DT Tech Cam Test - IQ250 vs IQ260 vs IQ280

[gerald.d]

The reason I write these long explaining posts of my position now is that it has come to my attention that my pessimistic view on the IQ250 with tech wides is by some seen as unfair, even to the point that some get angry.

I don't understand where this is coming from at all. No evidence in this thread that I can see? If people aren't prepared to challenge your evidence rationally and publicly, then that's a fairly convincing indication that they've lost the argument already.

FWIW, your long and detailed posts are extremely interesting to read, even from a purely educational perspective. My dealer approached me wondering whether I'd be interested in the IQ250. My reaction was "nope, purely down to the sensor crop". However, subsequent to Doug's files being made available, I did start to backtrack on that a little. In some respects, the IQ250 looks very good indeed.

But.

This cross-talk issue though is clearly a fundamental design flaw (as viewed from the perspective of a tech cam user). I was never even aware of the concept before you highlighted it, and I suspect nor were many others.

I'm sure I'm not the only one who is very grateful you have made the effort, and hopefully there will be some recognition of this issue publicly by "Team Phase One".

Simply put, if you shoot with wide angle tech lenses and colour accuracy is important to you, it would appear to be a no-brainer. Steer clear of the IQ250 - there are plenty of other great backs from Phase One to consider instead.

Kind regards,

Gerald.

 

[darr]

I sometimes complain about MFDB manufacturers not caring enough about tech camera users. I know that too is not a popular opinion among some dealers. Ok, fine, agree to disagree.

I think this is another sign of it. If P1 had a clear interest in tech cams would do the simple thing to check the data sheet of the Sony and the data sheet of the Rodenstock and Schneiders and make those numbers available to dealers so they could be formed into good lens recommendations. Guesswork from what C1 algorithms can hide in some situations I do not think is a serious take on the problem. I'm sure many dealers won't agree on that, but I think I'm free to have that view and actually I think mine should be considered less controversial if you look at it from a buyer's perspective.

When you design a camera system you make sure sensor and lens play together in concert. The Rodenstock wides have been designed with certain sensor properties in mind, probably what is found on the the 6um CCD sensors. The Rodenstock designers have known the critical crosstalk angle of these sensors and adapted their optical design to that. Sure there's color cast, but that's reversible unlike crosstalk.

When you then sell a new back with radically different sensor properties the original design criterias can break, and that is what's happening with the IQ250. If I had been a dealer I would find it very difficult to suggest a lens+sensor combination which I knew broke the design criterias and require software algorithm guesswork to hide. As said, while colorcast is 100% reversible (with the exception of a bit DR loss of course) crosstalk is not.

The reason I write these long explaining posts of my position now is that it has come to my attention that my pessimistic view on the IQ250 with tech wides is by some seen as unfair, even to the point that some get angry. I maintain that my points are both valid and fair, but sure for someone that think live view is more worth than color fidelity fine at the high end, go ahead use it with these combinations. My recommendation is however to pick the IQ260, IQ160, Credo 60, P65+ or any other 6um Dalsa sensor technology to use with these wides. Furthermore based on my results I do not think it's a good idea to upgrade P40+ or IQ140 (44x33 6um Dalsa CCD) to IQ250 if you are a tech cam wide angle user that actually use shift or tilt.

By actually having the IQ250 back and a lens lineup I could make much better tests and come with more nuanced conclusions. I would still use measurement-based test as an important aspect of testing, and I would still not recommend to use the sensor in crosstalk mode as it compromises the fine color rendition properties. But I could make a much better assessment of exactly how bad crosstalk will hurt your colors and complicate your post-processing needs, if someone still would like to push its limits. But I would point on an image circle diagram which show "no crosstalk inside this circle" and say "outside this border, you're on your own". The supposedly superior CFA design doesn't count when colors are mixed. I think that matters to users.

