January/February 2006 HelpLine

Sensor Dust

    * Keeping Dust From Your Photos
    * Camera dpi…Huh?
    * Standard-Sized Prints
    * EV Mystery

Keeping Dust From Your Photos

Q)  I just bought a digital SLR, and my images have been plagued with sensor dust. What can I do to avoid it in the future?

Jim S.
Houston, Texas

A)  Here are some good rules to follow to minimize dust:

1. Avoid leaving the camera body open for any length of time.

2. Turn the camera power off before you change lenses.

3. Keep a lens on the camera at all times.

4. Don’t change lenses in the wind or dusty conditions.

5. Keep the camera body pointing down when changing lenses.

6. Vacuum out your camera bag on a regular basis.

7. If you use body caps, don’t put them in your pocket or other dusty, dirty, linty places.

Camera dpi…Huh?

Q)  I hope you don’t mind two bonehead questions at once. My first question is about dpi. I’ve been told by a Nikon user that I should set my camera’s dpi to 300, but I can’t find anything to set dpi in any of the camera menus or in the manual that came with the camera. Do you know of a way to set dpi in a camera? I usually shoot RAW.

Susan B.
Via e-mail

A)  Your questions are not bonehead (unless you’re saying that you’re asking a bonehead to answer the questions…hmm). Let me tackle them one at a time.

Camera dpi (dots per inch) or ppi (pixels per inch) can’t be changed in your camera (the two terms are used interchangeably, though technically, ppi is the correct term for images). It’s a measure of how far apart pixels are and has nothing to do with image quality except when paired with a specific output. The camera does set a parameter in the metadata to a specific dpi or ppi, but that’s just metadata (instructions about the file) and doesn’t affect the image that the camera captures.

If you process your image in an image editor, you can adjust your ppi there (particularly, in Adobe Camera Raw). The ppi setting only relates to your output. The camera simply gives you a certain number of total pixels, such as 2048 x 3072. If you could set your camera to 300 dpi, it would still output those 2048 x 3072 pixels. The only time you’d need to worry about the camera’s dpi setting is if you took your media card directly to some output device that used ppi directly.


Standard-Sized Prints

Q)  My second question is how do I fit an image to a standard-sized mat or frame? For example, a pixel size of 2048 x 3072 has a document size of 6.827×10.24 inches at 300 ppi. How do I print this 6.827×10.24-inch image so that it will fit into the opening of an 8×10-inch mat without cropping it, adding more canvas and cloning more background (thereby changing the composition), or distorting it by unchecking Constrain Proportions in Photoshop’s Image Size dialog box?

Susan B.
Via e-mail

A)  Distorting the image isn’t the answer, and cloning would be a lot of work, besides not matching the original scene. So that appears to leave cropping.

You first need to get an image that fills the mat size, then, as you’ll see, you have to crop it because you’re dealing with two different aspect ratios. First, let’s look at getting the right size. Because you’re changing the pixel count of your image to 300 ppi, the resulting size isn’t quite large enough.

Go to the image-resize part of your image processor and be sure resampling of any kind isn’t checked. You want to use your original pixels. Then adjust your dpi to 250. That should be enough resolution and will boost your image printing size to a width slightly larger than eight inches. The length won’t yet be correct for the 8×10 size because it all comes down to aspect ratio-the ratio of width to height.

Digital SLRs’ aspect ratio is based on 35mm. The actual image size of a 35mm frame is 36x24mm. That equals an image aspect ratio of 1.5:1, more commonly called 3:2. When camera manufacturers try to design a camera that feels like, works like and acts like a 35mm film camera, you can be fairly certain they want the image aspect ratio to measure up.

Let’s take a look at an 8×10. This is a standard size based on formats used by photographers back in the 1800s. Rotating the dimensions so they read comparable to my 35mm calculations, you get 10×8, and dividing by two makes for a 5:4 aspect ratio. This isn’t 3:2.

While some might attribute this “rectangular peg in a square hole” printing problem to digital cameras, those cameras are really acting just like a film camera. A full-frame print from 35mm equals not 8×10 inches, but rather 8×12 inches, which means it has to be cropped to fit an 8×10-inch mat, just like the digital image must be cropped. (Note that some non-SLR digital cameras shoot in different aspect ratios, like 4:3 or even 16:9.)

EV Mystery

Q)  I’ve been in and out of photography for a number of years, and it seems that each time I get back into it, they change the rules. I got through the change from ASA to ISO. I’ve even gotten through the digital hurdle. But can you please explain EV to me and why I should care about it? It keeps popping up.

Clint T.
Via e-mail

A)  EV has actually been around for quite a while and isn’t a digital-specific term. EV stands for exposure value and is just another way of talking about exposure. Unfortunately, it’s not quite as simple as the film-speed name change from ASA to ISO. Though the math is a little complicated, EV is derived from a combination of lens aperture and shutter speed.

There’s an explicit value for EV of zero—a shutter speed of 1 second with an aperture of 1.0. But it’s more useful to consider exposure value in terms of how you might change your exposure.

Start with the understanding that an increase of 1 EV is equivalent to halving the light allowed to reach the sensor. A decrease of 1 EV is the same as doubling the light. (If you must know the math, EV is measured on a negative logarithmic scale.)

In terms of f-stop or shutter speed, an increase of 1 EV is the same as either decreasing your ƒ-stop by 1 (say, ƒ/8 to ƒ/11) or shortening your shutter speed a whole step (say, 1/125 to 1/250 sec.). Realize that you’re changing one parameter or the other-either one reduces the light by half. If you decrease your ƒ-stop from ƒ/8 to ƒ/11, but also increase your shutter speed from 1/125 to 1/60 sec., the exposure will stay the same, resulting in no change to your EV or the light reaching your sensor.

A decrease of 1 EV comes from the opposite change in your f-stop or shutter speed-either increasing your f-stop by 1 (say, ƒ/11 to ƒ/8) or increasing your shutter speed a whole step (say, 1/250 to 1/125 sec.). Again, you’d change one parameter or the other-in this case, either one increases the light by two.

And that’s where EV comes in handy. It’s useful for defining a change in exposure without explicitly calling for a change in aperture or shutter speed—you get to choose. If I say, “You overexposed by 1 EV,” you can decide whether to correct that by a change in your ƒ-stop or your shutter speed.

Now here’s a qu
estion for you: If the explicit value of EV=0 is ƒ/1 at 1 sec., can you think of another value for EV=0? Next month, I’ll give you an answer or two.

If you have any questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or helpline@pcphotomag.com.

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