September 2004 HelpLine

Monitor Vs. Photo Size

    * Photos On The Monitor
    * Digital Terms
    * Depth Of Field And Digital Cameras

Photos On The Monitor

Q)  If I open a high-resolution picture from my computer via any photo-imaging software, I can see the entire image. Yet if I attach that image to an e-mail, the recipient opens that picture and it’s huge, and one needs to scroll a mile in each direction to see the whole picture. Why does a high-res picture show so large on a monitor? Why doesn’t it just show full screen or less? I want a high-res picture to be able to be seen on a monitor without having to scroll.

Cindy
Via e-mail

A)  When you open a picture using an image-editing program, the software resizes the displayed image to fit on your screen. It does this no matter the actual size of the image. Your software has viewing adjustments that allow for zooming in or out, displaying the image in its actual size or “fitting” it to the viewing window. When you step out of the program, you have to deal with the fact that in a simple image display, “a pixel is a pixel.”

Check the display settings for your monitor. On a Windows machine, right-click on the Desktop, click on Properties and look at the Settings tab. On a Mac, check the Displays section of your System Preferences. You might see that your video display card is set for something like 1024×768.

 


For discussion, let’s use 1024×768. If you try to display an image that’s 2272×1704 outside of image-processing software, the pixels are going to map to the display one to one. Since it’s much larger than the actual display area, the image is going to go off the screen.

If you want to get the image to display directly on someone else’s monitor, you’ll need to resize the image so that the pixel count is under 1024×768. You can do this either by adjusting the image measurements or changing the number of pixels. When adjusting image measurements (inches, for example), you need to adjust the ppi/dpi setting also. In the case of computer displays, 72 dpi should work well. So if you size the image to 5×3.75 inches at 72 dpi and e-mail it, the receiver of the image will see your picture at about that size on their monitor. Although there are some variables—the actual monitor size, the display settings on the receiving end—this adjustment should work for most setups.

A more important point to consider when e-mailing images is file size. If you don’t resize the image before sending it, you could be sending a very large file, which might cause a few problems:

• It might take a long time to download. The recipient might only have a dial-up connection, or even if he or she has broadband, the recipient might be “on the road” and not have his or her normal high-speed connection.

• The recipient’s Internet service provider (or business e-mail account) might have a limit for attachment file sizes. This might mean the recipient wouldn’t get your image.

So before you send a file, pay attention to file size.

Digital Terms

Q)  I’m doing research about digital cameras, but sometimes I get stuck with terms. What does SLR mean? What’s the difference between an SLR and a regular digital camera? Some ads say a camera comes “body only.” Does that mean it doesn’t come with any lenses?

Marcelo Lucas
Via e-mail


A)  SLR is an acronym for single lens reflex, meaning there’s one path (a single lens) for viewing and capturing the image, with a mirror (reflex) in between the lens and image sensor. When composing, the mirror is down and reflects the image coming through the lens to your eye through a pentaprism (or pentamirror), a five-sided glass prism that reflects the image in the right orientation into the viewfinder. When you release the shutter, the mirror quickly flips up and allows the image to go to the sensor. This design allows you to change lenses on the camera.

Besides the obvious benefit of accommodating various lenses, a digital SLR offers some additional advantages:

• You see exactly what’s coming through the lens with an optical viewfinder (most low-priced SLRs crop the edges of the image in the viewfinder). With cameras that use one lens for image capture and another lens for the optical viewfinder, this isn’t true.

• The image seen in the viewfinder is a direct view of the subject. For digital cameras with electronic viewfinders (EVF), what you see is a little monitor showing what’s seen by the sensor. This view still doesn’t match the optical viewfinder.

• D-SLRs are faster than most digital cameras (with minimal to no shutter lag).

• D-SLRs use larger sensors than other digital cameras, which results in images with less noise and the capability of using higher ISO settings.

On the negative side, you don’t have a “live view” from the sensor on the camera’s LCD (the live view lets you see exposure and color balance, for example). Since the mirror is reflecting light to the pentaprism and into the viewfinder, it’s also blocking the light from hitting the image sensor when it’s down. The flip side of this problem is that when you actually take the picture, the mirror flips up and you can’t see the composition of the frame at exactly the moment the image is being captured.

“Body only” does mean no lens, not even a “standard” one. This is the way many SLRs have been sold for years. Why? A photographer may already have lenses from other cameras and may not want to buy another one. However, you’ll often find packages that include a lens.

 

 


Depth Of Field & Digital Cameras

Q)  I’ve shot 35mm for years and only recently ventured into the digital field. One thing I miss is the depth-of-field control that I had with my 35mm. Even though my digital has ƒ/1.8 lens capabilities, I don’t notice much change in depth of field from, say, ƒ/5.6 to ƒ/1.8. Is this inherent with compact digital cameras, in general, or is it a problem with the shorter fixed lenses?

Everett
Via e-mail

A)  Depth of field is usually defined as the range of distance from front to back in which the objects in the scene are in focus. In reality, there’s only one plane that’s in focus, but the depth of field is the range in front of and behind the focus plane that’s acceptably sharp in a photograph.

Depth of field is a function of four things: ƒ-stop, distance, focal length and the size of the print. Distance to the subject and the print size aren’t affected by the digital camera, but it’s worth remembering that the closer you are to a subject, the narrower the depth of field gets, and as print size increases, depth of field decreases.

With small digital cameras, the size of the sensor influences the size or focal length of the lens used. Lenses start getting very small. A lens with a 35mm equivalent of 28-200mm might actually be 7.2-50.8mm. In addition, lens designers have found ways to make these lenses extremely small and compact (especially compared to 35mm).

This affects both ƒ-stop (apertures) and focal length for depth of field. Shorter focal lengths (“wide-angle” compared to 35mm) will give more apparent depth of field, so these very short lenses will give more depth of field than typical 35mm lenses.

 

 


In addition, the smaller the lens opening (a higher ƒ-stop number), the more depth of field. This is modified because as this opening gets physically smaller, depth of
field increases (for example, a pinhole gives infinite depth of field), so with the physically small lenses used, the aperture is smaller, too, giving more depth of field. (Lens designers run into another problem from this smaller size: When the lens opening is too small, light diffracts around the edges of the aperture, dramatically reducing sharpness. This is why most small cameras don’t stop down past about ƒ/8.)

With the larger sensors used on D-SLRs, you use longer focal lengths, meaning depth of field is more restricted and defined. You can limit your depth of field with small cameras by using the telephoto end of the zoom and as wide an aperture as possible (you may need to use a neutral-density filter to compensate for exposure issues).

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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