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05-16-2019, 10:40 AM - 2 Likes   #16
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I like to pair my 20-40 with my 50-135 - that pair makes for a very fine all around combo.

05-16-2019, 12:03 PM   #17
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QuoteOriginally posted by BruceBanner Quote
I recently acquired my first zoom, the HD DA 20-40. I haven't really had the chance to fully play with it yet but thought I might have to do some homework to discover when using what focal length what might be the optimum aperture to use at those ranges. I then realised ephotozine does stuff like this already, and I think PF but not in a graph manner? (Maybe I'm wrong about that).

20mm


30mm


40mm


Summary

If I am to understand their findings correctly it looks like at each range going past f11 results in significant drop in IQ. F5.6 is the sweet spot for 30 and 40mm, however f8 is better at 20mm (but not by much, so for simple memory recall you can pretty much say f5.6 gives the best overall IQ for uniformed centred and edge sharpness).

f2.8 should not be shy'd away from using at 20mm as long as it's centre sharpness you only care about.

At 40mm you will get drastically better results if you can stop down to f5.6.


Would you all agree with those findings?


And now I'm wanting to know, is this not what PF review found themselves? What about our longtime HD DA 20-40 users out there, are their findings in line with ephotozine tests?


TIA!

Bruce
I bought this lens when it first came out and used it extensively until I went to FF. The lens for me was fantastic. All focal lengths... all aperatures. It had such wonderful character in a light standard zoom. It is one of lenses that I'd love for my K1 now. Without going back to my image library, I suggest it would be hard to tell a shot at f11 vs. f16. I don't think I ever shot an image at f22.
05-16-2019, 12:52 PM   #18
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diffraction losses are often overblown the increased depth of field may be worth it.
05-16-2019, 01:43 PM   #19
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QuoteOriginally posted by RGlasel Quote
I'm still trying to understand how centre resolution at 20mm and 30mm actually decreases as aperture narrows from wide open. Stopping down to f4 and f5.6 actually decreases resolution?
That's a characteristic of a very good lens. At 10 MP the 31 ltd. does the same.


Definitely won't see the same behaviour from your consumer zooms.

05-16-2019, 02:16 PM   #20
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QuoteOriginally posted by bdery Quote
Yes, I do that, with at least three focal lengths for zooms. I do not use a quantitative method, but rather a comparative method. I think for most readers looking at examples is more telling than looking at a graph.
I enjoy your reviews very much. Our aussie internet can be appalling (still on 5mp/s download speeds here ) so when I see the images and charts it can just lag and not work properly for moving around (especially when viewing on some mobile tablet devices). I have never really looked into charts that much, preferring actually to visit flickr and see if I like what I see etc, but perhaps it's possible to add some chart summaries from your findings? It might be a nice addition to the review? Just a thought anyway, as someone starting to pay attention to charts I can see them being a handy tool.

And so with your review and the chart findings of ephotozine, did you find similar results with your copy of the 20-40?

QuoteOriginally posted by Rondec Quote
The DFA *50 is excellent wide open. That's all I mean. If you want edge sharpness at f1.4, there aren't any other lenses in the catalog that probably match it.

As far as why lenses go to f22, I don't think there is any negative to have f stops you don't use very often, but occasionally, you want to drag your shutter and don't have an ND filter with you and you just stop down, even though you lose sharpness. I know that I've done that before and even though there is some softness, depending on the image it isn't the end of the world.
Yeah like in Tv mode and you want 1/4 second for some motion effect on a bright and sunny day etc, best to get that effect at the expense of quality, yeah I get ya.

QuoteOriginally posted by normhead Quote
I also like looking at graphs. What the charts that get published by manufacturers mean, I have no idea. Not only that, most manufacturers charts are derived from the design specs, not from actual shooting. Actual shooting brings sample variation into play. Whether that's here or there is another whole debate.

The one thing I'd mention....

The better the lens, the lower the -stop at which it achieves maximum resolution.