As co-author of Lumariver HDR and RawTherapee where I specialize on various algorithms I know a thing or two about raw processing. I'm also myself a tech cam user which shoot landscape on a recreational basis. This makes me a bit emotional about this subject, I have quite strong ideas on what makes up a well-behaved system, and what's fair to fix/hide in post-processing and not, and what the customer has the right to know about the substantial amount of money that goes into these systems. To me it's a big difference between color cast and crosstalk. Reversible vs not reversible. And it doesn't end there, green separation which is a crosstalk side effect makes the back sensitive to particular LCC and demosaicing strategies. I would not be surprised if Lightroom makes a much worse job on this particular back than C1. I think it's a bad idea(tm) to make a high end system dependent on advanced guesstimate algorithms. Therefore I think what's the "crosstalk-free image circle" is a very central piece of information.

I have a somewhat faint memory of that there were rushed tech cam upgrades from P65+ to IQ180, and they did not all turn out too well, as the IQ180 has much more wide angle issues than the P65+ and IQ160. Did the dealers give proper information to the customers about that before upgrade was sold? It would be sad to see people rush from IQ140 to IQ250 and be semi-disappointed in the longer term when it's clear that it actually doesn't perform as well as it seemed in the first tests with the favourite wides. Presenting the actual crosstalk free image circles where performance can be guaranteed would avoid that situation.

It is refreshing to have you on this forum. Thank you for your knowledge and experience. I get tired of the sales push that seems to follow anything medium format digital. It seems if you buy a digital back, you are on a marketing list FOREVER.

Kind regards,
Darr

 

[dougpeterson]

I'm not "angry".

I just wish you had the opportunity to do some of your own shooting with this back and make evaluations based on that experience and not solely on scientific evaluation of the raws. I'm sure you'd want the same thing if that was practical. But I realize that's hard given your location. If you're ever in NY...

Still I posted the raw files for the exact purpose of people examining them and posting their thoughts both good and bad, so I cannot complain. We'll just have to agree to disagree the exact size of the usable image circle for these various lenses for practical photographic applications.

I did reexamine all the files last night and lowered the sizes I listed by 5-10mm on a few of the lenses for the 250. Can't hurt to take a more conservative view.

 

[Paul2660]

5-10mm of useable image circle or useable shift?

Thanks
Paul

 

[Ken_R]

Guys I really do not know what the big deal is. I mean, it is not like Phase One is only making the IQ250. The other IQ backs are still available and are superb.

The IQ250 is the best back for available light work with an SLR in almost any available light condition. Simple as that. No other digital camera offers the same combination of low light performance, high resolution (50 MP!), tethering stability / capability (WiFi also) and Color Characteristics as the IQ250.

Some people micro focus on what the back does not do best and forget about what it can do amazingly well. Sometimes its best to pull back a bit and see the big picture.

 

[gerald.d]

Guys I really do not know what the big deal is. I mean, it is not like Phase One is only making the IQ250. The other IQ backs are still available and are superb.

The IQ250 is the best back for available light work with an SLR in almost any available light condition. Simple as that. No other digital camera offers the same combination of low light performance, high resolution (50 MP!), tethering stability / capability (WiFi also) and Color Characteristics as the IQ250.

Some people micro focus on what the back does not do best and forget about what it can do amazingly well. Sometimes its best to pull back a bit and see the big picture.

The thread title is "DT Tech Cam Test - IQ250 vs IQ260 vs IQ280".

 

[f8orbust]

Some people micro focus on what the back does not do best and forget about what it can do amazingly well. Sometimes its best to pull back a bit and see the big picture.

I agree to an extent, but what is one, if not the, selling point of the 250? True live view. And when would true live view on a DB be most useful? Shooting movies, or focussing on a tech cam. It can't do the first, and can do the second - but its performance with even the strongest retrofocus lenses is compromised, so even though you can focus the lens you can't shoot with it. For $35k you want perfection or as near as dammit. You don't want to be told, 'you know what, your client's will probably never notice.'

 

[RodK]

In essence then, if you took a vertical image and then reversed the back 180 degrees you could observe the phenomona in a sort of mirror like comparison on the same subject.
Might be interesting to do this some time and see how ones sensor performs and if one orientation is 'better',with one position or the other, and also what type of subject effects the color in what manner. A user might discover that vertical works better in one orientation, with a particular subject better than the other. In other words, turning left to vertical rather than right, or vise-versa etc...
Thanks for all your work Torger and Doug.

