Is Tamron 200-400mm f/5.6 LD a worthy upgrade?

Der Freischutz wrote:

I have an opportunity to go to Serengeti National Park this summer and want to go with sufficient camera equipment, but the budget is tight (graduate student).

For wildlife I usually use an EF 55-250mm IS with my EOS 40D. From what I've found on the net, more reach might be welcome, so I bought a used Tamron 200-400mm f/5.6. The price was right (

This is a complex thing to compare. The 55-250 might look sharper in 100% pixel view, but it can not capture the detail of a 400mm lens in focal-length-limited situations, so while the 55-250 might get more detail at shorter focal lengths, 250mm can never get more detail than the 400mm can in optimal situations, unless the 400mm is toy-lens quality, or the pixels are so small that both lenses are over-sampled, in which case an extremely sharp shorter lens could get more detail. However, IS vs non-IS can make a big difference in low light (but IS can not freeze the subject; only the camera jitter).

Does this seem right? The Tamron is so much bigger/heavier and lacks IS, am I better off just cropping pictures from the 55-250mm and getting rid of the Tamron? Thanks for any feedback/ideas.

Unless the Tamron has serious problems, at 400mm/5.6 on a tripod the Tamron should get more real detail, even if the 100% pixel view looks softer. Sharpness and detail, especially natural, non-pixelated detail, are two completely different things. The aperture of the 400mm even at f/8 where you imply it is sharpest, is 50mm. The maximum aperture on the 250mm is 44mm, so the Tamron gets more light at f/8 than a crop of 250mm gets at f/5.6, so don't think for one minute that you are compromising light using 400/5.6. 400/5.6 gives an aperture (71mm) unavailable on the 55-250.