Is this the best lens to use with the a7R for low light situations? http://amzn.to/2lExqnH

You didn't mention what sort of photography you wanted to use it for, so, in general terms a 50mm is a good starting point for general photography. IF you are planning on doing a lot of general low light shooting then save up your money and get a faster 50mm f1.4 lens which let's in approximately twice as much light. There are even faster lenses but are much more money or not as good optically. If you buy a 50mm lens, and want to stick with prime lenses, meaning no zoom lenses, then for future lens choices consider looking at lenses with focal length that are half or double your 50mm. In other words something like a 24mm wide angle and 100mm Telephoto lens. The 24mm would be good for taking photos of landscapes, architecture, or groups of people. The 100mm, or 105mm, would be good for portraits. Again, this is speaking in general terms and what was considered the rule of thumb prior to the 70's and the introduction of affordable zooms with good optics. Nowadays zoom lenses seem to be the norm with people starting out with a camera that came with a inexpensive "kit" lens or two. Some if those can be a good deal for beginners but they sacrifice light gathering ability for versatility of zoom. Even the professional zoom lens costing a thousand dollar, often much more, are limited to a maximum aperture of f2.8, fast for a zoom, but slower than a prime, or fixed focal length lens. Your first step should be to stop and consider exactly what you want to photograph, now and in the future, next your budget, then look at what is available. If you're undecided which lens to go with consider renting a lens first. In fact if you're only going to need a particular lens once or twice, say for a vacation, then renting is an option to consider.

When will Sony offer an uncompressed RAW format, so as to adress the image quality problems resulting from its current compression algorithm?

EDIT: It's just been announced Sony will be offering 14bit on new cameras and updating a7rii. Original post:I wouldn't worry about it. I've never had a problem with Sony's lossy RAW and I'm a professional photographer. If you're shooting RAW then you're most likely using Lightroom which supposedly compensates for Sony's 11+7 compression algorithm anyway.

Does it have overheating issue?

The problem plagues mirrorless Sony cameras in particular. It is rare on Canon DSLR cameras but when it happens the camera turns off without any warning message. (T3i is a great model if you have a good enough lens, and is extremely fast at writing to even low-class memory cards, which is insane how the DIGIC processor seems really robust.) Overheating issues are not on Panasonic cameras such as GH4. You probably want that camera for 4K if Sony gives you this problem, and I think that if videography is more important over photography on a budget, you probably want the Sony RX10 II. It is a lot better than the RX100 IV (its JPEG images are not very good even at low ISO--the lens is smaller than the RX10 II, which does play a role on a larger sensor camera). Maybe if you do not want a small camera and better battery life on a budget with greater zoom, Panasonic FZ1000 is probably the best choice. It takes really good ISO 6400 shots and very clean 4K video (remember that it is really UHD) in good to dim lighting. I cannot seem to achieve good ISO 6400 shots on any Sony model I own and samples on professional websites. Panasonic may be cheaper and is just as good, if not better. Just do not expect weather resistant protection and headphone jack on those models. Hope this helps.

Why usb 2.0 and not usb 3.0, which is the fram rate and how long the battery with video?

The reason Sony went with usb 2.0 and not 3.0 is because only with 2.0 would the camera be able to both be charged via external battery/wall charger and used to photograph at the same time. That was one of my major gripes with the A6000 as i was not able to do long time-lapse movies because of the small battery. My A7r2 solved that problem!

ASIN	B083R2V6DQ
Item model number	ILCE7RM2K/B
Manufacturer	Sony
Product Dimensions	33.5 x 16.36 x 15.24 cm; 2.01 kg

Sony a7R II + 28-70mm F3.5-5.6 OSS Zoom Lens Bundle, Black (ILCE7RM2K/B)

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