The idea I came up with, was to print out a lens test chart using my wireless Brother color laser printer MFC-9840CDW. Set @2400dpi, the chart came out pretty nicely. It doesn't compare to a 100-500$ professional chart, but I figured it was a start. The chart I got came from this website: http://www.graphics.cornell.edu/~westin/misc/res-chart.html
I then set-up the camera with the 75-300mm on a stand, zoomed in @ 300mm, then moved the tripod so the chart would fill in most of the image. Then I took pictures at 75, 200, and 300mm. I noticed that I was doing a better job in MF compared to the AF, so I adjusted the focus on each shot manually, spending a few minutes each time to ensure I had the best image possible. Afterwards, I swapped lenses, but did not reposition the tripod or camera. That way I kept the same distance between the chart and my dSLR sensor throughout the test.
The main question I was trying to answer with these tests was whether or not it was worth keeping my 75-300mm, or just take the pictures with the 50mm f/1.8, and crop them.
Here are the results I got. The tests are somewhat subjective, but it can give an idea.
75-300mm @ f8 & 300mm: 1100 hor, 800 vert (average = 950)
75-300mm @ f8 & 200mm: 900 hor, 800 vert (average = 850)
75-300mm @ f8 & 75mm: 500 hor, 500 vert (average = 500)
50mm @ f8 & 50mm: 300 hor, 300 vert (average = 500)
If you do some calculations based on that:
@75mm, you get around 500/300 = 1.67 magnification. 50mm x 1.67 = 83mm
@200mm, you get 850/300 = 2.83 magnification. 50mm x 2.83 = 141.5mm
@300mm, you get 950/300 = 3.17 magnification. 50mm x 3.17 = 158.3mm
So, compared to a 50mm f/1.8, and with both of them opened up @ f/8, the 75-300mm III would work as a 83mm to 158.3mm. On the other hand, you can see just how terrible the lens can get between 200mm and 300mm, where you only get a gain of 141.5mm to 158.3mm.
What does all of this mean? If you are happy with the image quality of a 50mm f/1.8, and want to get the same IQ out of a 75-300mm, you need to treat it as a 83-158.3mm lens. So instead of having an image 6 times larger with your 75-300 @ 300 compared to a 50, you can only print an image 3 times larger. It's still a gain, so meanwhile it's still worth keeping it, but it's only a marginal gain.
Unless I can figure out why my flickr account doesn't let me upload fullsized images, meanwhile here's the link to the large version of these images: http://www.flickr.com/photos/51787685@N02/sets/72157626861453596/
75-300mm III @ f8 & 300mm by Jean-Pierre La Forest, on Flickr
75-300mm III @ f8 & 200mm by Jean-Pierre La Forest, on Flickr
75-300mm III @ f8 & 75mm by Jean-Pierre La Forest, on Flickr
50mm/1.8@f8&50mm by Jean-Pierre La Forest, on Flickr
I'm open to suggestions on how we could do an even better job at comparing how much better is the 200$ 75-300mm lens compared to the 99$ 50mm f/1.8, and if it's worth keeping.