 

[torger]

You can get crosstalk with "tech cam friendly" CCDs with the SK35 which is a pretty extreme optical design in terms of angle. I have the SK35 and I do start to get crosstalk with my 33 megapixel back, but need to get quite far out (farther out than I want to use it because of its sharpness falloff so I don't see it as a problem). I did a quick color checker test and colors were affected as one would expect, some colors more than others. I might do a more detailed test and present some data at some point, info of how to differ between color cast and crosstalk etc.

I may have over-reacted a bit on offline feedback I got on my testing. In any case I just try to be as fair and correct as possible, and just want to raise a warning so people don't rush for an upgrade without knowing what compromise is being made. I think few knew about the difference between colorcast (reversible with LCC) and crosstalk (not reversible with LCC), and that it for this particular back is an important thing to be aware about.

If your lens and shift combination is not pushing the sensor into crosstalk, all will be fine and orientation should not make any difference. If your sensor does crosstalk, its orientation may affect the result, I do think it will for the IQ250, but it's hard to predict. The only thing that happens is that the crosstalk flow changes a bit, you change from one incorrect result to another incorrect result, one might look better for the particular scene shot, but surely one would not like to have this in a production system.

An interesting experiment would be to test well-established combinations like the IQ260 + 32HR and see if there is any color fidelity falloff. The measurements I've made are quite crude, ie I need considerable crosstalk to register it, and I register it through the side effect of green channel separation. On the IQ250 crosstalk was clear quite early on as discussed. When looking at the IQ260 I saw no crosstalk whatsoever, but possibly there could be a small amount anyway towards the edge. As the quality reduction is gradual it's thinkable that lens designers accept some minor crosstalk towards the edge of the image circle assuming it would not hurt color too much. Possibly there are optical effects that could affect color fidelity too if angle gets too high (coatings?), I don't know. Therefore I think the experiment would be interesting also for a well-established combo to get a baseline of how good it can get.

 

[torger]

Does the IQ250 Sony sensor have the same type of microlens offset the Sony in the A7r has? See image:

The results I get indicate quite strongly suggests that this is the case.

For example, the painting with the dress used in a crop further back in the thread have very different color in the shots 15mm shift up vs the 30mm shift up. The 15-up shot has considerably less crosstalk and thus looks better. The difference is that the painting is registered in different places on the sensor.

In the bad case (30-up) the painting is registered on the lower part of the sensor, where microlens offset would expect to see a low part of the lens projection, but instead it gets a high part and thus the light comes in at the opposite angle: worst case!.

In the good case (15-up) the painting is registered on the upper part of the sensor, where microlens offset and lens image projection work in the same direction: best case!.

This microlens offset makes the sensor hard to work with in shifted position, and explain why it works okay when located in the center of the image circle, but how drastic degradation you can get on one side of the sensor (and less so on the other) when shifted.

 

[torger]

Although there could be microlens offset as described in the previous post, there could be just regular non-uniformity of pixels, ie that the amount crosstalk for a certain larger-than-designed-for angle vary a bit over the sensor. The much worse top left corner of the 30 up is hard to explain otherwise. It could be a combination of both.

I have attached here small size "scientific" renderings of the 15mm up and 30mm up IQ250 32HR raws. They have been both LCC corrected, but reference color of the LCC was from the center shot, this to avoid slight whitebalance differences that otherwise could occur.

White balance setting is exactly the same for both shots, as well as the contrast and saturation parameter. The exposure has been adjusted manually to approximately equal, as LCC correction (vignetting etc) shifts the exposure, and I'd guess copal shutters have been used which vary too a bit. To make images brighter I've clipped the windows (would have to use tonemapping to bring them in, the DR in these images is impressive). Software used: RawTherapee, Lumariver HDR for the LCC (not yet released version), and Adobe's newly released IQ250 dcp profile (which is 2.5D, ie no exposure-related color corrections to worry about).

The purpose of these renderings is to show how the sensor generates colors before you've started with your Capture One and/or Photoshop magic to try to match them. If there had been no crosstalk the color would look the same on both images. Crosstalk leads to desaturation and/or casts which is still there after you have applied LCC.

