Tamron AF28-300mm XR
This lovely lens has been unfairly maligned by many. Those who have used it tend to find it a captivating lens... the size, the weight, the price... all very appealing. The optical performance is startlingly good for such a long zoom (almost 11X). There are, of course, compromises that one must make when building such a lens. The key to using one is to know how to use it most effectively. Hopefully this page will help you use yours to advantage.
Here we have several test shots of a detail from a Canadian $50 bill. These shots were taken by two different lenses - a Nikkor prime lens legendary for sharpness, and my Tamron XR super-zoomer, notorious among many for being too soft, and considered by some to be not worth the space it takes up in the camera bag. All of these shots were framed at the same size, within the limits of my ability to do so. Certainly they should be within a percent or two. This first series were all taken in the same setup using my Nikon D100 with the same lighting, same shutter speed, all at f/8. All shots are shown at 1:1 - the actual pixels coming from the camera. There is minor variation in zoom amount due to my inability to get the tripod in exactly the right spot to frame the detail in exactly the same way, particularly given the foreshortening of my framing references as the zoom increased, but it should be close enough to show you what you need to see.
Base shot - Nikkor AF50mm f/1.8D
Tamron XR @ 55mm
Tamron XR @ 100mm
Tamron XR @ 200mm
Tamron XR @ 300mm
I think the above does a pretty reasonable job of showing the relative sharpness of the center of the lens. However, where long zooms fall down is in the corners. So here are some more shots, all at 300mm, same setup, with different apertures, with the test shots showing details from near the center, and the corner of the shots. The top half of each shot is the center section, and the bottom half is the top right hand corner.
f/6.3 - This is wide open at 300mm on this lens, and the performance suffers because of it. MTF testing shows the sweet spot to be at f/22 for the corners. Certainly *this* isn't it. Even at f/6.3 the shot is quite salvageable using post-processing, and I have shots to prove it, but it really would be a good idea to not shoot this lens wide open at *any* length if you can help it. If you need the speed then that's life in the big city - you do what you've got to do to get the shot.
f/11 - This shows a great improvement, not just of the corner but even of the center sharpness. f/11 is probably the optimum for center sharpness at 300mm, but the corner is still rough. When taking a shot of the moon or something else where you want optimal center sharpness but the corners really don't matter, then f/11 is what you want.
f/22 - A very slight softening of the center, but by robbing Peter a bit we get a significant improvement in the corner. f/22 is your optimal all-around aperture when you need the corners sharp.
Now maybe you're thinking that even at f/22 that looks unacceptable. You feel that a lens should be clean right to the corners at all ranges, apertures, etc. Surely the legendary Nikkor 50mm f/1.8D will be much better, right? Well, I wasn't able to recreate the shot exactly because the Nikkor just won't frame that close on its own... I could have put it on the macro bellows, but then somebody might complain that the bellows caused some kind of loss of quality or something, so I just got the biggest shot that I could for display at 1:1. Here it is.
This is the center section, and below is the corner. Like the other shots, these are taken from the same exposure.
So, for me, the bottom line is that I get quite acceptable results from my ~11X Tamron 28-300XR. Especially in the long ranges no doubt a good 300mm prime would be better, but the convenience and just downright *fun* of being able to change from wide to super-tele in a heartbeat, with a lens that is a fraction of the cost, size and weight of a 300mm prime, is a powerful salve for that bit of anguish.