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Author Topic: 1Ds MKIII and Optical Low Pass filtering  (Read 154309 times)

Dave Millier

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1Ds MKIII and Optical Low Pass filtering
« Reply #140 on: November 02, 2007, 06:27:23 am »

John
 
I'm a Canon user, I got the Sd9 to test rather than to use. My reputation on the DPR Sigma forum is as a sceptic - I have few friends there and have jousted with Lin on a number of occasions, so I have no agenda to promote Foveon or Sigma.  

I prepared this comparison as best I could to show the differences. The crops are 100% crops of resized files. I compared every method I could think of to equalise the comparisons including uprezzing the Sigma, down rezzing the Canon and re-sizing both to some common size. Nothing really changes anything.

I'm in the middle of another comparison (Kodak 14nx vs SD14) and am using a different approach. Prints have been made from the two cameras and I'm making make high resolution flatbed scans of the prints to show.  

It's work in progress at the moment but I'm happy to share the conclusion: in prints up to 24 x 16 inches (injket and lightjet) of distant landscapes shot with finest primes at optimum apertures, it is impossible to tell which camera took the shot.

The SD14 competes 100% with 14MP bayer in landscape type shots.  

As I said in another post, evidence trumps theory. I'm happy to be challenged on this but only with hard evidence; you won't change my opinion by quoting sampling theory. It's science not philosophy...



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He said these are both 100% crops, which is a disadvantage to the 350D, as a smaller fraction of the image is used, which always results in lower quality.

The only fair way to compare on a monitor is to use the same FOV lens with both cameras, and upsample both so that they have the subject size, and step back and view from a distance, if necessary.  Even when printing small, images are resampled, as the brain-dead drivers resample before printing, so you must print large for a fair comparison.
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Graeme Nattress

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« Reply #141 on: November 02, 2007, 10:24:13 am »

Dave, I like the photographs you took to examine the SD9 and 350D. They show up a number of issues in both cameras quite clearly, and are wonderful diagnostic images. Thanks for sharing them with us.

I looked in the metadata and see the Canon was converted with Bibble - is this correct? The demosaic is awful. Now, as I'm saying that, it's only fair to say that such a comment comes from my experience with the the RED camera project, and we're currently producing images like this: http://www.reduser.net/forum/uploaded/9_2kpomona_01033.jpg at 72fps. I'm not going to say the images are perfect - there's always room for improvement, and I'd also point out that we're recording to CF card through an advanced RAW compression scheme that's lossy (uncompressed or losslessly compressed is not really possible to record to a CF card at the fps and frame sizes we're doing) with a remarkably low data rate. If you're interested, here's the forum thread about those images: http://www.reduser.net/forum/showthread.php?t=5624 if you're interested.

I started this thread to talk about aliasing. It's all the more important as moving images show up the issues of aliasing much more clearly than stills, and you can't in all honesty, go in and paint and fix up a movie the way you can on a still.

And then we took a track that started to talk about Foveons. That's an interesting topic in it's own right, but it's also a tad "religious". They get brought into any discussion on aliasing because the don't have an optical low pass filter. Most Bayer CFA cameras do, because without, the bayer pattern reconstruction algorithms don't always work as well as you'd like and chroma fringing artifacts are a problem. Interesting that Phantom, maker of high speed Bayer CFA movie cameras have announced a "upgrade" to put an OLPF in front of their sensor. And it's removable, so if you don't want / need it, you can remove it.

But, when we talk about images, and what we see in them, we are always talking about a "reconstructed" image. Look at a RAW Bayer CFA image, as I often have to do, and it looks awful - black and white, a mosaic of little dots, and very dark as it's linear light. It needs to go through complex algorithms to allow you to see the RGB image. Those algorithms are often proprietary, and include all kinds of processing from noise reduction, demosaicing, sharpening etc. etc. Similarly a RAW Foveon image would also look very dark, with funny colours, but it would not have the mosaic pattern as it uses co-sited samples. It also needs a number of steps to turn it into a viewable RGB image that looks like the scene you've photographed. The Foveon white papers hint at the processing involved, and pixel-peeping can tell you more about what they do, but for the most part, it's proprietary. I don't know precisely what they do, but I can make educated guesses as to the nature of the processing needed to generate the RGB image, and I suspect it's of the same order of complexity as we use for demosaicing Bayer CFA images, just different.

