Originally Posted by
NorthernFocus
Which, broadly applied, is flawed logic. For one thing, "longer" isn't necessarily more expensive nor technically difficult to build. Lenght comparison has to take into account maximum aperture. If two lenses are the same focal length AND the same aperture, and one sells for multiples less than the other, then yes, something is amiss.
There are some objective generalities that can be made regarding optical design:
1) Prime lenses are easier to design and simpler than zoom lenses. That lends itself to better optical performance.
2) Similarly, the lower the zoom ratio, the easier it is for the designer to achieve good optical performance across the zoom range. It is generally recognized that zoom ratios of 3x or less achieve optimal image quality. There are technical reasons that zoom ranges such as the traditional 24-70 and 70-200 are some of the only ones widely used by pro shooters.
3) As another general rule(and contrary to popular belief), lenses utilizing modern glass forumulations and optical coatings peform better than older ones of similar design.
So you to consider a lot more than maximum focal length. I've never owned either one but I'm willing to bet that the Tamron 150-600 will outperform the famous Sigma 50-500. Why so? Per above, you're comparing a 4x zoom to a 10x, modern glass to 15 years old, etc. Tamron would have to have blown the design pretty severly for it not to be so.
As another example, based on my own (admittedly flawed) subjective judgement, the $1500 Nikon 300mm f4 plus 1.4xTC (i.e.420mm) combination outperforms my $5000 Nikon 200-400mm VR shooting at 400mm. It pains me to say it, but having shot them side by side, so it is.
Making use of sample images on the web is highly dependent on ones reasons for doing so. But generally speaking, comparing random images posted by random individuals(myself included) in order to attempt and compare the relative quality of various lenses is at best a crap shoot.