The green rectangle shows the painting discussed in the previous post which could indicate that the sensor has microlens offset. The blueish top left corner of the 30-up (worst area) does not really match this (in that area lens and microlens offset should be in the same direction), but I think the angle there is too far into crosstalk to produce any sane result, ie it does not necessarily contradict the existance of microlens offset. If you look at the topmost edge of the 15-up picture it's much better color fidelity there than in the corresponding area in the 30-up (note especially the painting with the red dress in the top-left corner of the 15-up, huge difference!). Also this is a sign of that the sensor could have microlens offset.

So what does this say? It says that if you shift the sensor too much the whole surface suffers, like in the 30-up shot. In the green rectangle I'd think the color fidelity falloff is too large in the 30-up, but may be acceptable in the 15-up. It could be the case that a 20-up shot would still produce acceptable results, but say at 25 you start to see a sharp falloff. The exact capability of the back is hard to specify without very detailed testing. If you want to be on the safe side and not have any crosstalk at all, which is what I would recommend, the 15-up shift is already too much (the upper half of the image has some crosstalk). If you can accept crosstalk if color fidelity falloff is not too bad, there's a whole lot of "it depends". You can't specify an image circle where it works, but you must specify sensor positions that work.

 

[Paul2660]

From looking at the image in your post, it appears the the entire upper left suffers, much past 15mm. I base this on the blue hue?

I am still wondering where the horizontal shift falloff is, I am assuming somewhere between 7mm and 10mm of horizontal shift?

Paul C.

 

[dchew]

I agree to an extent, but what is one, if not the, selling point of the 250? True live view.

I think that's what we tech camera users hoped the selling point was, but I don't think that is what Phase thinks it is. Just look at their first marketing story and associated images: An action photographer shooting mountain bikers in low light. Not a technical camera user wandering in the woods and/or shooting architecture shifting and using live view. They are marketing ISO first, live view second.

Dave

 

[tjv]

That's a pretty significant difference in colour. I think Doug's post above (three images of same crop with green dress) is (unintentionally, of course,) misleading with regards to overall colour rendition and fidelity when employing movements. For example, if you correct for just that part of the image, the rest of the image suffers and visa versa. It seems to me that if you own a technical camera, the IQ2250 falls short of being an idea back. Bring on the next iteration!

 

[torger]

On the other hand if Doug had skipped the 30mm-up shot (where color breaks apart completely) and only stitched the 15mm-up the color problems would have been much much less. Haven't detailed-studied the 15-up-15-left/right yet though could be significant problems in the corner, but need to check that.

If you look relative to the sensor size 30mm is a very large shift for a 44x33mm. In my landscape shooting style I would rarely do more than 10mm shift on a lens this wide with a 44x33mm. For interiors I'd probably need a bit more though, and if you stitch to simulate a larger sensor area you'd need more too.

Since I got aware of this crosstalk issue I've noted that the old SK35XL introduce crosstalk on CCDs, even my old Aptus 75, and probably it's a bit worse on a P65+ or similar 6um back. The effect is desaturation and slight hue shifts towards the sides, can't get as drastic effects like with the IQ250 though (probably due to that the sensor's critical crosstalk angle is not violated as much), but the point is that people have made pictures before with some crosstalk in them and been satisfied with the results. Many sold their SK35XL though with the P65+ and other 6um sensors, most complained about lack of sharpness rather than lack of saturation and color accuracy though, probably because we did not know that one should look for that.

If I had the opportunity to in person test the IQ250 with some lenses I'd do it and try to find more well-specified limits, say 1) crosstalk free image circle, and 2) maximum shifts with crosstalk no worse than it was for a SK35XL and a P45+ which was a high-end configuration of the past whose preformance many can relate to. I think one could then end up with a usable system for moderate shifts on the wides, but you'd need to compromise color a bit, as we apparently did with the SK35XL.

