To the OP.
The 75-300 may work with a 1.4x TC but will be near unusable with a 2x.
300mm is not a bad focal length but not knowing what you need the extra focal length for it's hard to really make a recommendation.
Yes the optical collision is a factor when TC's are used with lenses that are not physically compatible. Glass damage? The user would have to be a special kind of moron to actually damage the glass in this situation. At worst you would be out several hundred dollars for something you can't use.
What?
Things that we've updated. Cameras, lenses, cars, TV's, etc.
TC's are still made new to this very day for almost everyone one of those new cameras that were NEVER designed for film.
I once bought a SIgma 70-300, went back year or so later looking to buy a TC and ws told the TC was designed for the SIgma 70-200 2.8 and could actually damages the 70-300.
I did try it once with the Pentax 1.4, and it clearly would have damaged either the glass in the TC or the glass in the lens.
I have a Tamron, 2x Pz-AF MC7 which is sold under other brand names. I can compare it with my Pentax HD DA 1.4 TC, for which the specs are published. 3% loss of resoution, 40% increase in resolution on the subject, for a 37% gain, if the lens is sharp enough. You can’t tell the sifference between an image taken with it or without it There is no visible loss of sharpness. The MC7 on the other a hand is only used on the Tamron 300 2.8 AF ID with the 36 MP FF K-1. The larger pixels on the K-1 keep it useful to a degree, but it’s images are never as clean as the HD DA 1.4 TC from Pentax.
SO, I can get to 600mm ff equivalent with the 1.4 and 300 2.8, in my APS-c K-3 or 600mm with the K-1 and MC7. I prefer the images from the 1.4 and APS-c.
I have an old Pentax 1.7x AF converter, which coupled with the Tamron 300 give me excellent images at 510mm. IMHO those images are superior to the MC7.
So there are two things here. The TC has to be good quality. And the lens has to be really good quality. If you are going to blow up the image to twice the size, there has to be enough resolution in the lens to be able to handle that level of magnification. No 75-300 type lens I’ve seen has that in the long end, where you need it. Even my preferred Pentax DA 55-300, that has been rated best in class for that type of lens, has to be dialed back from 420 to about 380 to be razor sharp.
MY old 70-300 wasn;t sharp enough after about 120mm to warrant a TC. However it works great with my star quality lenses. DA*200 2.8, DA*60-250 ƒ4, Tamron 300 2.8 AF ID, and works acceptably well with the DA 55-300 PLM, with a bit of restraint in the long end. Sorry, I don’t know enough about the hardware of other companies, but I’d seriously suggest sticking with the manufacturers suggestions. Phyiscal damage is possible in some cases.
15 years ago I was in the same position as the OP. I owned a 70-300, and was getting soft images at 200-300mm already. There’s nothing you can do with that. A TC just magnifies the softness and it looks even softer. My choice was to buy a premium quality lens and a TC at a cost of close to $2000 CAD. I don’t think there are any shortcuts. And the third party MC7 type TCs are just not the same quality optically you’d get from an OEM manufacturer.
There are lot of great 150-400mm tyes zooms availble that will support a TCs and OEM TCs are capable of great performance, and most modern glass is designed to be useful as the resolution of future sensors increases, so they have more resolution than needed for current sensors. That’s the resolution a TC exploits.. But whatever you do, you need to move up a level from the 70-300 type lens. I look at my old SIgma 70-300 images now, and they just don’t hold up, against even old film designs like my DA*200 2.8, with the old F 1.7x AF Adapter, which was also designed for film.
DA*200 and HD DA 1.4…280mm
2016-07-28_Cedar-wax-wing-F by
Norm Head, on Flickr
DA*200 2.8 and F-1.7x AF Adapter (340mm)
Bird-Cedar-waxwing by
Norm Head, on Flickr
My absolute best shot ever taken with my Sigma 70-300.
Bird-1-Common-Redpoll by
Norm Head, on Flickr
If you can’t see a difference, no problem, but if you can’t, you may as well stick with a 70-300 type lens. You won’t appreciate the better glass. You’d be wasting your money going better. It’s good to have a few comparison images to help with your choice. The sad thing being I’ve never had another chance to get a better image of this bird. History doesn’t always repeat itself.