So, when you compare an image, there's a whole host of factors you're adding into the mix, not least all the algorithms that conver the RAW to RGB. So when you start a discussion about aliasing, and try to keep it about aliasing, how it works, how OLPFs work, what positive and negative effects they have, you can keep it all quite reasonable. When you look at the real world on how cameras and photographers work with their images, the number of variables go up through the roof, and it's very hard to get to the bottom of it all.

I'd dearly like to see some Sigma images from a camera modded to include an appropriate OLPF. Lin, do you know if anyone has done this? There are enough examples of Bayer CFA cameras modded to remove the OLPF, but to see the other way around, I think, would be enlightening.

Graeme
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Ray

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« Reply #142 on: November 02, 2007, 11:01:01 am »

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I looked in the metadata and see the Canon was converted with Bibble - is this correct? The demosaic is awful. [a href=\"index.php?act=findpost&pid=150248\"][{POST_SNAPBACK}][/a]

Let's not beat about the bush here, Graeme. Are you implying, if the 350D image had been converted using ACR, the comparison would be quite different and the result would favour the 350D image?

If so, then this is a serious criticism of Bibble. Perhaps users of Bibble would like to defend their choice of converter   .
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Graeme Nattress

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« Reply #143 on: November 02, 2007, 11:10:03 am »

I'm not saying if ACR did it, it would  be better. I am saying that if I did it, then it may not be overall better (just becasue I have no way of telling), but we'd not see those particular artifacts. Those artifacts are most probably not in the data, and most probably in the demosaic algorithm, probably too aggressive on some gradient adaptive code to try and extract too much resolution in an inappropriate way from the image.

Graeme
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Dave Millier

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« Reply #144 on: November 03, 2007, 09:12:24 am »

Here's an alternative version from RSE with colour noise reduction to suppress the colour component of the aliasing (sorry, I forgot to sharpen the Canon crop).

NB: It's not visible in this crop but elsewhere in the image, the NR causes some local desaturation of points of colour so it can't be used without penalty.  

http://forums.dpreview.com/forums/read.asp...essage=23493083

What's interesting to me about this balustrade is that although RSE has cleaned up the colour fringing and superficially it looks good, there is still a problem. As you look to the left neither camera seem to be able to quite render the right number of vertical bars but for the first 3 panes the SD9 is more accurate and after that manages a reasonable approximation while the 350D rendering is inconsistent and gradually turns into mush.

Obviously, at the limit both cameras are aliasing and producing false detail in somewhat different ways. It's a matter of taste which is least bad but for my money the Sigma's attempt at least tries to approximate the underlying detail.

More resolution is the real answer, but I think at the very least this demonstrates that the Foveon approach holds up pretty well and some of the criticisms of its aliasing performance (often from people who have never used a Sigma) are a bit over-blown.  

Hopefully, people will accept that blanket statements along the lines of "this approach doesn't work because of xyz" need to be tested against the real world.

ps

I won't be abandoning my bayer cameras, however. At the current stage of Foveon development, it is competitive but I don't believe it is enough to compensate for the rather budget Sigma bodies and the fact I'm invested in other systems. But I think for someone starting out, the Sigma is worth a good look.




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I'm not saying if ACR did it, it would  be better. I am saying that if I did it, then it may not be overall better (just becasue I have no way of telling), but we'd not see those particular artifacts. Those artifacts are most probably not in the data, and most probably in the demosaic algorithm, probably too aggressive on some gradient adaptive code to try and extract too much resolution in an inappropriate way from the image.

Graeme
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Graeme Nattress

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« Reply #145 on: November 03, 2007, 09:54:17 am »

Dave, thanks for that update. The new RSE image certainly looks a lot better than Bibble. I still see issues with, what looks like, their local gradient based approach, but the artifacts are much more subtle than Bibble, and indeed, it does handle the chroma oh-so-much better.

Personally speaking, I prefer the Bayer CFA image, unless I pixel peep and look at a small area, when I begin to see some Bayer artifacts that I don't like. I think that can be solved with better algorithms, but who knows? The Sigma image, when viewed close up, doesn't have those issues. However, the roof tiles aliasing are as much an artifact as the confused gradient interpolation I see in the Bayer image.