If you today are used with the color performance and shift stability of a IQ260+32HR I don't think you'd want to downgrade that, but a user going from a SK35XL and P45+ to a IQ250+32HR could become satisified (note I haven't tested the P45+/SK35XL in person, there's a slight risk that my Dalsa Aptus 75 has more crosstalk than the Kodak in P45+, but I don't think so).

 

[torger]

Haven't detailed-studied the 15-up-15-left/right yet though could be significant problems in the corner, but need to check that.

Just rendered the 15 up + 15 left and right, and yes desaturation from crosstalk is quite visible in the corners, but you don't get any disaster blueish cast like on the 30 up image. Ie a stitch with +/- 15 mm shifts would probably be quite successful with only very minor color matching issues, but color fidelity on the sides would suffer visibly. With photoshop and localized saturation increase it could be masked.

I've attached an image that illustrates the desaturation, by using the IQ260 image as reference. Decide for yourself if this is acceptable as a starting point. As said, with localized edits in photoshop you could match the colors a bit better.

(The IQ260 and IQ250 were white balanced with a color picker on the same spot, but profiles differ a bit and there can be crosstalk color shifts too on the IQ250 so the color of the blueish green dress is different. The purpose of this image is however to see how little the pale yellow background differs on the IQ260 image, and how it has been desaturated due to crosstalk with the IQ250)

 

[tjv]

Wow, thanks. It's really interesting to see this stuff and makes me realise how insanely hard it must be to design, build and implement these sensors and related technology. Again, this shows a big difference in colour. The IQ260 is still king in my book, but that's because I'm a tech camera shooter. For others, I'm sure it's a God send.

 

[torger]

Here's a trick if you want to test your lens+sensor combination for crosstalk that is a little simpler and quicker than shooting a colorchecker in various positions and comparing:

1) shoot an LCC with white light (ie a normal LCC shot, ambient light is generally okay)
2) shoot an LCC with magenta light (or use a magenta filter on the lens, or a magenta LCC card if you can find one), the purpose is to strongly reduce the green channel content.
3) correct the magenta LCC-shot with the white LCC-shot
4) you will now have a flat magenta image, white balance it in a place that is as close to the center of the lens as possible (where we assume there is no crosstalk).
5) Result: if the whole image now is a flat gray without casts, you have no crosstalk, if you have a residual cast (should be green) you have crosstalk, the stronger color the more crosstalk.

The reason the cast is green is because we made a magenta shot (very little green content) and light filtered through blue and/or red color filters on the sensor has leaked into the green photodiodes.

The attached image shows the result with an SK35XL and Aptus 75 with extreme shift, which has some crosstalk issues. The crosstalk is much stronger for horizontal shift than vertical (with sensor in landscape orientation like here), as the sensor has tungsten bars horizontally that reduces crosstalk for vertical shifts.

I think this test could be a good one to do in addition to regular color cast tests when testing a new sensor on tech wides. Note: I've used Lumariver HDR for this, haven't tested if the test procedure works in Capture One (or Lightroom) but I think it should.

While there's no mathematical solution to make a true inversion of crosstalk, it might be possible to make a compensation which in practice yields a good improvement, ie fix automatically what you would otherwise have to tweak manually in photoshop or leave as-is. I might make an attempt for that. The workflow would probably require two LCC shots like in this example. I'm not sure yet though if there's enough information in the magenta shot to make a decent compensation. If you make shots for red and blue light too you have even more information to feed an algorithm, but at some point the workflow would become too complicated still without possibility to make a true inversion.

I'll keep you posted if I make progress with such an algorithm (would be included in Lumariver HDR). It can help out the IQ250, and bring better colors to your SK35 shots.

 

[torger]

Added some preliminary results from crosstalk cancellation algorithm development here:

http://www.getdpi.com/forum/medium-...osstalk-cancellation-preliminary-results.html

With some luck it may turn out useful for the IQ250, it's early in the process though.

 

[tjv]

Thought I'd resurrect this thread and ask if anyone out there has bought and is using the IQ250 with a tech camera and the 32HR in the field? I haven't seen any test images that show how all the above plays out in the real world where bright blue skies etc. might make the crosstalk and colour fidelity issues very apparent. Any more info of test images would be greatly appreciated.