Perhaps the Tamron is an F6.3 on paper, but its true F number will depend on its light transmission and maybe the light transmission is not as good as the Canons and the AF sensor tolerance of the older Canon cameras is too wide.
It should not a variable anyway. If you sell a lens that you claim is f6.3, it should pass the same amount of light as a Canon lens at f6.3. Especially if it is supposed to be Canon-compatible.
Imagemaster wrote:
It should not a variable anyway. If you sell a lens that you claim is f6.3, it should pass the same amount of light as a Canon lens at f6.3. Especially if it is supposed to be Canon-compatible.
Yes, but simply for argument sake, ISO 100 on many Canon bodies is not actually ISO 100 in reality.
"Perception can indeed become reality," but we do still need empirical data.
Perhaps the Tamron is an F6.3 on paper, but its true F number will depend on its light transmission and maybe the light transmission is not as good as the Canons and the AF sensor tolerance of the older Canon cameras is too wide.
Mike Engles
No, the amount of light transmission affects the T number. However, small differences won't affect the AF unless it is right at the edge of being too dark. OTOH, the F number is a mechanical (geometric) value and may affect the AF function depending on the optical path and design of the sensor.
The next poster that says the AI Servo issue is down to user error, or that we are expecting too much, will find my 150-600 lodged firmly where the sun don't shine!!
Ask yourself this - has anyone yet, in the 3 or 4 threads on this lens said;
"Yes, I have a 7D/5DII/1DIV (or older) and AI Servo works perfectly with my set up."
No?
Now I'm no detective, but it seems to me there is a common theme developing that maybe, just maybe, that AI Servo doesn't work as expected on anything other than the newer Canon bodies.
Surely we can all agree this has been established. We should be happy that, unless someone who has one of the cameras mentioned above or older and can conclusively prove AI Servo works with this lens for them, we can, with fair consideration, assume there is an issue.
Yes, it's a cheaper 600mm than a Canon 600mm, but if it only has full functionality on certain bodies (and not even between 'pro' and 'budget' bodies (be interested to hear how it performs on the latest x00D body)), I don't think it's unreasonable to expect to be informed of this issue before parting with my hard earned cash.
Especially as this lens will be used for the type of photography where this feature will be used more than any other.
I haven't seen any comments regarding what focal length these AI Servo issues are occurring at ? or maybe it's all of them.
Just got the lens the other day and while I haven't been able to any real AI Servo testing in the field with it, I did notice a consistent issue with the AF while testing in the back yard. Basically up to 425 mm worked pretty good but after that it's almost non existent in low light. I tried to focus on a brick wall that was in the shade and over 100 feet away. Up to 425mm no problem in One Shot as well as AI Servo. Go to 450+mm and I couldn't get a focus lock in either mode and I was on and off the focus button like crazy. Now while in AI Servo and the shutter pressed half way I moved from the brick wall to an area the sun was hitting and still no focus lock, I had to let off and repress to get focus. Camera is a 7D
That's about it, Diver Down! I think it's around the 350mm mark it starts. To reassure you, I have a Sigma 150-500mm here that will focus whereever you point it in AI Servo, but the Tamron won't (past the 350mm mark). Same on 7D & an old 30D body I have.
I remember we found the true fstop and true focal length somewhere back when the lens was just announced. I searched but can't find it on the web. Let's just say the true numbers weren't quite what the name of the lens implies but it wasn't that far off. I know I posted it in the original announcement thread. I will try searching for it.
surfinrobin wrote:
The next poster that says the AI Servo issue is down to user error, or that we are expecting too much, will find my 150-600 lodged firmly where the sun don't shine!!
Ask yourself this - has anyone yet, in the 3 or 4 threads on this lens said;
"Yes, I have a 7D/5DII/1DIV (or older) and AI Servo works perfectly with my set up."
No?
Now I'm no detective, but it seems to me there is a common theme developing that maybe, just maybe, that AI Servo doesn't work as expected on anything other than the newer Canon bodies.
Surely we can all agree this has been established. We should be happy that, unless someone who has one of the cameras mentioned above or older and can conclusively prove AI Servo works with this lens for them, we can, with fair consideration, assume there is an issue.
Yes, it's a cheaper 600mm than a Canon 600mm, but if it only has full functionality on certain bodies (and not even between 'pro' and 'budget' bodies (be interested to hear how it performs on the latest x00D body)), I don't think it's unreasonable to expect to be informed of this issue before parting with my hard earned cash.
Especially as this lens will be used for the type of photography where this feature will be used more than any other.
gocolts wrote:
I found my 7D/150-600 will run and keep tracking something like a car...but if you quickly switch over to, say, another car going the other direction and try tracking that...it needs the user to release the shutter, give it a second, and then it'll re-acquire focus.
But that could just be the 7D. Let's just say the 7D has some bizarre AF quirks when it comes to tracking.
Perhaps the Tamron is an F6.3 on paper, but its true F number will depend on its light transmission and maybe the light transmission is not as good as the Canons and the AF sensor tolerance of the older Canon cameras is too wide.
Mike Engles
They pretty all use high quality coatings on lenses to minimize reflections and maximize light transmission. DxO can probably tell you the T-stops values, but I seriously doubt Tamron is worse than other lenses. Also T-stop is not F-stop, it's true F number is 6.3
