Sometimes it helps to select a lower pixel count when taking a photo. It helps to reduce file size, allowing you to take a long continuous stream of photos or simply store more on your memory card. But what happens to the image when you do this? Does the camera crop the image, or does the image simply get resized?
About pixels and pixel counts
It’s pretty hard to explain image quality without talking about pixels first. A pixel is a tiny dot of color. Your computer monitor has a lot of these things running both vertically and horizontally. If you get up really close, you can see a bunch of differently colored little squares. Each square is a pixel.
All digital photos are a grid of horizontal and vertical pixels just like the ones you see up close on your computer monitor. Whenever you crop or resize an image, you reduce the total number of pixels in that image.
The real question is how do these two things differ? What does cropping do that resizing doesn’t do?
Cropping vs. Resizing
There’s a pretty big difference between a crop and a resize. Cropping is what happens when you take a picture and decide to keep a smaller portion of that photo. The following is an example of a crop.
I start with this photo:
...and then I decide that I want to keep the top half the photo, so I open up a photo editing program like Photoshop Elements, select the top half, and then copy it into a new image to this:
By cropping the image, I have also reduced its size. This new image is roughly half the size of the original image. It has half of the total pixels.
Now, if I were to resize the top image, I would still reduce the total number of pixels, but instead of selecting a portion I want to keep, I would keep everything, just at a much smaller size.
The following is the same image after being resized down:
If there’s anything to take away from this, it’s that a crop is selective (you select what you want to keep) while a resize is not.
What your camera does
If you choose a lower pixel count setting on your camera, it resizes the image. You can figure this out by taking two separate photos at different image size settings. If the smaller image looks just like the larger image but smaller, your camera has resized the image. (Note you'll usually need to look for the difference on a computer because cameras usually resize the image to the height of the LCD screen.) If the smaller sized image excludes sections that were in the original image, it has been cropped (I’m not saying this does happen, but if this were to happen, it would happen on the outside edges of the image).
To my knowledge, no camera crops an image without first asking you which areas you want to discard or keep. It wouldn’t exactly be fair to the photographer who takes a picture expecting to capture the entire scene. How disappointed would you be if you got back an image with areas missing around the edges? I’d be angry enough to demand a refund.
Back in the earlier days of digital photography, (and cheap film cameras) there were a few cameras where the image you saw through the viewfinder wasn’t the same thing that was captured. Even back then, the issue didn’t have anything to do with the camera settings. It had to do with the viewfinder not being properly aligned with the lens (it was a point-and-shoot and the two were separate). If you suspect your camera is cropping your images without first asking you how you want them cropped, check your camera documentation to see if something similar is going on.
Otherwise, it’s safe to assume your camera is simply resizing the entire image.
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