One does not have to be a landscape photographer for very long before the question arises “ Would a filter make my photos better “. ( Along with the questions about better lense, newer camera with more pixels, sturdier tripod, etc.) But let's just address filters today. ( Lens is singular, Lense is plural, I had to look it up )
The function of any filter is to change or adjust the light coming into your camera and striking the internal sensor. The common filters used in landscape photography are a UV ( Sky ) filter, a polarizing filter, a graduated neutral density ( ND )filter, an un-graduated ND filter, and occasionally a colored filter. There are other special effect filters but they are not commonly used in landscape photograph.
When you go to buy a filter, you must read the size on the front of your lens; 58 mm, 77 mm etc. You can get adapters for your filters to fit different size lense, but it is really less fuss just to buy the right size, in my humble opinion.
There are 2 physical types of filters in general photography; a circular filter composed of an outer metal ( brass or aluminum ) frame with threads holding a round piece of glass; this screws onto the front of your lens, and a square type comprised of a square piece of optical resin or glass that slides into a holder which also screws onto the front of your lens.
Filters vary greatly in cost, the un-coated resin being cheapest and the multi-coated glass being the spendiest. The coatings reduce glare and flares in your photos.
The circular filters are often the UV, polarizing, a variable form of the ND filter, and a fixed value ND filter. The square filters, often called Cokin filters because they were one of the first to produce them, are polarizing, graduated ND, ND, and colored filters.
Per Photography Life, the purpose of a UV filter today is to simply protect the front element of a lens. In the past, these filters were used to block UV from hitting the film. All digital camera sensors have a UV/IR filter in front of the sensor, so there is no more need to use UV filters on DSLRs. Think of UV filters as cheap insurance for your lens; I have a UV filter that saved my new $ 700 dollar lens. I slipped while descending a wet muddy trail and the front edge of my lens lightly hit a rock. I always keep a UV filter on my camera lens now and take it off only when I am set up to take a shot.
Polarizing filters do 2 things; they can make your skies darker blue, and reduce glare and reflection off water. The caveat with polarizers is they should be used when the light is at a 90 degree angle to you (coming from your side, not from front or back of you ). And when taking a panoramic shot that you will several photos stitched together in your software, they can cause vertical lines where the frames meet. So don’t use polarizers for panos.
A variable ND, or VND filter, cuts down the amount getting through the filter in a variable fashion. Both polarizing and VND filters are comprised of 2 halves, the back half screwed onto the lens, and the front half can be rotated to increase or decrease the effect of the filter. VND are rated by “ stops “, F-stops, often up to 8 stops. They are used to prolong exposures. I think of them as blurry water filters; I use them to make streams and ocean waves blur for effect.
ND filters come in both the square ( Cokin style ) filters and also a circular NON-adjustable style; they are often rated as ND2, ND4, ND8, ND 16 to indicate their darkness. They are uniformly dark from edge to edge. ND filters are also used to reduce the amount of light reaching the sensor consequently prolonging your exposure. They also are used for creating blurry water.
Graduated ND filters are as the name suggests. The upper edge will be the darkest and then fades to lighter and eventually clear at the lowest edge. The demarcation line where the darkness lightens can be a hard edge, very abrupt, or feathered with a soft edge. These graduated filters come in a variety of darkness, often designated as .3,.6, and .9. They can also be designated as S,M,L for strong, medium and light.
GND filters are used to balance the exposure of a scene; for example a mountain scene at very early sunrise will have the mountain peaks very well lit while the foreground in the shadows will be very dark. You slide the graduated filter into the holder with the dark portion at the top; you slide it down into the holder far enough to darken the mountain peaks and equalize the exposure between the foreground and the mountain peaks. This way the peaks aren’t blown out, over-exposed, and the foreground isn’t so dark that you can’t make out the details. You can adjust the filter up and down so the demarcation line falls where you want it in the scene.
Just to mention them, there are several other types of filters you may come across which aren’t often used in landscape photography but which you should be aware of. These are special effect filters, color filters, and close-up filters. Special effect filters are often “ star effect “ filters; these give a star appearance to any light source such as a street light etc. Color filters do as the name suggests, they give a color cast to your photo. They are available in a rainbow of colors. And lastly, the close-up filters that act like magnifying glasses, and are used in macro photography ( such as close ups of flowers ). They are designated as +1, +2, and +4, and are round screw on type. These are a viable option if you don’t want the expense of a macro lens.
I hope this Part 1 of my filters blog has given you a basic understanding of filters; in Part 2 of this blog I will try to explain how in Adobe Lightroom Classic CC photo processing software, you can create many of the effects of a filter in LR after you have taken the shot.