wow great topic!
Firsly a couple of points
a 28-300 with 2x tc to get to 600mm would be f11 no? and couldnt auto focus on any canon body? Its not the sharpest of lenses to begin with
It would be bordering on pinhole photography like that ! Would be funny to see the results.
Also on dof / ap / sensor size. Not sure its exactly a marketing ploy, for a given resolution of sensor a 2.8 p&s lens will put the same number of photons on a given area of sensor as a 2.8 dslr lens. The dof would be much deeper but the actual light would be correct in relation to a 35mm or crop lens. Put simple we could both take the same picture at 2.8 and get the same shutter speeds for the same iso. I routinely use my 50 1.4 at 2.8 on a 5d2 to meter for my 110 2.8 mf lens / camera, but the mf lens at 2.8 gives about the same dof as the 50 1.4 at 1.4. Simples
As for a monster lens, it would be such a dog. Sigma make a 200-500 2.8, its quality wide open is suspect and it weights the same as a clydesdale horse that just ate a cow, but the horse is smaller. 2.8 zooms dont do well above 4x mag, f4's tend to top out at 5x mag and over that you tend to be in the realm of variable max apat like the 100-400 or 70-300. Not sure why, I hate physics and maths scares me. It also costs about 28k
Not sure as a photographer if Im anal about f2.8 lol I tend to use f4 zooms as canon make some great ones. I have primes for low light (50 1.4 and 300 2.
and a 70-200 2.8. Why are they used? sometimes for thin dof to isolate a subject or for very dark conditions or when a fast shutter speed is needed. Trust me we dont buy them and lug them about for giggles or an ego boost. The 70-200 is a killer portrait lens as is the 300 , something I just cant get with something like a 70-300.
Nikon make a popular 200-400 which is a quality lens, but again its huge and expensive.
If the image quality was there I would go for a 24-500 f4 and live with the cost and weight but I just cant see them making a lens work at such a high magnification. The siggy 50-500 isnt known for prime like color / contrast and saturation. Its an amazing lens but only considering its versatility.
I settle for carrying two cams and a bunch of lenses, easiest way to do it
Plus if canon made a single lens that negated the need for you to buy 5 other lenses you know it would cost a fortune for that alone. I just want a sharp 100-400, making it f4 would be even better, i'd be ok with a 200-400 f4 is for 1500-2000. i can dream!
As for a step up camera, dont discount film or an elderly dslr such as a 10d or 20d or some of the rebels. I shot a wedding with 20ds not too long ago due to loaning my primary cameras to a fellow tog whose cameras had busted and I got a call from a bride at the last minute desperate for a shooter. Its not that newer cams arent better, just that the older ones are as good as they were when they were new which was good enough to make money then. keh is a decent place in the us, mifsuds is for the uk for used gear.