Looking at the DFA 50 1.4
you can see it's sharpest from 2 to 4. That's a hallmark of an exceptional lens.

Most consumer lenses are sharpest at 5.6. But 8 is still quite good if you need more depth of field. Less so on APS-c than on FF, the the diffraction limit is between 5.6 and 8 for both but the larger the format the lower the diffraction limit, which of course ends with Ansel Adams shooting 64 of 8x10 film. So you might want to be a little quicker to use 8 on your K-1 than you were on your APS-c sensor.

To me the whole argument for the DFA 50 1.4 is sharp edge to edge at wide apertures. Because landscapes need wide depth of filed, I'm not sure that even matters to me.

DA* 55 chart


For me, sharp edge to edge from 2 to 4 but falling off on the edges from 5.6 on would be of limited use. AT 5.6 to 8. the DA could be the better lens, if you're looking edge to edge. But not necessarily. You still have to do some image comparisons to be sure.

I use the graphs to tell me what I should be looking for when I test.

Why would I need further testing? From the graphs you know what the numbers are, but you don't know how that correlates to real world images. I don't think any of us are so smart we know how images will compare based on a bunch of numbers. IN the above examples I know the DA* is more even edge to edge at 8, but because they are on different formats, I really know nothing. I don't know if the DA*s edges fall off considerably after the APS-c circle. Where the DFA 50 looks weaker on the edge, for all I know it's still stronger on the edge than the DA* is in the middle.

The first thing I'm looking at comparing these two would be how many images do I take that would take advantage of the DFA's strength. Not including wildlife, of my 900 keepers this year, 15 would qualify as f2-4 images. For me the DFA would be a waste of money. It would be rare I could actually see a difference. So what I would be looking at on the charts would be 5.6 and 8, and those are the places I would look at comparison images.

To me the charts are a good way to narrow down your search and focus on those situations you are actually buying the lens for.

Looking at the DA 20-40 chart, its a traditional Pentax design. Soft edges where the depth of field is narrow, meaning large areas of the image will be out of focus and soft edges won't be as critical. Good edge to edge from 5.6 to 11 for landscape. A very functional lens for the way I shoot.

The DA 20-40 is definitely in the spirit of the FA 31 ltd


When I see a lens like the DFA* 50 1.4 I wonder "who shoots like that?" Who shoots from 2 to 4 and needs edge to edge sharpness? Do people shoot walls in dim light (and why?)

But everyone is not me. Having better edge to edge would effect 1.6% of my images. But I can imagine that other people might shoot more in that range. That's what you should be buying a DFA 50 for. If not, and you're like me, you're buying capacity that you won't use.

Of course people can buy lenses just because they can. No other reason is needed. I do wonder if anyone actually makes use of 2-4 on their DFA 50 1.4, or if it's just a prestige item. Personally I look at the DFA and DA* images and decided the DFA* wouldn't help many of my images.

And I wonder, who are the folks who benefit from the DFA and what are they shooting that makes use of it's qualities.

And I seriously like graphs , so I can quickly look for numerical differences that highlight what exactly I should look at when comparing the images taken by the lenses. But you still have to compare images to see exactly what the numbers mean in terms of how each lens design was executed. I look at the graphs as the key to intelligent evaluation, letting you focus your attention at lens differences, and not going through the areas where they are pretty much the same. Every tool used to filter information helps.

Especially for those of us with short attention spans.
We have to get to the important part quickly or we might forget what we're doing.
Good to know, I might test that theory out more when I use the 20-40 on the K-1, move up to f8, back to f5.6 on crop.

Yeah I've never found when using wide open apertures the need for sharp oof corners (for portrait use etc). Typically I want my edge sharpness for when I am stopped down for landscape stuff.

