Nintendo 3DS’s 1:1 display mode for older DS games, comparison shots against previous models.
Since the 3DS’s resolution (400x240) is bigger than that of standard DS games (256x192), the system stretches older DS games to fit its screens by default, resulting in what some have described as "blur filter" images.
If you prefer to play DS titles at their intended resolution and don’t mind seeing the images shrinked on your 3DS, you can hold down Start and Select when the game boots for the effect you see above. I think I would prefer just having the DS screens stretched.
Here’s a comparison shot of what a DS game looks like on a DS Lite and stretched on a 3DS (click to see a much larger version that better shows the scaling/artifacts):
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