How small can or should a pixel be

[ErikKaffehr]

Hi,

This article I have written a couple of years ago may be of interest and it has links to some good articles.

http://echophoto.dnsalias.net/ekr/index.php/photoarticles/24-how-many-megapixels-do-we-need

Best regards
Erik

Erik,

I agree with you.

I am not against pixel resolution. 12MP image a great and more MP can give the image more--I actually like lower rez sensors for the more 35mm filmish look or structure or rendering (it does not look like film, buy rather a limit in frequency). Our perception of images is far more complex than just reducing the problem to resolving power and pixel resolution. But likewise just going for more and more pixels has diminishing returns. When making calculations, how many significant figures do you use? Would you build a house with a ruler accurate to a micron? In images, how many pixels?

Personally, I don't know where the limit is to pixel size or to pixel resolution, at least in a simple number. I guess what I react to it a simple view the smaller the better, the more the merrier. Given a choice, I would take fewer, fatter pixels and larger sensors. But at work we do scientific imaging with 1.25MP to 4MP microscope cameras with 7-9um pixels. I have even made 24" prints from those images. So maybe I have a different perspective from most. The manufacturers will keep pushing pixels like drugs; that is what the people (and addicts) want. I am also noticing a loss of acutance in optics because they need more resolving power, so there are systemic changes that do affect the results.

My too sense.

[theguywitha645d]

Sorry Erik, but the concepts of standard viewing distance works really well and maximum print size has no baring on pixel resolution. When I publicly displayed a 24" print from a 4MP camera, everyone who saw it were amazed by it--and they did not know what camera it was from, they were just reacting to the quality of the image. Your model is too simple.

[bjanes]

This article is by a well known expert on printing: http://theonlinephotographer.typepad.com/the_online_photographer/2009/02/why-80-megapixels-just-wont-be-enough.html

Roger Clark has reached a conclusion similar to that of Ctein. Roger's experiments indicate that the highest print quality requires 600 pixels per inch on the print and this assumption is similar to the conclusion reached by Jeff Schewe in the latest Camera to Print and Screen tutorial. When printing with Epson printers with a native resolution of 720 ppi, Jeff suggests that if the native resolution of the image is greater than 360 ppi, one should print at 720 ppi to preserve maximal image quality. However, one must recognize that with ink jets using error diffusion technology there is a rather nebulous relation between the nominal dpi and the actual resolution.

To attain this resolution even with the Phase One IQ180, one would have to resort to stitching, and this is exactly what Jeff does in some cases. For prints viewed at average distances one can make do with less resolution, but as Bruce Fraser used to say, for some photographers the viewing distance is determined by the distance from the nose to the eye  .

Regards,

Bill

[madmanchan]

There are other image quality tradeoffs involved with small pixels besides noise and resolution.  For example, with smaller pixels it is often much tougher to deal with (undesirable) crosstalk between neighboring pixels and uniformity.

[ejmartin]

There are other image quality tradeoffs involved with small pixels besides noise and resolution.  For example, with smaller pixels it is often much tougher to deal with (undesirable) crosstalk between neighboring pixels and uniformity.

At what pixel pitch does this start to be a substantial problem?  Do you see serious issues with raw data from compacts having 2µ pixels?  Nikon 1 series (3.4µ pixels)?  Canon 7D (4.2µ pixels)?  Nikon D7000 (4.7µ pixels)?

[ErikKaffehr]

Hi,

Blueray has just two MP resolution and it's said that normal cinema film has lower resolution than Blueray.

I have made moderately large (A3-A2)  prints from images with low resolution in the 4-6 MP range that were OK. That is a good or great subject with a good sharpening can compensate for lack of detail.

I don't understand your reference to "my model beeing to simple", which model do you mean?

