To start, I'd like to say that because I currently only own two filters (technically three, but I'll address this later) that doesn't mean I don't want more, these are simply the two 'types' which I feel are irreplaceable through post processing. And that brings me to my main point - and also the short version of this blog post - if it can be done in post processing without sacrifices to image quality then I would chose not to spend the hundred(s) of pounds on each piece of glass. One final comment is that this is specifically for photography, not video, use of filters in video is a very different topic!
Which filters do you own?
Well I'm glad you asked! I have a circular polariser and a 6-stop ND filter and I'll begin by talking about the circular polariser. It is a 'Cokin 77mm Slim Pure Harmonie Multi Coated Circular Polarizer' and although that quite a mouthful, basically it'll set you back about £100 RRP (can be found for less) and for the price has stunning results. Cokin are a well established brand and produce quality products; they are not the best and do produce budget filters along with their top level filters which can damage a brand name, but from my dealings with their products, the expensive ones are of top quality! The 6-stop ND filter is a 'Fomatt-Hitech 77mm Firecrest Neutral Density 1.8 filter' and this one will set you back £130 RRP but can once again be found for cheaper. It is extremely well reviewed, available in all filter thread sizes and at a variety of densities.
So, what does a circular polariser do?
A circular polariser can do a couple of things to an image, but primarily (for me at least) they are used to remove reflections. If you currently own no filters and have money burning a hole in your pocket that you want to spend on something photographic then this is only type which is truly 'necessary', i.e. the effect cannot be replicated in Photoshop/Lightroom. I have used an example below to illustrate the effect - on the right is the non-polarised image and on the left is the polarised one. There's a pretty stark difference and you probably see now why this filter is irreplaceable by post-processing. Sure, you could darken the water and colourise the stream to make it closer to the polarised version, but you'd never retrieve the data of the river bed.
If you are interested in finding out about the physics behind polarisation, then there are plenty of resources on the internet, but there is not enough space for me to cover it here, plus it would be like the blind leading the blind if I were to attempt an explanation...
Okay, so if the polariser is the only 'necessary' filter, why own an ND filter as well?
Well, ND filters are extremely close to being irreplaceable and personally I would say that they are, however, there is a way to replicate the effect that an ND filter has on an image (covered further down). An ND filter is used to lengthen the shutter speed. It is a uniformly dark piece of glass which blocks a given amount of light from passing through, depending on the density that you choose. A 10-stop is often the go-to ND filter as its affect is much larger than my own 6-stop is on a scene, but I prefer the flexibility of a 6-stop as if I need a little bit of a longer shutter speed then I can usually fiddle with camera settings to achieve it.
If you are wondering about the '1.8' in the name of my filter, this applies to the optical density of the glass and you can go >here< for further understanding.
I mentioned that these filters are somewhat replaceable in Photoshop and I'll quickly explain how. As a longer shutter speed records data over a longer period of time and so you see the smoothing of movement, in clouds for example, you can replicate this by shooting a timelapse and blending all the images together into one. There are times when this works perfectly but there are also times when it fails and this is why I would say that you still, with the current state of technology, need some form of ND filter.
What filters do you think aren't needed?
ND graduated filters are what I feel are not needed in any way. I am yet to come across a scene that I thought I could produce a stronger image by having an ND graduated filter. If an ND filter blocks a set amount of light from an image, then an ND graduated filter blocks the stated amount at the very top of the filter and slowly fades this out as you reach the bottom of the glass, thus allowing you to balance skies and foreground in one image. You can buy many variants of the graduation, some that have a hard edge in the centre or are inverted, thus starting at middle and fading out towards the edge of the glass. However; you could also compose a scene, expose for the sky, take a picture, then expose for the foreground and take another which can then be blended in seconds in Photoshop or Lightroom. So in essence both methods do the same jobs and I have no problem with anyone using either, however there is one way in which the post-processing method is far superior than in-camera method and that is truly custom blending/gradients.
To explain, take a look at this example....
I have used a red paint layer to show how each technique works and the results are quite obvious. On the right you can see what an ND grad filter would do, in comparison the image on the left is how I would blend the sky and foreground in Photoshop. If you photographing a flat horizon then this effect wouldn't matter, but as there are object sticking out above the horizon they will be affected by the graduation on an ND grad filter. However, if I shoot (as I did for this image) multiple exposures, then I can blend the sky and the foreground much more carefully, thus not affecting the rocks but also making it look natural. Feel free to tell me in the comments if you can see how my method didn't work and looks strange, but personally I feel it looks more natural than having the tops of the rocks dark...
So what filters do you want?
There are some I haven't covered and one which I have yet to mention. I do have one more filter, it's a variable ND filter and is used to affect shutter speeds whilst filming, if you're interested it is a cheap Zomei filter and I actually wouldn't recommend it! I needed a filter quickly and didn't have much money to spare and so I got this ~£40 piece of glass, which makes the image slightly softer and has a slightly 'brown-ish' colour cast. I didn't cover it fully as I just wanted to cover the filters I use for my photographic work in this article.
A category of filter which I missed was colour filters. I am not a black and white photographer by any meaning of the words. I have converted a couple of images to b+w and shot a couple for purposeful effect, however I much prefer colour images and, unless there is a purpose for it, feel that black and white takes out what I like most about nature - the vibrancy. However, if you are a black and white shooter then colour filters have their purposes - in essence they change how colours are represented in grey. I won't say much on this topic as I don't know enough, but this is great article if you want to look into it - www.photographymad.com
Of all the filters out there, the only one I'm still wanting is a 10-stop ND; the effect it has cannot be replicated by my 6-stop at times.
To conclude; polarising filters are irreplaceable, solid ND filters are extremely useful for interesting effect and ND graduated filters will help you balance the exposure between sky and foreground, but their effect is completely possible to replicate and is even surpassed in effectiveness by Photoshop/Lightroom.
Thanks for reading, hope you found it informative. Please feel free to jump into the comments if you disagree with anything I've written, its always interesting to know other people's standpoints!