October 25, 2014, 06:13:25 PM

Author Topic: Speculations on the Mpxls race or "Do I need a + 32 Megapixel sensor?" - PART II  (Read 5128 times)

tjshot

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In a previous post ( http://www.canonrumors.com/forum/index.php/topic,1902.0.html ) I proposed
an in-depth analysis of the expected performance for a 36  Mpxls full frame sensor compared to actual 21 Mpxls and also a hypothetical 50 Mpxls one.

With the attached PDF file I'm extending a similar simulation-based analysis to print performance for the same full frame sensors.

Part I  includes the original lens+camera analysis; in Part II the new section about print performance is developed.

Differences between modern DSLRs tend to be less relevant in the final print than in the original image to be printed, as viewed on a screen at high magnification; this is  due to a degradation of performance at high spatial frequencies, depending on many factors, some of which mechanical (for ex. print nozzles and head carriage precision), some chemical (for ex. ink and paper quality), some purely information-related (for ex. dpi density  and contrast in the input image).

In other words printers will “flatten” the performance obtainable from the camera, both for low enlargements where performance is capped below the same level for different cameras, and for very big enlargements where the original image may not contain enough information for  an optimal feed to the physical print system.
The scope of this document is defining where those limits lay for different full frame cameras and how much performance difference is to be expected on the same printer.

The proposed method, as it involves some degree of judgement in assessing test data (mostly print MTF values), is not to be considered as an accurate description of absolute performance: other printers/paper combos could deliver different results; however I believe it offers a good estimate of delta in performance from different sensors on the same printer, which may be of interest for a user evaluating an upgrade of the  camera but not of his printing gear.

Summary of Results

An input density of 350  dpi is commonly considered enough for a sharp  printer output, but in practice it involves accepting a maximum resolution of about 120 lp/in in final print for an optimal input image; that's 50 lp/in short of the best  possible result from an inkjet printer, but still optimal for images taken with consumer-available camera equipment and enlarged to usual (8x10 inches plus) print sizes.
For inkjet prints of very sharp images that must stand up to close  inspection I suggest  a higher setting, 450 dpi or better for a Bayer sensor generated image.
(As a side note, tested but not included in this document,  350 dpi is enough for Foveon sensor generated images, due to higher pixel-level sharpness).
Performance differences for cameras with full frame sensors are relevant from 4x enlargement and up, where 4000 dpi sensors (EOS 5D MKII) start to lose sharpness and resolution compared to maximum printable values; from 6x  32-36 Mpxls sensors will start to give ground and from about 9x also a 50 Mpxls sensor will.
A very close (i.e. 10 inches) inspection distance is required to perceive those differences.
Up to 16x enlargements delta in performance (both MTF 50% and MTF 10%) is  more or less constant and equal to a +2 enlargement factor for each 1000 dpi increase in sensor density: a 21 Mpxls sensor at 8x enlargement will perform like a 32-36 Mpxls sensor at 10x and a 50 Mpxls sensor at 12x.
Within this range however, a 50 Mpxls sensor will hold very fine details better (print MTF 10%), showing a very gradual falloff, while lower density ones will show a somehow steeper curve; this could make for a visible quality difference from close inspection distance.
From 16x on, curves become more irregular (less than optimal information for the physical printing system) and differences start to be “flattened”; the denser sensor however will keep a significant edge over the others up to 20x or more (personal estimate, not tested: I don’t print that large).

As a quick reference, following chart illustrates test results for the simulated full frame Bayer sensors + excellent lens, printed on Epson R1900 inkjet printer on Epson Premium Glossy paper.


image host

Lines of same color are related to same sensor; dash lines are indicative of top resolution, solid lines of perceived contrast from 10 inches view distance.
Te graph can be used to asses performance for each enlargement factor / print size: both print resolution (MTF10%) and perceived sharpness (MTF50%, indicative for a close 10 inches view distance) are included for the three sensors; the bar on the right indicates required distance to match perfect 20/20 vision criterion.
In a similar way, for each required print performance level (lp/in), the maximum possible enlargement from each sensor is indicated.
« Last Edit: December 13, 2011, 06:54:11 AM by tjshot »

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Mt Spokane Photography

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The problem is that you have to put a lens in front of the sensor.  The lens limits the resolution of the light striking the sensor and the printer limits a already flawed image.  Then, there are AA filters and Bayer filters, and even cutoff filters in the camera processor to attempt to limit the frequencies beyond Nyquist.

Each additional element limits the MTF of the entire system, and not in a small way.  Even the paper used to print on has a effect.

tjshot

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The problem is that you have to put a lens in front of the sensor.  The lens limits the resolution of the light striking the sensor and the printer limits a already flawed image.  Then, there are AA filters and Bayer filters, and even cutoff filters in the camera processor to attempt to limit the frequencies beyond Nyquist.

Each additional element limits the MTF of the entire system, and not in a small way.  Even the paper used to print on has a effect.

The proposed simulation takes into account the contrast reduction from a real lense (order 2 MTF rolloff for wide F-stop range, close to 3 for smaller F-stops), antialias filters and Bayer matrix demosaicing (cumulative order 3 MTF rolloff); frequencies are Nyquist limited as a constraint in the simulation equations.
Modelization of printer rolloff however is hardly feasible, thus proposed values are from real prints of simulator-generated test targets.
I believe the model can provide a quite reliable prediction of real performance for a flawless imaging workflow.

tjshot

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Added the test results chart as a quick reference on print performance from simulated sensor (Bayer matrix + AA filter) + lens, printed on Epson R1900 inkjet printer, Premium Glossy photo paper.

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