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06-22-2019, 07:46 AM   #16
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QuoteOriginally posted by Madaboutpix Quote
Thanks for taking the time to answer, Mike, since I was genuinely puzzled. Also acts as some sort of confirmation that, with the DA35 Macro already in my kit, a limited budget, and a soft spot for primes, it is probably the HD21 that I want to close the moderate-wideangle gap, and not so much the HD20-40.
You're welcome I will say, there are plenty of folks who really like the DA20-40, and it's capable of taking great photos if you work with its idiosyncrasies (or if they don't matter to you). But there are enough who've noticed the same characteristics I have to reassure me that it's not just my imagination, or my copy of the lens. The HD DA21, on the other hand, is one of my favourite lenses and probably my favourite Pentax DA. I doubt you'd be anything other than delighted with it

06-22-2019, 04:06 PM   #17
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There are many variables at play when you're taking a photo. Yes, as Mike says, there are many of us who are big 20-40 fans, and there's a reason - it's a fine lens. The fact is that more or less all lenses have field curvature to some degree, even the most expensive, and this comes from projecting a 3D image onto a 2D plane. So I've evaluated many of my photos taken using the 20-40 objectively (pun - sorry) by running the 'select focus area' tool in Photoshop, which shows the pixels in focus in a photo across a range of zero to 7.5. Zero is the most picky where nothing is assessed to be in focus, to 7.5 where everything is. Photos that I've taken in the with the 18-55 kit lens had the range set at around 3.5 to 5 to get the focus areas to show across the frame; 2.5 to maybe 4.5 for the 16-45. With the 20-40, the range comes in at 0.5 to around 3.5 depending on the subject and how steady you're holding. Shots on a tripod are the ones down at 0.5 where Photoshop is being most picky, and according to the 'select focus area' tool, they are usually in focus across the frame. Photos of near objects, or scenes with different objects at different distances show the areas of focus being where they should be; in other words, where it looks like that I focused. And landscapes, or objects in the middle to far distance in roughly the same plane are also in focus, so using this tool, I don't see the blurriness in the edges from the 20-40 that others are indicating. If you have Photoshop, give this a try, it might give a better feel for whether you have an issue rather than trying to judge it by eye.

I'm not saying that the 20-40 doesn't have any field curvature - all lenses do - but at the apertures I shoot (predominantly f7.1 to f9), I don't see it beyond other lenses that I've used, and neither does Photoshop seem to, from a focus area selection perspective. Still, not to worry - with field curvature, you'll be ready for when curved sensors are introduced.
06-22-2019, 05:22 PM - 2 Likes   #18
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QuoteOriginally posted by BigMackCam Quote
As @ZombieArmy says, the HD DA20-40 exhibits fierce field curvature... one member here described it as "runway style", and I think that's a good description. As you move further away from the centre of an image, the distance in perfect focus moves nearer to the camera at quite an alarming rate (look at the foliage on both left and right edges of your photos and the areas in best focus are closest to the camera). And this is even the case if you stop down considerably, though of course it's less of an issue then. As such, if you're not absolutely perpendicular to your subject, that can give the effect of one side being softer than the other.

Looking at your photos, I can sort of see why you think the right side is softer than the left (I see a small difference, I think), but at the sizes provided, these aren't by any means conclusive. If I were you, I would switch to Live View and move the focus point to the far left, focus on whatever is there, and see how sharp that part of the image looks. Then, move the focus point to the far right, focus, and see how sharp that part of the image looks. If left and right are similarly sharp, the lens is probably fine.

I have a love-hate relationship with the HD DA20-40 because of the field curvature. For some time - a couple of years, maybe - it sat on my shelf unused because of my early experiences with it. Recently, I've been forcing myself to use it and it's growing on me - but it depends very much on the subject matter as to how satisfied I am with the results. It's not suited to landscape shots if edge-to-edge sharpness is required... but where the subject is mostly within the centre of the frame, it can produce fantastic images. As a photographer, it's up to me to understand the lens' characteristics and get the best from them...
I use mine exclusively for grandchild portraits* on KP. In that kind of shot the field curvature is actually a feature.

*Though it is an awfully expensive lens for such a limited use.

Last edited by monochrome; 06-23-2019 at 06:26 AM.
06-22-2019, 08:58 PM   #19
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QuoteOriginally posted by monochrome Quote
I use mine exclusively for granddchild portraits* on KP. In that kind of shot the field curvature is actually a feature.
Yes, the field curvature at the long end may be a feature and not a bug. A conscious design decision when photographing objects or subjects to really give that 3D pop, OOF falloff or whatever you else one may want to call it.

06-23-2019, 12:57 AM   #20
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