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Digital Cameras 101
Frequently Asked Questions
Updated 9-19-2009
Digital Camera Glossary | Frequently Asked Questions | Print Sizes | How Many?

Sensor size or number of megapixels—which matters more?

Neither. The most important variable is the size of the individual pixels. A pixel is like a light sponge—the bigger it is, the more light it absorbs. Greater sensitivity lets you shoot at higher ISOs with less noise, better detail, and finer color gradation. Of course, bigger pixels require a bigger sensor to hold the same number.

So that’s the reason you get better pictures from a DSLR than a compact that has the same number of megapixels?

That’s the main reason. DSLRs also tend to do better at image processing. They give you more control and, of course, a range of lenses. Other pluses: speedier startup time, lack of shutter lag, faster and more sensitive autofocus, clear and accurate viewfinder, faster burst rates, more powerful flash, many accessories, longer battery life, and typically greater ruggedness.

What’s the difference between Automatic and Program mode?

Automatic is pure point-and-shoot—the camera sets aperture, shutter speed, ISO, white balance, autofocusing mode, flash. We much prefer Av (Aperature Priority), where we set the aperture (for creative control of Depth of Field) and camera selcts the appropriate shutter speed. You can set some or all of the other parameters, apply exposure compensation, and decide when you want flash.

What does Program Shift do?

It lets you change the aperture/shutter-speed combination while maintaining the exposure. Say that in Program mode your camera chooses 1/250 sec at f/8, but you’re shooting a portrait and want to use a large aperture to blur out the background. With Program Shift, you set the aperture to f/2.8, and the shutter speed will automatically change to 1/2000 sec, which gets the same amount of light to the sensor or film.

What’s with “equivalent” focal lengths? Why do you sometimes call a 50mm lens a 75mm equivalent?

It refers to the corresponding focal length on a traditional 35mm film camera or full-frame DSLR. Because most DSLR sensors are much smaller than a 35mm frame of film, they record only the center of the image circle cast by the full-frame 50mm lens, producing images with the same field of view as those a 75mm lens would capture on a 35mm or full-frame SLR. (The difference in area is called the crop or lens factor.)

What makes one lens “faster” than another?

A fast (or bright) lens admits lots of light through a big maximum aperture. This allows you to use a faster shutter speed than you can with a small-aperture (slow or dim) lens. The f-number designation on a lens tells you the biggest aperture you can set, with lower numbers signifying wider apertures: An f/1.4 lens is very fast, f/2.8 is pretty fast, and f/5.6 is slow. Keep in mind that speed of the lens increases exponentially with speed. A Canon EF 50mm f/1.8 II sells for about $100, the Canon EF 50mm f1.4 sells for about $375, and the Canon EF 50mm f/1.2 L sells for about $1,600 (prices on Amazon.com as of 09-19-2009)

So is it better to use a full-frame lens on a DSLR that has a smaller sensor?

No. Since the smaller sensor records only the central “sweet spot” of the image circle, full-frame lenses theoretically should give you images that are sharper around the edges than digital-only lenses do—but there’s scant evidence of that in the real world. Full-framers have some benefits: Retaining their utility if you upgrade to a full-frame DSLR with the same mount, for instance. But the crop factor means they can’t give you an ultrawide angle of view—that 17mm lens becomes a 25mm equivalent on a camera with an APS-C-size sensor.

What does a range such as f/3.5–5.6 on a lens mean?

On less-expensive zooms, the lens gets slower as you move to longer focal lengths. On an 18–55mm f/3.5–5.6 lens, for instance, you can set an aperture as big as f/3.5 at 18mm, but at 55mm the widest you can set is f/5.6. At an intermediate focal length—say, 35mm—the maximum aperture will be around f/4.5.

Why should I buy a telephoto lens when I can just put a teleconverter on my kit zoom?

That will make your already-slow lens even slower. A 2X converter will make an f/3.5–5.6 lens, for example, effectively f/7–11, dim enough to prevent autofocus. And the teleconverters available for kit lenses tend to be of so-so optical quality. Better to put the money toward an inexpensive kit telezoom.

What’s the difference between evaluative, centerweighted, and spotmetering?

All camera meters try to make some part of the frame appear as a midtone—right between very light and very dark. Evaluative metering divides the frame up into segments, compares the readings in each section, and decides on an appropriate pattern for determining exposure. Centerweighted takes into account all the light, but puts a heavy emphasis on the middle of the frame. Spotmetering uses a small percentage of the frame—often less than 5 percent and typically in the center, though some cameras let you link it to the active AF point. While evaluative is best for most situations, centerweighted works well for subjects such as group portraits. For the most control, use spotmetering to choose the midtone.

How do I tell how far my flash will reach?

By its Guide Number, assigned to almost all built-in and shoe-mount flashes and listed—in feet, meters, or both—in the back of your camera or flash manual. The GN tells you how far your flash will carry at a given aperture or what aperture to set for a given distance. Just divide the GN by either distance or f-number. For example, if a flash has a GN of 80 in feet (at ISO 100), it can reach up to 20 feet with a lens set to f/4 (80÷4=20). If you want to shoot at 40 feet at the same ISO, you’d have to open your lens to f/2.

My flash has TTL and Auto settings. What’s the difference?