Everyone has a different tolerance for different kinds of artifacts. Some people will look at the Sigma image and see nothing wrong. Others will see everything wrong with it. Similarly for the Canon image. The criticism of aliasing is very much a two pronged thing: a) the technical aspect, where I think we can all agree we'd rather not have aliasing, all other things being equal and  the mind / brain side of things, where some of us can't stand the way aliasing looks, and don't mind so much the effects of the OLPF, and others who find the sin of commision - ie aliasing effects, is worse than the sin of ommission, ie ommission of detail through using an OLPF.

The thing with a) above, is that all things are not equal. Add and OLPF and your measured resolution goes down, so you need more pixels or smaller pixels, and that effects FOV, cost or noise. All things are never equal.

When you design your own camera, you get to pick your compromises. When you don't you've got to pick someone elses compromises. That's why Nikon and Canon and Sigma and MF cameras exist - everone has their own internal set of compromises they can make, and they're all different.

Graeme
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Ray

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« Reply #146 on: November 04, 2007, 03:21:46 am »

Perhaps the impressive point here is just how much detail appears to be captured by this Foveon sensor which has only 3.4mp in the one spatial plane as opposed to the 8mp of the 350D.

Neither image is perfect with regard to aliasing, but it seems to be nit picking to argue which is preferrable.

The Foveon design, without AA filter, would be the way to go in my opinion. However, perhaps a technological breakthrough is required before sensors with great pixel density can be constructed, considering the loss of energy which the red and green light suffers as it passes through the layers of silicon.

Bayer type sensor also block a lot of light by virtue of the CFA, causing the sensors to be less sensitive than they otherwise could be (with B&W images, for example). But the inefficiency of the Foveon system, with regard to red and green light, seems to be greater.
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Dave Millier

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« Reply #147 on: November 04, 2007, 05:42:41 am »

I'm in the middle of writing up the results of a Kodak 14nx vs Sigma SD14 shootout which will be published on my website shortly.

To cut a long story short, the image quality of the SD14 is more than just a pixel count advance over the SD9. Dynamic range, detail, noise and colour reproduction are all improved.  The conclusion: 24 x 16 inch prints from the two cameras are indistinguishable.

I actually think the 14nx has a couple of percent more resolution in certain very specific circumstances but you would be printing 10 foot wide prints and scrutisinising with a loupe to see it.

The Sd14 is a bit of a bargain from a resolution point if view; it's just a pity the overall package is a little bit "budgety".



Quote
Perhaps the impressive point here is just how much detail appears to be captured by this Foveon sensor which has only 3.4mp in the one spatial plane as opposed to the 8mp of the 350D.

Neither image is perfect with regard to aliasing, but it seems to be nit picking to argue which is preferrable.

The Foveon design, without AA filter, would be the way to go in my opinion. However, perhaps a technological breakthrough is required before sensors with great pixel density can be constructed, considering the loss of energy which the red and green light suffers as it passes through the layers of silicon.

Bayer type sensor also block a lot of light by virtue of the CFA, causing the sensors to be less sensitive than they otherwise could be (with B&W images, for example). But the inefficiency of the Foveon system, with regard to red and green light, seems to be greater.
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httivals

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« Reply #148 on: November 04, 2007, 10:05:12 am »

These are great posts about the Sigma SD14.  It's the  first time I've been tempted to buy one, and buy one I would if I could get it with a Canon or Nikon lens mount.  Bummer it doesn't exist.
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Mark D Segal

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« Reply #149 on: November 04, 2007, 02:19:08 pm »

I've been attracted by the Foveon concept, much as many other have, but one wonders why it hasn't captured a larger share of the market than appears. Could lenses be the key issue? How does the quality of Sigma lenses normally used with a camera such as the Sd14 stack-up compared with the better of the Canon L series? Would this not also be an important factor affecting comparisons? I must say, I don't find the comparisons of the images on DP Review determinative because they are small and hard to interpret on a display. Perhaps one of these days I'll seem some (at least A3) prints of real-world subjects captured and processed under good comparative test conditions. Meanwhile, I'm more persuaded by evidence of what users have found to pass muster in critical market segments.

Mark
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Graeme Nattress

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« Reply #150 on: November 04, 2007, 04:19:53 pm »

Sigma make great and affordable lenses, so I don't see quality of glass being the issue. I've got the feeling that ergonomics and speed of the bodies might be the major factor? Not that Canon get the ergonomics totally right either....

I've not seen a proper dynamic range test for the Sigma - anyone got a Stouffer chart shot they could share?

