Using "cheap" filters ... then you wasted your money on that nice lens. Here's why!
Well ... if you do not know about the effects of these CHEAP optics, please understand that I do not mean to insult anyone with this posting, so grow a thick one and prepare yourself for ... information.
"Cheap" filters are notorious in their degradation of image quality on DSLRs and you can argue until the cows come home or until you are blue in the face, I am not backing down on this. Film lenses have been quite a bit more tolerant of poor filters. In fact, if you had a manual focus SLR ... your own eye could easily compensate for a lot the optical distortion these "low-cost" filters present and hide them. Unfortunately, the AF systems in a DSLR are for more critical of focus through the lens. The distorted plane of the "cheap" filter will cause the autofocus to actually malfunction, focusing incorrectly on the subject, to the tune of being "off" by several centimeters.
So the question always comes up: "I have a kit lens ... and I notice no obvious degradation using a cheaper filter on it." This is like the worst observation and makes the argument that much harder to justify, but here's where we get to the ugly truth to all of it.
Yes, this observation between the kit with and without the "alleged cheap" filter could be quite true ... and it would make my argument and personal findings look, well, untrue and contrived. Problem is ... they are not. It is because of the sloppy and poor quality of the "kit lens" construction that it can tolerate the addition of the poor performing filter on it w/o noticeable effect. You just cannot tell the difference, because the light is already so corrupted ... there "effectively" is no difference.
The bottom line to all this discussion is that if you finally decide to improve your "kit lens" to a better grade optic, say a 17-50mm f/2.8, a 28-75mm f/2.8 or even a 70-200mm f/2.8 ... it is with these improved lenses that you will realize this insidious flaw and it will begin to show its true colors and poor optical quality. Now, admittedly, you will have to buy a new CP-filter/UV-filter for these lenses, because they are of a much larger filter-diameter than the 18-70. But, for argument sake, let's just say when you take the filter off the "kit lens" and happily place it on your new BETTER optic (of the same filter-size), thinking you just saved a buck on a filter ... you just committed the biggest faux pas we all can face.
Please, bear with me on this. If you actually did do this ... you have just converted your BETTER and definitely more expensive lens to something no better than the "kit lens" you replaced and, perhaps, made it something worse! No kidding. My advice (seriously): Dump that crappy filter or give it away with that crappy "kit lens", when you replace it. I am pretty sure that you can probably bet that if you paid less than $70 for your CP-filter, it is a poor quality tool. Chances are: if the lens you just bought cost more than $400, you're just ruining your work and destroying the capability of the new optic. Just smile, buy the better filter and move on.
Personally speaking, I am committed to this expense (although I do resent having to do it) and it is something I have done for my 49mm, 52mm, 55mm, 58mm, 62mm, 67mm, 72mm, 77mm, 82mm, & 86mm filtered optics. I should add that I feel if I am going to make use of better glass ... it deserves the best reasonable filter I can put on it. Heck, my work deserves no less, because I am going to a lot of trouble to make it look good. I am simply not going to let some stupid filter ruin it on me, because "I cheaped out." Believe me, for any filter diameter larger than ø77mm, we are ALL spending over $100, easy. Consider that it is a built-in cost of using better glass. (Pleasse know, that there is no money in this for me for this suggestion to your photography. I'm only interested in getting the "bang for my buck" from the lenses I use (Excuse my analogy, but it is hard to have respect for a man wearing a $1000 suit ... and a $3 tie to set it off.). Ruining a lens' performance, that you paid perhaps a $1000 for, by slapping a cheap-filter on the front of it ... c'mon. Don't do it! ("Ooooooh, Don ... it is sooooo tempting! I saw a $30 CP-filter, just the other day.")
Here is a quick ø77mm-filter solution, THAT DOES WORK across the board. Just simply buy the following:
1) a good, high-end ø77mm CP-filter (Hoya, B+W, Heliopan)
2) a ø77mm lens cap (to protect your filter),
3) a ø77mm screw-on rubber lens hood (to provide some lens flair protection)
4) a Step-down filter-ring set ø77/72/67/62/58/55/52/49mm (<- this is the REAL money saver and optical preserver)
This is a SONY AF 50mm f/1.4 (55mm filter-ring) with two incremental step-down rings and the 77mm CP-filter
This is a TAMRON AF 28-300mm f/3.5-6.3 (62mm filter-ring) with one incremental step-down ring and the 77mm CP-filter
Please understand many photographers have gone through this drill (it takes some experience, because we all hate spending the money ... but, you just cannot avoid it when you move to a better grade lens ... you just mitigate it as best you can) but, this ø77mm-filter solution works and will until you get your collection of CPs for all your various lenses.
Please note that when you fit this ø77mm-filter onto your various filter diameter lenses, using the step down rings, the original lens shield and lens cap will not be usable with the filter attached, that's why you buy the extra ø77mm lens cap and ø77mm screw-on lens hood.
Okay ... have at it. I hope this helps everyone to understand the problem of crappy filters, some solutions and I wish you luck.
This ... could get you started.
Hoya | 77mm Screw-In Rubber Zoom Lens Hood for 35mm to | H77RH
Bower | 77mm PRO SNAP-ON LENS CAP | CS77 | B&H Photo Video
77mm-52mm 77-52 Step Down Filter Ring Stepping Adapter - eBay (item 380152139786 end time Nov-22-09 00:49:57 PST)