The Pixel Peeping Trap
How did I arrive at 30% of actual size for the 5" x 7" print? Well, professional negative scanners will scan a negative at about 4000 dots per inch and, for a 35mm negative, that results in an image that's about 5512 x 3674 in digital terms. The average photo printer will print between 240 to 300 dots per inch, with 300 being the desirable one, producing a print that is between 15" x 22" to 12" x 18" in size, depending on the printer. Photographers (and not just the pros) with good technique and proper equipment for the job will produce sharp images printed at these sizes which are, based on our equipment to print, 100% size in effect.
So, how does all this relate to pixel peeping on the monitor? Well, many people, myself included, will tell you that when you print you'll want a minimum 240 dots per inch for a good quality result, anything less will start to look bad. Well, a monitor is substantially less dots per inch! The general LCD monitor for today's computer is displaying at 96 dots per inch or 40% of the resolution of your printer output (at 240 dpi) and if you were to print your film image at that resolution it would be 38" x 57" and look like, well, crap if we held it in our hands and looked at it. Now, if you stepped back quite a distance and looked at it, it would start to look better, but odds are that you're not sitting 15 feet back from your monitor and so viewing it at 100% on your display is a very innaccurate look at the image.
Now, most photo software, especially ones like Photoshop, Lightroom, and the like, do a very good job reinterpolating your image for display when you view it fit to screen. They are accounting for a lot of the differences in resolution and displaying according to the capabilities of the medium. According to some sites, and I haven't specifically verified this, Adobe recommends that you view your images at about 25% unless you have specific retouching requirements. That's probably a little too much to get a realistic impression, 30-40% is more in the ballpark based on our previous numbers and that is often what I'll use. Why? Because then the display on screen is a much more realistic approximation of the final output to paper. If the image looks good, sharp (if sharp is what I want), and attractive at that size on screen, it will, 99% of the time, look that way when I print it.
Net effect, if you insist on evaluating the sharpness and quality of your images at 100% on screen, you'll only come out of it unhappy. In fact, you may discard the images as a result of that and it is entirely possible that they would have been razor sharp when printed, so you lost out and maybe the rest of us did as well. This is a trap, avoid it.
Finally, it is obvious that not all of us are printing our images. I only print some, I have a very good printer for this, but I still only print what I intend to hang on my wall. Most of the time we intend to publish our pictures on the web in some capacity. If it's Facebook, it's going to be shrunk substantially (down to about 13% for me), but for other sites, such as Flickr, it will be resized several times and your original will be left intact. However, regardless of the site, you'll notice that many of the professional photographers never publish the full-sized image. There are some reasons for this:
1. Protection against theft.
2. People will misjudge the image because they'll pixel peep. Yep, they will, there are a lot of pixel peepers and they'll want to see how the pro is and judge him or her based on that. Inaccurately, I might add.
I'm not a pro, but the above two issues still apply to any amateur. In fact, the first one may be greater for us because we probably don't police the use of our images as strongly as we don't have a vested financial interest. Some, less scrupulous companies know that. However, you'll notice on my site that the longest size I upload is 1200 pixels, around 25% of the image size produced by my camera. I do that because it's a good size for my site and, more importantly, it's comparable in display to a printed version of the same image when someone views it on screen.
So, stop peeping at the pixels and just get out there and shoot!