If you are looking for information on effect filters (stars, rainbows, etc...): sorry, I hardly ever use them.
It's not only that I find using such "enhancements" slightly unethical - I also tend to feel that pictures that rely on such techniques are pretty boring.
Special effects like star-filters create pretty patterns and can be pretty exciting at first.
But use them ten times during one holiday trip and you’ll already feel tired of them.
My recommendation: try them out, but always take a version without such a filter. Chances are this will be the one you keep in the end!
If a scene cannot stand on its own, it's usually not worth shooting and if it is, the added bells and whistles may spoil it.
Having said that, I came across a number of great landscape shots that used graduated colour filters to enhance a given atmosphere.
I have used a "tobacco", "sepia" or "blue" filter myself - in very special situations.
I often was not really satisfied with the results the blue filter gave me -again, if you try it, make sure to take a shot without the filter as well. However, there are some filters that I always carry in my photo bag:
1.) neutral density filters can be a great help. They are supposed not to change the colours, just reduce the light.
There are different types of such filters: either the whole filter is grey or it's graduated, which means it's much darker at the top and clear at the bottom.
There are many filter systems (like Cokin) which permit you to use a big rectangular filter and bring it exactly into the right position. With a neutral density grey filter, this means that you can make the brightest part (in most cases the sky) darker, which automatically means that the dark parts will be exposed longer. A great way to deal with subjects that would otherwise be impossible to handle.
For example: a dark gorge under a bright sky has too much contrast to be photographed without a filter...
Also, a neutral grey filter can be used to prolong exposure time, for example to give a special effect with running water or to "empty" a public space of people.
2.) a polarising filter. I have one for each lens in my bag. It can enhance colours, bring out the clouds in the sky, eliminate or reduce reflections in a window - or even accentuate them. A (real) rainbow may appear much more saturated - or disappear.
It also helps against UV rays.
3.) a UV filter - you need it especially in high mountains and at the seaside to get rid of the blueish hue and blur brought about by intensive UV light.
4.) a "daylight" filter. A good protection for your lens making the image appear a bit "warmer" especially at noon.
5.) warming filters. If there’s much sky in your picture or your lens has a tendency to produce “cool” images, then a warming filter may be helpful.
6.) conversion filters may help you shoot "natural" images when using daylight film with artificial lighting.
7.) colour filters may (sometimes) help on rainy days and are almost indispensable for black and white photos, - but as my expertise with this medium is rather limited, I leave this to someone else.
In any case: a filter is a bit of glass (or plastic) between your lens and the object. This means that it will influence the "quality" of the lens.
Lens makers use very special glass and coating when constructing a high-quality lens. The exact distance between all those elements has, - of course -, been optimised, extensive tests have been carried out. Now you add some sort of inferior window pane to the finished product... What can you expect?
I would very much advise to rather buy only a few filters, forget all the possible "effects" - and rather invest in high quality that doesn't ruin your images.
Also, I'd rather buy a "medium-class lens" with a good filter than combine a top of the range lens with a bad filter...
Cheap filters will also add reflections. This effect is, of course, most pronounced when combining filters.
Another warning: especially with strong wide-angle lenses (or the wide-angle position of your zoom) you may actually "shoot" your own filter or at least create a shadow in the corners. This is esp. true for polarizing filters. There are special ultra-flat filters, for this application, which cost dearly, - but then, why invest in a sophisticated lens, when all you get are "pinhole" pictures...
And: even good filters cannot rescue a badly composed picture. You can take a great photo with a cheap camera and no filters at all when the conditions are right and the subject is worth it.
If it isn't, the best equipment will not produce an outstanding picture.
However, the limitations of the medium (e.g. contrast range of film) make it technically impossible to shoot some photos without a filter.
BTW: when it comes to "effect" filters: nowadays, computer software (e.g. Photoshop, Paintshop Pro,...) enables you to try out multiple filters on a digital or scanned photo. You save a lot of money, don't have to carry around all those filters, do not have to annoy the other participants in your group by endless fiddling with your equipment and can reverse any effect without throwing away valuable film...
It’s possible to combine all sorts of techniques and selectively apply them to only small parts of an image using the Photoshop or Paintshop Pro layer functions.
Whether it’s “ethical” to do so - it’s up to you to decide. At least you always have the unaltered original.
In any case: the pictures on these pages have not been digitally “enhanced”, although some have benefited from a bit of “cropping” as compared to the original slide.