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02-07-2021, 09:42 PM   #16
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Now that I've been able to compare my lenses on 24mp without an aa filter vs. 16mp with one, I've revised my opinion somewhat of my existing lenses. The 60-250 is still very good - not amazing on the edges/corners at 60mm but very good at most other focal lengths, and stopping down just one half or a full stop makes the results even better. It might not be perfectly consistent between sides and corners but it's very, very close. My 70-300mm Tamron is still pretty impressive in terms of sharpness in the center 2/3rd of the image or so, but now I can see that my copy isn't what it could be: at the short-to-mid range the right side and especially upper right corner is a little weak at wider than about f9 - f10. Now that I know that about the Tamron, I can go back to the 16mp with the aa filter and see signs of it being a little off in those areas, which is honestly disappointing since it's a FF lens and the corners on APS aren't really corners for the lens. At about 200mm or more overall the Tamron is pretty consistent across the frame. Of course it has much more fringing than the 60-250, but I'm comparing "net" results after processing, and while you can't completely recover from fringing with processing, it definitely helps and makes the comparison closer. The 55-300 DA... well it's the same story as the Tamron with less fringing and better contrast. For whatever reason I get better AF with the Tamron. It's just so frustrating to me that these manufacturers can't get their act together and build even "cheap" lenses to have the same performance across the frame. I don't mean the same as the center - I just want opposing sides and corners to match, no matter how good or bad they are. Honestly all of these lenses would be good enough for me if their poorer-performing sides and corners could replicate their better sides and corners at all their focal lengths. I'd be interested if someone has tested their 55-300 PLM the same way. I understand if I have to buy a 60-250 to get good sides/corners wide open with little fringing, but I know from their "good" sides/corners that these other lens designs are capable of being really good maybe a stop or so down if they were just built the way they were designed. Oh, my 60-250 was way off on AF when I bought it new, so it doesn't get a pass on quality control either... but Pentax adjusted it under warranty so it's better now. Viewfinder AF still isn't consistent across its focal length range, but it's closer than it was. When I first started with digital/AF I just assumed AF would work and be accurate, but now I have pretty low expectations for viewfinder AF, and try to use live view (not available on the early bodies, of course) AF as often as possible.

Oh, in case I didn't say so, all these lenses are pretty good in the center 2/3rs of the crop frame: at least acceptable to me wide open, and generally better stopped down a little. The 60-250 is pretty good wide open all over the frame; it's that last 1/3rd of the frame that kills the others. Now, the 100-300F... well that's pretty uninspiring everywhere, until you stop it down quite a bit.


Last edited by tibbitts; 02-07-2021 at 09:50 PM.
02-07-2021, 09:55 PM   #17
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I use my 55-300mm PLM even out of spec on the K-1 in full frame mode and corner/edges are consistent - as long as SR doesn't seriously have to engage engage. This naturally throws a lot of lenses with tight image circles off, so far that sometimes corners on e.g. the 28-105mm (FF) and 18-135mm (APS-C) are basically unusable.
02-07-2021, 11:03 PM   #18
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QuoteOriginally posted by JensE Quote
I use my 55-300mm PLM even out of spec on the K-1 in full frame mode and corner/edges are consistent - as long as SR doesn't seriously have to engage engage. This naturally throws a lot of lenses with tight image circles off, so far that sometimes corners on e.g. the 28-105mm (FF) and 18-135mm (APS-C) are basically unusable.
I disable SR for testing lenses, and only have the pre-5-axis SR, but I can understand if SR can move the sensor significantly it could push one side or corner into worse-performing territory.
02-09-2021, 02:10 PM   #19
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QuoteOriginally posted by JensE Quote
I use my 55-300mm PLM even out of spec on the K-1 in full frame mode and corner/edges are consistent - as long as SR doesn't seriously have to engage engage. This naturally throws a lot of lenses with tight image circles off, so far that sometimes corners on e.g. the 28-105mm (FF) and 18-135mm (APS-C) are basically unusable.
I'd never thought of this as a contributor to soft corners. Jens, are you saying that a fractional movement of the sensor for SR could produce an effect similar to decentering in a particular image?

The DA 18-135 has a lot of vignetting even on APS-C, so a shot will show the outline of the image circle on the frame (so to speak). If you are testing on a tripod with SR off, the location of the vignetting should be constant; presumably if the image circle is off to one side a bit, it suggests the lens is decentred. But as I understand what you are saying, when using SR, the outline of the image circle shown by vignetting may vary between shots because the sensor may be repositioned. So it should be possible to test the extent of the effect. I have never seen anyone try this.

I just did some quick and dirty tests using the 18-135 at 18mm f3.5 where it vignettes horribly. I used a slow shutter and refocused/re-acquired SR each time. There was a slight difference in the position of the image circle from one shot to another. Whether the difference is enough to turn usable corners into unusable ones, I'm not sure. (At 18mm f3.5 the corners are really bad anyway.) I would need to see some proper testing to be persuaded that the effect is something that matters.


Last edited by Des; 02-09-2021 at 03:34 PM.
02-10-2021, 01:33 AM   #20
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QuoteOriginally posted by Des Quote
The DA 18-135 has a lot of vignetting even on APS-C, so a shot will show the outline of the image circle on the frame (so to speak). If you are testing on a tripod with SR off, the location of the vignetting should be constant; presumably if the image circle is off to one side a bit, it suggests the lens is decentred. But as I understand what you are saying, when using SR, the outline of the image circle shown by vignetting may vary between shots because the sensor may be repositioned. So it should be possible to test the extent of the effect. I have never seen anyone try this.I just did some quick and dirty tests using the 18-135 at 18mm f3.5 where it vignettes horribly. I used a slow shutter and refocused/re-acquired SR each time. There was a slight difference in the position of the image circle from one shot to another. Whether the difference is enough to turn usable corners into unusable ones, I'm not sure. (At 18mm f3.5 the corners are really bad anyway.) I would need to see some proper testing to be persuaded that the effect is something that matters.
The easiest way to test this would be use the composition adjustment (shift) and horizon correction (rotation - but shouldn't matter) features.
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