This image is about 4 MP I guess and is quite enjoyable in A3+:
 

Best regards
Erik

Sorry Erik, but the concepts of standard viewing distance works really well and maximum print size has no baring on pixel resolution. When I publicly displayed a 24" print from a 4MP camera, everyone who saw it were amazed by it--and they did not know what camera it was from, they were just reacting to the quality of the image. Your model is too simple.

[PierreVandevenne]

BT

There are other image quality tradeoffs involved with small pixels besides noise and resolution.  For example, with smaller pixels it is often much tougher to deal with (undesirable) crosstalk between neighboring pixels and uniformity.

BTW, this page has a couple of nice movies and images about micro-lenses and cross-talk

http://www.lumerical.com/fdtd/applications/cmos_image_sensor_pixel_microlens.html

[Sareesh Sudhakaran]

Assuming 1000 mm viewing distance that would correspond to 0.42 minutes of arc, but you need at leas three point two resolve two. The two points and a darker point in between. That would be 1.2 arc minutes, but I'm not sure about that. Let's say 0.8 minutes of arc. Are you looking at 25 cm which is feasible it would be 3.2 minutes of arc.

I might be wrong Erik, but here is my line of thought:

The Snellen fraction does not talk about line pairs. At 20/20, a 'normal' eye subtends 5 arc minutes of an optotype. The most commonly used optotype (probably because it has 5 lines) is the letter 'E'. Each line therefore subtends 1 arc minute. For a 'perfect human', the angle would be 0.4 arc minutes, regardless of distance. If the object gets closer, the line width gets smaller, but the angle never changes. Even under magnification (microscopy or telescopy), the perfect human eye can only read at 0.4 arc minutes. But it is 0.4 arc minutes of one line only. Practically not very useful (which is why they use E), but makes calculations easier. It would be unfair to use line pairs on sensors.

How I arrived at 5 microns:
The sensor does not have the field of vision like an eye. The lens compresses the image it to a single plane - the back of the lens. It is as if the sensor is almost watching a movie in a dark hall. Sure, there's light scattering, and other 'leaks', but for our purposes, we can assume our camera lens to be perfect, and treat the back of the lens like an image plane. Therefore, it is not important to consider what is in focus and what isn't, etc. And I assumed the distance to be the focal flange distance (back of the lens to sensor). EF (44mm) and PL (55mm). I'm from the motion picture background. Some cameras like the RB67 (which I own) has a flange focal distance of 112mm. Obviously this figure varies as the lens zooms, focuses, etc. For simplicity sake, I have assumed it to be constant (I did say it was amateurish).

The smallest readable line that leaves the lens must subtend an angle of 0.4 arc minutes to a point behind the pixel - because the pixel is not a point object but a line itself. However, a pixel never has to resolve anything greater than 0.4 arc minute, so we can safely assume that a pixel must be smaller than the width of the line that is subtended on the back of the lens. Treating the sensor like the eye, and going by the fact that a pixel can always have one and only one value, it can be treated like a point object if only for theoretical calculation. In this case, if the angle is 0.4 arc minute and the distance is 44mm (for EF), say, then the width of the line is 5.112 microns.

For the EF mount:
@0.4 arc min, minimum line/pixel/dot width is 5 microns
@1 arc min, minimum line/pixel/dot width is 12.76 microns

For the RB67:
@0.4 arc min, minimum line/pixel/dot width is 13 microns
@1 arc min, minimum line/pixel/dot width is 32.5 microns

For the Pentax Q mount (the smallest I could find) (9.2mm flange)
@0.4 arc min, minimum line/pixel/dot width is 1 micron
@1 arc min, minimum line/pixel/dot width is 2.7 microns

Assuming a viewing distance of:

@ 12 inches (305mm): the line width @ 0.4 arc min must be 35.5 microns or 0.035mm. This corresponds to 726 ppi at 0.4 arc min and about 290 ppi (very close to 300 dpi isn't it?) for 1 arc min.

