For a budget $100 on reaching
300mm with AF, there are few choices as suggested and you have to consider the caveats for each.
I personally will pick Tamron 70-300mm or the Sigma 70-300mm for their good range as well as
good 1:2 close up, they offer the
best value for the $100 to $150 you spend. The
best consumer zoom will be the Pentax DA 55-300mm, the wider reach in 55mm is very useful for portrait.
If you can wait out a bit, I will suggest you to save up on the DA 55-300mm and use your DA 50-200mm for the immediate need, I often used DA 50-200mm in the zoo with satisfaction.
All the choices mentioned are worth considering. I highly valued my shameless zoom with
Tamron 70-300mm f/4.0-5.6 Di LD 1:2 Macro. This lens may overlap a bit with your
Pentax DA 50-200mm f/4.0-5.6. I for one love a zoom with 1:2 close up capability. I think you may have tried DA 50-200 in the zoo, but if you have not, I suggest you to use it as I have shot a bunch of zoo pictures with the DA 50-200mm. You can add close up to your DA 50-200 with a Raynox 150/250 or simply a 52mm close up filter in the +3 and +4 close up filter.
The caveat to watch for the Tamron 70-300mm zoom has always been in the
frequent showing of CA, you see purple fringing on high contrast area. I note that you have used lightroom, xnview and gimp, all those tool have handling for purple fringing to some degree. Most showing can be reduced to certain degree. And you can reduce it in stopping down to f/8.0 and shoot pictures with multiple angles to avoid the high contrast areas as in branches and reflective surfaces.
Some of my best pictures are coming from this zoom but you have to work with pp on the PF. See the tip of the wings of a vulture shot with the tamron zoom plus
Pentax F-1.7x AF TC. The combo gives me 510mm reach with lost of 1.5 stop.
If you find that the PF is easily fixable such as the above, then the Tamron 70-300mm can be considered. To me, the zoom is worth much more than the $100 I spent on it as a used copy. It strives also in other areas such as
Thanks,
Hin
Last edited by hinman; 04-07-2009 at 05:09 PM.