Graeme
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Jonathan Wienke

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« Reply #151 on: November 05, 2007, 04:17:39 am »

Could links the original Canon and Sigma RAWs be posted? I'd like to do a comparison converting both with ACR, and using my deconvolution-based capture sharpening approach.
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John Sheehy

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« Reply #152 on: November 05, 2007, 12:52:29 pm »

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I've been attracted by the Foveon concept,
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Which concept?  The disregard for aliasing issues, or the concentric RGB capture?  They are unrelated to each other, except for the fact that concentric RGB requires weaker AA filtering than a CFA, relative to pixel pitch.

Concentric RGB is not a concept that is unattractive to many people.  In fact, it's the way most people assume digital cameras work until they find out otherwise.  The Sigma implementation, however, has luminance aliasing (it doesn't have to; it's not part of the concentric RGB concept) and also has poor color discrimination in certain wavelengths, and a zero-sum blotchy noise between the blue and green channels (blotches are negative in one channel while positive in the other).

If some company came up with a concentric RGB sensor without the aliasing and color noise issues, as well as higher MP counts, it would be much more readily accepted than what Sigma is doing now.
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Mark D Segal

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« Reply #153 on: November 05, 2007, 01:08:43 pm »

Quote
Which concept?  The disregard for aliasing issues, or the concentric RGB capture?  They are unrelated to each other, except for the fact that concentric RGB requires weaker AA filtering than a CFA, relative to pixel pitch.

Concentric RGB is not a concept that is unattractive to many people.  In fact, it's the way most people assume digital cameras work until they find out otherwise.  The Sigma implementation, however, has luminance aliasing (it doesn't have to; it's not part of the concentric RGB concept) and also has poor color discrimination in certain wavelengths, and a zero-sum blotchy noise between the blue and green channels (blotches are negative in one channel while positive in the other).

If some company came up with a concentric RGB sensor without the aliasing and color noise issues, as well as higher MP counts, it would be much more readily accepted than what Sigma is doing now.
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John, what I had in mind is the concentric RGB capture. Thanks for the insight into the issues with the present implementation.
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Graeme Nattress

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« Reply #154 on: November 05, 2007, 03:37:46 pm »

The co-sited nature of the Foveon sensor is indeed a most wonderful advance. However, it doesn't come without a price. The price is that silicon is not the best colour filter and the processing needed to extract colour (not luma) from the sensor is a lot more extreme than for a Bayer CFA, and that exaggerates any chroma noise.

Bayer sensors rely on us being less sensitive to chroma resolution than luma. But so does Foveon in the noise reduction used on the chroma.

With a good Bayer sensor you can do the image processing without any noise reduction of any kind, and still get a very nice looking image at base ISO. I, obviously, have access to a lot of Bayer pattern RAW data. I don't have such data for Foveon, but the Foveon white papers outline the processing needed and show how the luma and chroma get treated seperately, and the noise reduction in the basic image processing path.

Graeme
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Mark D Segal

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« Reply #155 on: November 05, 2007, 05:24:14 pm »

Quote
The co-sited nature of the Foveon sensor is indeed a most wonderful advance. However, it doesn't come without a price. The price is that silicon is not the best colour filter and the processing needed to extract colour (not luma) from the sensor is a lot more extreme than for a Bayer CFA, and that exaggerates any chroma noise.

Bayer sensors rely on us being less sensitive to chroma resolution than luma. But so does Foveon in the noise reduction used on the chroma.

With a good Bayer sensor you can do the image processing without any noise reduction of any kind, and still get a very nice looking image at base ISO. I, obviously, have access to a lot of Bayer pattern RAW data. I don't have such data for Foveon, but the Foveon white papers outline the processing needed and show how the luma and chroma get treated seperately, and the noise reduction in the basic image processing path.

Graeme
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The 1Ds, 1DsII, 1DsIII and 5D, from all I've seen and experienced, allow us to expose far above "base ISO" without a need for noise reduction. From what you and John are saying it looks like a present-day situation where we win some and lose some in different ways with Bayer versus Foveon, the net result being the kind of "real world" comparisons discussed above.

