Actually it would be nice if Klaus just ran through the distances and testing procedures. (please!)
Some say fifty times the focal length; .6mts X 50 = 30mts .... seems quite a lot although it's a real world ideal distance, one would need a large testing lab.
Overall an ideal shooting distance for small birds is around 2.2- 12mts and 20-40mts for large ones, what is nice with the G2 is it's MFD is 2.2 mts, close enough to catch all but the smallest critters.
I've not worked out how the focus breathing (being IF) is, but testing at close distances would maybe influence resolution figures, it's really sharp close up.
I'm sure Marcus will be be "very interested" at any tips that may come his ways about testing tele-zooms especially relating to the F mount:
Err, Marcus where are you?? Ah yes I see you.......trembling under the table!... :o 
It depends on how the charts are designed on the distance. The 51x focal length came from me I think, as I thought PZ used such charts, but they do not. Klaus in some post did state the distance, but I do not remember.
Usually, focus checker charts are set up in 25...50×FL which means the target is somewhere distant in between 15 and 30 meters from a 600mm lens.
Most birds are shot at closer ranges.
One of the upcoming problems to solve in a test is stabilize the whole unit enough and avoid mirror/shutter slap. Next will be, get a big enough target. And keep it safe an clean. And light it up evenly enough. Do that in a room without atmospherical distortion. Dust, wind, humidity will lead to worse results.
After that you can look a t systematic problems. I know that my Nikon 300/4 E will getting me blurred images in bright daylight at shutter speeds between 1/180 and 1/500 when I forgot to switch off Image stabillsation. Next, if AF is slow , not reliable or unpredictable, even an otherwise great lense loose value.
Usability, ergonomics, weight, weather and meanchical resistance also come into equation, but cannot be rated. A phsysically trained person can hold a 3 kg lens longer without too much of shake than another. Stabilisation is great but we tned to forget the limits. In-lens stabilisation is constantly changing the optical axis as far as I understood. Comes bad to worse, the corners will suffer more.
Also, at the end of the day experience counts a lot. 15% more sharpness of one lens under test conditions? Forget it. Using it not careful enough, I loose quickly 30...70% of that advantage - and I'm only talking about static objects. Birds in flight benefit a lot more from experience than from IQ of a certain lens.
SLAC, the Stanford Linear ACcelerator, for creating subatomic particles has a straight indoor room 1 mile long.
...maybe in their down time, some arrangement could be made!
Quote:SLAC, the Stanford Linear ACcelerator, for creating subatomic particles has a straight indoor room 1 mile long.
...maybe in their down time, some arrangement could be made!
That'll do it!
1 mile is not ∞ B)
I suggest to put yourself on the moon with the Tamron and try the shadow side to focus on Mars. 
Just to be o the safe side.
You were close, the distances are 40x the focal length on FX and 60x on DX. Yes... that requires a lot of space  And hopefully explains why ASP-C based reviews of these lenses are quite unlikely to happen 
-- Markus
Editor
opticallimits.com
Ok, When I returned the Sport to the seller, I said I would find out how his lens was fairing after it's first return from Sigma service, apparently this time with accompanying images Sigma looked at it again and indeed found it was de-centered, they promptly returned to Japan being unable to repair it in France!
The Tamron "Tapin console" to give it their "posh" name for it, arrived yesterday and I just plugged it in to the lens, it recognized the lens and displays four categories:
First off the Firmware was up to date when I clicked on it.
1; AF: 150/200/300/400/500/600mm at three distances 2.2 mts...20 mts...Infinity from-20- 0 +20 (can't find infinity symbol JoJu!)
Mainly the AF is accurate but I noticed a small back focus (I think) at about 50 mts @ 600mm so as a starting point I dialed in -1 at infinity.
2; Focus limiter: First position; is factory set.
Second position;.adjustable from 2.2 to infinity....I chose 5 - 10 mts;
Third position; adjustable from 2.2 to infinity... I chose 5 mts - infinity. Very useful!
3, Manual focus override sensibility;
From: low---0---high. (I didn't touch this setting)
4, VC Shutter/ viewfinder priority.....Standard......Viewfinder priority.....Capture priority.
This section is not so clear as the third choice is greyed out...so unsure at this stage I left it in standard.
Some AF and other settings are greyed out according to the lens used.
It requires an internet connection to make any changes to the settings so unless you can find the internet wherewithal you might as well leave it at home. it connects quickly and any changes are made before saving the settings, these can be done one at a time to avoid confusing programing, the process updates the lens in 15 seconds. I left the D500 AFFT (-2)set in the camera as it's different from the D750 (+2) and console changes will pivot around that.
The G2 basically has pretty accurate AF and not wishing to scramble my brain I shan't do a complete AF recalibration, but will modify any settings as and I go.
∞ on Windows: character table > mathematical operators. Unicode-ID would be U+221E, I just don't didn't know how to type it 
HTML would be:
∞
You need to set the editor of the forum to HTML (little switch top left, right of the eraser) and type & # x + the Unicode number. Not a bug, just Windows...
I don't know about the Tap.in console, but maybe it's helpful to know:
3. Manual focus override: I think it's the sensitivity the lens react when you turn the focus ring and the camera is set to AF-C.
0 could mean "no manual override at any time," Low could mean, you need to turn it i.e. 5° in short time to make the manual focus work, high could mean you need to turn it only 0.5 or 1° (I don't know the numbers, I'm guessing). So that means, Tamrons fucking manual is no bit better in that aspect than Sigma's fucking manual? Because here Sigma is also not clearly describing which parameter does what and how much of it.
4. VC At VC tests the most effective setting was "VC only when shutter is in action" - continuously VC would mean, depending on your movements VC comes occasionaly to the actuator's limits when using viewfinder priority. Because here it tries to stabilize the finder "image". While shutter priority basically starts fresh, before shutter is open. So it has more freedom to move and might not touch the limits of the actuator's maximum ranges, resulting in less motion blurred images. Pretty cool to be able to choose this setting. Will need to look into my last OIS lens from Sigma, the 24-105. That is already the best VC in my house, maybe I also can change the settting to "only act, when needed - don't act to smooth the handshake while composing."
I think the internet connection is needed to check firmware. Sigma USB-dock does that as well, but I never tried with a disconnected internet.
Thanks JoJu, however when I click on the box right of the eraser, I get a box drop down with BBCode and a multi choice, the only connection with infinity is the time it will take me to understand it, ie. infinitum!
Never mind I've been shooting birdies today "blue tits" (no connection with the cold weather) B)
More time is needed for my tired and damaged brain, you know softly softly catchy monkey! as they say.
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