[Serkevan]

For example...softness sue to other factors.



I love it when a bit of vocabulary is thrown into the discussion. Thank you for a concise summary that will send a few in search of a dictionary.


Steve

Sorry, I had just been reading a couple papers for work and "stochastic" came up in my head before "random". Darned scientists

About the softness, it can be so many things that one can't list them. Definitely not diffraction though.

[normhead]

The da* is sharp sharp sharp and it's sharp from wide open. The sharpness is good at all focal lengths.

And it was originally designed as an FF lens, even if the edges don't meet the current Pentax spec.
I would expect the DA* to be slightly better in the centre 80%, and the DFA 70-210 to have slightly better edges (20%) based on what I've read, but I'm waiting for bdery's tests.

[repaap]

the dfa*50 is softer at f14 than the dfa50mm macro at f14, (subject is at same distance from lens for each) because if suffers from diffraction differently. but at f4 the dfa* destroys all other lenses.

or is diffraction the wrong word?

I think that Digitalis did explain that properly. Macro lenses have flatter focusing fields. So it is designed in mind of having that thing in focus in close up, and stopped down, because that is what you have to do when you do macro. Pleasing bokeh and pop in wide apertures is not the main aim with macro.


DFA*50 has been designed for portraiture and some details, and what one uses classic normal 50 mm lens for. for some purposes I'd say it might be stopped down to f 8. But usually these lenses are around f 2-4. That is why it exell around there. shooting wide open is not that common either.

[WorksAsIntended]

With your camera, that point is between f/8 and f/11 regardless of lens mounted if evaluating at 100%. That said, if you are using in-camera lens corrections and evaluating from an in-camera JPEG or TIFF, extra sharpening has probably been applied for the 50/1.4 and the 100/2.8.


Steve

This is not true. The diffraction limit is way more complex than it gets cooked down often these days in camera forums.
First of all, diffraction at the aperture clearly depends on the optical formula, as not all lenses provide a plain wave at the point of the aperture and with converging or diverging light the diffraction is another story. All those claims done are using the Fraunhofer diffraction model which fits very poorly for some lenses. In fact, Fresnel does not work either for some, a clear case of mid field diffraction with no algebraic solution and no easy proportional dependencies as quote many times.
The "theoretical diffraction limit" also does not mean every lens behaves the same and has the sweet spot at the same aperture as this is only one of many diffractional limits in the system.

For most lenses it works as a rule of thumb, for others it fails miserably. The bokeh with "onion rings" is a sign for non plain wave forms at the aperture for example, it is a thing for a lot of lenses and points to the assumptions being not true for every lense (although the wave forms of the focus plain will look differently of course).

About the thread opening post, I would expect the 70-210 to be "at least" on the level of the 150-450 based on the Tamron lens (pleas do not read at least as if the 150-450 is a bad lense, I never used it but reviews show it to be a good lense).
If this is not the case, I expect either a problem with focus or a bad copy, or the 150-450 being a exceptional good copy.
I am looking forward to read more about this lens. If it is at the level I hope and the price comes down a bit it may be my future tele lens.

---

Last edited by WorksAsIntended; 03-11-2020 at 12:47 PM.

[bdery]

Thanks for that info Bdery!!

A pleasure

Please be specific about 60-250 / 4 vs 70-210 / 4 such as:

What you're asking for is a complete review. That doesn't happen overnight.

I hope you understand what interests me as end user in the new DFA lens and as advanced amateur / enthusiast user

I will try to make my review as useful as possible, while keeping to the same guidelines I always follow. I cannot, on the other hand, satisfy everyone's particular requirements, and answer each and every question In your case, especially if you're in a hurry, you might be better off taking the time to test the lenses on your side.

I was hoping you might drop in. Thanks for your early impressions.

Happy to!

I'm waiting for bdery's tests.

Seems like this review will be a popular one!

[normhead]

This is not true. The diffraction limit is way more complex than it gets cooked down often these days in camera forums.

Almost everything is more complicated than what gets posted on camera forums.

However diffraction limits, just for a general idea can be easily identified from lens charts.

A typical lens chart...

Looking at the DA 70 on a 10 MP camera


It gets better and better until you get to ƒ5.6 then diffraction kicks in.

At 16 MP, same density as a K-1 the diffraction limit would appear to be somewhere between ƒ4 and ƒ5.6. The smaller pixel sites are more affected by diffraction. ƒ8 and ƒ11 are over the diffraction limit and are losing resolution even though without diffraction they'd be the sharpest.


