Ask a National Geographic Photographer

Q: How can I calibrate my monitor for digital photos? My monitor is an HP model; the operating system is Windows XP-home, and I use Photoshop CS to retouch or correct the photos.

A: This is a very complicated subject which I think is beyond the scope of this article. I have no experience calibrating monitors, so I'm afraid I'm not much help with this one. Here is an article that describes in detail the process. It involves purchasing one of many calibration tools: http://www.imaging-resource.com/ARTS/MONCAL/CALIBRATE.HTM

Question submitted by Sujit in Kolkata, India

Q: What is the best way to bracket and blend if, for example, I need a different exposure for my sky than I do for my foreground? Is it easier, or more appealing, to use a graduated neutral density filter for this purpose?

A: In situations where the sky is much brighter than the foreground, a graduated neutral density filter can be an option, but it's not a perfect solution. These filters are dark at the top and graduate to lighter at the bottom and are most often used for shooting landscapes. When positioned correctly in front of your lens, it will darken a bright sky while keeping detail in the lower part of your photo, which usually contains the foreground.

There are a few problems with this. First, your composition needs a straight, unbroken horizon line to disguise the graduated filter. If, say, a person is breaking the horizon line, then the part above will be noticeably darker than the bottom. Second, if you are moving around while shooting, you'll need to constantly adjust the filter to line up with the horizon line.

Another option is to shoot at another time of day when the contrast ratio between the sky and foreground won't be so high, such as the dusk hour right after sunset. Depending on what you are shooting, a fill flash might be an option as well.

Question submitted by Jamie in Toms River, New Jersey

Q: How do you tackle the problem of dust spots on your digital camera sensor? My camera (Canon Rebel XTi) has both a dust-removal mechanism and dust-deletion software. I've used both, but have had limited success. Since you interchange lenses and do so outdoors, how do you minimize this nuisance? And how do you clean the sensor when you have to?

A: Digital photography has made the lives of photographers better in many ways. Dust on the sensor is not one of them. Dust is a real problem, especially when changing lenses in dirty conditions. It's most visible as dark, fuzzy spots at smaller apertures when you get more depth of field, or shooting against solid backgrounds.

There are many techniques and kits for cleaning your sensor. This can be very risky though, and you could do irreparable damage to your chip. You have been warned!

That said, here is a run down of a few cleaning methods. Before you use them make sure your camera has freshly charged batteries and is in camera-cleaning mode, which exposes your sensor for cleaning.

Bulb blower: Hold the camera upside down and hold the tip of the bulb blower close to the sensor. Squeeze the bulb several times to force air across the sensor. This is the safest method but is not effective for stubborn dust spots.

Adhesive pads: These are small foam pads with adhesive that you press against the sensor to pick up the dust. It is the most risky method, but works best on stubborn dust spots that can't be blown off.

Liquid solvents: Place a small drop of solvent on an applicator pad and swipe the dust off the sensor. Needless to say, this is also a risky technique but can remove dust that wasn't removed by other methods.

Question submitted by Soumya in Greenbelt, Maryland

Q: What is the function of the AE Lock, and how can I use it?

A: Metering can cause problems if the camera isn't metering the main subject or when the main subject is very dark or light. The AE Lock button's function is to set the auto exposure so you can recompose the shot but use the exposure you locked in. It can be very useful technique when you want to be sure that one part of a scene is exposed correctly.

For example, if you photograph a barn in the middle of a snow-covered landscape, the image will most likely be underexposed and too dark because so much of the scene is white snow, which the camera will capture as middle gray. Using AE Lock, you can move in on the barn so it fills the center of the viewfinder and lock-in the exposure. You can then move back to shoot the barn with the exposure you locked in from close up. These occasions are uncommon, but when they occur, the AE Lock feature will ensure accurate exposures.

Question submitted by Faheem in Indian Kashmir

Q: How do you get good shots of misty trees—or any other situation in which mist is present—without making the picture look too dark or too dull?

