took at 40mm before trying another lens, or may be I would have more chance with the limited 21+40mm instead of the 20-40...

Thanks Mike!

01-11-2017, 04:44 AM - 1 Like   #8
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QuoteOriginally posted by superdave Quote
I have to find sharp photos took at 40mm before trying another lens, or may be I would have more chance with the limited 21+40mm instead of the 20-40...
Well, there are some users of the 20-40 that say they get as good or better performance at the wide end as they do from the DA21. That hasn't been my experience, though.

I have both the DA21 and DA40 Limited lenses, and I prefer them to the 20-40. Both are sharp in the centre from wide open. The DA21 benefits from stopping down to f/5.6 for best edge-to-edge performance, but the DA40 is remarkably consistent at any aperture (though still best at around f/5.6). I've seen some great pano-stitching from the DA40

Good luck!
01-11-2017, 04:55 AM   #9
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QuoteOriginally posted by BigMackCam Quote
Well, there are some users of the 20-40 that say they get as good or better performance at the wide end as they do from the DA21. That hasn't been my experience, though.

I have both the DA21 and DA40 Limited lenses, and I prefer them to the 20-40. Both are sharp in the centre from wide open. The DA21 benefits from stopping down to f/5.6 for best edge-to-edge performance, but the DA40 is remarkably consistent at any aperture (though still best at around f/5.6). I've seen some great pano-stitching from the DA40

Good luck!
In my case, I have good performance at wide end f/8 with the 20-40.
Do you have any field curvature issues with 21 or 40mm ?

I just look at the review of the 20-40 on ephotozine and there have 1 sample at 40mm f/6.3.
If you look at the trees near the edges, there is the same problem I got.

full size sample

I may go for the 40mm Ltd ... instead of the 20-40... but the question is smc or HD I was interested by smc in first place for starburst, but know that I tried the 20-40 and see how flare resistant it is, I am not sure sure about the smc

Last edited by superdave; 01-12-2017 at 01:25 AM.
01-11-2017, 05:32 AM - 1 Like   #10
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QuoteOriginally posted by superdave Quote
Do you have any field curvature issues with 21 or 40mm ?
...
I may go for the 40mm Ltd ... instead of the 20-40... but the question is smc or HD I was interested by smc in first place for starburst, but know that I tried the 20-40 and see how flare resistant it is, I am not sure sure about the smc
DXOMark tests show there is a small but fairly negligible amount of field curvature with the DA21, whilst the DA40 Limited shows virtually none. I haven't experienced any problems in normal use with either lens.

As for SMC vs HD... I have the HD versions and I'm delighted with them. They're very resistant to flare, that's for sure. Having said that, if I was buying again, I'd probably try to find the SMC versions in good used condition - precisely because of the starbursts.
01-11-2017, 08:47 AM   #11
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QuoteOriginally posted by superdave Quote
Here are samples pics:
Crops (center vs. edge vs. corner) are more effective to demonstrate resolution problems. Your full-resolution images are wasted bandwidth and for many users display at much lower resolutions than you uploaded (depends on platform, browser, and user forum settings). Making a full-resolution crop is easy.

Steve
01-11-2017, 09:54 AM   #12
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QuoteOriginally posted by superdave Quote
I may go for the 40mm Ltd ... instead of the 20-40... but the question is smc or HD
Neither. Get the XS instead. Its a collection-ist lens. Besides it performs very well and is cheaper and FF compatible. But it is just my opinion.
01-11-2017, 03:58 PM - 1 Like   #13
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You've got a lot of variables to nail down and control before you can make a conclusion.

Shooting through a glass window and blinds at a city-scape is probably not the best starting point.
If the first city-scape is soft and the later city-scape is sharp at the same Focal length/ISO/Aperture then most likely the focus point is not the same.

Exactly what was the focal point? A third of the way into a scene like this could mean a foreground rooftop, a midground lamp post or something 500 yards down the street.
It looks like the first shot was focused much closer than the later one and you may be seeing a bit of field curvature.

If you haven't already I would suggest trying the same scene with manual focus (using the distance scale on the lens) at a variety of different focusing distances (near, medium, far) on a tripod at F8.
01-12-2017, 01:24 AM   #14
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QuoteOriginally posted by caliscouser Quote
You've got a lot of variables to nail down and control before you can make a conclusion.

Shooting through a glass window and blinds at a city-scape is probably not the best starting point.
If the first city-scape is soft and the later city-scape is sharp at the same Focal length/ISO/Aperture then most likely the focus point is not the same.

Exactly what was the focal point? A third of the way into a scene like this could mean a foreground rooftop, a midground lamp post or something 500 yards down the street.
It looks like the first shot was focused much closer than the later one and you may be seeing a bit of field curvature.

If you haven't already I would suggest trying the same scene with manual focus (using the distance scale on the lens) at a variety of different focusing distances (near, medium, far) on a tripod at F8.
Hi,

I already packaged the lens to return to the vendor so I can't make more test; however the lens I tried do not work close to all my other lens... I never have edges out of focus on any other lens when the focus is done properly.

For example, if I focus at 40mm on something at 50m at f/8, according to a DoF calculator everything farther than 17m (up to infinity) should be acceptably sharp. In practice with the lens I tried, the left and the right third of the frame are soft, not because of CA but because they are not in focus... I don't know why... If I close aperture to f/11, edges are sharper. If I close to f/16, they are sharper, so it is clearly a focus problem.

On the Paris cityscape photo I posted, I focused at 100 meter in the water using liveview, so every piece of building are behind the focus point and should be sharp.... however, the edges are very very soft. That cannot be the "normal" behavior of this lens, isn't it ? or the lens is totally useless for landscape.

Once I got refund, I think I will try another copy and cross fingers...

Last edited by superdave; 01-12-2017 at 01:29 AM.
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