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Photo Cropping, Camera Resolution, Depth of Field, and More Q&A

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 and August.

Cropping Photos to Fit a Frame

My camera's image size doesn't conform to any standard frame size--the aspect ratio is off and photos need to cropped before printing. Is there a chart that tells me how to crop photos so the machine doesn't crop the edges for me?

--Ken Rowland, Fort Worth, Texas

It wouldn't be all that useful to make a chart, Ken, because that would probably tie you down to cropping your photo to a particular pixel size. Instead, if you're using any modern photo editing program, you can rely on the program's Crop tool to crop the photo to the right aspect ratio. Just dial in the frame size you want, like 5 by 7, and then use the crop box to choose how and where to crop within your photo while maintaining a perfect 5 by 7 aspect ratio, suitable for printing and framing.

To do this in Adobe Photoshop Elements, for example, click the Crop tool and then, in the Tool Options palette at the top of the screen, set the aspect ratio from the drop-down menu. Choose 5 by 7 and then drag the crop box in the photo. It'll stay perfectly proportioned while you make the crop box bigger or smaller to capture just the part of the photo that you want.

What Determines a Camera's Resolution?

I am confused about how to determine a camera's true resolution. Do I look at megapixels? Sensor size? Something else? I want to choose a camera that lets me capture as much detail as possible.

--Joe Rogers, Madison, Wisconsin

If you are interested in capturing the most pixels, the camera's megapixel rating is the easiest number to compare. Megapixel is, of course, a measure of the number of millions of pixels embodied in a photo--so a 10-megapixel photo will generally have more detail than a 6-megapixel photo. An even more precise way to compare the resolution of various cameras is to look at the pixel dimensions of the highest resolution photos each camera captures. For example, my Nikon D7000 is a 16-megapixel camera, and it takes photos that are 4928 by 3264 pixels.

There's more to the story than pixels, though, which is why you shouldn't buy a camera based on specifications alone. Raw pixels aren't going to help a lot if the camera has poor optics, high digital noise at low ISOs, or other limitations that keep it from taking great photos. That's why I recommend that you read a lot of reviews, like those here at PCWorld.

In general, if you're comparing two cameras with similar megapixel counts, the one with the larger sensor will generally take better photos. That's why some photographers pay a premium for digital SLRs with full-frame sensors. I discussed full frame versus APS-C sensors in my September photography FAQ.

For more on camera and image resolution issues, read "Everything You Want to Know About Megapixels, Megabytes, and DPI."

Understanding the Depth of Field of Point-and-Shoot Cameras

I have heard that my small pocket-sized point-and-shoot camera exhibits much greater depth of field than my digital SLR. I assume that this is due to the optics and sensor size, but I don't understand why. Could you please explain?

--Phil Gentile, Orange, New Jersey

Unfortunately, Phil, to fully answer your question I'd have to delve into a fair bit of math and graphs. Rather than do all that, let me steer you to an excellent discussion of this topic on Photo.Net.

Nonetheless, I can give you a short answer. The oft-repeated observation that point-and-shoot cameras have greater depth of field than digital SLRs is misleading. That statement is only true under very specific conditions. For example, if you compare a digital SLR to a point-and-shoot camera and set both to an identical field of view--not an identical focal length, but a identical image in the viewfinder--then it's true that the point-and-shoot camera will have relatively more depth of field.

Here's one way to think about this: Because point-and-shoot cameras have smaller sensors than DSLRs, they have a "crop factor" that makes their photos seem to have a higher magnification. So if you have two cameras set to the same focal length, the camera with the smaller sensor will behave like it has a higher effective focal length. To get the same field of view through the two viewfinders, you need to get physically closer to your subject when using the camera with the larger sensor. As a general rule, the closer you get to your subject, the more narrow the depth of field becomes. So your point-and-shoot camera, with the smaller sensor, has a higher apparent magnification and lets you stay further from the subject. As a result, it delivers more depth of field.

Can I Take Photos of Stars Without a Motor Drive?

I have a digital SLR with a standard kit lens, a tripod, and a wireless remote control. What is the best way to achieve clear images of stars and the Milky Way without a motorized mount? The best I have been able to manage so far are very poor indeed.

--Mike Hoffman, Federal Way, Washington

I have some bad news for you, Mike. What you're trying to do isn't really practical. The earth is spinning under the stars, and that motion creates a blurry photo if you leave the shutter open long enough for the sensor to soak in sufficient light to capture the stars. That's why astrophotography is generally done with a camera mounted on a motor drive that can compensate for the earth's rotation during a long exposure.

Instead, you might want to consider a style of night photography that is easily within your grasp: star trails. I recently explained several techniques for taking gorgeous star trails that show the relative motion of stars in the night sky.

One more thought: I've never tried this technique, Mike, but if you're willing to experiment, you might try setting up your camera in a location that is utterly devoid of artificial light, pointing it at the sky, and taking some short exposures with the camera's ISO setting cranked to its maximum position. It's possible you could get some serviceable photos, but I suspect not. If you are willing to try, please do let me know how it goes.

Choosing a Neutral Density Filter

You mentioned neutral density filters in "5 Accessories to Take With You (and Your Camera) on Vacation." The question is, which one? When looking up neutral density filters, there are many different degrees to choose from.

--Gordon Falise, Ontario

Neutral density filters--which screw into the front of your lens and reduce the light reaching the sensor--come in a wide variety of strengths. There's a standard notation to make it easy to choose the filter you want: ND2, for example, lets half the light through, which translates into a 1-f/stop change of exposure. ND4 admits a quarter of the light, which is a 2-f/stop change of exposure. ND8 gives you an eighth of the light, for a 3-f/stop reduction, and so on.

So which one should you get? In my experience, I've found that if you want to carry only one filter, an ND4 is the all-around best neutral density filter to have. If you don't mind carrying two filters, I'd get both an MD2 and an MD4. You can use either one as conditions dictate, or even combine them if you need to block a lot of light.

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