Frequently Asked Photo Questions for October

Have a question about digital photography? Send it to me. I reply to as many as I can--though given the quantity of e-mails that I get, I can't promise a personal reply to each one. I round up the most interesting questions about once a month here in Digital Focus. For more frequently asked questions, read my newsletters from July, August, and September.

More About F-Stops and Polarizers

In the September Digital Focus FAQ, I answered a question about whether using a polarizer affects a camera's depth of field. (To learn how to intentionally tweak depth of field, read "Four Ways to Get Stunning Depth of Field.")

In a nutshell, my answer was that polarizers reduce the light entering the lens, but they don't actually change the diameter of the camera's aperture. Verdict: no change to depth of field.

Boy, did I get a ton of mail about that. Almost everyone who replied disagreed with me, echoing the following assessment.

If you place a polarizer on the camera lens, you lose two f-stops in exposure. To correct for the loss you have two choices: decrease your shutter speed two steps, or open up the lens two stops, which would absolutely result in a loss of depth of field.
--Tom Pagett, Westland, Michigan

That's true Tom; but I'm still right, and I stand by my original answer.

The question was whether polarizers affect depth of field. They don't. By itself, apolarizer doesn't change the aperture any more than simply buying a lesson book makes me a better drummer.

Of course, you can choose to open up the aperture to make up for the loss of light--and that will reduce depth of field--but you could just as easily opt to lengthen the shutter speed or increase the ISO, and neither of those alternatives affect depth of field. Or, I suppose, you could do nothing and have an underexposed photo, proving definitively that polarizers do not affect depth of field.

Thanks for all the letters (even the ones that questioned my IQ), but keep reading, because I've got four more opportunities for you to disagree with me this week.

Can I Use an Old Flash?

I have an old Vivitar flash from my film camera days. I'm planning to buy a digital SLR soon. Can I use the old flash on the camera, or will I need to buy a new flash for the digital camera?
--Ron Nichols, Salem, Oregon

If only this question were as easy to answer as whether polarizers affect depth of field!

Here's the problem in a nutshell: Many older flashes generate very high trigger voltages--like 200 volts and sometimes even more. Modern digital cameras are typically designed to accommodate a maximum voltage of just 24 volts. Put a 400-volt flash on a 24-volt camera, and you risk damaging the electronics in your expensive camera.

Unfortunately, it's a bit more complicated than that. First of all, it's rarely obvious what the trigger voltage of your old flash really is. And then there's your new digital camera to consider: What maximum voltage can it handle? Camera manufacturers are cagey, sometimes putting one value in the specs but implying that higher voltages are allowable in other venues.

My advice is to avoid this circus. Don't put your camera at risk: Just buy a new flash that's designed to work with your camera. After all, older flashes can't take advantage of all the new flash synchronization features in your digital SLR anyway.

But if you're not willing to spring for a new flash, you can do a little investigation. The Strobe Trigger Voltages site does a great job explaining the issues and rounding up numbers for a huge number of flash units. The site even gives you simple recommendations--yes, no, or "your call"--for whether each flash will work with modern cameras.

Screen-Capturing Photos

I sometimes use screen capture software to grab images from Web pages rather than using the context menu in my browser to save the photo. Is the metadata from a photo saved when captured in this manner?
--Dave Paulson, Santa Maria, California

No, I'm afraid not. Your screen capture software doesn't know anything about the image it's capturing; it's just "taking a picture" of your Web browser. So if you capture a screen grab of a photo on the Web, any metadata that was in the photo won't be transferred to the screen capture.

Correcting the Date on Digital Photos

I recently took some pictures with a digital camera. The camera's clock hadn't been set, so all of the pictures came out with a date of two years ago. Is there a way to correct this date information in the photo's metadata?
--John Lydon, via e-mail

Certainly! Several photo organizers let you tweak the date and time stamped into the photo's metadata. My favorite program for this task is Windows Live Photo Gallery, a free program that's an excellent photo organizer and basic photo editing program--plus it has a superb panoramic stitching tool built in. To change the date in Windows Live Photo Gallery, select your images, then right-click and choose Change Time Taken.

The Best Way to Scan Black and Whites

Should black-and-white photos be scanned using color or black-and-white settings? Then, should the scanned photos be treated as color photos during image enhancement?
--Arnold Seiken, West Perth, Australia

If you're scanning old black-and-white photos, you'll generally get similar results whether you choose your scanner's grayscale or color mode. But don't use your scanner's black-and-white mode. What we usually call a "black and white" photo is really considered grayscale on the computer, because it has hundreds of varying levels of gray. A true "black-and-white" image, captured using a scanner's black and white mode, has just two tones--pure black and pure white. This mode is mainly designed to capture text documents.

Whether you choose color or grayscale depends upon whether you're going for accuracy--replicating the actual print you're scanning--or getting a great black-and-white experience in the edited digital photo. Personally, I recommend using color mode, because that gives your scanner more flexibility when capturing photos that may have faded, have some sepia tones, or that have otherwise taken on a color cast. In fact, I'd keep the photo in full color mode throughout your workflow to maximize your editing options.

Hot Pic of the Week

Get published, get famous! Each week, we select our favorite reader-submitted photo based on creativity, originality, and technique.

Here's how to enter: Send us your photograph in JPEG format, at a resolution no higher than 640 by 480 pixels. Entries at higher resolutions will be immediately disqualified. If necessary, use an image editing program to reduce the file size of your image before e-mailing it to us. Include the title of your photo along with a short description and how you photographed it. Don't forget to send your name, e-mail address, and postal address. Before entering, please read the full description of the contest rules and regulations.

This Week's Hot Pic: "Fowl Sunset," by Gary Brown, Riverside, California

Gary says: "I took this photo with a Canon Sureshot at a pier in Oceanside."

This Week's Runner-Up: "Nature's Merry-Go-Round," by Julie Finley, Easton, Pennsylvania

Julie says that she took this photo of an Astra African Daisy with a Fuji FinePix S8100FD.

To see last month's winners, visit our Hot Pics slide show. Visit our Flickr gallery to browse past winners.

Have a digital photo question? E-mail me your comments, questions, and suggestions about the newsletter itself. And be sure to sign up to have Digital Focus e-mailed to you each week.

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