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Author Topic: 1Ds MKIII and Optical Low Pass filtering  (Read 150335 times)

Graeme Nattress

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« Reply #160 on: November 08, 2007, 11:00:37 am »

A Bayer CFA with AA will measure somewhere between 70% to 80% of the rated linear resolution, depending on algorithm and AA. A combination I know well comes out at 75%, so I'll use that figure. Therefore our 10mp Bayer CFA would give about the same measured resolution as a 10 * 0.75 *0.75 = 5.6mp Foveon without AA. That 5.6mp Foveon needs 5.6 * 3 photosites to get it's resolution, which would  be: 16.875mp. Our Bayer CFA needs 10mp photosites to get it's resolution. The difference is that shrinking the Bayer CFA down to 75% reduces noise a bit, and sharpens up the image a bit (assuming a good downsampling filter is used). The downsampling doesn't add aliasing (noticibly) as we're well anti-aliased from the OLPF. Now, theoretically, the Foveon should have higher chroma resolution. However, the noise reduction used in the Foveon processing might not let the sensor keep that advantage to it's full. All this leads to, is a justification of the x2 factor that many people use to compare Foveon MP with Bayer OLPF MP. It's a rule of thumb, but a reasonable guide.

Graeme
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Mark D Segal

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1Ds MKIII and Optical Low Pass filtering
« Reply #161 on: November 08, 2007, 11:43:28 am »

Graeme,

A few things I don't understand:

(1) Why are you multiplying 10MP by 0.75^2 rather than simply 0.75?

(2) Also, why does a Foveon need 3*5.6mm photosites to get its resolution? Is there a separate photosite for each primary for each pixel, or is it three filters on top of the same photosite?

(3) When you knock 25% off the Bayer, you aren't really shrinking it are you, you are just discounting its resolution because of the "rounding the edges" impact of the AA? If so, where does this sharpening effect come from? I thought the AA blurs, and that is why you discount 25% of its native resolution.

Hence I don't really understand your conclusion.

Mark
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Mark D Segal (formerly MarkDS)
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Ray

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« Reply #162 on: November 08, 2007, 12:53:39 pm »

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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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Mark,
I did have a look at those hot-rod images with AA filter removed and I didn't think there was a significant increase in resolution. The differences were rather subtle. I don't believe the lack of an AA filter alone can explain the extra resolution of the Foveon sensor. Rather I think it's a combination of the absence of an AA filter plus the fact the color values are not interpolated as in the Bayer system.
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Mark D Segal

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« Reply #163 on: November 08, 2007, 01:03:09 pm »

Ray, maybe I'm a bit thick today, but frankly I just don't see how interpolating colour information between neighbouring pixels to calculate the hue of any one pixel should affect resolution. Each photosite corresponds to a pixel and captures a certain amount of greyscale information depending on the light that strikes it from the lens - I would have thought that defines the resolution, not the calculation of the hue.
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Mark D Segal (formerly MarkDS)
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Dave Millier

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« Reply #164 on: November 08, 2007, 01:25:24 pm »

At the risk of sounding like a broken record, trust the evidence, not the theory (there are endless numbers of factors overlooked in simple theories).

I have a draft article comparing Sd14 (4.7MP X3) and Kodak 14nx (14MP CFA).

In prints up to 24 x 16 inches there is no practical difference in resolution. I would love to show you the article but it is with my collaborator for comment at the moment but here's one example:
 

http://www.whisperingcat.co.uk/scans/big1.jpg

These are flatbed scans from prints. I don't know which is which...


