seeking advise on filters

I am currently using a Canon 7D with a sigma DG 120-400mm, 1:4.5-5.6 APO/HSM lens, the lens filters I am using are both Zeikos, Hi Def, Professional, Digital/multicoated filters...the UV filter I leave on all the time and the circular polarizer I use in bright sunshine and when taking photos of birds on water. Without any filter at all, I get great crisp photos with good to great detail. With the uv filter, some minor, although noticable loss of crispness and with the polarizer, the camera focuses poorly and many pics are unusable. How can I address this problem? Is taking pics with an unprotected lens common practice or a no-no? Of course, all my eqpt is impeccably clean.

If your UV filter is downgrading lens performance, and thus image quality...don't use it.

If AF doesn't work well with the polarizer, switch over to manual focus This assumes you have eliminated other causes of the lack of sharpness, e.g camera or subject movement, a defective polarizer, or an issue with the AF functionality of the lens.

--P

Are you using the polarizer in low light levels? If so, does the same thing occur in bright light?

This is just my guess, but I think taking photos with an "unprotected" lens is at least as common as having a UV filter on the lens. Use the lens hood at all possible times, and keep the caps on when not in use. I, personally, don't use UV filters.

The focusing issue is because when you add a polarizer you cut the light by at least two stops, most camera-lens combinations need a minimum of f/8 and some even f5.6 to auto focus, so when you add a filter to your lens, which is 5.6, it becomes f11, not enough light for you auto focus.

You shouldn't notice any image loss with a high quality UV filter. Try a B+W MCR or Heliopan. However, I don't use them unless its raining, snowing or really windy.

Carl (and Joseph) - Auto-focus is only actually disabled in the 7D when the camera knows that it has something less than a f/5.6 lens attached (an f/5.6 lens with a teleconverter, for instance). In this case, while the polarizer does cut the light, the camera doesn't know that, so it should still try to autofocus. It will certainly be worse at it and slower though. In decent-to-good light, it should still be acceptable.

That's why I asked what the light was like and if this happened in very bright light as well. If it doesn't happen in bright conditions, then I think it's just an issue with the amount of light the auto-focus sensor is seeing.

I don't cover my lens unless I feel it will improve the image or in extreme cases protects it example would be a shooting in a sandstorm. Most specs, fingerprints ect will clean and an impact directly to the front will push broken filter glass into you lens. As mentioned caps and lens shades offer protection.
For the polarizer I agree with the others.

I just want to add a thought on the polarizer. Are you sure it's a circular polarizer and not a linear polarizer. Most modern polarizers are circular but linear polarizers are still available. Linear polarizers cause problems for auto-focus cameras. Double-check yours.

Cheers,
Mark

On Mark's comment - hold your polarizer up to a mirror, oriented the same way as you would if it was on your lens. Make sure the room is well it. The order is you, the back of the polarizer (which would be next to the lens), the front of the polarizer, the mirror. If it is black, it is circular, if there clearly is light coming through, it is linear.

Also, if is was cheap, it was probably linear. A circular polarizer costs in the neighborhood of twice as much as a linear polarizer.

I think the problems are probably due to the quality of your filters. If you put a $40 filter on a $1000 dollar lens, you'll get the lowest common denominator. I use B+W filters ; Other brands are also good, but I don't know about Zeikos. I don't use any filter on a daily basis. I just treat my lenses like precious cargo, often use a hood in case I have a moment of carelessness. I did once, at night, put my 100-400L down lens first on a gravel driveway (I thought I had put the lens cap on) when changing lenses. Smacked myself over that for weeks even though it did not appear to get scratched.

When conditions make it necessary I use a filter. Otherwise I shoot without them. When a filter is necessary I use B+W glass and it seems to work fine.

Thank you so much for your responses. I think that, given the lens that I am using, shooting without a filter whenever possible and using the hood will best suit the capabilities of the canon 7D.

I don't use UV filters either unless in very bad conditions, such as at the bottom of a large waterfall (spray contains grit.) I keep my lens cap on when not actually taking a shot, and also use a lenshood. My lenses are perfect.


Kent in SD