@ 2.5 feet (762mm) (normal computer monitor viewing distance): @ 0.4 arc min, the ppi is 287 ppi and @ 1 arc min the ppi is 115 ppi. Right now, the average screen is only 100 ppi. Professional monitors aim for higher ppi to justify this resolution.

@ 1000mm the width of one line is 0.12mm or 116.3 microns. This corresponds to 212 ppi @ 0.4 arc min, and 88 ppi @ 1 arc min.

@ 6 feet or 1830mm - the 'normal' viewing distance for home television. The ppi required is 120 @ 0.4 arc min. @ 1 arc min the ppi required is only 48 ppi. My LCD screen (42") has a horizontal ppi of 64. This is why 2 MP is okay for home viewing, even on a large monitor. Smaller TV sets need lesser ppi, and larger sets will be viewed farther away. The 72 dpi standard for TV is okay. The same standard is used for film projection (where half the resolution is lost in projection).

Side note: Line pairs per mm for EF mount:

Since the minimum required line width is 5 microns for an EF mount lens, a line pair would be 10 microns. Therefore, the lp/mm required would be 100 lp/mm corresponding to 0.4 arc min. @ 1 arc min, it is 40 lp/mm. The highest possible Canon EF lens (according to DXO) is the 85mm L which has a lp/mm of 67. Of course, it all depends on the testing and cameras used, etc.

Printing:

I'm not an expert on printing, but the maximum 720 ppi limit seems justified, if all things are perfect @ 1 feet. However, the Epson 9900 is capable of 2880 ppi. Paper resolutions are easily greater than 60 lp/mm or around 720 ppi+ to 1440 dpi for Epson Fine Art papers. Of course, it is important to avoid interpolation when making these calculations. A drum scanner is capable of 3000 dpi to 24,000 dpi, and a flatbed scanner can top 5000 dpi.

As I've mentioned before, my calculations make many assumptions and are amateurish. I did these calculations just to understand how the standards that are being used have come about. I probably have made mistakes along the line (hopefully not grave ones). But I'm also secretly glad they correspond to the standards currently used in photography and cinema today.

For the full text with links to sources, please visit: http://sareesh.com/2011/12/resolution-visual-acuity-and-lpmm/

[madmanchan]

Hi Emil, it depends on the sensor design (e.g., filter stack height on top of the recording layer), but generally I've seen more and more crosstalk and uniformity issues in the progression towards 5 and 4 micron pixels over the past couple of years.  Some of this can be addressed in software, but it is a tradeoff because of potential impact on resolution ... which of course is one of the motivations in going to smaller pixels.

[ErikKaffehr]

Hi,

Thanks for sharing your calculations.

In my view you are ignoring a few factors. To begin with I don't think that sensor to flange distance is the key parameter. The optical center of the lens may be in front or behind the lens mount. I'd suggest that you could simply use the focal length in your calculations.

The other issue I have is, that according to my understanding, that you try to compare the angular resolution of the eye with the angular resolution of the sensor. But it has never been the case that we would opt for the same angular resolution in the imaging system. Let's just look at some glossy magazine. An article may be illustrated with some image, next page perhaps would contain a full page image and a third one have double spread image. All these images would be seen at the same distance having the same angular resolution, but very differing field of view.

The images would be observed using the same angular resolution, but the magnification would vary, like a factor of ten.

Another observation I may have made that the pixel size of the eye seems to be around 4 microns (cones are said to have a radius of 2 microns), that is pretty close to your estimate of 5 microns.

Best regards
Erik

I might be wrong Erik, but here is my line of thought:

The Snellen fraction does not talk about line pairs. At 20/20, a 'normal' eye subtends 5 arc minutes of an optotype. The most commonly used optotype (probably because it has 5 lines) is the letter 'E'. Each line therefore subtends 1 arc minute. For a 'perfect human', the angle would be 0.4 arc minutes, regardless of distance. If the object gets closer, the line width gets smaller, but the angle never changes. Even under magnification (microscopy or telescopy), the perfect human eye can only read at 0.4 arc minutes. But it is 0.4 arc minutes of one line only. Practically not very useful (which is why they use E), but makes calculations easier. It would be unfair to use line pairs on sensors.