A: If your shots of trees in the early morning mist turn out dark and gray, it's because your camera's meter is being fooled by the white mist and underexposing the image, trying to make the white mist a neutral gray. When I'm confronted with this problem, I bracket my exposure, shooting incrementally overexposed pictures until I get something I like.

There are several ways to override your camera's meter. The approach I like in this situation is to use auto exposure compensation, because the camera meter can stay on automatic. Look for a "+/-" icon. If you press it, you should be able to adjust the scale from -2 to +2. This means that your camera will bias the automatic exposure by the amount you have adjusted; -2 means two stops underexposed, and +2 means two stops overexposed. Take pictures of the misty trees while changing the auto exposure compensation dial to the “+” side of the scale. You will see your pictures getting lighter. There is no hard-and-fast rule as to how much compensation you should give your exposure. Experiment with different settings and see what happens.

Question submitted by Colin in Vestal, New York

Q: When I use external flash on my digital camera for indoor photography, sometimes the skin tone of people looks washed out. If I do not use flash and use ISO 200, the photos look yellow. If I increase ISO to, say, 1000, the pictures get an orange tone. What is the right way to get the correct skin tones?

A: There are several issues here. First let's tackle the orange skin tones. Presumably you are shooting indoors and the scene is lit by tungsten lights. These are the round light bulbs we have in lamps at home, and the light that comes from them is very warm. If you decide not to use flash and use the available light in the scene, you are doing the right thing by increasing your ISO to 1000. Next, to prevent the orange cast, set your white balance to automatic or tungsten (the light bulb icon) and see which setting gives you the desired skin tones.

Now let's tackle the washed-out skin tones when you use a flash. This is happening because the meter in the camera that controls the flash is trying to get a pleasing exposure by balancing the foreground and background. If the person in the foreground is quite a distance from the background, the camera's attempt to maintain detail in the background will cause the foreground to become overexposed. Because you are using an external strobe, you probably have the ability to point the strobe head in different directions. Try pointing the strobe straight up in the air so it bounces off the ceiling. This will light both the foreground and the background and create a more natural photo without the harshness and black background of a direct flash.

Question submitted by Chapekar in Herndon, Virginia

Q: I would like to take portrait shots, but am unsure how I should go about it to get extra sharp, clear results. Is there certain equipment you need, or does it rely solely on settings?

A: It's interesting that you want sharp portrait results. Most people feel softer portraits are more flattering because they de-emphasize more severe features. On the other hand, if your portrait subject is an old fisherman, you might want to focus attention on his weathered face.

One way to get sharper portraits is to use strong sunlight coming in from a low angle, like during the early morning or late afternoon. This kind of light is great for showing the texture of your subject, whether it is a rocky landscape for a weathered face. You could also sharpen the portrait in image-editing software like Photoshop. But don't go overboard. If you aren't careful, you can over sharpen.

Another problem may be that your pictures are soft because of camera shake. If you think camera shake might be a problem, try to brace your camera against a wall, tree, or railing. You can also use a tripod.

Question submitted by Nadine in Edmonton, Alberta, Canada

Q: I've bought myself a UV filter, but am ashamed to say that I'm not entirely sure what benefits it may have for my shots. I was wondering how to use the filter properly.

A: A UV filter blocks ultraviolet light from entering your lens. UV light causes film to have unnatural colors, but in this digital age, the results of using a UV filter are negligible. One benefit of a filter is that it protects your lens without seriously altering your photographs, changing colors, or making things look blurry.

Imagine you're on a hike and grind the front of your lens against a rock. Without a UV filter in place, you're going to have to replace the lens. This can cost anywhere from $300 to $3000 (depending upon the lens). With a UV filter in place, the only thing you've damaged—and need to replace—is the filter for about $30.

There are some professional photographers who argue that a UV filter affects the quality of your photos regardless of the filter's clarity. This argument also makes sense. For the clearest possible images, don't put something on the front of your lens and leave it there all the time.

