EOS R Video Recording, looking for a better low light lens

[jades]

I use a Canon EOS R to record 4k video, I know it has a 1.7x crop while doing so. If I purchase an adapter and a EF lens, will that crop increase or will there be no change? My main reason for asking is that I'm looking for a better low light lens than what I have now, which is the 24-105 f4-7.1 rf. Any information is appreciated. Thanks!

[shadowsports]

Greetings,

This is a good question.  This may help you understand better.  Adapters such as a speed booster can reduce the amount of crop, increasing the FOV, and light gathering capability when shooting in 4k on the EOS R. The 0.71x adapter would effectively reduce the crop from 1.75x to 1.24x approximately.  These adapters are actually intended for super35 sensors, but can work on the EOS R in 4k due to its crop.  You would not want to use it in 1080p since this mode uses the full sensor.  In this mode the speed booster could cause vignetting.

You may want to consider the following.  Buying the speed booster will also mean buying an EF lens. I would recommend buying used in this case.  The Canon adapter is about $650 new. I've seen others made by metabones and viltrox, but I have no experience with them.  To maintain best overall performance and compatibility I suggest you stick with Canon brand gear.

As far as what lens to choose.  Any with a constant aperture f4 or faster would be great in low light.  

[p4pictures]

If you use the standard or control ring mount adapters with your EOS R you can use almost any EF lens with your camera. Since the 4K crop of the camera uses a smaller area than the 1.6x crop that the EF-S lenses cover, then the EF-S lenses could also be used, but I have not done so. For example if you use an older EF 24mm f/1.4L II USM lens on the adapter you will gain 2-stops of light by shooting at f/1.4 over your current lens which is f/4 at 24mm. However be careful as depth of field will also reduce, though this is often thought of as more cinematic looking. 

Don't forget that there are a few keenly priced RF primes that offer fast apertures and these might be better purchases - even used - than EF ones at this time. I'm thinking of the RF 50mm F1.8 and RF 35mm F1.8 as examples.

RF50mm F1.8 STM 

RF35mm F1.8 Macro IS STM