May 2006 HelpLine

ƒ-Stops And Bad Crops

    * Digital Camera ƒ-Stops
    * Straight-Line Troubles
    * Cropping Casualty
    * PPI Canon Vs. Nikon
    * Out-Of-Shape Histogram?

Digital Camera ƒ-Stops

Q)  Is there a scientific reason that all or most digital cameras go down only to ƒ/8? I’ve been a photographic nut for more than 60 years and always loved being able to stop way down to increase my depth of field.

Pete W.
Via e-mail

A)  When you say “all or most digital cameras,” I assume you’re talking about fixed-lens compact digital cameras, not digital SLRs. And yes, there’s a scientific reason for the limitation. The image sensors for the compact digital cameras are much smaller than those of digital SLRs and 35mm film cameras. With the smaller image sensors come smaller lenses (both physically smaller and shorter in focal length) with physically small lens openings.

With these smaller lens openings comes increased diffraction (the tendency of light to change intensity and direction when going around a barrier like an iris-the opening in the lens that creates ƒ-stops). Diffraction causes softening of the image, and this diffraction limits the ƒ-stops that you can have on a compact digital camera. (Diffraction is also the reason why it’s never a good idea to completely stop down a lens-the sweet spot of a lens is always less than the maximum ƒ number.)

I should mention that depth of field at a particular ƒ-stop is inversely proportional to a sensor’s diagonal dimension. A compact digital camera at ƒ/8 with an 11mm diagonal sensor is equivalent to ƒ/32 on a 35mm camera (using equivalent focal lengths). This, then, is a challenge for small digital cameras-limiting depth of field!



Straight-Line Trouble

Q)  What causes a scanner scanning to Adobe Photoshop Elements to create bad scans on brick and clapboard houses? The bricks don’t keep a pattern, and the clapboards don’t keep a straight course, but sometimes a jagged one.

Jim Chatfield
Via e-mail

A)  Are you looking at the scanned image in Elements at full resolution? One thing to keep in mind with Photoshop and Photoshop Elements is that displays of non-full-sized images can sometimes look poorly. For example, when you zoom into an image and display it at 33.33%, the display isn’t very accurate; 50% or 25% will give you a better idea of what the image will look like.

Cropping Casualty

Q)  When I crop a little 1600 x 1200 image, say, a 10% crop, and send it to print at 4×6 inches, it appears pixelated. Also, when I resize at 1024 x 768, there’s a loss of quality. Can these problems be avoided?

Pierre Michel
Via e-mail

A)  One thing to check is whether your printing process is okay to start with. If you print without cropping, does the image look okay? If it does, then the problem might be related to how you’re accomplishing your crop. Photoshop allows you to crop and resize the image at the same time. You may be resizing the image, or changing the ppi, at the same time as the crop. When you select the crop tool, the tool settings appear just below the menu bar. Make sure that your crop tool setting doesn’t have a figure in the resolution box. You can also check your image size to be sure you have a printable resolution between 200 and 360 ppi.

PPI Canon Vs. Nikon

Q)  I have a Canon EOS 5D and a Nikon D70. Nikon JPEG images load at 300 dpi, while those of the Canon load at 72 ppi. I’m printing the majority of my work on my Epson R800 or through at 300 ppi with the Nikon. Since my 5D is new but the total pixels are more, do I need to change the Canon to 300 ppi in Photoshop CS2?

Jim J.
Via e-mail

A)  Many assume that if the Nikon is outputting a file at 300 ppi and the Canon is outputting at 72 ppi, the Nikon is a higher-resolution camera, but this interprets the information incorrectly. (Also, many people interpret dpi and ppi as the same thing, though technically, ppi, or pixels per inch, is more accurate.)

A digital image file by itself doesn’t need a ppi; ppi only comes into play when you’re outputting that file to something that has a specific size, such as a print or an image in a magazine. In your case, set the ppi to an appropriate value for your use. But you won’t be changing pixels at this point. When you first change the image size, be sure interpolation is off. In Photoshop, for example, make sure the Resample Image check box is unchecked. That way, you’ll be changing only the metadata of the file (instructions to the computer on how to read the pixels), not the actual image.

Say you start with a 3072 x 2048 image from a camera. When you open it in an image editor, the image size dialog box might show that the resolution is 72 ppi. This would yield a document (if printed) of about 42×28 inches, but the resolution of the printed image wouldn’t be very good. If you were to resize the image just by changing the resolution box from 72 to 200 ppi and make sure that Resample Image is off, the new print would be about 15×10 inches. The picture would be much better because inkjet printers generally do best with a resolution of 200 to 300 ppi.


Out-Of-Shape Histogram?

Q)  How do you tell from the histogram if the photo you’ve just taken is all right? Should the histogram look like a bell curve, a straight line or some combination?

Tom Bear
Via e-mail

A)  Histograms can be confusing, but first you need to consider what the histogram represents. The histogram’s horizontal scale is the measure of brightness the image sensor can resolve, from dark (on the left) to full white (on the right). The vertical scale is the number of pixels at each brightness level, with the bottom being no pixels at that brightness level and the top being many. There’s no such thing as a perfect shape for the histogram because its curve depends on what the scene is like. Bell curves, straight lines or curves with multiple mountains may all be proper exposures.

The key to working with a histogram is to avoid the spikes on the far left and far right. To ensure proper exposure, avoid anything that’s under- or overexposed-this means avoid having a large pixel count on the far left or far right, especially if it “clips” or is cut off at the ends. These spikes amount to detail being lost in the shadows or highlights of the image.

In some situations, the scene you’re trying to photograph has too great a contrast ratio, and no exposure setting will help keep those spikes in bounds. In such a case, your options include using a fill-flash or graduated filters, or taking multiple exposures (using a tripod, of course) and merging them in an image editor.
For more information on histograms, check out “The Magic Of Histograms.”

If you have any questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or [email protected].

Leave a Comment