Looking at these two charts you can draw a few conclusions not only about where the diffraction limit is, but see how it is affected by pixel size and location on the sensor, as the edges seem more affected by diffraction than the centres on the smaller pixel cameras, which is no doubt what led to the DFA's made for full frames being so much more heavily corrected.

It's unfortunate Klaus has never done a 24 MP comparison. And if you look through the lens charts it's interesting to see, how different lenses have different characteristics at different MP counts.

So when you're adding in all the things you need to add in, lens design and characteristics etc. etc. be sure and deal with pixel density as well.

These lenses are tested at a specific distance from the target image. What they'd look like tested at different distances from the target is a whole different issue.

Really, for so many reasons, this is not a subject you want to try and analyze mathematically. "I like this lens on this camera." Is all you need. Start trying to explain why and you're in for a world of over simplified information, and possibly misleading information.

Couple that with the fact that a bit of diffraction may make your image more pleasing to look at, and you quickly realize, not knowing what you don't know can quickly make your conclusions meaningless. You have know what the lens does on the camera you use. That's all that's important.

My guess is that the empirical knowledge of lens design contains elements so far removed from what you can establish without years of producing and evaluating lenses, it would make any attempts at mathematical analysis pointless.

Somewhere there was a description of one Pentax lens where the design process was based not on the test charts, but on which variant of the lens produced prints most enjoyed by members of the general public leading to the philosophy "lenses for the way people take pictures, not for the test charts". IN other words the pure test chart lens was not the lens selected for production, based on not as many people liking it's images. I'm fairly certain of a couple things with regards to that. The DFA* 50 1.4 was designed in part with that criteria. The change being they designed a lens to excel both on the test charts and in actual usage. I have no doubt Jun Hirakawa took some of that with him to Tamron, and that you're seeing something similar in the adopted tamron lenses. They have math they aren't explaining to us... and if they did, they'd have to kill us.

Or to return to 1966 philosophy, find the lens you love, buy the camera it goes on.

[WorksAsIntended]

Almost everything is more complicated than what gets posted on camera forums.

However diffraction limits, just for a general idea can be easily identified from lens charts.

A typical lens chart...

Looking at the DA 70 on a 10 MP camera


It gets better and better until you get to ƒ5.6 then diffraction kicks in.

At 16 MP, same density as a K-1 the diffraction limit would appear to be somewhere between ƒ4 and ƒ5.6. The smaller pixel sites are more affected by diffraction. ƒ8 and ƒ11 are over the diffraction limit and are losing resolution even though without diffraction they'd be the sharpest.


Looking at these two charts you can draw a few conclusions not only about where the diffraction limit is, but see how it is affected by pixel size and location on the sensor.

It's unfortunate Klaus has never done a 24 MP comparison. And if you look through the lens charts it's interesting to see, how different lenses have different characteristics at different MP counts.

So when you're adding in all the things you need to add in, lens design and characteristics etc. etc. be sure and deal with pixel density as well.

These lenses are tested at a specific distance form the target image. What they'd look like tested at different distances from the target is a whole different issue.

Really, for so many reasons, this is not a subject you want to try and analyze mathematically. "I like this lens on this camera." Is all you need. Start trying to explain why and you're in for a world of over simplified information, and possibly misleading information.

I agree completely.
My first few papers, quite some years ago, were about very specific diffraction effects in lenses, although not about photography lenses. What I learned back than is that even a pretty simple lens design needs a lot of itchy assumptions to be calculated algebraically. Today numerical solving is pretty easy by utilizing a bit of physics knowledge, but trying to solve the integrals directly numerically mostly gives wrong results due to the high oscillatory behaviours and the modern methods required too much calculation power up until the early 2000. If you look at the modern designs with complex coatings, anormal elements, aspherical elements, many groups and 20 lenses inside, mathematical models become very hard up to the point that they only barely describe what is going on. The idea to calculate the limits, even do a good approximation, without considering any of this will fail on some lenses badly.

[mbukal]

What you're asking for is a complete review. That doesn't happen overnight.

yep, i'm waiting with a lot of desire to read that text, may for example on 1st april, maybe someone even upload your review text 60-250 / 4 vs 70-210 / 4 as a first April joke, connect the cozy with the helpful and the forum members would have something to hyperactively discuss (not knowing if it's an actual review or as a first April joke),
bdery i believe in you that you will make that description and requested review in a short time of a few weeks, cheers

with this review all interested Pentaxians (and beyond) would be able to choose to buy the desired lens according to the known and presented capabilities of the same

I am convinced that I am not speaking solely on my own behalf for this desire

[newmikey]

Possibly a stupid question so apologies ahead of time but what would be your expectations (IQ as well as performance-wise) of the new 70-210/f4 against the DA*50-135 with the HD DA 1.4x converter (which would make it almost the same zoom range and quite the exact aperture)?