In the end, the answer has a lot to do with how you use your camera. If you enjoy taking your SLR on your travels, hikes, and other excursions, where there is a high chance of smacking the front of the lens on something, then always use a UV filter. If you take photos in more controlled environments (indoors, studio, etc.), then you may opt to leave the UV filter off to ensure the highest possible image quality.

Question submitted by Nadine in Edmonton, Alberta, Canada

Q: I've just bought a compact Canon PowerShot. A wonderful camera, and I can adjust the f-stops on it if I choose to. Can you tell me exactly what do I use the f-stops for, and how do they affect my shots?

A: The f-stop is one of the more confusing parts of how cameras work. Let me explain what it is and then how to use it. The f-stop setting is also sometimes referred to as the aperture. While the shutter speed affects the duration of light hitting the sensor, the aperture affects the amount of light hitting the sensor. To put it simply, the aperture is the part of a lens that dictates how much light is let through to the sensor. If it's wide open, lots of light gets through. If it's closed down, not much light gets through. In essence, it performs the same function as the pupil of an eye. If you are in a dark room, the pupil is open. In sunlight, the pupil is small.

It turns out that when the aperture is closed down (letting in less light), your image will have more depth of field than if it's wide open (letting in more light). You can set your camera on "Aperture Priority" mode to select the f-stop, and the camera will automatically select the correct shutter speed based on the camera's meter. If you are shooting landscapes, you probably want lots in focus from foreground to background. Set your f-stop to a smaller opening to get that extended depth of field, but don't set it too far or your shutter speed will be too slow, resulting in blurry images from camera shake.

Question submitted by Maureen in Limerick, Ireland

Q: What is the best camera trap system to use with a digital camera like a Nikon D3?

A: TrailMaster (www.trailmaster.com) makes some excellent camera traps that we have used at National Geographic for years. They have several different products, but they all basically work on the principle of an infrared beam that is broken by the animal which sets off the camera. TrailMaster's products range from basic to very sophisticated, and they have a track record of proven technology. These will work with the Nikon D3 and any other digital camera with a remote input. You just need the right cable.

Question submitted by Dave in King of Prussia, Pennsylvania

Q: I try to get sharp focus, but most of the time, I fail. I have tried using a tripod, and even using the timer to avoid vibration. Does lighting influence the sharpness of the image more than depth of field? I have taken shots at f/22 and higher, but still can't seem to get sharp focus throughout the image.

A: Lighting: Lighting can influence the sharpness of an image. If it's harder light from the sun coming in at a low angle like early morning or late afternoon, then the textures of the subject will tend to jump out. If it's an overcast day, then the light will have a softer effect on the textures.

f/22: Changing your aperture to f/22 can give you more depth of field, but it can also slow down your shutter speed—so slow that you can't hand hold your camera without introducing camera shake. Although your image will be technically sharp, it appears blurry because the camera isn't held still during the longer exposure. Use a tripod when you stop down all the way and see if that helps.

ISO increase: If you increase the ISO, you can use a higher shutter speed and reduce camera shake.

Lens: A wider lens such as a 24mm has much more depth of field than a 70mm lens at the same f-stop. So if you are using a 70mm lens, don't expect to have everything in focus from foreground to background even if you are stopped down to f/22.

Also the quality of the lens makes a difference. Basically you get what you pay for. A more expensive lens will take sharper photos than a cheaper lens.

Enlargement: It could be that your image is plenty sharp, it's just that you are viewing it at an extreme magnification in your photo-editing software. I feel today's digital tools allow us to measure our photos in more precise ways than ever before, so precisely, in fact, that we start paying attention to things that don't really matter. If you enlarge your image to the pixel level, 100 percent, and it doesn't appear sharp, that is normal. At that level, you can't really tell if any image is sharp or not.

Question submitted by Andy in Lakeland, Florida