What would really intrigue me as a 1DsIII without the AA filter. You can get a taste for this issue here: [a href=\"http://MaxMax.com]http://www.maxmax.com/hot_rod_visible.htm[/url] They don't seem to offer this service for the 1 series  - probably too risky for them, but fascinating nonetheless. The Mamiya ZD is available in a version without the AA filter, but noise becomes a problem above 100 ISO - those nasty trade-offs again!
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Mark D Segal (formerly MarkDS)
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« Reply #156 on: November 08, 2007, 02:50:18 am »

The other great thing about the Foveon sensor is that it demands less resolution from the lenses because you're not wasting any of it.  So if you have, for example, a full frame 35mm Foveon 10 megapixel sensor (i.e. 30 mllion photosites), the sensors would be less dense than with a Canon 5D (thus stressing the lenses resolving power less than the 5D), but should produce an equivalent of somewhere between 20-30 megapixels of Bayer data.  This is the big problem with the Bayer sensors at this point -- they demand too much from imperfect lenses, which is why, that I surmise Canon and Nikon will all eventually go the Foveon, or similar, route, but probably not until the 1DsIV comes out in 4 years.
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Mark D Segal

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« Reply #157 on: November 08, 2007, 08:12:22 am »

Quote
The other great thing about the Foveon sensor is that it demands less resolution from the lenses because you're not wasting any of it.  So if you have, for example, a full frame 35mm Foveon 10 megapixel sensor (i.e. 30 mllion photosites), the sensors would be less dense than with a Canon 5D (thus stressing the lenses resolving power less than the 5D), but should produce an equivalent of somewhere between 20-30 megapixels of Bayer data.  This is the big problem with the Bayer sensors at this point -- they demand too much from imperfect lenses, which is why, that I surmise Canon and Nikon will all eventually go the Foveon, or similar, route, but probably not until the 1DsIV comes out in 4 years.
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I wonder whether this line of reasoning is correct. Is there not a mixing-up of resolution and interpolation here? I'm under the impression that resolution is a luminosity-based concept which depends on the number of megapixels (strictly defined, and devoid of perceptual factors); hence a 10MP sensor has less resolution than a 16 MP sensor. The interpretation of colour, on the other hand, is concentric with Foveon and Bayer matrix with other sensors, meaning that the ultimate hue of each pixel is read and calculated differently, but I question whether that has an impact on the sensor's basic resolution.
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Ray

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« Reply #158 on: November 08, 2007, 09:37:20 am »

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I wonder whether this line of reasoning is correct. Is there not a mixing-up of resolution and interpolation here? I'm under the impression that resolution is a luminosity-based concept which depends on the number of megapixels (strictly defined, and devoid of perceptual factors); hence a 10MP sensor has less resolution than a 16 MP sensor. The interpretation of colour, on the other hand, is concentric with Foveon and Bayer matrix with other sensors, meaning that the ultimate hue of each pixel is read and calculated differently, but I question whether that has an impact on the sensor's basic resolution.
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Well, Mark, Phil Askey of dpreview certainly appears to be of the opinion that the 3.4mp of the SD9 and SD10 equal in resolution and detail the 6mp of the Canon 10D. Check out his review at [a href=\"http://www.dpreview.com/reviews/SigmaSD10/page18.asp]http://www.dpreview.com/reviews/SigmaSD10/page18.asp[/url]

Whilst the Foveon sensor appears to resolve slightly beyond the Nyquist limit, such detail is represented at a lower frequency, as shown in the example of the vertical struts in the balustrade from Dave. Even if you set aside the issue of just how relevant that detail is beyond the Nyquist limit, the fact is the Bayer type sensors do not resolve up to the Nyquist limit. I think the figure is something like 2.8 pixels per line pair.

It would seem therefore that a 10mp Foveon sensor should be at least equal to an 18mp Bayer type. Since there's a law of diminishing returns at work with regard to system resolution as one increases the resolving power of just the sensor without simultaneously increasing the resolving power of the lens, then it would be reasonable to expect a 10mp full frame Foveon to have greater resolution than a full frame 18mp Bayer type.
« Last Edit: November 08, 2007, 09:39:23 am by Ray »
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Mark D Segal

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« Reply #159 on: November 08, 2007, 09:50:01 am »

Ray, thanks for recalling Phil's review. On re-reading the conclusions, it seems to be that the most distinguishing feature in terms of resolution is perhaps less the method of determining hue, but more the absence of an AA filter on the Foveon sensor. I can relate to that. Did you see what a difference the AA filter makes (on www.maxmax.com)?

Mark
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