CF for XZ-1?/Diffraction?

Started Mar 6, 2012 | Discussions thread
Mike Davis
Contributing MemberPosts: 690
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Re: Nice post mosswings!/lp/mm?
In reply to Randyflycaster, Mar 13, 2012

Hi Randy,

Randyflycaster wrote:

Mike,

Thanks so much.

I'm not quite sure what lp/mm means.

I guess it refers to resolution.

Randy

Yes, "lp/mm" means "line pairs per millimeter."

If you imagine alternating black and white stripes of equal width, viewed at a viewing distance of 25 cm (roughly 10 inches), it would be easy to distinguish the white stripes from the black stripes in just about any lighting if each stripe was 25mm wide (about an inch). But as you the width of each stripe is made smaller, you will eventually have a hard time distinguishing black stripes from white stripes, even in good lighting, when viewed at a distance of 10 inches, with the naked eye. The stripes can be made so small that, together, they will appear as a uniform grey - you will no longer be able to detect individual black and white stripes. This is generally agreed to occur when 8 line pairs (16 stripes) occur in one millimeter, as viewed with healthy vision at a distance of 10 inches.

8 lp/mm is equivalent to an image density of 400 dpi.
6 lp/mm is equivalent to an image density of 300 dpi.
Etc.

But thanks to roughly a 30% loss in resolution caused by a typical CMOS sensor's RGBG Bayer algorithm and the AA (anti-aliasing) filter, your sensor must actually capture 30% more pixels than the resolution at which you hope to render subject detail in the final print.

Thus, if you really wanted to render subject detail at the limits of what human acuity can appreciate (about 8 lp/mm when viewing a print from a distance of 10-inches), your enlargement factor must not be any greater than is possible with an unresampled image density of 576 dpi (instead of the 400 dpi that would be possible but for the aforementioned 30% loss in resolution.)

So, if you have a 12 Megapixel sensor, capturing 4000x3000-pixel images, if it's your goal to render subject detail at 8 lp/mm, you cannot make prints as large as 10-inches wide (4000 pixels / 400 dpi = 10 inches). Thanks to the RGBG Bayer algorithm and AA filter, you can only make prints no larger than 6.944 inches wide (4000 pixels / 576 dpi = 6.944 inches).

Yes, a 12 MP CMOS sensor cannot render subject detail at the limits of human acuity in prints any larger than 6.944 x 5.2 inches (at an uncropped and unresampled image density of 576 dpi).

That's just the limit imposed by your sensor's pixel count. You've also got to prevent defocus, diffraction, subject motion, camera motion, lens aberrations, high ISO noise, and other factors from inhibiting your desired print resolution (whatever you personally hope to achieve in terms of subject detail in the final print.)

It's very hard to actually achieve 8 lp/mm in the final print, after enlargement - and unnecessary for most audiences, as the average person is perfectly content with prints that exhibit acuity (edge sharpness) in the absence of resolution (subject detail.) Most people have never seen prints that render true subject detail at 8 lp/mm because most photographers have no clue how to get there, nor the patience or self-discipline (obsessiveness) to make it happen.

But that's perfectly OK - different strokes for different folks. I'm just trying to equip people with the ability to take control of the factors affecting final image resolution - to suit their personal requirements - be that 8 lp/mm or only 1 lp/mm - the latter being a perfectly reasonable print resolution goal, in my opinion, when viewing distances are expected to be at least 40 inches, for example (instead of 10 inches).

A roadside billboard photo taken with a 35mm camera looks great when viewed from a distance of 50 meters - it has plenty of resolution!

So, keep in mind that if you are willing to assume that no one will examine your prints any closer than 20 inches, shooting for a print resolution of 4 lp/mm will deliver the same apparent detail as an 8 lp/mm print that's viewed at 10 inches.

Thus, a 12 MP CMOS sensor could be used to make 13.89 x 10.4-inch prints at an uncropped, unresampled image density of only 288 dpi (instead of 576) dpi, if you want to achieve the look of an 8 lp/mm print viewed at 10 inches and are content with assuming that your print will be viewed no closer than 20 inches. Your actual print resolution goal would be 4 lp/mm instead of 8 lp/mm and as long as no one views the print any closer than you anticipated (20 inches), in terms of limitations imposed by your sensor's pixel count, the print can deliver every bit as much subject detail as a healthy pair of human eyes are capable of appreciating at that distance.

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