Quote
Ray, maybe I'm a bit thick today, but frankly I just don't see how interpolating colour information between neighbouring pixels to calculate the hue of any one pixel should affect resolution. Each photosite corresponds to a pixel and captures a certain amount of greyscale information depending on the light that strikes it from the lens - I would have thought that defines the resolution, not the calculation of the hue.
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Mark D Segal

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« Reply #165 on: November 08, 2007, 01:38:21 pm »

Dave,

You are not sounding like a broken record, and what you are saying reflects very much what's going through my mind. It's not self-evident how stacking pixels on top of eachother improves RESOLUTION compared with arraying them side by side and interpolatinfg colour information. I've seen the technical papers on Foveon's website, can't say I really understand it all, but yes, clearly there are numerous variables at play and in the final analysis what counts are (i) the measurable resolution in the file, and (ii) the quality of what emerges from the best printers and papers we have for reproducing the resolution of high frequency images.
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Mark D Segal (formerly MarkDS)
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Ray

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« Reply #166 on: November 08, 2007, 02:02:47 pm »

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In prints up to 24 x 16 inches there is no practical difference in resolution. I would love to show you the article but it is with my collaborator for comment at the moment but here's one example:
 

http://www.whisperingcat.co.uk/scans/big1.jpg

These are flatbed scans from prints. I don't know which is which...
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Dave,
In resolution comparisons, shouldn't you be comparing subject matter with fine detail such as cat's whiskers?  
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Dave Millier

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« Reply #167 on: November 08, 2007, 04:04:06 pm »

Ray

The sample comparison is of around  1 x 1 inch section from a 24 x 16 inch print scanned at 240ppi.

On a typical monitor it is equivalent to examining the print with a low power loupe.  

We shot 12 pairs of prints in total, most being landscapes, forests, trees, foliage. This was the lone sample of architecture. There were a couple of macros, too.

All indistinguishable. Three of them (including this one) particularly so for me as the photographer didn't label them!

Of course there are lots of variables in testing so I don't claim any kind of scientific veracity but for the purpose of making your mind up whether the Sd14 is for you, the tests are good enough.

The main lesson for me from all the tests is not that the Foveon sensor competes well with high end cameras but that the end results don't look particularly different. All this stuff about aliasing problems is clearly irrelevant unless you are aiming for ridiculous enlargements (several feet wide).

I would like to add, though, that there is a clear difference in quality from both cameras in 19 x 13 and 24 x 16 prints. IMO, 24 x 16 is a size too far for 12MP class cameras if optimum quality is required.  




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Dave,
In resolution comparisons, shouldn't you be comparing subject matter with fine detail such as cat's whiskers? 
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Mark D Segal

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« Reply #168 on: November 08, 2007, 04:24:38 pm »

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I would like to add, though, that there is a clear difference in quality from both cameras in 19 x 13 and 24 x 16 prints. IMO, 24 x 16 is a size too far for 12MP class cameras if optimum quality is required.

[a href=\"index.php?act=findpost&pid=151375\"][{POST_SNAPBACK}][/a]

Of course, because once the long dimension gets to be 24 inches wide, the PPI count sinks below 180. Bound to be sub-optimal I hazard to suggest, no matter how many pixels you pile on top of eachother - the linear resolution just gets too low for optimal quality.
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Mark D Segal (formerly MarkDS)
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wolfy

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« Reply #169 on: November 19, 2007, 10:44:02 am »

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..., I can walk and chew gum at the same time because I don't use my legs for chewing. 
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Aren't your legs chewing away at the remaining distance to your destination?  

Great thread from the tech-gods, ... thanks to all from someone down at the mortal level who is trying to learn!
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Graeme Nattress

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« Reply #170 on: November 19, 2007, 10:53:41 am »

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Graeme,

A few things I don't understand:

(1) Why are you multiplying 10MP by 0.75^2 rather than simply 0.75?

(2) Also, why does a Foveon need 3*5.6mm photosites to get its resolution? Is there a separate photosite for each primary for each pixel, or is it three filters on top of the same photosite?

(3) When you knock 25% off the Bayer, you aren't really shrinking it are you, you are just discounting its resolution because of the "rounding the edges" impact of the AA? If so, where does this sharpening effect come from? I thought the AA blurs, and that is why you discount 25% of its native resolution.

Hence I don't really understand your conclusion.

Mark
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1) 0.75 is the linear factor. Because we're dealing with mp, which is a  measure of area, we need to multiply by 0.75^2.

2) Each "pixel" on the Foveon has three sensors, one each for the three depths it senses, which is later converted into RGB colour.