[rawr]

That correction, isn't it limited to jpg?

On paper, yes. And usually tied to specific on-brand lenses too.

But as the incoming image data in modern computers digital cameras makes it's way from sensor to image processor to SD card, lots of clean-up work is always going on to varying degrees.

[stevebrot]

In-camera diffraction correction, as introduced with the K-3II onwards for example, uses a capability built directly into the Milbeaut imaging chip. And that chipset and capability is shared by Nikon and others. So oftentimes we may not see an optical problem at all, if the correction is taken care of in-camera [optionally, or sometimes non optionally].

This claim has been made several times in the past and I was wondering (at the risk of being way off-topic) if you have a link to documentation at Socionext discussing the Milbeaut lens resolution correction feature and how it relates to diffraction losses and where in the image processing pipeline it is applied?


Steve

[mtngal]

I’m not the OP, but I’m still struggling with this lens. I’ll readily admit to being someone who doesn’t understand anything about optics, so much of the conversation above is way over my head.

However, I look at my photos and either like or dislike them. I know enough to be able to recognize many of my self-induced problems, and some of my dissatisfaction with this lens stems from the ergonomics of the lens, but some of it isn’t.

My copy front focuses a lot (-8 on the KP, fine adjustment, just used it this morning on the K1 and it isn’t as bad but will need adjustment).

The lens seems to underexpose more than any of my other lenses that I’ve used recently (mostly the DA 16-85 on the KP and the 28-105 on the K1, which hasn’t been used much in months). With the KP I didn’t notice it so much while I was shooting for the past three days, but I saw it on the computer monitor. On the K1 this morning I could actually see that the scene was darker in the viewfinder than it was in real life - I thought that was weird.

The one problem my copy of the lens does NOT suffer from is sharpness, IF it is in focus.

Every so often I’ll see a little CA in high contrast situations, but not always. I don’t see it most of the time.

Mine seems to play better with the K1 than the KP, where the focus seems to be more inconsistent.

I’m not entirely sure I’ll be able to get over the ergonomic problem though - while it’s not a heavy lens in the grand scheme of things, it is heavier than what I’ve used in a long time. I wondered if shooting with a bigger, heavier camera with a bigger grip than what I use on the KP would help, but not really - there’s a reason why I’ve been using the KP almost exclusively for a year.

[rawr]

have a link to documentation at Socionext discussing the Milbeaut lens resolution correction feature and how it relates to diffraction losses and the point in the image processing pipeline where it is applied?

I had to dig to find an old 2014 thread here on PF, but there is some useful material there:

More details about diffraction correction - PentaxForums.com

including a Fujitsu PDF (link still works, amazingly):

http://jp.fujitsu.com/group/fsl/en/release/20130904-1.html

No sensor logic diagrams or details though.

[stevebrot]

I had to dig to find an old 2014 thread here on PF, but there is some useful material there:

More details about diffraction correction - PentaxForums.com

including a Fujitsu PDF (link still works, amazingly):

http://jp.fujitsu.com/group/fsl/en/release/20130904-1.html

No sensor logic diagrams or details though.

Cool! I am familiar with the old thread (was a participant at the time) and will take a look at the PDF. Thanks!


Steve

(...still curious as to how the K-3 released October 3 could have been made with a chip released only a month earlier, but since these are all custom applications, anything is possible... )

[biz-engineer]

Looking at these two charts you can draw a few conclusions not only about where the diffraction limit is, but see how it is affected by pixel size and location on the sensor, as the edges seem more affected by diffraction than the centres

What you see in resolution charts is the total contribution of diffraction and optical sharpness. There are other charts (https://www.lenstip.com/574.4-Lens_review-Sigma_A_24-70_mm_f_2.8_DG_DN_Image_resolution.html) on excellent lenses that show that beyond f8 the lp/mm of center to edge converge to the same figure, with resolution dropping proportionally to aperture by something like -10lpmm per stop (diffraction limited system), which means at that point of aperture onward you are not getting much for you money by buying a larger camera system unless it's tilt shift capable.