I looked into the DFA50, especially against the FA43. I decided (I think like you) that all the value and goodiness from the DFA50 is from f1.4-2.8ish. Once you stop that lens down and pitch it against the competition things start to look less 'night and day different'. I mean the DFA 50 is usually still the winner, but by the kinda cash value it costs? But from 1.4-2.8 I'm not sure there is a better lens to sit on Pentax bodies right now, and add the WR component, better AF... you can see some value. Which leads me to who would want the DFA50? Any portrait studio shooter will want it. Any product photography shooter would want it. Those are where it's strengths easily lie. Niche tho imo.

I decided when comparing the DFA50 to FA43, even though the DFA50 was doing things better, I didn't feel it justified the costs, and where the FA43 was doing well it was doing well in a more 'unique' way (i.e. it kinda renders bokeh just... differently than to a lot of other glass I have owned).

Clinical is the way I would describe the DFA50, Arty is the way I'd describe the FA43.
05-16-2019, 02:26 PM   #21
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I wish Ephotozine would do more tests on the old glass. Amazing how well it compares. The Tak 55 f2 -- 50 year old glass that was the kit lens of the time.
05-16-2019, 02:33 PM - 1 Like   #22
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QuoteOriginally posted by normhead Quote
the 31 ltd. does the same.
But it's not the same. The 31 Ltd. is about 1800 wide open, which increases to 2200 at f2.8 and 2350 at f4, with borders following close behind. That makes sense. At 20mm, the chart the Hulk posted shows 1.1 bars better than Excellent wide open at f2.8, which decreases to 0.9 bars at f4 and 0.5 bars at f5.6, before levelling out at Excellent for f8 and f11; edge starts at 1 bar below Good wide open and steadily improves to 0.9 bars above Excellent at f8 (noticeably better than centre resolution at f8). At 30mm, something similar is going on from wide open at f3.5 up to f8. At 40mm, things look like expected, centre performance is higher than edge performance all the way and both improve going from wide open at f4 to f5.6.

It seems that ephotozine is applying grade school methods for assigning marks by avoiding numbers, but the graphics suggest that at the shorter end of the zoom range, you need to keep aperture wide open or your centre will go soft, which is counterintuitive to most photographic techniques.

---------- Post added 05-16-19 at 03:43 PM ----------

Checking this forum's in-depth review, it is clear the editors at ephotozine screwed up. Centre sharpness at 20mm and 30mm is worse wide open and improves noticeably at f5.6.
05-16-2019, 02:52 PM - 1 Like   #23
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My 20-40 is sharp enough, but it's not the sharpness that really stands out. (Not in the same class as my FA 50 macro or DFA 100 macro - or, I would guess, the DA 35 macro.) It's the colour and microcontrast and the lovely bokeh. That's where it's a true Limited.

I agree with Paul: up to the point where diffraction really bites, choose the aperture to get the DOF you want, not because of sharpness. I wouldn't shy away from f11 for a landscape if I wanted more DOF just because centre and edge sharpness drops from Excellent+ to Excellent! It won't make any real difference.

The really tricky thing about the DA 20-40 is the high field curvature. To illustrate, here's a shot at 20mm f2.8. It's not cropped.



My focus point was the eye of the koala on the left. I used centre point focus and re-compose. The focus is slightly off. That might be because I didn't nail it, but more likely it's the field curvature. The plane of focus is not even. The paws of the koala on the right are in reasonable focus even though they are closer than the face of the koala on the left. The plane of focus is like a bowl.

The effect is still there even when you stop down.

You need to bear this in mind all the time with this lens. It could work to advantage. If you are shooting a group of people, stand them in an arc, so the ones on the outside are closer.

05-16-2019, 04:38 PM - 1 Like   #24
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QuoteOriginally posted by Des Quote
My focus point was the eye of the koala on the left. I used centre point focus and re-compose. The focus is slightly off. That might be because I didn't nail it, but more likely it's the field curvature.
As you noted, best focus in the recomposed shot is at the suspender/paws to our left on the right side koala, a point significantly closer than the intended point of focus. Assuming the image sensor center position was stationary during the recompose, I would expect the actual plane of focus to be slightly behind the measured point assuming a flat field. For the suspender/paw of the right koala to be sharp due to field curvature would require an amazingly strange field geometry.

