Frequently-asked questions about choosing and handling filters

We answer seven important questions about filters.

How do I keep my filters clean?

Whatever kind of filters you use, remember that they are precision optics and must be treated with care. Most filters come with protective plastic boxes, and they should be stored in these boxes after use, not left loose in the bottom of your gadget bag. Protective filter wallets and cases are also available. Clean filters as you would a fine lens, using a lens brush or blower brush to remove loose dirt, a microfiber cloth to remove fingerprints and grease, and lens tissue moistened in lens-cleaning fluid for general cleaning.

Filters seem to come in all kinds of sizes. How do I know which to choose?

Simply look at your lens's specifications and you will find the filter ring size listed. That's the size you need. Got more than one lens with different sized filter rings? You can mount a filter that's larger than the front ring of your lens by using an inexpensive step-up ring to allow your lens to accept the larger filter. Example: a 62-to-72mm step-up ring will let you mount a 72mm filter on a lens having a 62mm threaded front ring.

I already use step-up rings, but when I shoot with filters this way, the corners of all of my images are dark. Why?

You may be using a step-down ring, which is a no-no: you cannot mount a smaller-diameter filter on a larger-diameter lens without cutting off the corners and/or edges of the image (i.e. vignetting). That's why you should always buy filters that fit your largest-diameter lenses, And you should also consider buying filters that are somewhat larger than your largest-diameter lens so they'll be able to fit any future lenses you may acquire. This is especially important with wide-angle lenses, which can sometimes vignette slightly even with filters that seem to fit. If you do lots of shooting with ultra-wide-angle lenses, check out the wide-angle filters offered by some makers, and consider using oversize filters.

What are the advantages of square filters?

If you do a lot of shooting with different cameras, including digital, 35mm and medium-format cameras consider using square plastic or acetate filters that slide into a special filter holder that attaches to your lens. Several different makers offer good ones. While not as durable as glass filters, these systems are used by many pros, perform very well, offer a wide variety of filters types, and provide maximum mounting flexibility.

Do I need to adjust my camera's auto exposure settings when I use a filter?

No. Today, the overwhelming majority of cameras feature through-the-lens autoexposure and autoflash systems so you no longer have to calculate of filter factors--that is, the increase in exposure required with various filters or combinations of filters. In most cases, the additional exposure (if any) will be calculated and set automatically by the camera, and you'll get a perfectly exposed shot. So long as you use the camera's built-in metering system, you will get the right exposure even when metering manually. If your exposure is slightly off (most common with stacked filters and very dark filters) you can easily lighten or darken it using your camera's exposure compensation control. Make sure to check the result carefully on your LCD, especially if your camera displays a histogram, which will tell you if exposure is accurate.

What about filters and auto white balance?

Depending on the nature of the filter you're using, auto white balance could be misled by your filter. If it is a neutral density filter or polarizer (which is effectively neutral density) then it won't affect white balance. But if there is any color in your filter--a split ND tobacco filter, for example--it's better to set white balance manually to avoid unwanted color shifts.

I own an all-manual camera. How do I make sure my filter exposure is correct?

If you're a traditionalist using a manual camera and a separate handheld exposure meter, just remember that filter factors are the same as f/stops, and are additive. For example a circular polarizing filter has a filter factor of 2, so to get the right exposure when using it, you open up two stops from the metered exposure. Add a .3 neutral density filter (filter factor of 1), and you have to open up 3 stops from the metered exposure. Simple!

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