Quote: Which 2X TC are you using?
Tamron 2X F AF Review Quote: Does your camera get enough light with it attached to a more restrictive lens {say f/4, or even f/5.6} to autofocus?
No
Quote: added: I'm trying to learn what the limits of this kind of TC are.
For the 1.4 TC the limit is ƒ4 (but works better on 2.8 glass. For a 2x the limit is ƒ 2.8. This isn't a hard limit, you can extend the range, but you'll feel like you are futzing around.
Quote: Your 300 f/4 can act as a f2.8 at 6400, and still create commercially viable pictures.
ƒ4 can never act as ƒ 2.8. The AF system depends on ƒ2.8 DoF for accurate focussing. You want the image calibrated with 1/3 in front, two thirds behind and the subject in the sharpest focus. Using other than ƒ2.8, you get wider DoF and the camera can't be as precise in selecting the focus point. When you focus with 2.8 glass, the lens selects the correct focus point, then stops down from there. The focus point is the best the AF system can select. When I put the 2x TC on the Tamron it never stops searching. WIth the 1.4 TC, the focus lock is accurate and immediate. With the 2x , the point at which it wants to focus is less defined, and many times I interrupt the "searching for the perfect spot" algorithm with the shutter release, because I've found if I don't it never settles on a focus point. The range for acceptable focus is just to wide. The AF can search back and forth within acceptable focus looking for the "right" spot. For that reason, looking at the DoF of the final image, I may not be happy with where the images is sharp with a 5.6 lens.
Or more briefly, your focus point is selected more accurately with 2.8 glass.
The salient point here is the word "can". You can create commercially viable images, but they won't be as good, I find noise starts creeping in and results are unpredictable. Sometimes I get good images with the K-3 at 1600 ISO, sometimes I don't with the K-1 at 800 ISO. So ya, I can also get commercially viable images from my Optio W90 waterproof point and shoot. In fact our best selling image was taken with one. I'm not sure that says anything. You should be able to get a commercial viable image with almost any camera on the market.
I'm quite happy with my 55-300 PLM 4.5-6.3 But let's not pretend it gives me images like the 300 2.8. My standard procedure is to go out and grab a few shots with the K-3 and 55-300, then go back to the car and get the DA 300 2.8 and set up... the subject has to ba around awhile for that to happen, but, you don't want to miss the opportunity because you took time to set up a tripod and long lens when you had the chance.
From a day when I used both. (4 flickr favs)
K-3 and 55-300
K-1 and Tamron 300 2.8 (5 flickr favs)
My flickr followers have spoken. You need ƒ2.8 to maximize your "favs".
I can do both images with the Tamron 300 2.8. It's my choice. With the 55-300 PLM I can only do the top image. But if you do what I did when I used to have only ƒ4 (or slower) telephotos you can always just say " I prefer the top image anyway." I fully expect the ƒ5.6 guys to go and "fav" the top image just to prove me wrong.
From my perspective, I don't take the 6 pound hunk of glass and metal with me for nothing. I take it because it's more functional even if it's less convenient. But as long as It's being carried around in the back seat of the car, it's not even inconvenient. And when I was guaranteed good shooting opportunities, I've carried it as far as 5 miles (with my bad hip and two torn rotator cuffs.)
Last edited by normhead; 04-27-2019 at 05:55 AM.