Thanks guys for your feedbacks! I apologize for the delay, but I've been very busy these past days. Now I'm back, so let's see:
If you have a chance to try it (just in a shop would do), go ahead... After that, if you think it is slow at all, do not even consider the 70-180mm...
Actually I did consider the 70-180mm, but finally gave up about it. Thanks anyway, Davi!!!
The new 105 VR focuses quick and will zoom past the focus point on land, but more so underwater. It needs contrast to lock on the focus point, so if a land shot does not have good contrast it will have trouble locking on. But you can compensate for this by manual focus.
I do not do much above the water macro photography now, but it may be easier for you to just get the 105 VR since you may have trouble getting a good used 70-180 mm micro lens. Also if focus speed is an issue the 70-180 mm micro lens is a lot slower than the 105 VR.
Besides the money issue, you need to be happy with the way the housing feels in your hands and what controls are accessible to you. The ergonomics of the housing is very important and good housings are expensive. I can feel the half click of the shutter and every click as a change f stops or shutter speed with my housing controls. Other housings I have tried did not have such a feel, they are just water proof boxes around your camera.
Yep, Elmer, surely I know the focus is based on contrast. Just in case on land I use to set my lens on M/A for a quick manual focus if needed. And when the situation is really critical of course I change it for just manual.
I had a look searching for the 70-180mm and I could find some, but it's a bit expensive for an used gear which I don't know how long may it rest in good conditions (I must admit: I don't like at all to buy used photo gear). Adding to the difficulty for finding new ports to this lens, I indeed decided to go for the new 105VR although the versatility of the 70-180mm have in fact attracted me a lot.
"[...] they are just water proof boxes around your camera": I definitely agree with your statement! Feel the housing on my hands in a completely intuitive way (I mean much more than just able to shoot) should be as a friendly experience as a good camera is expected to be too!
I also started the thread on the 200mm Nikon and also suggest that you audit that thread. There are some very interesting comments and shots and i fear I will be purchasing this very expensive lens and making the underwater modifications necessary.[...]
I read your thread about the 200mm and every comments on it, Tom. Thanks for the indication forwarding me to that, but I don't like the idea of such a long lens + ports, adapters and so on for an underwater use. I'm fighting against an inflammation on my ancon exactly for being carrying around so much weight while working. Obviously there's a considerable reduction underwater but again it seems to be very bulky on the frontside of the housing. And besides you ever need to carry it until arrive to the boat or whatever leads you to the dive point…