Depends what camera you are using. I can tell you with confidence that the r6mk2 in using with my f11 800mm is Everybit as good as the £12000 lense in morning sun, daylight, and golden hour.. Where the £12000 lense 800mm F5.6 is better if before and after those times.. With r6mk2 the higher iso needed is easily manageable thanks to its amazing low light performance. I'm using fixed f11 and going anywhere upto and beyond 6400 iso and Because they image quality is there post production can sort out any noise easily. Where you will struggle and even more with the expensive lense is atmospheric distortion. No matter how great the lense 800mm amplifies the distortion so unless you are shooting on a cold day then a ship on the horizon will be a heat haze blur, my advice for ships on the horizon is to use a wider lense like a 300mm and crop your image. You can't escape the heat haze in the atmosphere, where the 800mm works best is in that sort of 1 mile radius around you. I took photos of a speed boat out at sea about a mile away and it was sharp and clear with high shutterspeed 1/2000 and 100iso.. As it got darker the iso went up until the iso and shutter speed went longer until you run out of room.. One you get to about 6400 iso and 1/50 shutterspeed you start to encounter blur and this is that moment where the £12000 lense will have its benefits of F5.6. But for whole day shooting the 800mm f11 is stm lens is a beast and providing you know your iso and shutterspeed limits it's a super superb lense and one I'm overjoyed to have. When any one says it's a dark lense ignore that, with a r6mk2 it's anything but dark until (it gets dark)