January/February 2008 HelpLine

This Article Features Photo Zoom

• Inadequate Compensation
• Learning To Polarize
• Is It Stabilized?

As I start this column, the first of the new year, I’d like to thank all of you who have submitted questions to HelpLine through the years—they have helped make this column a useful resource for our readers. Keep them coming!

Inadequate Compensation

Using exposure compensation is kind of like setting a toaster dial from light to medium to dark, where medium is the exposure that your camera’s metering system determined. Positive EV values will overexpose (lighten) the image, and negative values will underexpose (darken) it.

Q) I’m trying to figure out how the exposure compensation works on my digital SLR. It doesn’t seem to affect the metering. Am I using it the right way, or is there a problem with my camera?

Tim T.
Via the Internet

A) Exposure compensation doesn’t affect your camera’s metering. The light meter built into your camera is going to do one thing: measure the light coming into the camera. You can change how the meter reads the light by adjusting the metering method. Some cameras offer a center-weighted metering system that averages the entire scene but, as the name indicates, places emphasis on the center of the image. Some offer a spot-metering mode that allows you to pinpoint objects in the scene to measure. Most cameras default to a sophisticated metering system that takes into account the subject on which you’re focused.

But none of this has anything to do with exposure compensation. Exposure compensation adjusts how the camera reacts to the meter reading.  If you think that the camera meter reading is correct, then you leave exposure compensation alone.

Look at a test shot. If you think the image is underexposed, then you want the camera to “open up” the exposure a bit to let in more light. This could be done by a slower shutter speed or larger aperture. Underexposed images should be compensated for by a positive number and overexposed images by a negative number. These values actually represent exposure values (EV), but are more easily thought of in terms of “stops.” An increase of one stop would be like going from a 1?250 sec. shutter speed to 1?125 sec. For aperture, it would be the same as going from ƒ/11 to ƒ/8.

Once again, when you adjust exposure compensation, you aren’t changing the metering. You’re telling the camera to react differently to the meter reading. If you’re in aperture priority, the camera changes the shutter speed to adjust the amount of light that reaches the sensor during the exposure. Conversely, when you’re in shutter priority, the camera adjusts the aperture. If your camera’s program or auto mode allows for exposure compensation adjustment, whether the shutter speed or aperture (or both) is adjusted by exposure compensation is dependent on the camera. But when you’re in manual mode, exposure compensation doesn’t fit into the exposure equation—you’re already explicitly setting both the shutter speed and the aperture.

Learning To Polarize

Q) I’m finally getting around to getting a polarizer for my camera. I’ve read your explanation about circular polarizers in a previous HelpLine, so I know what to get. Now I would like to learn more about using them.

D. Franklin
Via the Internet

A) Polarizing filters can be used in many situations. You can use them to remove some glare and reflections in your scenes, but you can also use them to increase contrast and saturation in skies. It’s just a matter of mounting the filter on the lens, rotating the ring and looking in the viewfinder to see which reflections are reduced. But before I talk about specific uses, I have two suggestions to prevent damage to the filter.

First, identify the part of the filter that turns and get comfortable adjusting it. If you look at the edge of a polarizing filter, you’ll find that there’s a part of the “ring” that freely turns and a part that’s stationary. The stationary section is used to screw the filter on the lens, while the free-turning section is used to adjust the effect. Since you’ll often adjust the polarizer while looking through the viewfinder, it’s important that you turn the right thing. If you continually reach for the stationary part, you could end up screwing the filter on so tight that it’s difficult to get it off the lens.

Second, it’s a good idea to turn the filter in the same direction as you’d turn it to mount the filter on the lens. (This would be clockwise as viewed from the front of the camera and counterclockwise when your eye is up to the viewfinder.) Getting into this habit helps to keep the filter securely mounted on the lens. If you were to turn the filter in the “loosening” direction, there’s a chance that eventually the filter would unscrew and fall off the lens.

Now, on to using a polarizer in your photography. The typical application for polarizers is for outdoor landscape shots with a blue sky and some clouds. With a polarizer, you can increase the saturation of the sky to create a deep blue that will make the clouds pop out. To achieve the greatest effect with the polarizer, shoot at right angles to the sun in order to take advantage of the polarizer’s ability to block light that’s in a different “phase.” Too often I’ve run across people trying to use a polarizer to deepen skies while shooting into the sun. Then they wonder why the polarizer isn’t working.

When shooting skies, you should consider how strong an effect you want. I see images all the time that shout out “polarizer” because the sky is an unnatural blue. Obviously, that’s a creative call on your part, but it’s a good thing to keep in mind when you’re “dialing in” the polarizing effect.

If you shoot panoramas (multiple shots that you stitch together later in the computer), you’ll have to forgo the polarizer. Since the effect of the filter is dependent on the angle to the light source, as the camera is rotated, the sky will be affected differently with each shot. This makes it difficult to achieve smooth seams between images when you stitch them together.

A polarizer can be useful for water shots where there are a lot of reflections that dimi
nish your images. Once again, the polarizer is dependent on the angle of reflection. Although it can be hard to predict its effectiveness, that shouldn’t make you think twice about trying the polarizer.

If you just want to quickly check how well the polarizer will work in a composition, you don’t have to mount it on the camera. Just hold up the filter to your eye and look through it while turning it back and forth to see the effect. Watch out if you’re wearing polarized sunglasses because the combination of two polarizing filters can black out the whole image or half of the image. (Actually, polarized sunglasses can be useful for previewing filter effects. Just rotate your head back and forth to see the effect.)

A polarizer isn’t just for outdoors. If you’re shooting shiny objects with reflections, or even objects that you don’t perceive to be shiny, a polarizer can help minimize the glare that reduces detail and contrast in your image.

My final tip about when to use a polarizer has nothing to do with glare or reflections. In a pinch, a polarizer can be used as a neutral-density filter to reduce the amount of light coming into the camera.

Is It Stabilized?

Q) I just bought a lens that has built-in vibration reduction. Recently, I was at a friend’s house, and they had a little compact camera that they said also had some sort of vibration reduction. Is it the same as what I have in my camera?

Bob D.
Santa Fe, New Mexico

A) Your lens and your friend’s camera could indeed use a similar technology. Vibration reduction is just one term that refers to technology used to compensate for inadvertent camera motion. Other terms that different manufacturers use include image stabilization or shake reduction. The basic concept is to measure how much the camera is moving and then move either an element in the lens or the image sensor to cancel out the camera movement.

You might have noticed that I said it “could” be similar technology. This is because there are a few compact cameras out there that are said to have image-stabilization capability, but in fact don’t employ any mechanism to counteract movement like that used in your lens. Instead, this feature simply boosts the ISO speed setting, allowing the camera to use a faster shutter speed. This isn’t true image stabilization in the way the term is commonly used.

If you have questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or [email protected].  Visit our website at www.pcphotomag.com for the web-exclusive HelpLine Weekly and past HelpLine columns.


Leave a Comment