Here are just a couple comparison shots taken by my K10D using (a) the Pentax FA 50mm f/1.4, and (b) the Tamron 28-75 XR Di f/2.8.
|20070425 lens tests|
I actually took a couple dozen shots, indoors, outdoors, at different focal lengths and with different apertures. I used same aperture in comparative shots. I shot Raw+JPEG; the shots I've put up for review are the unprocessed JPEGs. Both seem to qualify as "prosumer" lenses. While the Tamron is more expensive, the Pentax 50 f/1.4 has an excellent reputation. So I think the comparison is fair enough.
I am aware that my tests were not "scientific". I didn't use a tripod. I didn't use completely controlled lighting. I was not photographing a test chart. This wasn't just because doing more careful tests involves more work - it's also because I do not believe that one or two shots prove very much. What matters most isn't how the camera performs in some carefully controlled test, but how it performs over and over and over again in something like "real life conditions." After reviewing the couple dozen photos that I took, I think these four shots are fairly representative. I've taken a hundreds of photos with both of these lenses, and the tests I did today are more or less in line with what I've been seeing in real shoots.
At the pixel-peeping level - and of course at 50mm - the Tamron zoom lens is not as sharp as the Pentax prime. In the picture that has my checkbook sitting on some magazines, the grain in the checkbook is slightly sharper in the Pentax shot - viewed at 100% - than in the Tamron. In some photos of flowers taken outdoors, I THINK that the Tamron may be slightly oversaturating the colors, as compared to the Pentax lens. But I don't usually view my photos with a magnifying glass. The differences from one lens to another are so subtle that I had to study the photos carefully to find them.
And I'm not even sure that it would be right to say that the Pentax is simply superior at 50mm. I prefer the Tamron result in some photos - in these examples, I think the Tamron picture of the Pellegrino bottle is slightly "better." I also suspect that the lenses may have slightly different sensitivities. It occurs to me now that perhaps a better test would have involved bracketing the exposure on all the shots and picking the best result for each lens; but I don't have time now to go back and do that. The picture of the checkbook and magazines was taken at f/2.8. That's pretty clearly a good aperture for the Pentax 50mm f/1.4 - and equally clearly not the best aperture for the Tamron - so this might be a case of the Tamron being asked to play tennis with its weaker hand.
And that gets to the nub of the issue: how DO you compare lenses, really, I mean, when the results are worth comparing? I acknowledge that sometimes lens A clearly isn't as good as lens B. But I don't think the superiority of the Pentax prime here is overwhelmingly obvious. And if the prime does prove superior when the image is viewed under a microscope - the zoom lens can come back and say, "Okay, now lets shoot at 35mm, or 65mm!" In other words, the real issue is, is the superiority of the prime in its limited area of competence great enough to outweigh the fact that the zoom lens is so much more versatile? I wouldn't say that zoom lens's versatility should matter if the results of the zoom were lousy. But they aren't.
There are no doubt times when the advantages of the prime are worth the loss of the zoom. The Pentax 50mm can't shoot at 35mm or 65mm; but the Tamron 28-75 can't shoot at f/1.7. If I was shooting portraits, I'd use the Pentax lens, no second thoughts. But I don't shoot portraits very often.