How I arrived at 5 microns:
The sensor does not have the field of vision like an eye. The lens compresses the image it to a single plane - the back of the lens. It is as if the sensor is almost watching a movie in a dark hall. Sure, there's light scattering, and other 'leaks', but for our purposes, we can assume our camera lens to be perfect, and treat the back of the lens like an image plane. Therefore, it is not important to consider what is in focus and what isn't, etc. And I assumed the distance to be the focal flange distance (back of the lens to sensor). EF (44mm) and PL (55mm). I'm from the motion picture background. Some cameras like the RB67 (which I own) has a flange focal distance of 112mm. Obviously this figure varies as the lens zooms, focuses, etc. For simplicity sake, I have assumed it to be constant (I did say it was amateurish).

The smallest readable line that leaves the lens must subtend an angle of 0.4 arc minutes to a point behind the pixel - because the pixel is not a point object but a line itself. However, a pixel never has to resolve anything greater than 0.4 arc minute, so we can safely assume that a pixel must be smaller than the width of the line that is subtended on the back of the lens. Treating the sensor like the eye, and going by the fact that a pixel can always have one and only one value, it can be treated like a point object if only for theoretical calculation. In this case, if the angle is 0.4 arc minute and the distance is 44mm (for EF), say, then the width of the line is 5.112 microns.

For the EF mount:
@0.4 arc min, minimum line/pixel/dot width is 5 microns
@1 arc min, minimum line/pixel/dot width is 12.76 microns

For the RB67:
@0.4 arc min, minimum line/pixel/dot width is 13 microns
@1 arc min, minimum line/pixel/dot width is 32.5 microns

For the Pentax Q mount (the smallest I could find) (9.2mm flange)
@0.4 arc min, minimum line/pixel/dot width is 1 micron
@1 arc min, minimum line/pixel/dot width is 2.7 microns

Assuming a viewing distance of:

@ 12 inches (305mm): the line width @ 0.4 arc min must be 35.5 microns or 0.035mm. This corresponds to 726 ppi at 0.4 arc min and about 290 ppi (very close to 300 dpi isn't it?) for 1 arc min.

@ 2.5 feet (762mm) (normal computer monitor viewing distance): @ 0.4 arc min, the ppi is 287 ppi and @ 1 arc min the ppi is 115 ppi. Right now, the average screen is only 100 ppi. Professional monitors aim for higher ppi to justify this resolution.

@ 1000mm the width of one line is 0.12mm or 116.3 microns. This corresponds to 212 ppi @ 0.4 arc min, and 88 ppi @ 1 arc min.

@ 6 feet or 1830mm - the 'normal' viewing distance for home television. The ppi required is 120 @ 0.4 arc min. @ 1 arc min the ppi required is only 48 ppi. My LCD screen (42") has a horizontal ppi of 64. This is why 2 MP is okay for home viewing, even on a large monitor. Smaller TV sets need lesser ppi, and larger sets will be viewed farther away. The 72 dpi standard for TV is okay. The same standard is used for film projection (where half the resolution is lost in projection).

Side note: Line pairs per mm for EF mount:

Since the minimum required line width is 5 microns for an EF mount lens, a line pair would be 10 microns. Therefore, the lp/mm required would be 100 lp/mm corresponding to 0.4 arc min. @ 1 arc min, it is 40 lp/mm. The highest possible Canon EF lens (according to DXO) is the 85mm L which has a lp/mm of 67. Of course, it all depends on the testing and cameras used, etc.

Printing:

I'm not an expert on printing, but the maximum 720 ppi limit seems justified, if all things are perfect @ 1 feet. However, the Epson 9900 is capable of 2880 ppi. Paper resolutions are easily greater than 60 lp/mm or around 720 ppi+ to 1440 dpi for Epson Fine Art papers. Of course, it is important to avoid interpolation when making these calculations. A drum scanner is capable of 3000 dpi to 24,000 dpi, and a flatbed scanner can top 5000 dpi.

