Ok, I'm officially baffled, need your opinions...

[coastalconn]

Kris it really comes down to you, do you like it?

I got better results today, so the quick answer is hell yeah!

I can't tell which lens these are shot with, even after reading a couple times. It doesn't matter, the photos are fine.
You could be seeing sample variation, or cleaner air one day then another, or more contrast from a softer lens, or any number of things. Subjectively 'sharp' correlates only loosely with the relevant properties of the lens MTF.

Sharpness is very subjective. I've noticed that by the comments on my FB page. Seems 50/50 what people thought was sharper..

Two things.
1. I don't know if it's sharper than the 120-300 or not, but it is plenty sharp enough. If I could achieve pictures like that, happy would not even begin to describe how I'd feel. But then, I've always said I think you could get sharper pictures using a Coke bottle for a lens than I can get with my Sigma 150-500. That Common Yellowthroat is just spectacular.
2. That last one isn't a Mockingbird. It's a Gray Catbird. And a nice shot of it, too!

Thanks Sharon, I should get a 600mm PVC pipe and a few coke bottles and see what I can work out Catbird for sure. They are super abundant up here this year for some reason, the only thing outnumbering them in the woods is squirrels!

I love number 5. The DoF is a tad less than the bird in one or two, but I'd expect that. Are the "fuzzy-wavy" areas on top of the egret's neck (not the reflection) stray feathers or a Flickerism?

WTH is a Flickerism?

After reading your review I would have to agree with you that the Tammy appears to be sharper. I don't think it matters though when it comes to the lens that you have on your camera body and your ability to get so close to the birds. I think you could take any lens and produce beautiful images with it. It's like many have said here before, it's not the gear, it's the photographer.

Edit, I also noticed that the Tammy seems to be brighter.

I think the key is put the birds down and pick up a dull brick wall and some ideal lighting and a tripod.
At least for a pure sharpness test - yeah its dull, its not going to be anywhere near as pretty as the fantastic bird photos you get. But its informative and isn't going to be swayed by things like shifts in settings, lighting and fatigue.

Thanks everyone for all your thoughts... At some point I will do a real test on a pod with live view. But it might be a mute point now. Think I figured a few things out..

The light on this day was beautiful and crisp, and that ALWAYS makes a lens appear to be sharper and more-contrasty. I dunno...I looked at the other thread and on the POSTED sign, the Tamron appeared to produce the better image quality. I am wondering if on duller-light days, if the Tamron just doesn't look quite as snappy as it does on a bright-light day?

That is a good part of the issue I think..

I guess i would expect the tamron to do better than the sigma with the 2x on it. I guess it comes down to the focal distance you use the most of. If always over 420mm ish than the sigma and a 1.4 converter cannot be used and the tamron is probably going to be more contrasty and more accurate at auto focusing.
I think there are very few lenses if any that can match performance of a lower end zoom like the tamron 150-600 when used at the same focal distance with a 2x tc. Maybe the prime 2.8 like the 300mm or 400mm or 200mm f2.0. But even with them i found af to be not very accurate at times. But when it was accurate it was plently sharp

Matt, according to Nasim's test the Sigma 120-300 with 2x at F8 is as sharp in the center as the 300 F2.8 VR, so it can be very sharp, but focus accuracy is a big deal for sure. I think analyzing my test I came to the conclusion that I was struggling in post because the Tamron produces more shadow noise, I'm sure it is one of those sharpness things, so I went with ETTR, and less processing and got better results. I think the Tamron at 600 easily tracks better than the Sigma with the 2x TC..

 

[Overread40]

I wonder if the 2*TC you use affects focus tracking accuracy? I know Canon revamped their MIII Teleconverters for a new autofocus setup, although for some reason this is supposed to only work with their new MII supertelephoto lenses (ie big white ones).

 

[sm4him]

WTH is a Flickerism?

I think they are referring to one of those weird things that can happen to a photo's IQ as a result of uploading it to flickr, thus "flicker-ism".
The egret's neck, however, just looks to me like little tiny bits of neck feathers sticking up.

 

[snowbear152 82]

WTH is a Flickerism?

I think they are referring to one of those weird things that can happen to a photo's IQ as a result of uploading it to flickr, thus "flicker-ism".

^ This, exactly.

Yes, I think they are feathers, but it almost looks like imprecise clone tool placement.

 

[coastalconn]

WTH is a Flickerism?

I think they are referring to one of those weird things that can happen to a photo's IQ as a result of uploading it to flickr, thus "flicker-ism".

^ This, exactly.

Yes, I think they are feathers, but it almost looks like imprecise clone tool placement.

LOL, got ya, I didn't use any cloning.. So I think it is the wet feathers...

 

[Overread40]

Feathers and fur can do odd things sometimes, especially when resizing for the net. This owl chick I got a shot of almost looks like its got a white halo around it, but its just fluffy feathers not laying smooth
https://farm8.staticflickr.com/7345/14198694313_076e901c99_b.jpg

 

[Derrel]

At f/8 there ought to be enough diffraction softening that any lens looks "decent", so the 120-300 zoom and 300/2.8 are more or less equally messed up by the diffraction effects. Just thinking out loud.

Overread's comments about feathers and fur also seems accurate: fine, high-frequency detail oftentimes looks VERY weird when re-sized, both in PS< and also by different computer monitors, at different screen resolutions. I've seen some AWFUL-looking photos on one computer, then switched to a different machine, and the images have looked much better, so, I think sometimes the computer display can color our impression of a lens or a camera's performance; if the images are not sized and sharpened "just so", the quality can appear pretty low.