contrast sensitivity, visual acuity, and Photoshop

Argh!

I've been reading about "contrast sensitivity", "visual acuity", and
wonder what the difference is. When I try to find this out, I am
immersed in "cycles per degree". When I try to find out what that is,
I get math. It's so abstract.
:-(

Are these two things different? It seems that both are measured in
"cycles per degree".

Is there a way of explaining how CS differs from VA (if it does),
using terms used in photography?

If you have poor CS or VA, what do you not see?
Resolution, shadow detail, sharpness? ...... help!

thanks,
Liz
Indianapolis IN USA


thanks,
Liz

http://www.google.co.uk/search?hl=en...meta=&aq=f&oq=
http://www.google.co.uk/search?hl=en...G=Search&meta=

Easy.

Liz <(E-Mail Removed)> wrote in part:
> Argh!
>
> I've been reading about "contrast sensitivity", "visual
> acuity", and wonder what the difference is. When I try to find
> this out, I am immersed in "cycles per degree". When I try
> to find out what that is, I get math. It's so abstract. :-(

Some of us like that stuff !

> Are these two things different? It seems that both are
> measured in "cycles per degree".

People often use terms imprecisely. I would say "visual
acuity" presumes some sort of standard contrast, while
"contrast sensitivity" is somewhat more general.

> Is there a way of explaining how CS differs from VA (if it
> does), using terms used in photography?

Oh, a photog -- look up "Modular Transfer Function" ...
maybe not if you dislike abstraction more than ignorance.
"cycles per degree" is similar to "line pairs per mm"


> If you have poor CS or VA, what do you not see?
> Resolution, shadow detail, sharpness? ...... help!


It is all the same thing! You can see wires against sky
with very high resolution but cannot see something big
camouflaged. Contrast is what drives vision, and the
more of it there is, the smaller details you can see.


-- Robert

> Sorry Liz. If you are trying to quantify these terms to an objective
> standard, I do not see how you can avoid math.

I'm not complaining about the measurement aspect. Of course you have
to use math to measure it.
I'm saying that I can't figure out WHAT is being measured. I mean in
terms of anything concrete that I know what it is.

Unfortunately I don't know what "spatial frequency", "transfer
modulation function", are. Sorry! Yes, I looked them up and read
several times.

Liz

Mike, your explanation is SO much better than those articles!

> > I've been reading about "contrast sensitivity", "visual acuity", and
> > wonder what the difference is.
>
> Pretend your eye is the center of a circle. If you look at a letter "E"
> several feet away, you could measure the height of the letter in degrees,
> couldn't you?

Degrees as in 0 to 360? The way astronomers say something is "15
degrees above the horizon"? I think so. In other words, straight
ahead is 0, straight overhead is 90, and the space in between is
divided into 90 equal pieces...

> The nice thing about degrees is we don't care how far away something is, as
> long as it covers the same angle.

Yes, I see how the distance wouldn't matter.
OK.

> Since an "E" has three "arms" we can say, going vertically, it's black, then
> white, then black, then white again, then black. That's 2.5 black/white
> "cycles", or a 2.5-cycle "square wave grating." If the E is one degree tall,
> it's 2.5 cycles/degree.

So a "cycle" is about changes in appearance. From black to white, or
silver to pink, or any other change that's visibly different from
what's next to it?
And the number of "cycles" is the number of pairs of distinguishable
objects?
Would black green white pink white grey orange blue be 4 cycles?


> If the letter E is very blurry, you have something that isn't solid
> black-white-black, but rather lighter-darker-lighter etc, plotting a "sine
> wave" pattern if you graph the brightness. Patterns of darker-lighter-darker
> are called "sine wave gratings."

OK. Are they described using cycles too?

> "Contrast sensitivity" explores how well you see the pattern when it's made
> up of fainter and fainter shades of gray. It's typically tested with
> sine-wave patterns but you can also gray out a pattern of solid bars without
> blurring it, or use a gray snellen chart, for a similar effect.

OK.

Try this:
You have a white piece of paper with horizontal black bars on it.
Someone walks further and further away with this piece of paper.
Finally they are so far away that you cannot see the bars; you only
see the whole paper as grey.
That is an example of a limit of your visual acuity.
Right?
(In Photoshop I would call this "resolution".)

Suppose instead of black and white, you have black and dark grey. You
can tell them apart when the paper is near you.
The person walks away.
They haven't walked very far - nowhere near as far as they went with
the black and white paper - when you can no longer see any barring.
The whole paper looks one dark color.
This is a failure in contrast sensitivity?
(In Photoshop I would call this "tonal contrast".)

But..... hmmmm.

I feel as though there is a lot more but I have to think of how the
situations differ.
I think there must be interactions between being able to discern a
sharp edge and being able to see a tonal difference.

Liz