Gradient based upon data

jjs wrote:

I am positively stumped. I don’t think PS is supposed to do this. But here it is – I have a table of data that when plotted creates a flat, almost ‘S’ figure … Oh heck, here it is:
http://elearning.winona.edu/jjs/f22.gif

(That is relative light transmittance of a flat optical filter, measuring from center to edge (light to dark, top left to bottom right, respectively.)

I want to depict this as the filter it represents using a gradient. The subtelties of the curve are critical.
If I can just get the gradient curve right, then I can play with transparencies and so-forth for classroom demonstrations.
Must be the holiday blues, or perhaps I really am this stupid. Help?

John,

If I understand correctly, it seems like the gradient editor would do the job nicely.

I’m guessing that you would be using this as a radial gradient to superimpose on an image? For this, I would specify only the transparency of the gradient, and use it as a mask for a correction layer. —
Mike Russell
www.curvemeister.com
www.geigy.2y.net

I agree, using it as a mask and applying a gradient to the mask would have a slick transparency effect on it. Good Luck!

"Mike Russell" wrote in message

If I understand correctly, it seems like the gradient editor would do the job nicely.

I’m guessing that you would be using this as a radial gradient to superimpose on an image? For this, I would specify only the transparency of
the gradient, and use it as a mask for a correction layer.

Yes, Mike, I would be placing it over various images. (I am trying to mimick the light fall-off of various lenses.)

However I’m stuck. The gradient editor’s ‘stops’ don’t seem to give me a fine enough outcome, or they skew the entire range. Gosh, what am I missing here? I’ve played with them a lot. Yes, I’ll do the mask, but I can’t noodle out the darn stops.

I will continue to try.

wrote in message

I agree, using it as a mask and applying a gradient to the mask would have a slick transparency effect on it. Good Luck!

Will do.

jjs wrote:

"Mike Russell" wrote in message

If I understand correctly, it seems like the gradient editor would do the job nicely.

I’m guessing that you would be using this as a radial gradient to superimpose on an image? For this, I would specify only the transparency of
the gradient, and use it as a mask for a correction layer.

Yes, Mike, I would be placing it over various images. (I am trying to mimick the light fall-off of various lenses.)

However I’m stuck. The gradient editor’s ‘stops’ don’t seem to give me a fine enough outcome, or they skew the entire range. Gosh, what am I missing here? I’ve played with them a lot. Yes, I’ll do the mask, but I can’t noodle out the darn stops.

I will continue to try.

How about using curves. Start with a radial 0-255 linear gradient, then apply a curve with a suitable number of control points to the gradient. This should get you within half a percent or so. If you need more accuracy than that, I have another idea.

Also, what about photographing a carefully lit surface with the lens in question, and then applying that as a layer to the image. The tricky part will be getting an evenly lit surface – a cloudy sky should work fine, and should simulate the lens exactly.
—

Mike Russell
www.curvemeister.com
www.geigy.2y.net

First:
I think you might find some useful information if you look up

center filter

center ND filter

Or … check out the following web page:

http://www.luminous-landscape.com/tutorials/panorami.shtml look about 1/2 way down for "Digitally Simulating a Center ND Filter"

Second:
On August 24, 2004, Tom Nelson made a reply in this forum to a thread titled ‘Re: Center Filter Procedure?’ The following seems to work well in my trials, and is quoted in its entirety:

= = = = = Begin Quote = = = = =
There’s a MUCH simpler way to do it:

1. Do a good white balance and then shoot a medium-gray target completely out of focus. For instance, put a peice of translucent Plexiglas right in front of the lens, disable any autofocus and shoot using the built-in meter. If it’s a digital camera and you normally use several resolution settings, do a photo at each resolution. If you’re shooting film, shoot the film you normally use with that camera.

2. If you shot film, scan it taking care to keep the color balance neutral, at the resolution you’ll use for your final image. You might want to blur the image slightly to minimize film grain.

3. You have now reproduced the light fall-off without reference to any specific photo. In Photoshop, invert the image
(Image>Adjustments>Invert). You have now produced a counteracting image. Save this image for later use.

4. Drag this image onto your photo and set the blending mode to Soft Light. Soft Light lightens and darkens according to the layer’s deviation from neutral gray. Presto! an instant and universal center filter. Unlike the technique mentioned above, there is no need to manually match your gradient to the characteristics of the lens; the lens has done the work for you.

Tom Nelson
Tom Nelson Photography

= = = = = End Quote = = = = =

"jjs" <john&#064;xstafford.net> wrote in message

"Mike Russell" wrote in message

If I understand correctly, it seems like the gradient editor would do

the

job nicely.

I’m guessing that you would be using this as a radial gradient to superimpose on an image? For this, I would specify only the

transparency

of
the gradient, and use it as a mask for a correction layer.

Yes, Mike, I would be placing it over various images. (I am trying to

mimick

the light fall-off of various lenses.)

However I’m stuck. The gradient editor’s ‘stops’ don’t seem to give me a fine enough outcome, or they skew the entire range. Gosh, what am I

missing

here? I’ve played with them a lot. Yes, I’ll do the mask, but I can’t noodle out the darn stops.

I will continue to try.

"Mike Russell" wrote in message

[… snip the good stuff …]
Also, what about photographing a carefully lit surface with the lens in question, and then applying that as a layer to the image.

The lenses in question are breathtakingly expensive.

Thanks for the [sniped] stuff. I’m saving it to review tomorrow morning and will try again.

"RSD99" wrote in message

First:
I think you might find some useful information if you look up
center filter

center ND filter

Or … check out the following web page:

http://www.luminous-landscape.com/tutorials/panorami.shtml
[…]

Yes, indeed, I do know about graduated ND filters. I had better after all this time. And I do appreicate the pointers above, but each presumes the ownership of the lens so that a picture can be made from which to make the gradation mask. And I do not want to fall into the trap many make – the gradation is not linear, however while that’s ‘good enough’ for many people, it defeats the project.