As I've mentioned before, my calculations make many assumptions and are amateurish. I did these calculations just to understand how the standards that are being used have come about. I probably have made mistakes along the line (hopefully not grave ones). But I'm also secretly glad they correspond to the standards currently used in photography and cinema today.

For the full text with links to sources, please visit: http://sareesh.com/2011/12/resolution-visual-acuity-and-lpmm/

[Mark D Segal]

I might be wrong Erik, but here is my line of thought:

...............................

Printing:

I'm not an expert on printing, but the maximum 720 ppi limit seems justified, if all things are perfect @ 1 feet. However, the Epson 9900 is capable of 2880 ppi. Paper resolutions are easily greater than 60 lp/mm or around 720 ppi+ to 1440 dpi for Epson Fine Art papers. Of course, it is important to avoid interpolation when making these calculations. A drum scanner is capable of 3000 dpi to 24,000 dpi, and a flatbed scanner can top 5000 dpi.

As I've mentioned before, my calculations make many assumptions and are amateurish. I did these calculations just to understand how the standards that are being used have come about. I probably have made mistakes along the line (hopefully not grave ones). But I'm also secretly glad they correspond to the standards currently used in photography and cinema today.

For the full text with links to sources, please visit: http://sareesh.com/2011/12/resolution-visual-acuity-and-lpmm/

I think you may be inappropriately commingling the concept of PPI (pixels per inch) sent to the printer and DPI (dots per inch) at which the printer is specified for reproducing the image. The 2880 you mention is printer DPI. Even that metric is impossible to anchor as an end-point to the argument you are making, because the screening and dithering algorithms of these printers make such an exercise anything but straightforward; there is no simple relationship between PPI sent in and DPI coming out because of those algorithms, which are part of Epson's "secret sauce".

As a practical matter, I am now making prints with excellent properties of tonal gradation and resolution of fine detail from my 16.5 MP Sony NEX 5n sending the image over to my Epson 4900 at anything in the range of 240~296 PPI (no resampling) and printing it at about 16.3*11 inches on a Super A3 sheet, with printer set to High Speed OFF (not necessary but I do it anyhow) and Finest Detail (2880 DPI) ON (also not necessary - 1440 would probably do just as well). I just go all the way with these settings (per the Epson manual) to be certain the printer is doing the best it can for the PPI going in. So far I am very pleased with the quality of the results, whether viewed at normal viewing distance for this size image or examined in more detail at reading distance. The prints compete with output from my FF Canon 1DsMk3, which latter of course provides more leeway for cropping without compromising on the sweet range of PPI (no resampling) sent to the printer.

I think one can get carried away on theoretical arguments - which may or may not have their internal validity - but in the final analysis what matters most is what comes out of the printer and what your eyes see. There may be quite a range of theory-driven specifications over the which the real appearance of the image wouldn't differ either at all or enough to constitute an issue.

[Sareesh Sudhakaran]

In my view you are ignoring a few factors. To begin with I don't think that sensor to flange distance is the key parameter. The optical center of the lens may be in front or behind the lens mount. I'd suggest that you could simply use the focal length in your calculations.

My aim was to avoid factoring the lens into the equation. I could be wrong, but the assumption I made is that the back of the lens acts like a screen to the sensor. Also, once the light crosses the focal point, it can still be affected by the 'rest of the lens elements' on its way out. But the extent of my knowledge regarding this subject ends here, and I probably took the easy way out.

The other issue I have is, that according to my understanding, that you try to compare the angular resolution of the eye with the angular resolution of the sensor. But it has never been the case that we would opt for the same angular resolution in the imaging system. Let's just look at some glossy magazine. An article may be illustrated with some image, next page perhaps would contain a full page image and a third one have double spread image. All these images would be seen at the same distance having the same angular resolution, but very differing field of view.

