God, redheads, dpi, microns and lp/mm

[Neal Shields]

God, redheads, dpi, microns and lp/mm

Sitting in church today when I should have been paying attention to the sermon, I was marveling at the fact that God have given me eyes that could resolve individule flyaway hairs on the Redhead two pews in front of me. Counting floor tiles, I determined that she was 6 feet away from me, and remembering from a filtration project that I worked on, I knew that her hair should be about 36 microns in diameter.

I reasoned that if I could resolve a 36 micron detail at 6 feet I should be able to resolve a 9 micron detail at 18”

Now I keep hearing that 300 dpi is plenty of detail for a print. Unless I am doing my math wrong each of those dots should be about 80 microns in diameter. (About 12 dots per millimeter and 1000 microns per millimeter, so 1000 divided by 12 equals 80 each.)

So if my poor old tied eyes can resolve 9 micron detail how can 300 dpi (6 lp/mm) be enough?

P.S. In our camera club meeting the other night a speaker brought in some excellent 11x14 contact prints. While standing in front of those prints, I was told that you could get results just as good from a D-70. Maybe so, but I think I will keep my "Dorff" all the same.

[Hans Berkhout]

now I'd like to hear about rednecks- no, better discuss that with your minister, that should keep you focused.

[Emmanuel BIGLER]

Neal.
Are you sure that you do not make a confusion between resolution/separation of two adjacent objects, and individual perception of a sub-resolution feature ?

The resolution of the human eye is at best about 1 minute of arc, 1/3440, this is known from studies done in the second world war. This means : separating two features distant, centre to centre, by less than 1/3440 radians in angular terms is not possible.

However looking at a single black electrical line on clear sky background, you can see the individual electric line even if its angular diameter is much small than 1/3440 radians. However a pair of such adjacent electric lines will be seen as a single wire if the angular distance, centre-to-centre, is smaller than 1/3440 radians.

The classical example in astronomical images, since we have started this discussion by looking to the Heavens is that most distant stars have a sub-resolution diametre and appear all as the similar diffraction spot with the same image diametre, however double stars can, or cannot, be separated according to the actual f/number of the main telescope lens.. except that down to Earth, the images are blurred by the atmosphere, this is another story...

Another source of admiration is that the perception of a misaligned pair of lines or a broken line like e.g. in the split-prism rangefinder or microprisms focusing system is much better than 1/3440.

And Neal, you are not alone to think about photography during the Sunday service. In my church, there is a piece of stained glass missing just in front of everybody in the middle of the main stained glass church window. I am always thinking to make a nice LF picture of this stained glass. Unfortunately a small piece of glass was broken during some work done to clean the outside of the church ; behind the stained glass clearly appears a protective metallic grid which has been renewed and replaced. So instead of listening to the sermon ;-) often my questions are : could it be possible by any means to remove the image of this grid on a photograph, grid that appears not only through the small broken part but more or less as a backgroung behind the stained glass itself ? My conclusion is that the grid will be easily resolved even by a modest LF lens, and that it is located too close behind the window to be blurred by a reduced depth of fiedl. So we are left only with digital processing to fill the hole and selectively remove the periodic structure the background grid ;-);-)

[Ralph Barker]

I, too, am pleased that God invented both redheads and eyes with which to see them.

As to that 300 DPI sufficiency thing, it may help to think about what the digi-printer and its driver are doing with that 300 DPI input. (It's the interpolation, Baby!) D-70 output as good as 11x14 contacts? No comment. ;-)

[Brian Ellis]

I'm not an optical scientist but there surely is a difference between what you could see sitting in the pew and what a print would have looked like. Next time you go to church bring your camera with you, set it up in the aisle, and make a photograph of the redhead during the sermon.

I print digitally (scanned 4x5 film) and have a Nikon D100 that I use for "non-serious" stuff so I'm no fire-breathing, anti-digital film nut. But I've also made many 8x10 contact prints. Much as I like the D100 for some things, only a fool would say the technical quality of an 11x14 print from a D70 is equal to an 11x14 contact print from a technical standpoint.

[Roger Scott]

A year or so ago I had a crisp 4x5 transparency professionally scanned and a 10"x12.5" print done at the usual 300 dpi. I wasn't very impressed as it didn't even compare with my own optical prints done from 645 transparencies and printed at similar sizes. I haven't had any prints done digitally since as at my usual viewing distances my eyes can obviously resolve better than 300 dpi. Then again I was recently asked to do some B&W prints from colour negatives for a cousin. The prints turned out terrible and I said as much yet they were quite happy with the results - it's all subjective.

[Emmanuel BIGLER]

my eyes can obviously resolve better than 300 dpi
Roger : you are certainly lucky enough to have excellent eyes, but you should mention whether you look at the print from a 300 mm (12") distance or closer.
The resolution limit of the human eye is not a value in dpi, it is a value in angular units. And it is an average value of course.

300mm/3400 yields 87 microns ; 300 dpi is 84 microns per dot. 2 dots are required for one period if we want to speak in terms of line pairs.

[Roger Scott]

Perhaps I shouldn't have used the word 'resolve' and instead kept it more subjective however my usual viewing distance for prints of this size is around 600-700mm. Yes, my eyes are quite good although as I age that's changing. At this distance and with my eyes 300 dpi prints do not look as good as MF or LF optical prints. I'm also a private pilot so perhaps I have some self-preservation programming which looks for small objects at the limit of my vision.

[Leonard Evens]

I have a D70 with the kit lens. If I scan one of my 4 x 5 negatives at 3200 ppi with my Epson 3200 and then rescale the image to 2000 x 2500 (comparable to the D70's 2000 x 3000 image), the result is clearly superior to a direct D70 image.

[John D Gerndt]

Numbers lie! I teach math and I have done quite a look into human visual perception. Dots per inch and linepairs per milimeter do not describe prints any more than total harmonic distortion describes the sound of a stereo. These are crude measurments but all we have to go on until we face the real thing.

If you can see a difference, there is a difference*. Testing whether you can see the difference takes someone who knows which is which and who can show them to you without you knowing which is which then checking if you can repeatedly pick out one from the other, simple.

Cheers,

*if you cannot see a difference that does not mean that there is not a difference, it just brigns up the question of: "What's the point?"