[emalvick]

I think the issue with field curvature is related to depth of field. If you have a large depth of field, then you won't have major focus issues because you'll be able to better accommodate the field curvature. I think they both go a bit hand in hand.

The focusing and recompose technique I'd argue is similar to field of curvature unless you are adding some element of forward backward movement, which I know can easily happen if you are dealing with a narrow depth of field.

The other major difference is probably just due to the lens construction itself. There is certainly some aspect of image quality outside of field curvature that is due to the lens itself. I think that becomes especially the case with zooms since these are the lenses where I see quality issues discussed the most. I'm guessing that constructing a lens to be sharp across the frame at multiple zoom lengths is not the easiest or cheapest thing in the world to do.

If we go back to the DA* discussion, I do see where at least for the zoom lenses, the performance is about as good as I've ever seen a zoom lens. When I look at my 60-250 at 250 mm, the images are about as perfect as any image I've ever taken at that level, and the difference between that and a 300 mm isn't much (besides focal length) considering that one is a zoom lens. I don't own the 16-50 or 50-135, but both those lens seem to have similar characteristics.

I'm not certain that if you could conduct a blind comparison of the DA zooms with their DA prime (* or limited) counterparts that the average or even above average viewer would be able to tell which lens the image came from, except perhaps if the two images were side-by-side (assuming all shot parameters were as equal as possible). I do think that if you throw in the FA limited, more of us would see a difference. Of course, as soon as you bring in the lens speed and wide opened shots, things might change, but I think it's more important to think about these from an independent standpoint, which means no side-by-side comparisons.

i.e. would you be happy with the output on its own? can you really only see a difference if you have the images side-by-side?

Not knowing much of the 20-40 lens, I would have the same questions?

And... that is all just related to image quality. I think that when it comes to lens selection, there are real questions of lens size and use that users should consider. Is the focal range adequate? (i.e. 20-40 vs. 16-50), do you vary focal range a lot? (prime vs. zoom), does lens size matter? (prime vs. zoom, limited vs. prime), lens speed? cost? can you zoom with your feet? etc.

I've been asking myself some of the questions as I've looked to improve upon my consumer level zooms (Tamron 10-24, Tamron 17-50, Pentax 55-300). At the wider end, I've gotten the FA limiteds and the DA 15 and 21 to substitute for the two Tamrons. At the telephoto end, I opted for the 60-250. I ultimately emphasized size, to an extent knowing I could take any 2 or 3 primes and cover my wide end adequately. And, as much as I wanted the 300 mm, I chose to cover the full 60-250 range with one lens that is about the same size as the one prime. And, I'm keeping the original zooms for those instances where I need a more minimal kit, except perhaps the 10-24, which I might end up selling at some point soon.

[IchabodCrane]

That depends on how important lens contrast is to you. I have come to agree with Mike Johnston, who contended that



I used to believe that sharpness was the primary determinant of "subjective optical quality." But having shot with more than 30 lens of various grades of quality over the last five years, I have come to appreciate the importance of lens contrast. And while it is true that contrast can be "simulated" via post processing, I have found that the kind of contrast you get out of a lens just looks better than the kind you get from moving sliders in Lightroom or other image processing software. PP merely extrapolates from a given data set. With lens contrast, you get a better, higher quality data set to work from. For example, with a less contrasty, flare prone lens, if I try to recover details in shadows, I often do so at the expense of contrast. When exposure values in shadows are raised they end up looking washed out. With a contrasty lens, I can recover those shadows while more easily preserving a decent contrast level.

Lens contrast also improves color rendition, leading to more brilliant, saturated colors combined with greater variety of tones. If I'm shooting the sky reflected on water, the DA 15 will produce a richer color palette, with more aesthetically piquant tones, than my DA 17-70.

I have found through trial and error that lens contrast tends to produce images that are easier to produce stunning prints. With lens contrast, you get images that have a lot of brilliancy and pop and yet at the same time feature very good tonal gradations throughout the entire image. Simulated contrast and saturation via PP, while it sometimes can look pretty impressive on screen, doesn't always transfer well to prints. Cranking up the contrast in post can damage the sensitive tonal gradations of an image, leading to areas of poorly differentiated tones -- something that doesn't always print up well. And over-saturation can sometimes push the image into tones that printers can't handle, leading to washed out-looking prints.

When it comes to lens contrast, I'm merely a pragmatist. I just find that the more contrasty lens tends to make it easier for me to produce images that I'm satisfied with than a less contrasty lens. To be sure, there are some types of light where the differences are insignificant. If I'm shooting redwoods in fog, my DA 17-70 can more than hold its own with my DA 15. But if I'm shooting reflections on water with sunlight involved, the DA 15 will give me a better data set to work with in post.

Interesting comments, for sure. How do you rate your DA 17-70 for contrast opposite other lenses you have? I ask because mine gives what what I perceive to be very good contrast, certainly better than my DA 18-135 and, I think, better than my DA 35/2.4.