Just wanted to clear a doubt: Isn't magnification mainly the job of the lens? Even if a scene were magnified, the pixel (or eye) would still have the same built in angular resolution. The point was not to match the angles, but to find the minimum required angle, in this case 0.4 arc minutes. A pixel whose angle is lesser can out-resolve the eye, and help in interpolation to large print or display sizes. Have I understood correctly?

Another observation I may have made that the pixel size of the eye seems to be around 4 microns (cones are said to have a radius of 2 microns), that is pretty close to your estimate of 5 microns.

Best regards
Erik

One thing I read was about Hyperacuity. To quote wikipedia:

Under optimal conditions of good illumination, high contrast, and long line segments, the limit to vernier acuity is about 8 arc seconds or 0.13 arc minutes, compared to about 0.6 arc minutes (20/12) for normal visual acuity or the 0.4 arc minute diameter of a foveal cone.

. Link here: http://en.wikipedia.org/wiki/Visual_acuity

[Sareesh Sudhakaran]

I think you may be inappropriately commingling the concept of PPI (pixels per inch) sent to the printer and DPI (dots per inch) at which the printer is specified for reproducing the image. The 2880 you mention is printer DPI. Even that metric is impossible to anchor as an end-point to the argument you are making, because the screening and dithering algorithms of these printers make such an exercise anything but straightforward; there is no simple relationship between PPI sent in and DPI coming out because of those algorithms, which are part of Epson's "secret sauce".

As a practical matter, I am now making prints with excellent properties of tonal gradation and resolution of fine detail from my 16.5 MP Sony NEX 5n sending the image over to my Epson 4900 at anything in the range of 240~296 PPI (no resampling) and printing it at about 16.3*11 inches on a Super A3 sheet, with printer set to High Speed OFF (not necessary but I do it anyhow) and Finest Detail (2880 DPI) ON (also not necessary - 1440 would probably do just as well). I just go all the way with these settings (per the Epson manual) to be certain the printer is doing the best it can for the PPI going in. So far I am very pleased with the quality of the results, whether viewed at normal viewing distance for this size image or examined in more detail at reading distance. The prints compete with output from my FF Canon 1DsMk3, which latter of course provides more leeway for cropping without compromising on the sweet range of PPI (no resampling) sent to the printer.

I think one can get carried away on theoretical arguments - which may or may not have their internal validity - but in the final analysis what matters most is what comes out of the printer and what your eyes see. There may be quite a range of theory-driven specifications over the which the real appearance of the image wouldn't differ either at all or enough to constitute an issue.

You are absolutely right, Mark. It was an overly simplistic assumption on my part.

[ErikKaffehr]

Hi,

Look at the enclosed images, both are taken at the same time (within a few minutes) at the same spot (within a few meters) but with different lenses, one of the images is taken with 16 mm lens and the other with a 70 mm lens.

The 16 mm lens has a horizontal FOV of 132 degrees, the sensor here resolves around 6000 pixels that is 1.3 minutes of arc

The 70 mm lens has a horizontal FOV of 54.5 degrees, the sensor has still 6000 pixels so it resolves 0.54 minutes of arc.

If you would print both images at say 60 cm width and looked at them at 80cm the angle of view covering one pixel would be 0.43 minutes of arc for both images.

Best regards
Erik

Just wanted to clear a doubt: Isn't magnification mainly the job of the lens? Even if a scene were magnified, the pixel (or eye) would still have the same built in angular resolution. The point was not to match the angles, but to find the minimum required angle, in this case 0.4 arc minutes. A pixel whose angle is lesser can out-resolve the eye, and help in interpolation to large print or display sizes. Have I understood correctly?

