The key reason for buying this lens for my Canon R is to get more detailed and closer shots of the moon than my Sigma 600mm (now sold) could.
Unfortunatley the moon where I live has been hidden by clouds for the last few days, so I decided to use a street lamp at night for the example shot to explain the feature I am going to talk about now.
The Canon R has a built in 1.6 crop.
So what I hear you say.
In essence 800mm becomes 1280mm, the f11 becomes f18 (ish), the file size drops from 22mb to 9.5mb and the sensor from 30 to about 18 Megapixels.
In the sample shown this street light is about 20ft away. The left hand image is a standard shot ISO 2000, f11,1/1000 the right hand is with the 1.6 crop on using the same settings.
I hope you can see that this feature could be useful when the next full moon comes around by giving an even closer image of the moon, no loss in detail & I will post a full video on my youtube channel showing this feature off in full.
I also want to comment that both these pictures were taken handheld with no tripod, straight away the fantastic IS built into the comparably light in weight lens has already proved its worth.
I wish you well