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Re: [OM] Resolution? Which resolution? [was Value for money?]

Subject: Re: [OM] Resolution? Which resolution? [was Value for money?]
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 14 Jul 2009 11:46:34 -0400
You make very good arguments which it will probably take me about a 
month to study and absorb (probably starting with Koren).  I'll just say 
that I hope you're right.

Chuck Norcutt

Moose wrote:
> Chuck Norcutt wrote:
>> What got me on this point to begin with was DPreview's test of the Canon 
>> 70-200/2.8L  ...  So, if this lens can just barely handle a 5D it certainly 
>> can't handle the pixel density of a 50D (15 MP on an APS-C size sensor).  At 
>> f/5.6 to f/8 the 5D Mk II will pull a bit more out of the lens than the 5D 
>> but not a great deal.  To utilize the resolving power of the 5D Mk II you 
>> need very good primes. ...
> 
> I'm not sure I agree with your conclusions. I suspect that the following 
> considerations aren't meaningful for the vast majority of actual image 
> making and display. However, for those few working at large 
> magnifications and/or the larger number simply afflicted with terminal 
> technophilia or untreated addiction to pixel peeping, they may be, or at 
> least seem, significant.
> 
> First, I have issues with coming to such broad conclusions considering 
> the methodology used.
> 
> 1. Judgments/measurements are made with a camera with AA filter in 
> place. That means one could adapt some industrial super lens capable of 
> resolving much more than the sensor, but the system resolution they 
> measure, while better than regular lenses, would be nowhere near the 
> lens's true ability. As long as there is an intentional bandwidth 
> limiter in the system, no fully meaningful conclusions may be drawn 
> about individual parts.
> 
> 2. The lppmm standard, while a good method to compare cameras with 
> different sensor sizes, is not ideal for lens comparisons or judgments 
> about lens vs. sensor resolution.
> 
> 3. A nit pick. How can the system out resolve the "theoretical 
> resolution of the sensor", Nyquist frequency? If theory doesn't agree 
> with practice, why even mention it? My real complaint though is that it 
> gets the reader thinking about absolute limits, in a test environment 
> where they don't exist.
> 
> There are also a few other considerations that make coming to such 
> conclusions from these test results questionable to me:
> 
> 1. Back in the area of #1 above, system bandwidth is a complex function 
> of the bandwidths of the components. It's not like hooking a garden hose 
> onto the end of a fire hose, where throughput is strictly a function of 
> pressure and the physical characteristics of the garden hose. Even the 
> formula someone posted a little bit ago is probably too simple in the 
> real world. In addition to which, we don't have the numbers for 
> individual components to plug into it.
> 
> Increase the resolving power of any one component and system resolution 
> goes up. If the improved component was the poorest, it goes up a lot, 
> but not as much as the % increase in the component. If it was already 
> the broadest component, bandwidth increases only a little. But it does 
> change.
> 
> 2. These luminance only, test pattern tests don't say a thing about 
> spatial color resolution. If Foveon sensors had fulfilled their apparent 
> potential, this wouldn't be an issue. However, all but a tiny proportion 
> of digital cameras use a Bayer array. So depending on color and 
> directional orientation, the color resolution may be quite a bit less 
> than B&W. Demosaicing algorithms are quite good, but the fact is that 
> only one of the color channels for any pixel is a actual measurement. 
> The rest are interpolated guesses.
> 
> Since there are a few Foveon sensor cameras about, it's possible to make 
> actual comparisons. <http://www.ddisoftware.com/sd14-5d/> The SD14 vs. 
> 5D results convince me that larger Bayer array sensors will have better 
> color IQ than smaller ones, regardless of luminance test target tests. 
> As the lenses we are working with project the magnificence of their 
> images on all sensor sites equally, an improvement in sensor color 
> resolution will show up in visible image quality.
> 
> 3. There is an unspoken assumption in most web discussions of color 
> accuracy in digicams, but it is wrong. When we talk about sensors, we 
> act like the array filters are at least close to perfect. It just ain't 
> so. Look at the spectral responses of even the best quality photographic 
> filters. They aren't sharp cut-off tools, not even close. How good do 
> you think the gazillions of tiny little, mass deposited Bayer array 
> filter in our cameras are?
> So the fact is that wavelengths of light falling on sensors are only 
> majority reported by the appropriate sensor sites. There are also 
> minority values misreported by adjacent sensors for other colors, with 
> the amount of error depending on how far the actual color is from the 
> center of the intended sensor for it.
> 
> I don't know how big these errors are, but suspect that they are 
> significant in an absolute sense. We tend to be unaware of them because 
> we have been viewing the same sort of errors on color film and  because 
> nobody is publishing about them. In any case, more, smaller sensors will 
> tend to reduce these errors.
> 
> 4. No matter what the resolution chart tests say, all digital capture 
> involves sampling and that sampling always reduces 
> sharpness/contrast/clarity/whatever along any edge that is not exactly 
> aligned with the sensor array. If you work out the angular requirements 
> for that on a large sensor, you will see that it applies to essentially 
> all edges. As I've written about a while ago on Zone-10, that means all 
> images captured digitally have poor edge contrast relative to the 
> nominal resolution. {Starting with pp6 here. 
> <http://zone-10.com/cmsm/index.php?option=com_content&task=view&id=149&Itemid=1&limit=1&limitstart=1>}
> 
> Sharpening generally can do a wonderful job of improving apparent edge 
> sharpness and visible resolution. However - for exactly the same reasons 
> the digital image has edge clarity problems in the first place, 
> sharpening has to work on adjacent pixels as well. So while an 
> improvement, sharpening is not a true replacement for simply increasing 
> the sampling frequency, i.e. more pixels.
> 
> When you add the issues in #2 & 3, sharpening will usually be working 
> with significant numbers of pixels with already compromised color data - 
> and changing it still further.
> 
> In conclusion, I suspect that, whatever the test targets seem to 
> indicate about resolution of simple B&W bars, more sensor sites will 
> continue to improve overall IQ in other images areas, including color 
> resolution.
> 
> I'm not suggesting that such tests are not quite useful. I think the 
> simple tests are pretty good analogs of overall lens performance and a 
> good practical way to compare them. That they don't say it all is 
> testified to by the decades of testimony of practical photographers 
> about meaningful difference between lenses that aren't reflected in 
> simple resolution tests.
> 
> I am suggesting that a simple conclusion about whether a give sensor 
> "out resolves" a given lens, and therefore whether there will be any 
> benefit from a higher resolution sensor, based on these tests, is not 
> supported by the methodology.
> 
> Moose
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