What filters can't be simulated in photoshop?

[aaron_good]

Ok,

So I have been trying hard in Photoshop to simulate various filters

that would be something that could be done (and that I have done) by

placing a physical filter over the lens of a film camera. <BR>

<BR>

I seem to be able to approximate most of them, but there is really

only one filter I can think of that can't be easily reproduced in

photoshop: the polarizer.<BR>

<BR>

Is that correct? Can anyone think of any other filters that can't be

approximated in Photoshop? Ultimately I am asking as I shoot mostly

in digital these days and am wondering what I'm missing as far as

altering the image BEFORE it gets to the CCD.

-Aaron

[lwg]

Infrared.

[joseph_wisniewski]

The gradient neutral density filter, when it's used to keep the sky from actually blowing out (overloading) in the digital picture. Once somethign is blown out, it's gone. Just like with slides.

 

The enhancing filter. This works by altering metamerism. Two objects that appear the same color without the filter may appear different colors with it. You cannot duplicate that in PhotoShop.

 

The infrared filter. There are lots of PhotoShop actions floating around that can produce some IR effects, but they can't really reproduce IR translucent skin and hair in a portrait, IR haze cutting ability, or the differences you see between different kinds of vegetation under IR.

 

The UV filter, when it's holding back haze on a UV sensitive camera. Not all cameras are sufficiently UV sensitive for this to be a problem.

 

Here's a series of posts I wrote on filters for digital cameras.

 

http://forums.dpreview.com/forums/read.asp?forum=1021&message=3758445

 

Ciao!

 

Joe

[gordonr]

A digital graduated filter will only work if you have sufficient dynamic range in your sensor, and store the images as RAW or 16-bit Tiff. An 8-bit Jpeg does not have sufficient <a href="http://www.photo.net/learn/drange/">Dynamic Range</a>.

[johnlund]

<p>There's no substitute for a polarizer when it comes to cutting glare. I'm talking about looking into the water, not enhancing the color of the sky.</p>

 

<p>You can mimic some of the <a href="http://forums.dpreview.com/forums/read.asp?forum=1021&message=3758445">IR effect</a> even though it's not the same. For ND grad you can stitch two photos together (same scene, different exposure), and adjust -- even along a jagged line.</p>

[Troll]

My box from Adobe just arrived, and sits as yet unopened on the hall table. Is it part of PS, or is there a plug-in, which will allow me to easily conver an image to B&W as seen through a particular filter. (Sort like "click on convert to B&W with #15 filter")? Or am I going to have to learn about layers and curves, etc, etc, etc?

[richard_cochran]

You can't <em>precisely</em> reproduce any filter

that has a sharp spectral cutoff, though you can

sometimes approximate some of them. For example,

a skyglow filter used by astronomers blocks the

specific spectral lines emitted by mercury and

sodium vapor lamps, while passing the spectral lines

emitted by glowing hydrogen nebulae. Astronomers

love these things because they're very effective at

cutting out light pollution

without cutting out the objects being observed.

See <a href="http://www.sciastro.net/portia/advice/filters.htm">

this link</a> for a few more details.

<p>

Once an image is captured digitally, all you have

are the relative levels of R, G, and B. You've

lost the detailed spectral makeup of the various

light sources, so you can't filter out specific

wavelengths while passing other nearby wavelengths.

<p>

The UV and IR filters already mentioned are

special cases of filters with sharp spectral

cutoffs. Even more garden variety colored filters

used for B&W photography often have fairly sharp

cutoffs, and if the spectrum of the object being

photographed isn't smooth and continuous, filtering

digitally post-exposure won't give precisely the

same effect as filtering at exposure time, even though

it will often be close enough for photographic purposes.

[richard_cochran]

Oh, almost forgot. You can't reproduce the crosstar filter. It

creates spikes around specular light sources that are brighter

than your CCD (or film) can register, and the lengths of the spikes

are related to the brightness of the highlight. When said brightness

wasn't captured at exposure time, you can't recreate it afterwards.

<p>

Nor can you reproduce the diffraction grating. It

separates bright light sources into the spectral colors.

<p>

Both these filters are a bit ugly, IMO, so perhaps it's not

such a bad thing that they can't be reproduced digitally.

[qdb]

A graduated ND filter cannot be duplicated in a single shot in Photoshop, but you can take two shots of the same scene with different exposures, and then blend them in Photoshop (easiest with two tripod mounted digital shots). You then have a range of possibilities that far exceed the limitations of an ND grad filter.

[michelle_cox]

Bill -

<p>

I don't know about PhotoShop as I don't have that kind of money. :) But I've got a fun little shareware program that does just that. It has a filter simulator and you pick a filter from a drop down list and it adjusts the photo to look like it was taken with that filter.

<p>

You can find the program here: <a href="http://www.mediachance.com/pbrush/">http://www.mediachance.com/pbrush/</a>

[roger krueger]

Even simple color filters can't be replicated exactly if there's a situation in the scene where the color filter reduces a burnt-out highlight to something with detail or causes you to increase exposure so that you do record shadow detail you would have otherwise lost it. (The latter being the reason tungsten images shot with an 80A or tungsten film look so much better than images shot straight and corrected later.)

[john_luke]

I thought I heard that using a polarizer on a digital camera can be dicey because it is basically a monochromatic chip and the color is interpolated by an array with microscopic R, G, & B filters on top of the chip and use of a polarizer can sort of wreck that interpolation.

[Mike D]

I was about to download a demo of Nik Color Efex Pro until I read this posting. Nik claims that their Photoshop plugin can digitally duplicate most photographic filters including grads and polarizers. Some posters say this is impossible. Has anyone used the plugin and is it worth spedning the time and hard disk space to give it a try?

 

http://www.nikmultimedia.com/usa/products/maincontent/all_products/products.shtml#colorefexprod

[johnlund]

Michael Dougherty - The Nik Color Efex Pro "polarization" effect is for simulating the color enhancement properties of polarizers ("Create crisp blue skies..."), not for cutting through glare. Digital alternatives to ND grad are mentioned earlier in this thread.

[roger krueger]

John Luke: CIRCULAR polarizers work fine on any digital I've seen, but, like most AF cameras, simpler linear polarizers cause trouble.