Richmond, US
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> not suggest getting a teleconverter because it would mess up the body of my camera, something to do with the mount.
Probably that's an overly general statement, because there exist some that they clearly do not think will mess up the camera. In particular, they would certainly sanction the combination of the D3100, 105/f2.8 AFS VR and TC-14eII - all of which are currently offered products and which are known to be supported as such. (There are a number of other combinations in this category, most of which are quite expensive.)
What's more precise is that older TCs, possibly even some Nikon ones, do require some modification to avoid problems. Most of these are from the 1960s and early 1970s, but they do exist. Some third party ones from roughly the same age also could create problems. But we're not mostly focused on stuff from 30-40 years ago.
What's more problematic today is that your Tamron 70-300 is an f/4-5.6 lens, and adding a TC, even a mild one, takes it beyond your camera's reliable AF range. More precisely, the camera is spec'ed to AF reliably at f/5.6, which it does. If you add even a 1.4x TC, you'll be at f/8 even with the lens wide open, because the tradeoff of a TC is essentially focal length for image quality and aperture. A stronger TC, such as 2x one, gives up two stops of aperture (eg f/11 maximum) and even more image quality. Regardless, all of them are slower than f/5.6 and even if the AF works, it won't work particularly well, nor will it work at all in lower light conditions. I even have trouble with things like this on a D3 that has a much more advanced AF system.
Nikon TCs are specifically designed so that only fast telephotos (f/2.8, or f/4) or telephoto zooms from Nikon can even mount, mostly for these reasons. Your Tamron won't mount physically mount on any modern Nikon TC - and that may be part of what Nikon told you.
If you persist on this path - and I wouldn't recommend it - you can try a Kenko 1.4x TC. It will mount safely; in fact it will mount quite a few lenses, whether or not it's a good idea to do so, photographically. You will discover that a 1.4x is a fairly mild improvement in focal length, and you'll find out for yourself whether or not the AF works to your satisfaction. I can assure you that it won't work well, say, indoors, where there just isn't enough light. If will probably work decently well on a bright summer afternoon for a kids' soccer game. And somewhere in between will be your tolerance. Remember that the D3100, as the entry model of the family, just doesn't have the most sensitive AF system.
You could also try the Kenko 2x TC. It may work decently in high noon light; and it will not even attempt to AF in most indoor conditions. You are likely to also find that the results are not so good, even if you achieve focus on an f/11 lens. That's because f/11 is a pretty dark aperture, and image quality is significantly impaired by most 2x TC's. (The only one I can really recommend is the Nikon TC-20eIII Aspheric, a $500 item that only mounts high end Nikon lenses ranging from $2400 to $10700, plus the above-mentioned 105/f2.8 AFS VR at $900. On most of those lenses it works splendidly, though.)
Other options? I'd first evaluate what you want to do with this. The moon is an interesting shot, but you really won't get too much variation, since our relationship here on Earth doesn't change much. If that's the reason for this purchase, I'd go rent something. If you have other interests, perhaps your kids are getting really into sports, or you're into dog trials, or wildlife, there are other approaches. However, I'll warn that few of them are really very viable under the $500 level. Even the "inexpensive" options typically start at nearly $1000 and go way up from there. _____
Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
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