[Rondec]

The DA * versus limited discussion is difficult. It seems like the DA * lens design isn't too far removed from the DA limited design (goal of edge to edge sharpness and neutral out of focus rendering) versus FA limited design (goal of center sharpness and edge sharpness not quite as good).

DA *55





FA 77

[Trudger1272]

Interesting comments, for sure. How do you rate your DA 17-70 for contrast opposite other lenses you have? I ask because mine gives what what I perceive to be very good contrast, certainly better than my DA 18-135 and, I think, better than my DA 35/2.4.

How do you rate your DA 17-70 overall?

@ Rondec: Those are some great images! Thanks!

[IchabodCrane]

How do you rate your DA 17-70 overall?

@ Rondec: Those are some great images! Thanks!

I like it a lot. It's probably a little stronger at the wide end than the long end but still good there nonetheless. As I mentioned, color and contrast are good. Biggest negative is it's a big lens... bigger than the DA 18-135 and Sigma 17-70 C.

[Nicolas06]

I was talking about using the focus-and-recompose technique with an off-center subject and a wide-angle lens. When recomposing, the distance to your subject (perpendicular to the sensor plane) will change from the distance at which the lens is focused. This can - and in some cases will - lead to the subject being out of focus. Field curvature has nothing to do with that.

You were responding to my comment explaining that the number the reviewer give by refocussing the border are accurate. You explained that it was possible to use the camera body correction to avoid bad/front focus.

I then responded that it was not a consistent back focus that was the problem then but more a camera body that might not focus that well with all lenses (just try K5 with FA50 in low light as an example) or that some lenses do have really visible field curvature (just use a DA15) and thus get weaker border performance than the number indicate. On photozone, the DA15 border are good.

Let look at this DA15 shoot at f/8, you can click on it if you to view a bigger size.



The center is really sharp, the border are soft.

This is something I never get for example out of FA77... That sound logical as field curvature is more a problem of wide angle.

[Nicolas06]

I like it a lot. It's probably a little stronger at the wide end than the long end but still good there nonetheless. As I mentioned, color and contrast are good. Biggest negative is it's a big lens... bigger than the DA 18-135 and Sigma 17-70 C.

I would say that it is overall good. (was my first lense) Still It has more difficulties than primes like DA35 f/2.4 or FA77 to keep great amount of detail on distant landscapes like a forest on a moutain. The micro contrast is not as good as theses prime I would say.

It is noticably prone to flare in difficult conditions. That not necessarily anoying in most situations, but for some landscape (for example in backlight) it will show. Indoors, you'd would have wanted it to be at least f/2.8. For portraiture too, outdoor f/4 is ok with distant background but indoor that a little restricting.

Colors and contrast, difficult to say. To me it was overall ok but all the lenses I brought after were prime or the 50-135 and all did seem to have better colors/contrast overall. The most visible through being DA15 on one side and FA77 on the other side.

A good lense overall and in no way worse than other zooms with the same range of focals and apperture. I mean even if many here want the best or lenses that really do what they like, in reality if you are not too picky, most lenses will do the job just fine. Right choice of subject etc is more important than the perfect lense. A basic 17-50 f/2.8 of sigma already allow one to go very far.

[kh1234567890]

that some lenses do have really visible field curvature (just use a DA15)

Really ?

[Nicolas06]

Really ?

On your link the corners of your picture but also the top are softer, this is more visible by zooming but also directly on the snapshoot you included.

[starbase218]

You were responding to my comment explaining that the number the reviewer give by refocussing the border are accurate. You explained that it was possible to use the camera body correction to avoid bad/front focus.

Maybe you haven't read my post fully, but I also explained the focus error that can occur when using a wide-angle lens, focus-and-recompose technique and an off-center subject, esp with big apertures. I brought this up because if you don't use such a technique, you will get soft borders in those situations and maybe then test results from a well-done review are not so relevant. But the fault is then your technique to start with. If you improve that then you should get better results. And then, reviews that are carried out meticulously will have more relevance.

Anyway, I think that when a reviewer wants to assess lens sharpness, he should not make any assumptions as to how I focus. PDAF, CDAF, focus adjustments, manual focus, lighting conditions that may have impact on one AF system but not on another... what is he really testing then? What do the numbers say? They could say anything, you just don't know.

Maybe tests carried out like this are relevant to you. But even in that case, the reviewer should describe his testing methodology, so you don't have to make up assumptions on how he is testing. At least then you can determine what unknowns there are and make up your own mind.

But I'm tired of trying to explain, so I'll stop here. Do with this information what you want.

[Nicolas06]

Maybe you haven't read my post fully, but I also explained the focus error that can occur when using a wide-angle lens, focus-and-recompose technique and an off-center subject, esp with big apertures. I brought this up because if you don't use such a technique, you will get soft borders in those situations and maybe then test results from a well-done review are not so relevant. But the fault is then your technique to start with. If you improve that then you should get better results. And then, reviews that are carried out meticulously will have more relevance.

