I already have the Sony 70-400mm G lens for the above camera. However I am looking to get a longer lens. I do have a 3rd party 2x converter, but this will only work with manual focus (not good for sports or action - e.g aircraft, etc). I have scoured the internet but as yet only found 1 lens - Sigma 150-500mm f5/6.3 HSM DG OS - in Sony A fit. It is from a reputable store and according to 'their' description: 'Average condition telephoto zoom lens for use with A mount SLR's. It's pretty spot on condition wise with light signs of general use, however it's been marked down a touch as there's a small spot of fungus on a central element unfortunately. This won't affect it's use but is worth mentioning. Comes unboxed with caps, hood and a 6 month guarantee'. It's priced at £449.99. I would welcome thoughts (pros and cons) on this lens as opposed to my G lens. Also if anyone knows where else I might be able to get the 200-600mm Tamron in Sony A fit, which would be better as it is longer. I am reluctant to go down the eBay route, or upgrade to Sony E mount, as I already have a number of lens in A mount fit and do not particularly want to 'start again', or trade all of my Sony gear to another make. Thanks in advance.
If it has fungus then I’d stay clear unless you will dedicate a body to it and keep it well away from any other gear. Fungus can sometimes clear up if exposed to UV but if you are swapping lenses on a body you risk moving it to another lens.
Alternatively Tamron did 2 versions of it's 150-600mm lens in Sony A-mount, but I can't help with where you can get one. https://www.dyxum.com/index.asp is a good source as to what lenses were released for A-mount, includes user reviews and may provide a few alternatives to search for. (edited to remove incorrect information)
I would keep on looking, avoid anything with fungus, things can only get worser! Unfortunately used Sony A fit longer lenses seem relatively uncommon. Iirc the Sigma 50-500 was reckoned to be rather better than the 150-500.
I've owned one for a very long time and hardly use it. At 500mm it's not so good but it might be down to handling. I paid £499.00 and I'm talking pre digital or at least early digital. This price was for new. Sometime later I bought the 50-500mm second hand for £300. It's a lot heavier but I found it preformed better across all focal lengths. I would suggest you look out for one but it is a heavy beast .
Not a zoom but worth a look is the Minolta 500mm F8 AF reflex, around £200 on Ebay or for silly money Minolta 600/F4 Sigma did an A mount 600 reflex but MF not AF.
Over many decades I've learned the following about choosing and using lenses with long focal lengths... Camera shake increases with weight and length, so stabilisation, either by placing the lens on a support or by using optical/electronic technology is extremely useful. Photographing distant objects places you at the mercy of atmospheric conditions such as turbulance or polution about which you can do little other than to chose your subject and circumstances with care. All optical technology is not equivalent, especially when it comes to magnification factors. A 720mm focal length lens on 35mm (giving a magnification of 14.4x compared with the standard 50mm) is much harder to make and use than a 129mm lens to cover the 1/2.3 format and giving the same effective magnification. So far, my least bad compromise lens for telephoto work has been the Panasonic 100~400mm. Mounted on a G9 body it gives surprisingly good shake control and general optical qualities. Some example ground to air shots from the 400mm end of the above combination. These were taken on a very hot day with the aircraft flying over the sea. All were cropped to about 1/8th of the 20MP frame...
I did have the 500mm, it wasn't bad, it wasn't wonderful either, a lot easier to handle than a heavy bulky zoom though.
You may be disappointed by the difference between the 'magnification' of a 400 mm and 500 mm lens, so try to test a lens at 500 mm on your camera body before spending any money. The heavier lens may be more difficult to handle, so the results may not be any better than a shot at 400 mm that has been cropped to the angle view of a 500 mm. A few seconds of searching found this: https://www.nikonians.org/reviews/fov-tables Look at the DX column (Nikon's name for an APS-C sensor, as in your Sony camera body), and you will see that the angle of view at 400 mm is 3.4 degrees (horizontal) and at 500 mm is 2.7 degrees, which I calculate to be about 80% of the width of the 400 mm image and matches what I would expect for a 25% increase in focal length. I suppose this is an example of the 'law of diminishing returns', in that the jump from 100 mm to 200 mmm is dramatic, but the same 100 mm jump from a 400 mm to a 500 mm lens is much less so.
