lp/mm l/mm ?

<p>Hi, I've been looking through posts and online and was just hoping for clarification on the difference between line pairs and lines per mm. <br />I have read that traditionally resolving power was measured as lp/mm, but the specs on current films (Kodak and Fuji) are given as l/mm. <br>

I understand that a line pair is a pair of one light and one dark line, why are they sometimes counted as pairs and sometimes as lines? I read somewhere (but I can't validate the source) that l/mm was actually just counting the dark lines, as to be able to distinguish a dark line there obviously has to be a light line either side. In which case lp/mm and l/mm would be the same number. (i.e. 50 pairs of lines or 50 dark lines). </p>

<p>Specs for T-MAX 100<br>

63 lines/mm (TOC 1.6:1)<br>

200 lines/mm (TOC 1000:1)</p>

<p>So at 1000:1 is that 100 line pairs/mm ? <br>

<br />Thanks I'd appreciate any help clarifying this I am getting more confused the more I read. </p>

<p>As you know, light rays passing through a lens aperture are not perfectly shadowed by the blades of the iris diaphragm. It is the nature of light waves that some light bleeds into the shadows. The result is a circle of confusion that is an illumined disk surrounded by dark and light rings. This is the Airy Disc.<br>

<br>

Now resolving power of an optical system can be described by the weather it is possible to view two separated Airy discs. Well studied by Sir John William Strutt 3tdh Baron Rayloigh (Nobel Prize for Physics). His work in this area is call the Rayleigh Criterion. “The resolving power of a lens decreased with the aperture, for this increases the Airy disc diameter which decreases the resolution”.<br>

<br>

The Rayleigh criterion gives the maximum resolution which is different for each wavelength, twice as grate for blue vs. red. Generally 589 millimicrons is stated.<br>

Thus Resolving Power (R.P.) = 1392 / f/number<br>

f/1 = 1392 lines per millimeter (l/m)<br>

f/2 = 696 l/m<br>

f/2.8 = 497 l/m<br>

f/5.6 = 249 l/m<br>

f/8 = 174 l/m<br>

f/11 = 127 l/m<br>

f/16 = 84<br>

Note: Resolving power at f/8 exceeds what is pictorially useful for film emulsions. Thus the resolving power is a combination of lens and film. </p>

<p>As well as I know it, line pairs are the same as lines. </p>

<p>The place it starts to get complicated is with digital imaging where we count pixels per mm.<br>

It takes two pixels, one light and one dark, to make a line or line pair.</p>

<p>Thanks for the responses Alan and Glen.<br>

Having looked at quite a few data sheets for various films now, it seems that l/mm is the standard. Agfa, Fuji and Kodak all specify resolving power as lines/mm. And looking at old Modern Photography lens tests, their results are given in lines/mm as well, but I have seen results given as lp/mm, and always thought that lp/mm was the standard (one black one white)...<br /><br />Is lp/mm still relevant if it's not used by the film manufacturers? <br />And is l/mm in fact the same unit as lp/mm, or should any data specified as lp/mm be doubled to make it l/mm? Does a white line actually count as a line or just as negative space? <br /><br />Thanks again</p>

<p>I`m just wondering how you can separate two black lines from each other unless there is some color between them ;)<br /><br />It´s obvious that one "line" consist two lines. Therefore I prefer <strong>lp / mm</strong>.<br /><br />BR<br /><br />Esa Kivivuori</p>

I think everyone's sense that a physical line "pair" is necessary is necessary to distinguish individual lines is correct, so the two terms are

actually the same thing.

 

If you want it from the horses' mouth, ANSI PH3.50 (1970s) says, "Line pairs per millimeter" may also be expressed as "Lines per

millimeter."

<p>It is nice to have the official reference.</p>

<p>I presume there is some lost history somewhere in the story, maybe 100 or so years ago, when people started measuring these things.</p>

Line pairs per mm give me the impression that both the white and black lines are equal. Lines per mm could be the lines on a note paper, wide separation between the black lines. But since I know what both mean it is okay.

<p>Ditto that Glen, thanks very much Bill C for an official reference, so glad to have it cleared up. <br />Very much appreciated input thanks everyone!</p>