CS2 - centre filter effect?

[Paul O]

I've been racking my brains - didn't take long :D but I seem to remember a thread that discussed how to create a "pseudo" neutral density grad centre filter effect in photoshop? Any ideas on a (simple) how-to tutorial?? TIA Paul

[Marko]

Hi Paul,

The easiest and fastest way to do it in CS3 is to use Filter - Distort - Lens Correction - Vignette.

I can't quite remember, but I think Lens Correction filter was introduced in CS2, so you should have it.

[z_photo]

radial gradient fill layer. open image. add gradient fill layer. make sure you use radial. set opacity and size of the 'center filter'.

[Leonard Evens]

I've done it with moderate success, at least for bw. I generally use the GIMP, working under Linux, but I do have CS2 and I'm certain you can do the same thing there.

I use a radial gradient on an adjustment layer. I first locate the optical center, which because of movements may not be the same as the center of the frame. I measure the values there and compare them with the values in the corner furthest away. I then create a radial gradient which compensates for the difference. I could take you through it step by step, but I don't think you can do it by following someone else's recipe. You have to have a firm understanding of what you are doing and also how gradients work. In particular, you should understand the different mixing modes. I find that using additive or subtractive produces the best results. I usually use a gradient somewhat larger than the difference between the readings and then adjust the transparency of the layer to get it right.

The effect of fall-off is to reduce values in the corners relative to the center. When you use a center filter, you have to increase exposure to compensate. That brings up the values in the corners with the filter decreasing them in the center, so the end result is more uniform. If you plan to do it in Photoshop, you need to do the same thing when you make the exposure. How much you should increase exposure depends on the lens, how you develop your negative, and what you want to accomplish in the photoeditor. You can start off by using the exposure correction which you would use with a center filter and see what happens, but in the end you will have to experiment to see what works best. If you don't increase exposure, you can to some extent deal with this matter by increasing the overall contrast in the digital image, but that may change the nature of the picture dramatically.

You can also do the same thing with color negatives, but it is harder because the gradient layer has to correct each color separately. I've had trouble doing that, and, although I reduce color shifts from center to corners, I haven't been able to eliminate them entirely.

If you shoot color slide film, I think you will find it extremely difficult to correct in a photo editor . Increasing exposure is not really an option because of limited latitude vis-a-vis color negative film. You will do much better with a center filter.

I use bw and color negative film, and in the end I broke down and got myself a center filter. It is much easier that way.

[z_photo]

i just did a quick example of what i said. perhaps i should have made it more blatant but here it is:
http://i164.photobucket.com/albums/u4/kzpictures/b.jpg

[QT Luong]

Here is a thread where I asked that question three years ago:
http://www.largeformatphotography.info/forum/showthread.php?t=12645

Since then, I've often used the Lens correction filter mentioned by Marko.

[Paul O]

Thanks Tuan - that was the article I was referring to! I'll give it a try! Thanks to all other suggestions too!