The TTL setting controls flash output by measuring how much flash reflects back through the lens (TTL) to the camera’s built-in lightmeter. It’s usually the most accurate means of determining flash exposure, and it lets you aim your flash in any direction to bounce the light. Auto Flash controls output by measuring how much flash reflects back to a sensor built into the flash itself. It’s accurate enough for most scenes and usually will function on many different cameras, while flash units that offer just TTL control work only with compatible cameras.

I love my old Vivitar 283 flash. Can I use it with my new DSLR?

It depends on when your 283 was made. If it’s one of the originals made in Japan from 1972 to 1987, it’s not safe. Their trigger voltages vary up to a reported 600 volts—enough to fry your DSLR’s circuitry. More recent models (marked “Made in China” or “Made in Korea”) generally use a 9-volt trigger charge, so they’re safer on new cameras. Still, we’d attach a Wein Safe-Sync adapter ($50, street) to its foot—or just buy a current TTL flash. It costs much less than replacing your DSLR, and you can still use the 283 as an off-camera background or fill light.

Can my camera keep pace with the latest high-speed memory cards?

The newest models can, and manufacturers are always working on improving image transfer speeds and reducing the time it takes for ever-larger image files to clear the camera’s buffer, says Jeff Cable, director of marketing for memory-card maker Lexar. But another benefit of high-speed cards is their ability to transfer images to your computer quickly. To make them run at top velocity, always use a compatible card reader. Here at AgentZ we recently upgraded from a USB2.0 Compact Flash card reader to Firewire8 version. Downloads froma 4 GB card went from 45 minutes to 3 minutes. The investment was well worth it.

Does it harm flash cards to reformat them frequently?

No! In fact, the experts encourage it. Reformatting scrubs images, file names and other image-related data from the card, freeing up memory so you can keep shooting. It’s best to reformat the card in the camera, rather than on your computer, to ensure they work together properly.

Is it true that JPEGs lose detail each time they’re opened? Should I work only with TIFFs or PSDs?

No! If you simply open your JPEG, do nothing but view it, and then close the file, you will not lose detail. If you use the Save As command in Photoshop and choose to save as a lower-quality JPEG, cause ugly artifacts that diminish detail. Frequently re-saving JPEGs at the same quality level may introduce some artifacts, but you won’t see them unless you zoom way into the image.Always work on copies of your original image, not on the original image.

Why would I need a monitor calibrator?

Every monitor is different, and if yours is, say, very bright and high-contrast, you’ll be disappointed when your prints come out dark and dull. So before you edit your pictures on your computer, calibrate your monitor. Once it’s set to the prescribed standard, you can count on your screen to display your pictures as they really are. At that point you'll probably want to calibrate your printer so that the prints match what you see on screen.

I love shooting sunsets, but when I expose for the sky, the foreground is too dark. What can I do?

Use a split neutral-density filter. The color, a neutral gray that won’t affect the colors in your photo, goes from dark to clear either abruptly (hard-edge) or little-by-little (graduated filters). A split ND is the perfect solution for situations where the foreground and background are under vastly different light. The filter allows you to expose properly for the dark foreground without blowing out the highlights of the lighter sky.
The other way to do it is to take 3 shots and bracket your exposures and combine them in Adobe Photoshop CS4 (HDR Merge) or a program like Photomatix Pro .

How many images fit on a memory card?

This is a tough question to answer as cameras use different jpeg compression methods. For typical image storage capacity follow the link "How Many?"

What size image do I need to get a good quality photo?

Generally for the best prints the standard image resolution is 300 ppi (pixels per inch). However, good quality prints rely on other factors as well. See our page on"Print Quality"

What is the difference between optical zoom and digital zoom?

Optical zoom will actually magnify the image the camera sees. A 3x optical zoom would make the object appear 3x closer. Digital zoom is a trick the camera does where it selects just the center portion of the image and then blows it up to full image size. Because there are fewer pixels to make the same size image there is always a degradation of quality.

What is resolution?

Resolution, simply put, the number of pixels the camera can capture. The higher the number, the better the potential for a high quality print that can be made.

What does "Good, Better, Best" (or Fine, Normal, Basic) refer to in my camera settings?

This refers to the amount of compression that is applied to the image when the camera processes it for storage. The higher the compression level, the smaller the file size, the more images that can be saved to a memory card. Compression comes with a trade-off. As you increase compression the quality of the images decreases.

What is compression?

Compression is a method of packing information in the image into a smaller file size. Because compression discards "un-needed data there is a reduction in image quality. Excessive compression can cause distortion of colors and "artifacting" to appear.
Compression also has cumulative effect, the more often and image is opened and saved (not just viewed) the more the image quality will be affected. Some higher end cameras now offer a "RAW" (or TIF) mode that stores the images uncompressed. These images are very large and not many will fit on a typical memory card.

What are the differences in image formats (bmp, jpg, tif, etc.)?

JPG (or JPEG) files are the most frequently encountered file types. JPG images are compressed files for easier storage. TIF and BMP are uncompressed file types. These are best used for files that will be opened and re-saved frequently so that there will not be a cumulative effect of compression on the image.

Digital Camera Glossary | Frequently Asked Questions | Print Sizes | How Many?