Originally posted by Zephos How much can I zoom on a photo
That depends on the medium you view the image on and the conditions you view it under, especially the distance between your eyes and the image.
That's not a helpful answer, but if you are viewing your photos taken with a K-50 on an LCD computer monitor, you will probably notice any upsampling of the original 4928 x 3264 pixels image. At 72 pixels or dots per inch (the original standard for Apple/Macintosh systems, the entire image will cover 68" x 45", but most modern LCD displays will be between 100 and 120 PPI, so expanding the image beyond 41" x 27" is not going to look good. That's a baseline, you can view the image on monitors with higher PPI values, but a 240 PPI display doesn't mean you can zoom twice as much and have the image look as good. Your brain can't distinguish individual pixels at higher than 120 PPI, but it can do a better job of blending adjoining pixels with a higher PPI display, so there is value to higher pixel density displays, but not as much as you would expect.
The maximum resolution of your display device should be where images will look their sharpest, and 100% crop or 1:1 zoom should be the maximum for viewing. On a 1980 x1080 display, the screen will only show about one-ninth of the entire image at 1:1. Depending on the composition and contrast of the image, you might be able to zoom in even more than that and see a sharp image, but that is a result of the post-processing your brain does. You can also upsample the image to a higher resolution with software, and depending on the characteristics of the image, you may not be able to tell the difference.
Bottom line: If you get your eyes close enough to the display to see individual pixels, and each pixel in the display represents a single pixel in the photo, you won't be able to see any more detail.