3) You  can shrink the image, if a good downsampling filter is used. I don't seem to think Photoshop or image editors normally use good downsampling filters though. In the video world I'd use an app like Shake which has user selectable downsampling filters.

Graeme
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ashdavid

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« Reply #171 on: November 20, 2007, 12:52:25 am »

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Optical low pass filters (OLPF's) or Anti Alias filter don't come in different "strengths". The filter works in two passes, each layer splits the light in two either horizontally or vertically, so by combing them together, you get vertical and horizontal filtering. The distance of seperation of the two rays of light is governed by the thickness of that layer, so if you want, (or need to as you don't have square  photosites) you can adjust the filter accordingly. You choose the thickness of the filter in relation to the spacing of the photosites on the sensor.
How do you do this? (the writting in bold)
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ErikKaffehr

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« Reply #172 on: November 20, 2007, 01:05:10 am »

Hi!

I guess that "you" is "you camera maker" and not "you photographer". :-)

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How do you do this? (the writting in bold)
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Erik Kaffehr
 

Ray

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« Reply #173 on: November 20, 2007, 01:32:43 am »

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Optical low pass filters (OLPF's) or Anti Alias filter don't come in different "strengths". The filter works in two passes, each layer splits the light in two either horizontally or vertically, so by combing them together, you get vertical and horizontal filtering. The distance of seperation of the two rays of light is governed by the thickness of that layer, so if you want, (or need to as you don't have square photosites) you can adjust the filter accordingly. You choose the thickness of the filter in relation to the spacing of the photosites on the sensor.

In dpreview's LPPH results following the test charts page, there's usually a discrepancy between vertical lines per picture height and horizontal lines per picture height. One is often quite significantly less or more than the other.

It is explained that both horizontal and vertical height are the same dimension, ie. a square, in order to avoid confusion due to different aspect ratios amongst cameras.

If the width is the same as the height, one wonders why the resolution discrepancy.

Could the above quote from Graeme be the explanation?
« Last Edit: November 20, 2007, 01:34:01 am by Ray »
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ashdavid

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« Reply #174 on: November 20, 2007, 02:17:05 am »

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Hi!

I guess that "you" is "you camera maker" and not "you photographer". :-)
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As I was thinking, I thought I missed out on something when ordering my 1Ds MKIII!    Being able to choose the thickness on the AA filter would be kind of good though.
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ashdavid

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« Reply #175 on: November 20, 2007, 09:01:55 am »

Does anyone know how strong the AA filter is on the 1Ds MKIII compared to, 1DsMKII and 5D?
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Mark D Segal

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« Reply #176 on: November 20, 2007, 09:21:49 am »

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Being able to choose the thickness on the AA filter would be kind of good though.
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I hope you were joking about this too? I mean, if that choice were offered would you know how, or put otherwise, what would it take for anyone to know how to do so intelligently?
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Mark D Segal (formerly MarkDS)
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Graeme Nattress

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« Reply #177 on: November 20, 2007, 09:53:19 am »

My "you" was referring to the camera maker. And in that respect, it's the specification you send off to the filter manufacturer. Once it's made, the OLPF is fixed.

Some sensors have non-square pixels. In that case indeed you'd need different OLPF spacing in the different dimensions.

Resolution measurements are usually in lines per picture height, to account for the varying aspect ratios of sensors.

Graeme
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Ray

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« Reply #178 on: November 20, 2007, 10:38:34 am »

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Resolution measurements are usually in lines per picture height, to account for the varying aspect ratios of sensors.
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Graeme,
I understand there are two figures. One for vertical lines and the other for horizontal lines. The horizontal distance is the same as the vertical distance, to account for varying aspect ratios. In other words, a square is taken from the sensor with the dimensions of the picture height.

Why the sometimes significant difference in number of resolved lines for height and width?
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Graeme Nattress

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« Reply #179 on: November 20, 2007, 10:53:39 am »

Experimental error? Lens effects? OLPF tolerances? Who knows?

Graeme
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