Anyway, I think that when a reviewer wants to assess lens sharpness, he should not make any assumptions as to how I focus. PDAF, CDAF, focus adjustments, manual focus, lighting conditions that may have impact on one AF system but not on another... what is he really testing then? What do the numbers say? They could say anything, you just don't know.

Maybe tests carried out like this are relevant to you. But even in that case, the reviewer should describe his testing methodology, so you don't have to make up assumptions on how he is testing. At least then you can determine what unknowns there are and make up your own mind.

But I'm tired of trying to explain, so I'll stop here. Do with this information what you want.

I do understand how reviewers perform their review. I understand why they do this. While it use scientific method, try to isolate one factor from the others it also become really theoretical. Something that you get on studio light with tripod, manual focussing with contrast AF refoccusing border and analysing resolution on test charts, often in black and white with very basic shapes and 2D. Only a few get it to the next logical level, does the measurement on a statistically representative number of samples to compound the QC and sample variation... Even through we all know decentered and defectives lenses are pretty common.

This has little in common with how most shoot in real life. I agree one can use contrast AF and tripod (to everybody check honestly with yourself if this is your main usage). I do avoid focus/recompose myself but this doesn't prevent a bit field curvature that can hapen to you working manually with hyperfocal distance too.

The numbers are interresting but give not that much alone. You need to compound with their relative importance (corner performance wide open is not that important..., perfect sharpness in center only make sense for heavy cropping and huge prints) and many other characteristics more or less reviewed (bokeh, optical aberations, flare) or not at all (contrast / micro constrast, colors, overall rendering, "poping" of subjects, performance at close focus and infinite focus, how the lense work with deph and 3D objects...). This is to be finally compounded with other very important characteristics like size/weight and price.

We get measurement of maybe 10% of what make up a lense and measure it very precisely, meaning in conditions that are not really representative of the typical photographer, even the great ones. For the rest we get almost nothing.

We are in a typical case where many look much too far into some metrix and consider it as key because it is the only info that is easy to measure and get. In the film era for that point it was much better. You could not see 100% crop of your shoots, you would not get reviews of sharpness all over the net so what counted in the end was how pictures actually looked, something that look much more pragmatic to me than the mess we have now where reviewers, photographers and lenses makers concentrates on resolution numbers almost exclusively.

[GeneV]

How do you rate your DA 17-70 overall?

@ Rondec: Those are some great images! Thanks!

It used to be my favorite in the wide-normal range until I owned the DA*16-50. The contrast on the 17-70 is good, and its overall IQ is noticeably better than the kit lenses. However, to the thread title, the DA*, stopped down one stop to F4, is far better than the 17-70 wide open at F4, and for interior shots, F4 gets used a lot. My 17-70 also stops focusing at 70mm from time to time, which may be a precursor to SDM death.

[starbase218]

I used to believe that sharpness was the primary determinant of "subjective optical quality." But having shot with more than 30 lens of various grades of quality over the last five years, I have come to appreciate the importance of lens contrast. And while it is true that contrast can be "simulated" via post processing, I have found that the kind of contrast you get out of a lens just looks better than the kind you get from moving sliders in Lightroom or other image processing software. PP merely extrapolates from a given data set. With lens contrast, you get a better, higher quality data set to work from.

Interesting, I hadn't thought of this. I think some sites actually say that the subjective term "sharpness" has to do with both resolution and with contrast. Actually thinking about it though, some sites like EPhotozine actually relate MTF20 measurements to contrast like in this review.

For example, with a less contrasty, flare prone lens, if I try to recover details in shadows, I often do so at the expense of contrast. When exposure values in shadows are raised they end up looking washed out. With a contrasty lens, I can recover those shadows while more easily preserving a decent contrast level.

I know that flare can cause significant drops in contrast, but the two are not necessarily related, are they?

[Nicolas06]

Interesting, I hadn't thought of this. I think some sites actually say that the subjective term "sharpness" has to do with both resolution and with contrast. Actually thinking about it though, some sites like EPhotozine actually relate MTF20 measurements to contrast like in this review.



I know that flare can cause significant drops in contrast, but the two are not necessarily related, are they?

Don't know if they are always related but the thing about flare is that in many case the only symptom might be reduced contrast on the whole shoot overall. It is only in extreme case you'll see the photo completely washed out or with artifacts.

To me the issue is prevent in many more situations than what we think of and a lense that key high contrast in thoses situation will perform really better, even through it might not show in resolution numbers of reviews.

I don't know... maybe reviewers could give resolution and contrast number in studio with lights in different position like center/corner ... ?

[lytrytyr]

the mess we have now where reviewers, photographers and lenses makers concentrates on resolution numbers almost exclusively.

To their great credit, Pentax still seem to design lenses for photographers, rather than just for internet bragging rights.
Even to the extent of offering distinct viable alternatives, like the * vs. Limited question that got this interesting thread going.