Upgrading to a Full Frame Camera, is the Canon 5D Mark III still a viable choice?
The 5D Mark III is the same camera it has always been. At the current selling price for new examples, it's a lot of camera for the money. But the more current EOS 6D Mark II is selling for about the same amount and, other than a few minor differences that may or may not be important to you, is more or less in the same league as the 5D Mark III.
If I were looking for another FF Canon camera today and couldn't stomach the price of the 5D Mark IV, I'd take a serious look at the 6D Mark II. As it is, I've been shooting with a 5D Mark III for several years now and plan to continue using it (and my 5D Mark II as a "wide" body along with a 7D Mark II as a "long/action" body) for the foreseeable future. Sure, there are a few things the 5D Mark IV offers that I would like to have, but the 6D Mark II offers most of them. Neither offers enough to me for what I shoot to spend the money, (though the flicker reduction feature of the 6DII comes awful close, and I may eventually bite). What you shoot now or plan to shoot in the future may differ significantly in this regard.
Moving from an APS-C camera released in 2015 to a full frame camera released in 2012 should give you some improvement in terms of noise when shooting at high ISO in low light situations. Moving to the 6D Mark II released in 2017 should give you a little more.
I find that I can use my FF cameras at about one stop faster ISO than my APS-C bodies with the same generation sensor technology and get about the same signal-to-noise ratio in low light.
If your fastest lens has a maximum aperture of f/2.8 or f/4, though, you're still going to be hamstrung by the speed of your lens a bit. A FF sensor is not a magic bullet. There will still be times when you will get noisy and blurry pictures using a FF camera with any lens.
Look into an f/1.4, f/1.8, or f/2 prime lens along with a larger sensor if you really want to be able to shoot in low light. Of course the narrower depth of field you'll get from using a wider aperture will be compounded by using a camera with a larger sensor - so you'll really need to up your game in terms of shooting technique and controlling autofocus.
As to whether the 5D Mark IV is worth the premium over the 6D Mark II or the 5D Mark III all depends on how useful the extra features offered by the higher priced camera are to you. You might find this comparison of the 6D Mark II and 5D Mark IV from The-Digital-Picture useful. Here's The-Digital-Picture's comparison between the 5D Mark III and the 6D Mark II. Just for completeness, here's the 5D Mark IV vs. 5D Mark III comparison.
Before we wrap it up, let's look at a few things included in the question that aren't exactly so.
Bigger sensor = more light = higher shutter speed.
Not unless you are willing to increase the ISO. We measure exposure in terms of light per unit area. At the same aperture and shutter speed, the "extra" light a FF sensor collects is distributed over an equally larger sensor area. The exposure is the same. It is true that most FF cameras demonstrate about the same noise level shooting the same scene at one stop higher ISO than their APS-C counterparts with the same generation of technology. But to shoot at a faster Tv you still need to open the aperture or increase the ISO.
The Higher ISO on full frames seem to have less noise in the RAW files in lower light than my rebel camera.
There's no real way to know for sure, because you can't really view a "raw" file. When you open a raw file in any raw conversion applications you're looking at one of two things:
- A jpeg preview generated in-camera and attached to the raw file at the time the photo was taken
- A conversion of the raw data by the application to a jpeg-like preview that is sent to your 8-bit or 10-bit monitor.
In either case, some noise reduction is likely being applied before you see what you see on your monitor. Even though you can't really see a raw file, though, you can see that under similar shooting conditions and processing choices FF cameras will demonstrate less noise than cropped sensor cameras when both have the same technology in them.
I can use much more professional lenses like the L series lenses with their intended sharpness like the 70mm to 200 mm and the 24 to 70 mm 2.8. I like to shoot with a wide aperture.
You can use those same lenses with any EOS camera. Assuming the resolution of the lens rather than the resolution of the sensor is the limiting factor, larger sensors do tend to give higher resolution with the same lens because the larger sensor means less enlargement for the same display size compared to an APS-C camera. But it also means you need a longer focal length to get the same field of view. That can work in your favor for wide angle lenses, it can work against you for longer focal length lenses.
Most Full frames have the Autofocus Microadjustment for lenses as opposed to the Rebel line. Which is huge for me and one of the primary reasons why I hate my rebel.
Other than the 77D, which is the successor to the Rebel T6s/760D and could just as easily have been named the Rebel T7s/810D, much of the x0D series (50D, 70D, 80D) and the 7D series also have AFMA. Those are all APS-C cameras. A FF camera is not required to join the AFMA club.
But I do like the 1080p 60fps video and the 4k video options on the MK IV.
Keep in mind that 4K video with the 5D Mark IV is highly cropped. It only uses the part of the sensor in the middle, for a 1.74X crop factor, that is slightly smaller than Canon's APS-C sensors. So for 4K video you're not gaining anything in terms of total light gathering over the 80D. 1080p video does use the full sensor of the 5D Mark IV.