AF can be used until about ƒ9.3. I can't get AF to work at all with the the 1.7x AF adapter an A-400 ƒ5.6, but F5.6 works fine and as far as I know f-8 works pretty good as well. It seriously depends on the bird in flight. I've practiced on seagulls with the MF A-400 and managed to nail a few shots, so the AF has to be pretty much inoperable before it's useless. There's not AF system anywhere slower than me cranking that A-400. Find if the widest aperture of the lens is 9.3, the lens simply won't focus.
With regards to underexposing, that also increases noise. Less light, more noise, it doesn't really matter whether you do it with the EV dial, or the ISO setting.
For the images posted above, the 1.4 TC will make huge difference used on a DA*300. Not necessarily on a 55-300. You are already well beyond the range at which you are going to much detail in your photos because your subject is so small. There is going to be a 37% increase in resolution on your subject, with the 1.4 TC and that will be visible. But at the distance, it's still not going to be a great image. maybe a 1200mm lens would be better. A 300 with TC really isn't getting you into the ball park if the subject is the bird.
For BiFs, I never go under 1/1000s. You don't know what your lens is capable of shooting under that.
Originally posted by Alex645 Is cropping a DA 300mm the equivalent of using the same lens with a 1.4x TC better or worse?
Worse. The TC gives you a 37% increase in subject resolution. Cropping to the same size reduces resolution 40% As long as you have good lens that is out resolving your sensor the TC is going to give you a better image, unless you are right on the edge of your ISO limit, and it still may. YOu have to take a really big his in resolution from noise reduction etc to equal a 40% resolution drop.
I have a few lenses that are not sharp enough to take advantage of a TC, my F 70-210, my VIvitat M 135 are examples, old film lenses that were consumer lenses not DA*s. Most long lenses are fine with a TC and our Sigma 70, Tamron 90 and Pentax 100 macros all work great with he TC. For the lenses I bought the TC for, the Tamron 300 2.8, the DA*200 2.8 and the DA*6-250 ƒ4, there is no noticeable penalty using the TC. I suspect any difference is so small as to be difficult to measure.
Here are 7 images taken with the DA*60-250 and F 1.7x AF adapter. I would be pretty hard to argue you'd get better taking the TC off, and cropping the image.These images are on an overcast day except for the woodpecker at the end. Low light isn't as much of a problem as people often make it out to be, as longs you are under your ISO limit (for me on a K-3 that's 640 ISO, and 1/1000 of faster.
http://s1132.photobucket.com/user/Norm_Head/slideshow/Pentax_forum/Sample_by...-7%20TC?sort=3