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A friend of mine wants to buy a warm filter for her Canon 450D.
I'm wondering if there's any real benefit there, as the color casting can be done in post.
Also, for people more familiar with the 450D than me, can't the same effect be done by tweaking the WB or some other parameters?
For more context, she's a casual photographer trying to get better at it. Not sure why she wants the warm filter though.
Some, yes. Most of the time filters are not needed, but sometimes they can make a visible difference. The reason why color correction filter can make the difference is that on digital cameras color correction is done after the image has been captured to the sensor, and often after it has already been digitized.
If the color correction is done before A/D conversion, not using color-correction filter means you get more noise to the channels that were compensated in color correction. So, for example, if you took your shot in very orange street light, your blue channel will go only to some 25-50% of the maximum value. This in turn means that when the blue channel is amplified to correct white balance, the sensor noise on that channel gets amplified 2-4 times more than it would have been if you had used proper filter.
Not all noise is amplified even if the white balance is corrected after recording the image to sensor, but it goes beyond scope of this question. See this question to learn more: What is noise in a digital photograph?
If the color correction is done after A/D conversion all the above is true, but in addition you can get some banding to the image. For example, your blue channel values might now be mapped from 0 to 63 instead of 0-255, and this when it's multiplied to balance colors, you get on average three "unused" values between each actual value. This shows as banding, ie. gradients are not smooth, but color changes in steps.
Using the filter obviously reduces the amount of light hitting your sensor, so you need longer exposure time to compensate. So it can be a tricky trade-off sometimes. Also, if you are shooting with ISO level where your camera has lot of S/N headroom, it might not be worth the effort.
However, if the point is to skew the colors, ie. get artistic results by coloring the scene differently, this can as well be done on computer at post processing stage. During capture it's often best to aim to capture the scene as well as you can, ie. capture all the information you can get as accurately as you can. This then enables you to choose, and change your mind, later about how to distort and skew the image for artistic effects. You can of course get make the effect all with filters, then you just lose the ability to change your mind later.
If your aim is to get technically as good results as you can, color correction filters can make sense. If your aim is to keep the art of analog photography alive, then color-changing filters can make sense. And these apply to lenses; for flashes the rules are all different.
So if your aim is to learn and revive the secrets of old school photography, go for it; just do it knowing you can lose information that way. So, on shots that you can't easily retake trying to get the information as cleanly as possibly to the memory card and applying artistic touches later on often makes more sense.
Your hunch is right: colour cast filters aren't very much use on digital. Many people have a fondness for them from their days using film, but nowadays it's very easy to apply the same effects either by using a warmer white balance setting (e.g. select cloudy on a sunny day - any DSLR will let you do this) or in post-processing (using anything from iPhoto/Picasa at the consumer end up to Lightroom/Aperture/Photoshop at the top end).
My recommendation would be to do it in post rather than tweaking white balance settings during the shoot: it gives you a bit more creative leeway after the fact which can be handy. Also you won't always have a WB setting that's a bit warmer than the current light (e.g. if it's already cloudy).
The only likely use you'd have for these filters on a DSLR is where no post-processing is going to take place. This might be either because the user is unfamiliar with photography software and prefers to take their photos just as they come ("straight out of camera"), or at the other end of the scale in a professional shooting environment where photos are transmitted straight to an editor (e.g. fast-breaking news reporting).
In either of those situations, you'd need to make sure the camera's white balance setting is set to the current light conditions: if you use automatic white balance the camera will just cancel out the filter's colour cast.
Instead of correcting the white balance in Photoshop or with a filter, I prefer to use the custom white balance setting, along with a gray card or suitable substitute. (Technique)
In theory, it is better to correct a white balance problem with filters, before capture. My experience of filters, though, is that they're never exactly right for the situation you find yourself in. Maybe studio photographers can find exactly the right filter for their light setup, but for those of us working with available light, filters are more of a pain than they're worth. The beauty of working with custom white balance settings and a gray card is that you can make whatever correction is needed at the time, no matter the color of light you're working with.
If you accept that, and resign yourself to white balance correction after capture, you might wonder why bother with white balance settings in the camera. Why not just make the whole correction in your imaging application? (Photoshop, Lightroom, Aperture, whatever.)
With raw files, the white balance setting merely writes some metadata into the file; it doesn't change how the sensor captures the image. Nevertheless, my experience is that it is easier to dial in final white balance correction if the image arrives in my imaging application close to correct to begin with. The larger the move required to correct the white balance, the harder it is to get it exactly right. For whatever reason, my camera's ability to analyze a gray card image is outstrips my ability to make the same moves manually. I, on the other hand, am better at making the final tweaks to white balance by eye than my camera is. I think we make a good team.
If you're shooting in raw, and using the right software (one that processes in high bit-depth) then it's unlikely using a warm filter will produce a better photo. As long as you capture the photo as well as you can, setting the white balance on your raw files in post is going to be a much easier solution.
After all, raw data has no white balance.
