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01-11-2020, 08:22 AM   #1
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De-centered element? Nope - but what should I expect from the DFA 150-450

I've been struggling with soft and/or OOF shots with my new (replacement) DFA 150-450.

I decided to test for front/back focus and the results have me wondering if there isn't a bigger issue.

I'm pretty ignorant on stuff like this so inout is appreciated.

Not all shots are bad, but maybe 40-50%.

Setup:
Tripod
Imagesync shutter release
1/640 shutter
f5.6
450mm (results are same at various FL)

Straightedge/ruler aligned with bottom of frame (observe hash marks at bottom are equidistant from bottom edge)



Straightedge aligned with camera



55-300 PLM for comparison



Thanks for any help.


Last edited by simple1; 01-11-2020 at 03:18 PM.
01-11-2020, 08:34 AM   #2
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Those look good to me. If you want to check for a decentered element you should take pictures perpendicularly, not at an angle. You should also have rulers (or something similar) at the edges/corners.
01-11-2020, 09:40 AM   #3
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Look for a distant stationary object, and take a picture with it in all four corners - top left, top right, bottom left, bottom right. Have it all on a tripod. Then compare and see if they are close on sharpness.
01-11-2020, 11:43 AM   #4
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There are a few things you can do to test for decentering
How to check if your lens is decentered
A Simple Test for Decentering

01-11-2020, 12:10 PM   #5
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Your focus looks fine to me. As far as decentering or element tilt, the shots you posted would not show much of that even if it's there.

I too would recommend rogerstg's recommendations.
QuoteOriginally posted by rogerstg Quote
There are a few things you can do to test for decentering
How to check if your lens is decentered
A Simple Test for Decentering
I have tested my lenses using the 2nd link's method from Roger Cicala at Lensrental.com, and found none of them are absolutely 100% perfect, but they are all very acceptable, and I'm sure they are within the manufacture's tolerances.
01-11-2020, 01:47 PM   #6
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Thanks all for the replies.

I'm going to go with user error on my issues, just getting used to the lens and longer FL.

These are both ~150% crops and seem fine.



01-11-2020, 03:16 PM   #7
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So...playing around a bit more it seems that at longer distances and FL I have to stop down to f7.1 for really good results. after that the number of shots where I figured I'd missed focus went to almost zero. And overall IQ is much better.

With the 55-300 PLM I can shoot wide open at any FL and distance with no noticeable difference in IQ. I figured this lens, being a good bit more expensive, would be at least as good.

Maybe an unreasonable expectation?

Here are a few 200% crops at f7.1





These are also 200% crop. First image is f5.6, second is f7.1. f5.6 is noticeably softer. Shutter speed for both is 1/800s. Not that I'd crop at 200%, it just better highlights the difference in IQ.





01-11-2020, 04:44 PM   #8
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Shooting a long lens can be very unforgiving when it comes to user error. And sometimes the AF just grabs the wrong thing to focus on. With my D-FA 150-450 mounted on my K-1, I use TaV mode, shutter speed minimum is 1/2000, aperture is F8 (I found that that's the sweet spot for my particular lens) and ISO in on AUTO (set from 100 - 6400), and I use either Select Single or Select 9 for the AF points (Select Single seems to give me a better rate of keepers in general). I often hand hold the lens and find that combination is pretty good (its even better on a tripod). Attached is a straight RAW image (one cropped and one not cropped, otherwise no edits), 100% crop, 1/3200 F8 ISO 800 Select Single AF at 450mm.


Personally I find shutter speed to be the biggest factor to out of focus images with my D-FA 150-450. Once I figured that out, it made a bid difference in my shooting workflow.
Attached Images
View Picture EXIF
PENTAX K-1  Photo 
View Picture EXIF
PENTAX K-1  Photo 

Last edited by david94903; 01-11-2020 at 04:50 PM. Reason: adding additional image
01-11-2020, 06:59 PM - 1 Like   #9
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Thanks Dave - great input.

Playing around a fair amount this afternoon I found it was a consistent issue at f5.6 and f6.3, but practically never happens at f7.1. Using TAv mode, AF.C and spot focus with 1/800 for all. When trying to work it out I was careful to use a sturdy rest, not a tripod but steady. Being very conscious of point of aim with focus confirmation.
So it seems f7.1 or above is the key.

Appreciate the response.
01-12-2020, 10:41 AM   #10
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QuoteOriginally posted by simple1 Quote
Playing around a fair amount this afternoon I found it was a consistent issue at f5.6 and f6.3, but practically never happens at f7.1
This seems like a good illustration of the importance of using a lens' aperture sweet spot. Back before the internet, I recall seasoned pros telling of a rule of thumb, that ƒ8 was usually the sweet spot for most lenses. The good news is that you need to pixel peep extreme (at 200% crop) to really see it.
01-12-2020, 04:15 PM   #11
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Would be interesting to see some comparative shots made between the DFA 150-450mm on the K-1 and the 55-300mm PLM on a 24mp crop body, adjusted to a FL for a comparable image size, and both at a mid aperture, then both wide open.
01-12-2020, 05:16 PM   #12
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QuoteOriginally posted by rogerstg Quote
This seems like a good illustration of the importance of using a lens' aperture sweet spot. Back before the internet, I recall seasoned pros telling of a rule of thumb, that ƒ8 was usually the sweet spot for most lenses. The good news is that you need to pixel peep extreme (at 200% crop) to really see it.
I get what you're saying but he difference was noticeable at normal viewing, which started me down this path. Though I will confess to being a serial pixel peeper
I guess I'd gotten spoiled with the 50-135 and 55-300 PLM. Old and/or cheap lenses I anticipate having to stop down for acceptable results, that's part of the trade-off in cost. But a modern expensive lens I (perhaps wrongly) expected to perform better.
01-12-2020, 05:35 PM   #13
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QuoteOriginally posted by simple1 Quote
But a modern expensive lens I (perhaps wrongly) expected to perform better.
You might be right of course. But before making a decision, you may want to test using a proper tripod, mirror up, 2 sec timer, etc to eliminate any issues with vibration. One of your "soft" photos seems to also reflect vibration.
01-12-2020, 06:12 PM   #14
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QuoteOriginally posted by rogerstg Quote
You might be right of course. But before making a decision, you may want to test using a proper tripod, mirror up, 2 sec timer, etc to eliminate any issues with vibration. One of your "soft" photos seems to also reflect vibration.
Good points.
What I finally did is set up on a tripod and used aperture bracketing to take from f5.6-f11 (multiple times since bracketing on KP only does full stops as far as I can tell). 1/800s shutter and used Imagesync for release, but not mirror up ( though I think Imagesync does live view which is the same?). After several iterations of this it was very consistent that f5.6-6.3 aren't good at all. F7.1 looks better, and f8 is very good.

Appreciate the feedback!
01-12-2020, 06:38 PM   #15
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QuoteOriginally posted by simple1 Quote
it was very consistent that f5.6-6.3 aren't good at all
Based on all you've done, I tend to agree. Hope you can exchange it.
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