This last of a series of what glass you will need. Now all lenses are through, it is just left what we can put in front to them.
If, why …
First screw off the UV/protection filter and tuck it deep into your bag for the visit of a steel work in full production. Filters degrade IQ. In daylight it is less visible, but once you botched some night shot in the winter where you stood freezing at -10°C, because all exposures caught an inexplicable flare you know better.
If you are really concerned, get a high quality filter from Tiffen, B+W, Heliopan or Hoya (Hoya sells different grade, do yourself a favor and buy the most expensive one). UV is not an issue, clear filters are what you want. Prepare to spend €50 for a 77mm filter.
Modern glass and coatings are hard to scratch and easy to clean, but when sparks are flying around or you are in adverse conditions where you will have to wipe the lens often they can save your lens and your shooting.
This is the main motive with filters: They serve a purpose. If there isn’t any, screw it off!
I think there are essential filters, advanced filters and special filters. You will work yourself down this stack over the years.
Essential - Polarizer
The only filter you can’t imitate in post-processing. It suppresses unwanted reflections or enhaces reflection of non-metallic surfaces:
This works best if the angle is ~53-56°. It as well can improve the blue of the sky and plants - this will work best if the sun is roughly on your side. A polarizer will by principle swallow half of the light (=1 stop) and the technical limits cost a third to half a stop on top of that. Don’t worry about it - the two exposures merge into the image above were more than two stops apart. Some cameras tend to meter incorrectly through a polarizer, you can’t correct this through bias as this might vary with the orientation of the polarizer
Essential - NDs
There is no ISO25 setting on your camera, it is bright daylight and shooting in A-mode wide open makes “Hi” blinking up in the viewfinder - this doesn’t mean that you camera gets nice with you, it just means: “I can’t do 1/16,000s”. Throw on an ND4 and you are at 1/4,000s so that you can profit from a shallow DOF. When shooting with flash outside you will encounter a similar problem already at speed slow as 1/250s, the so-called sync speed. With modern speedlights you can go faster, but you loose a lot of power. Flashes work most efficiently around sync speed, so adding an ND (aka grey filter) will help. A ND4 (=2 stop filter) that lengthens the exposure by a factor 4 will do the job in almost any situation, for 1.4 lenses you might consider an ND8 or stack your polarizer on it (which doubles as a ND2.5) 2.5×4 =10 voila, you just made an ND10 from what you have.
Shooting flowing water looks best at slower shutter speeds (>1/60s), a ND will help you here without stopping down to f/32.
Advanced - GNDs
GNDs or graduated ND allow you to block light in the scene selectively. Screw in GNDs are rarely useful because they limit the composition. Filter systems like Cokin and Lee allow you better control, but still you need different graduations and strengths. You can combine even several GNDs and there are colored ones as well …. you get the idea. It is almost impossible to get right and if the line where you want to split is not straight through the frame you compromise somewhere. Do it in the darkroom or in post-processing by shooting a highlight and a shadow frame and merge these - You can use HDR software if you can’t help it. Cheaper and better, but if you really insist….
Ah, did I mention they don’t work on long lenses and the graduating depends on the aperture?
Advanced - CCs
Color Correction filter require that you can create a profile for them (something like ColorChecker). The idea is that you compensate the skewed spectrum of your light source so that your can efficiently use the dynamic range of all RGB channels of you sensor. Caveat: You loose light for gaining dynamics in the reds or the blue of the spectrum. Tungsten and Fluo filters are from film and the Tungsten filter is still welcome, but the Fluo is now indispensable!
Why? Fluo filters are magenta colored glass (when shooting in fluorescent light, add a a green gel to your flash). We also have more fluorescent light in out homes which appears to be “whiter”, but actually still is greenish. To make things worse, the camera sensors are a full stop more sensitive to green than to red and blue. In fact it is even in daylight a good idea to use such a filter and correct by a profile afterwards. What a Fluo filter does is blocking 1 stop of greens, this will even out the sensitivity curve, thus full dynamic range in all channels, but in fluorescent light where greens are over-represented (actually some reds and blues are missing) the situation is worse. It can be corrected via the white balance, but the waste in dynamic range (or the clipping in the greens) stays.
Special - Colors
The same idea like with CCs, but when you either go for selected colors or work with colored light. In most cases you want to block one color by choosing the complementary (hue +/- 180°) color. You still preserve details in that color, but you will gain luminescence in the others.
Special - Hi NDs
ND400, ND1000 - these are for long exposures - several seconds in bright sunlight. Don’t stack these. Don’t use them for shooting the sun or solar eclipses! They are not safe for your eyes! For this you need a certified astro-filter or Shade 13+ welding glass. For black&white welding glass (€2 a piece) glued on a step-up ring give you a ND4000 - ND16000 - strong green tint, but correctable up to Shade 12 (for stringer shades glue a CCM50 gel on the read side - don’t use screw in Fluo, this creates flares.
Special - IR/UV
Infrared photography is severely hampered by almost all cameras having an IR filter built in. The Leica M8 hadn’t and that caused strange color casts. Some have the IR filter removed from their cameras sensor. I wouldn’t recommend such a thing unless you have an old DSLR to spare. I don’t like the results, they just look weird.
UV photography is another thing, because it reveals invisible structures, but only selected lenses have enough UV transmission, so be prepared for an adventurous journey.
Obsolete - All other
Everything else can be done in Photoshop, only better, with no upfront investment and the chance to experiment for free.