That being said, the EXIF indicates:
Focus Mode : AF-C (Focus-priority)
AF Point Selected : Fixed Center; 0
AF Points In Focus : Fixed Center or Multiple
AF-C (Focus-priority) does not work well for focus and recompose in that as focus is lost during recompose, the AF system will attempt to regain focus.* The EXIF will say focus was acquired even though it was not retained. For center-focus-recompose, AF-S (Focus-priority) is a better choice.


Steve

* At least that is the default. Changing the Hold AF Status to one of the ON settings will defeat that behavior.
05-16-2019, 06:27 PM   #25
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Thanks for your comment Steve. I was using back button AF (shutter button AF disabled), so assuming I released the back button when re-composing and didn't move the focus ring and didn't move the sensor distance to subject (and admittedly I might have screwed up in one of these ways), why would the focus change?

Last edited by Des; 05-16-2019 at 09:23 PM.
05-16-2019, 09:38 PM   #26
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Bruce - I'd say your charts match pretty well to my 20-40, except I find mine to be good to great at f4. Interestingly, I've been shooting in MTF-line program mode and the apertures suggested are what Pentax typically suggests.

Des - that field curvature would definitely explain some of the occasionally unsatisfactory results I've gotten from this lens which of course get me cursing myself and unselecting photos in lightroom... for once, maybe it is the gear! Ha!
05-17-2019, 01:40 AM - 1 Like   #27
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QuoteOriginally posted by Des Quote
My 20-40 is sharp enough, but it's not the sharpness that really stands out. (Not in the same class as my FA 50 macro or DFA 100 macro - or, I would guess, the DA 35 macro.) It's the colour and microcontrast and the lovely bokeh. That's where it's a true Limited.

I agree with Paul: up to the point where diffraction really bites, choose the aperture to get the DOF you want, not because of sharpness. I wouldn't shy away from f11 for a landscape if I wanted more DOF just because centre and edge sharpness drops from Excellent+ to Excellent! It won't make any real difference.

The really tricky thing about the DA 20-40 is the high field curvature. To illustrate, here's a shot at 20mm f2.8. It's not cropped.



My focus point was the eye of the koala on the left. I used centre point focus and re-compose. The focus is slightly off. That might be because I didn't nail it, but more likely it's the field curvature. The plane of focus is not even. The paws of the koala on the right are in reasonable focus even though they are closer than the face of the koala on the left. The plane of focus is like a bowl.

The effect is still there even when you stop down.

You need to bear this in mind all the time with this lens. It could work to advantage. If you are shooting a group of people, stand them in an arc, so the ones on the outside are closer.
Not doubting the curvature thing but I would say the biggest culprit is the focus/recompose tactic. I never EVER focus/recompose for the very reason what you experienced. It might work ok with other lenses and other apertures, but in my experience it's a flawed way to do business for a lot of shooting conditions. I do a lot of portrait work, nailing f1.8 or 2.0 with a FA77 or FA43 with focus/recompose and wanting that perfect eye focus is just not going to happen with that glass and close quarters. It's a different thing when stopped heavily down, but even then...
Where you can use focus/recompose a bit more (even wide apertures) is when the thing yer focused on is very far away, then the DoF is larger and more forgiving.

Looking at your shot, and knowing what a 20-40 now looks like at 20mm on a crop sensor, it looks like you would actually have been pretty close to the backpack. f2.8 at this distance from the focus point will equate a very thin DoF, narrow margins of error that a focus/recompose just cannot handle, especially that much of a deviation. Any lens that is 20mm on crop (regardless of glass curvature) and trying to do the focus and recompose stuff would likely amount in the same miss of focus tbh.
If you can, I would move away from focus and recompose at wide apertures and instead change the focus point in SEL mode, or switch to LV and manually focus in. Or... use the excellent Quick Shift feature, use the Spot AF point to get you to the general ball park and then recompose and adjust the manual focus ever so slightly whilst keeping your eye on the focus thing, then take the shot when it appears the sharpest to your eye, that's why Quick Shift is so great and I actually think quite important for this lens.
So after saying all of that to you I am not in disagreement, I've had some time to play with this lens and it is actually quite a tricky lens to nail focus regardless of area and technique. I've been using Live View a lot however and I have had a few jaw dropping moments when I finally do manually steer that focus in, it looks really sharp even in LV mode when zoomed in before taking the shot! I think this is a relatively difficult lens to master and might account for quite a bit of deviation of review opinions regarding sharpness rather than 'bad batches' existing etc.

