Hi Folks...
Ok, here is my question..
About 8 months ago I purchased this lens. AF28-300mm F/3.5-6.3 XR Di LD Aspherical (IF) Macro; Tamron USA, Inc. (http://www.tamron.com/lenses/prod/28300_di.asp)
It worked ok, but very soft after 275mm

Well, about a month ago I purchased this lens.
Sigma - Lenses (http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3305&navigator=2)
Why, because I thought the Sigma would be a better lens as all my other sigma lens are really nice and am very happy with them...Besides the sigma lens looks very cool...Anyway, the sigma lens was simply awful everyplace but at 29-150mm, so I took it back and exchanged it figuring I got a bad one...Well, the replacement was the same bad lens, so I shelved it...Well, I was pondering the two lenses and thinking, that both have the same specs but the tamron rocks the sigma...Well i decided to compare the two visually and what I found was the glass on the tamron is larger by quite a bit...This to me explains the better quality of the Tamron but wonder how both can have the same specs...??
Here is a picture...
Thanks for reading my rant...

http://img91.imageshack.us/img91/3489/lenscomparisonet5.jpg

the shape of some elements and they way they group them is probably what would make one better than the other. and the coatings they use on the glass.

The rear element is just that the rear element the F stop is set by the size of the front element of the lens. The light path after the front element or other element sizes have all to due with the corrections needed color and other correction.

Are you sure that the sharpness problem are not DOF issues? At wide open even a hundred foot out the long lenses have still a very small in focus area. Your subject will be in focus but your foreground and background will be out of focus.

No that sigma lens for the most part has a bad rep...I have 4 other sigma lenses and they are great...In this case the Tamron is better and I am thinking it is the back glass..Bigger to me means more light for the sensor

I am thinking it is the back glass..Bigger to me means more light for the sensor

The rear element diameter is just one tiny part of an overall lens design. I've never heard anyone debating it's size - larger vs smaller - and really can't believe that it would affect image quality. Really, it's like comparing the diameter of lug nuts used on different cars... :)

Lens coatings matter but I'd say almost any new lens will be fine with this. Long lenses can benefit from APO, ED or some other name for glass that keeps the reds, greens and blues focused to the same spot instead of giving it a bit of a prism effect - basically what happens is that if the reds are in focus, the blues might be soft, etc. But - the overall lens should be considered; not just the ingredients.

28-300mm, somewhat of a macro mode, small physical size, low weight and low price (comparitively) are an awful lot to expect from a lens. Like anything else, it's all about compromises. The Tamron sounds pretty darn impressive. A lot of lenses with a huge zoom range that get into 300mm or more can be a little soft at the long end.

However, the longer the lens the more important shooting technique becomes. I hear a lot about soft lenses but very little about technique. Handheld 300mm shots (especially without VR or IS) are going to be tough even at 1/500. Depth of field gets really shallow so focusing is critical and if the camera is bouncing around how can you be sure that focus is correct, much less take a stable shot? Are you using a tripod? Try this and also use a self timer and a shutter speed of at least 1/125 - and carefully focus - to compare lenses.

Thanks AV
I guess I need to look at the inner workings of a lens...When I see some of my primes and I see the size of the glass, it does not surprise me that they shoot sharper pics...

These two lenses with the same specs are different animals...

I guess I need to look at the inner workings of a lens...When I see some of my primes and I see the size of the glass, it does not surprise me that they shoot sharper pics...

I'd really compare images rather than lens guts. Just make sure to remove any sharpness variables (tripod, cable release or self timer, etc) so that the test is as fair as it can possibly be. Older zooms used to be a lot physically bigger and heavier - look at older 80-200 variable aperture zooms in the 4-5.6 range like the Vivitar Series I. It's almost the size and weight of an 80-200 f2.8 (plastics have something to do with it, but lots more glass in the older lenses). I'd take a new inexpensive 80-200 over one of the old ones any day. They've come a long ways!

I'd really compare images rather than lens guts. Just make sure to remove any sharpness variables (tripod, cable release or self timer, etc) so that the test is as fair as it can possibly be. Older zooms used to be a lot physically bigger and heavier - look at older 80-200 variable aperture zooms in the 4-5.6 range like the Vivitar Series I. It's almost the size and weight of an 80-200 f2.8 (plastics have something to do with it, but lots more glass in the older lenses). I'd take a new inexpensive 80-200 over one of the old ones any day. They've come a long ways!

av
I am going to take you up on your idea.
I will try and do that this week sometime and post the pics in here. :)

Before you test the lenses, check out this link: http://www.dudak.baka.com/dofcalc.html.

DOF at focal length of 300mm at F4 you have a DOF at 50 feet of only 1.7 feet and at F8 its only 3.4 feet. Unless you can shoot at 1/300 of second or faster at the higher F stops you don't have much DOF.