One thing I read was about Hyperacuity. To quote wikipedia:. Link here: http://en.wikipedia.org/wiki/Visual_acuity

[Sareesh Sudhakaran]

The 16 mm lens has a horizontal FOV of 132 degrees, the sensor here resolves around 6000 pixels that is 1.3 minutes of arc

The 70 mm lens has a horizontal FOV of 54.5 degrees, the sensor has still 6000 pixels so it resolves 0.54 minutes of arc.

If you would print both images at say 60 cm width and looked at them at 80cm the angle of view covering one pixel would be 0.43 minutes of arc for both images.

Best regards
Erik

Thanks for sharing, Erik.

Now I know where we differed. I did not consider the FOV angle of the lens because I wanted to disregard the lens from the equation. In my calculations, since I used the back of the lens as a screen, the angle does not matter at all - since it is the compressed result that is more important. E.g., for a 18-200mm lens, no matter what the angle is when the light enters, when it exits, it is compressed to the same plane, and the lens has the task of compressing resolution. The sensor remains an "independent" observer!

[BJL]

Roger Clark has reached a conclusion similar to that of Ctein. Roger's experiments indicate that the highest print quality requires 600 pixels per inch on the print ...

My reading is that his experiments show an advantage of 600ppi over 300ppi, which is consistent with the threshold being anywhere above 300, not necessarily all the way up to 600. At one stage, laser printers moved to 400DPI, which seemed to improve the sharpness of text; after that there was a move to 600DPI and beyond, but driven at least in part to support the dithering needed to reproduce gray-scale images. Note my shift to DPI, because once the dots of ink or toner are being used to produce finer tonal gradations than the dots themseves have, the DPI needs to be higher than the PPI of the input image file. But with pure black on white text reproduction, PPI=DPI is reasonable.

So let me throw out for discussion and testing the hypothesis that for the most demanding case of pure black and white (where jaggies are most noticeable, due to the exteme edge contrast) 400PPI is enough while 300PPI is not always enough. And for almost all photography, other than photographs of text and high contrast test patterns, the visually distinguishable level is likely to be a bit lower.

By the way, at 400PPI (16pp/mm), A4 and US letter paper are about 16MP, so A3 is 32MP and so on.

P.S. on this viewing distance thing: with many large prints (including all billboards!) the effect of size is to lead to viewing from further away, like several people viewing and discussing a large print in a gallery. Even with huge 4'x5' from LF film, I most often see people staying four or five feet away, so not much closer than "normal viewing distance". But that is not always the case, so the photographer planning on large prints has to work out what the viewing style is likely to be, or what viewing style he/she wishes to support, not us online theoreticians.

[hjulenissen]

P.S. on this viewing distance thing: with many large prints (including all billboards!) the effect of size is to lead to viewing from further away, like several people viewing and discussing a large print in a gallery. Even with huge 4'x5' from LF film, I most often see people staying four or five feet away, so not much closer than "normal viewing distance". But that is not always the case, so the photographer planning on large prints has to work out what the viewing style is likely to be, or what viewing style he/she wishes to support, not us online theoreticians.

But is it a failure if the viewer is able to spot image "flaws" by walking up to image distances that he/she was never supposed to do? What if she brings a magnifying glass? Is the value of a Picasso any diminished by knowledge about the limitations in the process of making it, or knowledge that the master changed his mind a few times, revealed by x-rays or multi-spectral imaging? I think not.

Someone will always find a flaw. I imagine that those people would never approve (or buy) the image in the first place, they are just looking for excuses.

I believe that a good image processing chain should look good at a range of distances, and that the consequence of walking "too close" should be a lack of expected details (lowpass filtering), not "stair stepping", "aliasing", "mosquitoes" or other nasty artifacts.

-h

[BJL]

But is it a failure if the viewer is able to spot image "flaws" by walking up to image distances that he/she was never supposed to do?

