How do I find the right size of filters for a lens?
I was looking to get a circular polarizing filter and an UV filter for my Canon 550D, 18-135mm lens. When I checked online I found these filters came in different sizes, like 62mm, 67mm, 72mm, 77mm. How do I know which is right?
Also, will there be any degradation of photo quality if I put both these filters on the lens?
Hi sfactor! No need to apologize for lack of knowledge. This is a Q/A site, so we're all hoping to learn. As for the degradation of quality issue, see this question: http://photo.stackexchange.com/questions/57/are-there-any-downsides-to-using-a-uv-filter
@mattdm +1 for kindness and sheer coherence. Anyway, while it must be appreciated that it offers many different points of view, I think the question you linked to shows the typical bias we were talking about in meta. @sfactor you will hardly need to use both filters at the same time, and you should avoid that.
As per other answers, it is usually marked with the ⌀ symbol on the front and, if not, on the barrel. Some specialty lenses do not accept filters, in which case you won't find any markings.
For your lens, the thread is 67mm
This is the thread size which means you can attach that size of filter directly. This convenient but costly. Instead, I buy my filters in the largest size (77mm usually) and have step-up rings to bridge the gap. A step-up ring costs about $12, so if you buy 77mm filters and have a 58mm, 67mm and 77mm lens, you need 2 step-up rings: 58->77 and 67->77. The only catch that you can't use a step-up ring and a lens hood at the same time. It saves lots of money considering a good polarizer costs over $200. Even if you have just two lenses with different filters you'll save. My lenses have 8 filter sizes so you can imaging how much money I saved on polarizers alone!
There will be a degradation in image quality if you use a filter. See my answer to this question. Generally, the less you pay, the more degradation there will be. UV filters are usually sold for protection but polarizers have a genuinely useful photographic purpose, attenuating glare, surface reflections and increasing color saturation of the sky and some other surfaces as a side-effect.
+1 for noting the use of step-down filters. BTW, a *great* polarizer doesn't have to cost $200 (although some do). In objective tests the Marumi filters tend to perform as well as the top-price ones but cost only 1/2 to 1/3 as much.
Although it should be ⌀ — http://www.fileformat.info/info/unicode/char/2300/index.htm :)
You can also get unwanted vignetting if you stack up multiple lenses!!! I found this out the hard way after taking around 50 photos with the UV filter and the Polarizer on. Had to go back and retake all the photos will just the Polarizer.
@JohnBubriski: Yes, and the same thing applies to filter + step-up ring combos. However, it's generally only an issue with wide angle lenses. Roughly 24mm and wider, I'd say. With a *really* wide angle lens, you may have to be careful to get un-stackable filters, the sort that only let you put a single filter on the lens at a time, because the extra set of *threads* can cause vignetting!