# Film grain size as dpi

Discussion in 'Photoshop' started by Don, Jul 25, 2003.

1. ### DonGuest

Does anyone happen to have handy a table of dpi values corresponding
to various ASA/ISO film ratings?

In other words, express film grain size as dpi.

TIA!

Don.

Don, Jul 25, 2003

2. ### J CGuest

I remember reading that film grain ranges from 2500 to 3500 per inch
(someone feel free to correct me if I'm remembering wrong).

However, from my darkroom experience the same film stock can yield
different results depending on the temperature and time spent in the
developing solutions.

I've never seen a chart that gives the ranges for different film
ratings though.

-- JC

J C, Jul 25, 2003

3. ### i.perrymanGuest

I thought film grain sizes were measured in microns.
A film with a rms granularity of 9, for example, would have grains 9 microns
across
I would have thought therefore that you could apply the math as follows

For a 35mm film with G=9 you would get
36mm/9microns across = 4000 grains
24mm/9microns down = 2467 grains

Presumably if you were to scan at this resolution (4000X2467) each grain
would be individually represented. Scanning at a higher resolution would,
at least in theory, produce no improvement in quality.

If you want to work this in inches
1 inch = 25.4mm
for a film with G=9
25.4mm/9microns =2822 grains per inch ( = maximum dpi of 2822?)

For a higher definition film with smaller grains - say G=8
25.4mm/8microns =3175 grains per inch (= maximum dpi of 3175?)

i.perryman, Jul 26, 2003
4. ### DonGuest

My 2700 dpi scanner puts me in the lower half end of that, then.
Indeed, so I was really looking for a rough estimate.
Over in "comp.graphics.apps.photoshop" the opinions as to why that is
range from physical inaccuracies of the analog domain (variable grain
size, 3D nature of emulsion, etc) to "shy" marketing departments
reluctant to admit the truth...

Don.

Don, Jul 26, 2003
5. ### DonGuest

Excellent! Thanks very much for that. What film speed is the 9 micron
figure for?

And you're probably going to guess my next question now... ;-)

Do you happen to have a chart of how rms granularity relates to other
common ASA/ISO ratings?

Don.

---

Don, Jul 26, 2003
6. ### J CGuest

"Shy?" -- way too kind.

I was once in a CompUSA store talking about a laser printer with an
HP sales rep who was there for a promotion. Another HP rep walked up
to him and asked whether when scanning with HP scanner model XXX if
the image was digitized. The response from the rep was, "No, it's just
in there. It's saved and he can print it out."

True story.

-- JC

J C, Jul 29, 2003
7. ### J CGuest

Not really, because there is actually detail IN the grain. If you look
under a microscope you see a crystal in, for example, black and white
emulsion. That crystal has somewhat irregular shape and contains
details and imperfections. Additionally, not all crystals are the same
size.

As another poster commented, to scan the detail in the film grains
you'd need to scan at a MUCH higher resolution.

But since the human eye cannot resolve the grain details in prints
scanning all that detail is (in my opinion) futile.

Generally, you should scan at 300 dpi for every inch of final print
size. Period. I suggest you run a test. Take the same 8x10 image at
150 dpi, 300 dpi, 600 dip, and 1200 dpi and print four versions on
your inkjet. Compare them. You *might* see a difference between the
150 and 300 dpi images, but you will not see any difference between
300, 600 and 1200, even though your injet is capable of 1400 or more.

Rather than the march of technology making today's scans obsolete, it
will be your decision to print the image greatly enlarged that is
more likely to cause you worry.

-- JC

J C, Jul 29, 2003
8. ### DonGuest

Oh, it's just my subdued understatement... ;-)
Oh, I believe it! There's nothing more frustrating then, when trying
to get some specs or information out of a company, you realize that
the person you're dealing with knows even less than you do. And yet
the incompetent continues to pretend they know what they're talking
about. "Yeah, it goes up to 11!" type of thing... (Spinal Tap).

Don.

Don, Jul 29, 2003
9. ### DonGuest

I guess the only exception is aliasing. I was surprised at how grainy
my test film scanner images were but someone pointed out that this may
be due to aliasing, which makes sense.
I found a very clever and revealing resolution test which comes at it
from a slightly different angle. It's based on the fact that
interpolation does not create any new detail. The test comprises
scanning the same image twice, once at high resolution (say, 2400) and
once at lower resolution (say, 600). The low resolution scan is then
interpolated, or should that be extrapolated? ;-) to 2400 dpi.

Putting these two images side by side (now of identical sizes and so
much easier to compare), examining them closely and even going down to
pixel level will show that the genuine 2400 dpi scan does not reveal
any more detail than the "make believe" pixels "invented" by the
interpolation process in the other image! Game, set and match!

Don.

Don, Jul 29, 2003
10. ### J CGuest

BUT I have two rhetorical a question about that...

1. Is the judgment just a visual inspection or did you actually
measure the color value of pixels in the same position in each scan?
The reason to ask this is that a greyscale image pixel can be any of
256 colors and subtle changes would mean that your 2400 dpi scan would
still be more accurate (though how significant the difference I'll
leave up to statisticians).

2. I wouldn't expect that the majority of pixels would be different in
the two scans cited above. After all, adjacent pixels in, for example,
the white area in a cloud would not be that different in the native
2400 dpi or in the 600 to 2400 dpi conversion. The real differences
only come into play at the edges where the pixels are changing color
to convey a new structure (detail).

AND NOW... Just as a complete aside, here's another completely
different image size conundrum:

We often publish photomicrographs and electron microscopy images. The
authors typically submit them as 5x7 prints and in the captions to the
photos they specify the microscopic magnification (i.e., X200, X400,
etc.).

We've has a few authors that insist that if we resize the image from
5x7 to 3x5 then the magnification in the photo's caption should change
in the same proportion.

In an explanation to one author of why not to do that I gave the
example of viewing the 5x7 image from 2 feet away then from 4 feet
away (would the magification change?). How about if the image had been
submitted as an 8x10, would they have increased the magification used
on the microscope?

Then when you layer the fact that the image is going to be printed
with a 150 halftone line screen, some authors seem to get really
confused about exactly what resolution that the microscopes lens is
conveying.

-- JC

J C, Jul 29, 2003
11. ### Donald LinkGuest

You think the reps in CompUSA are ignorant. Go to Best Buy and ask
different sales person the same question in the computer department if you
really want a chuckle.

Donald Link, Jul 30, 2003