I've been shooting Canon SLRs for years and having bought into a system it is hard to give it up. But, a few months ago I switched to a7R.
Gear selection can be a very sensitive and highly subjective matter, so it matters not why I decided to move away from Canon. The real question is why a7R? Well, there are several reasons my heart swayed this way.
It is not always possible to bring a whole DSLR kit on a trip. The camera and the lenses are quite big and heavy, and if you want to bring three lenses with you it becomes cumbersome to carry. With Sony, I can bring three lenses and the bag will still be lighter than 7D with 24-105 f4 L lens. It is a perfect travel companion for someone who wants a full frame sensor, great image quality and interchangeable lenses in a compact package.
Ever since I switched from film to digital I have been using APS-C sensor cameras. Initially because of the lower cost, and later I upgraded to Canon 7D because I didn't want to wait for 5DmarkIII, and 5DmII was outdated to spend ££ on. I always wanted a full frame camera, but I felt that my photos looked good enough with the APS-C sensor and if anything I needed to work on being a better photographer and not spend money on gear.
I would probably get Nikon D800 or D800E if I didn't decide on the Sony. But combination of full frame sensor with lens selection available to mirrorless systems through adapters made this camera an attractive option.
And let's be honest, a7R packs one of the best full frame sensors made to date. Same sensor used in Nikon D800E. Yes, sensor is only part of the equation, but for someone looking for a full frame camera this is a good start.
Despite the amount of APS-C cameras on the market none of the DSLR manufacturers have a strong line of lenses designed with APS-C sensor in mind. All the high quality lenses released are for full frame. They work on crop sensor, but they mostly weren't designed with one in mind. There are some exceptions to this rule, like Tokina 11-16 f2.8 (which by the way is THE best ultra wide angle zoom for Canon EF-S and Nikon DX) that wouldn't work on a full frame camera. Or like advanced 18-55mm zooms from Canon and Nikon, a great replacements for standard kit lens. But majority of good lenses are designed for a full frame. As a result you find yourself struggling with lens choices, especially when buying primes.
In terms of size, modern autofocus lenses are large and heavy, especially the zooms. 35mm and 50mm primes are probably the smallest DSLR lenses available, but on APS-C sensor they frame as 50mm and 80mm. If you want a 50mm equivalent - you need a 35mm prime. Want a 35 mm equivalent - buy a 24mm prime. As focal lengths get shorter lenses get heavier and more expensive, especially if you are after a large aperture as well. If you want a 24mm equivalent you are out of luck, you have to get something like Zeiss Distagon 18mm, we are talking expensive slow lens.
My point is that once you move up to some of the best lenses manufacturers offer you either need to upgrade to full frame or you to adapt and make compromises, and you still end up with a large and heavy kit. Mirrorless full frame has the potential to give you the best of both worlds. You can keep the weight down, enjoy native 35mm framing, and have access to some of the best lenses ever made.
I'll talk more about the lenses in a separate post. But I am very pleased with some of the lenses I tested on a7R so far. There are issues to work around while picking the wide angle options, but those are not deal breakers.
And a final thought. If you shoot a lot with long focal length, 100mm and up, you might benefit from using crop sensor camera. So weight your lens preferences carefully before committing to full frame.
When upgrading to full frame the options are limited to DSLRs, Leica M or Sony a7/a7R. Beside a few consumer options from Canon and Nikon all other options are significantly more expensive than the Sony. And like many stated before, digital backs are a depreciating asset. It is a money you can't recover.
I know some will say that if you are switching systems you probably won't be able to recover more than half of the cost of your whole kit (unless it's Leica :)), plus you already have lenses you can use on a full frame camera from the same brand, so why would you switch systems?
Well, I don't think there is one answer to that. The answer is what is right for you. For me it was the combination of all the factors above that made this package a cost effective option. I was looking for full frame capabilities, and a smaller kit that will be easy to travel with while still delivering great performance.
For the price of Canon 5DmarkIII body I got a7R and FE 55 f1.8 lens. I recovered enough money by selling my 7D, four lenses and some accessories, to buy a7R with two lenses and a Lee Seven5 filter system.
In the next few month I will be posting review sections about a7R and some of the kit I've got for it or tested. Hope you will find it interesting.