I agree; for my photography, such close viewing is "user error" (I call it "map reading"). That is why for my purposes, I am comfortable with about 12MP after cropping, no matter how big i might display the image, and aspire to more pixels than that solely to facilitate some macro and telephoto work when I cannot get close enough to fill the frame of the lens I have with me. I was however allowing that others have different intentions for their prints. (Hopefully there is better evidence than Roger Clark putting a big print on a staircase where it can only be viewed from close quarters, and then claiming this as evidence that people want to look at huge prints at very short range!)

P.S. I prefer to look at extreme resolution needs in terms of line pairs per picture height, avoiding the complications of how many pixels one needs to fully resolve a line pair, and issues like over-sampling to avoid moiré. For another day, I would argue that there is very, very little use for more than about 4000 lp/ph, due to the collision of diffraction and DOF needs: as the desired apparent image size increases (bigger prints/closer viewing) diffraction control requires larger aperture sizes while maintaining equal perceived DOF requires smaller aperture sizes until you are crushed between conflicting needs. 4000lp/ph could mean about 100MP, but nowhere near Ctein's 400MP.

[ErikKaffehr]

Hi,

My lab suggests 200 LPI for sizes above A3. The largest prints I have 70x100 cm and 50x100 cm. One of the 70x100cm prints made from 67 Velvia scanned at 3200 PPI is extremely sharp, but has clearly visible "staircase" effect close up. Another 70x100 cm is made from 10 MP APS-C camera, it is OK at 80 cm viewing distance.

I would say that the philosophy that 12 MP is all you need has some merits, but the other philosophy saying that you need all pixels you have is not without merit.

Sometimes I feel that I want to walk "into" a picture and be immersed in it. Panoramas can be like that,

Best regards
Erik

My reading is that his experiments show an advantage of 600ppi over 300ppi, which is consistent with the threshold being anywhere above 300, not necessarily all the way up to 600. At one stage, laser printers moved to 400DPI, which seemed to improve the sharpness of text; after that there was a move to 600DPI and beyond, but driven at least in part to support the dithering needed to reproduce gray-scale images. Note my shift to DPI, because once the dots of ink or toner are being used to produce finer tonal gradations than the dots themseves have, the DPI needs to be higher than the PPI of the input image file. But with pure black on white text reproduction, PPI=DPI is reasonable.

So let me throw out for discussion and testing the hypothesis that for the most demanding case of pure black and white (where jaggies are most noticeable, due to the exteme edge contrast) 400PPI is enough while 300PPI is not always enough. And for almost all photography, other than photographs of text and high contrast test patterns, the visually distinguishable level is likely to be a bit lower.

By the way, at 400PPI (16pp/mm), A4 and US letter paper are about 16MP, so A3 is 32MP and so on.

P.S. on this viewing distance thing: with many large prints (including all billboards!) the effect of size is to lead to viewing from further away, like several people viewing and discussing a large print in a gallery. Even with huge 4'x5' from LF film, I most often see people staying four or five feet away, so not much closer than "normal viewing distance". But that is not always the case, so the photographer planning on large prints has to work out what the viewing style is likely to be, or what viewing style he/she wishes to support, not us online theoreticians.

[BJL]

Hi,

My lab suggests 200 LPI for sizes above A3. The largest prints I have 70x100 cm and 50x100 cm. One of the prints ... has clearly visible "staircase" effect close up.

... but the other philosophy saying that you need all pixels you have is not without merit.

Sometimes I feel that I want to walk "into" a picture and be immersed in it. Panoramas can be like that,

On the staircase effect: can that not be dealt with by upsampling and then dithering/smoothing?

As to "all the pixels you can get" ... as I hinted in my last reply, I can see thtomsk a considerable degree, but only up to the diffraction/DOF limit. Once you get the resolution that the pixels promise by using a large enough aperture, the pleasure of close examination will be limited to elements of the scene that are very close to the plane of exact focus, due to the magnification of OOF effects as you view at such great enlargement. Though I suppose that the dedicated big print landscapist can use focus stacking or such to overcome that.