QuoteOriginally posted by skierd Quote
Bruce - I'd say your charts match pretty well to my 20-40, except I find mine to be good to great at f4. Interestingly, I've been shooting in MTF-line program mode and the apertures suggested are what Pentax typically suggests.

Des - that field curvature would definitely explain some of the occasionally unsatisfactory results I've gotten from this lens which of course get me cursing myself and unselecting photos in lightroom... for once, maybe it is the gear! Ha!
I no longer do the MTF program line, it appears to me that with a lot of lenses it's f5.6 that it wants to get you too.
05-17-2019, 04:44 AM   #28
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QuoteOriginally posted by BruceBanner Quote
I enjoy your reviews very much.
Thanks!

QuoteOriginally posted by BruceBanner Quote
Our aussie internet can be appalling (still on 5mp/s download speeds here
I thought you guys had fiber-to-the-home everywhere, and that your government had supported deployment?

QuoteOriginally posted by BruceBanner Quote
perhaps it's possible to add some chart summaries from your findings?
I try to do a pretty thorough summary in the text that follows the images.

QuoteOriginally posted by BruceBanner Quote
It might be a nice addition to the review?
The tool we're using does not allow this kind of measurement easily.

I do include a repeat of each chart test with an actual subject, which sometimes shows things with a different angle, and often influences the conclusions of the analysis. I think these are also very helpful.

QuoteOriginally posted by BruceBanner Quote
And so with your review and the chart findings of ephotozine, did you find similar results with your copy of the 20-40?
The 20-40 was not my review. I wish it was, it's excellent work I think! I do write a lot of reviews, but not all of them.
05-17-2019, 04:54 AM - 1 Like   #29
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QuoteOriginally posted by Des Quote
Thanks for your comment Steve. I was using back button AF (shutter button AF disabled), so assuming I released the back button when re-composing and didn't move the focus ring and didn't move the sensor distance to subject (and admittedly I might have screwed up in one of these ways), why would the focus change?
Good question, Des! Thanks for the clarification. The short answer is that I can't say. What I can suggest is that if the focus settings were preserved through the recompose/expose, your lens has some crazy field flatness issues at that combination of aperture, distance and focal length (very abrupt and wavy?). I have fairly shallow experience in this area, but have to ask...Centering defect?

BTW...I found your thread from February discussing the same photo and others.


Steve

Last edited by stevebrot; 05-17-2019 at 05:26 AM.
05-17-2019, 05:38 AM   #30
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QuoteOriginally posted by bdery Quote
Thanks!



I thought you guys had fiber-to-the-home everywhere, and that your government had supported deployment?



I try to do a pretty thorough summary in the text that follows the images.



The tool we're using does not allow this kind of measurement easily.

I do include a repeat of each chart test with an actual subject, which sometimes shows things with a different angle, and often influences the conclusions of the analysis. I think these are also very helpful.



The 20-40 was not my review. I wish it was, it's excellent work I think! I do write a lot of reviews, but not all of them.
We do have fibre, just slow to roll out, my village/suburb is finally getting it currently, the town is a mess with men working on the curbs all around the place haha. Soon! Soon!

I'll try and pay more attention and read the reviews better, I am a manchild, I need pictures

Ah, well... the review seems to knock the sharpness down a bit... like it's not that good? Thus far (and even on the K-1) I am not at all feeling that vibe... HD DA 20-40 on K-1 (Full Frame Mode) | Flickr
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