Originally posted by Ian Stuart Forsyth Only if that lens you have selected has shallower DOF or you are forced into buying a faster lens than you need
200 F/2 on cropped compared to FF 300 F/2.8
200 F/2.8 to 300 F4
cropped 300 F/2.8 to FF 400 F4
all about the same size, cost and weight
You are talking about shooting wide open. That's the only place you need ƒ2 to be comparable. So at one f-stop. For most of us we'd be shooting at least ƒ 5.6 on APS_c and ƒ8 on FF, but we could both be using an ƒ4 lens, or we could both be shooting ƒ2.8 lens. Shooting at 5.6 the advantage is to APS-c as for every aperture where the lens is stopped down a stop or more, we'd be using the same lens, but getting more reach with the APS-c camera. SO essentially your comparison is only valid for that percentage of your work where you are shooting wide open.
For many of us, that is irrelevant, for other reasons as well. We just don't shoot wide open very often. I shoot wide open in many sets, because I shoot at at least 4 ƒ-stops and select the one I think best suits the image, but rarely is the wide open image judged to be the best shot, or even worth keeping. So for many shooters, the question is , what will the lens do on APS-c and what will the lens do on a full frame and it just gives us more reach on APS-c. That is true for almost every f-stop, except for when the lenses are wide open. But Shooting ƒ2.8 on APS-c 200 you're likely to prefer the image to the ƒ2.8 FF anyway, so the point is a moot point applying to maybe 2% of your images. The rest of the time APS-c allows you more reach from the same lens.
It is kind of surprising though that a Nikon 300 ƒ4 weighs less than the DA* 200 ƒ2.8. The Canon 200 ƒ4 however weighs quite a bit more. So even that is somewhat dependant on the lens design.
AT $2000 USD, the 725 gram Nikon 300mm f/4E PF ED VR AF-S Nikkor is three times the price as the DA *200 2.8 and 825 grams.
The Canon EF 300mm f/4L IS USM Lens is $1300 USD , weighs 1,300 grams, so is way heavier and almost twice the price of the DA*200 2.8
The DA* 200 ƒ2.8 is $725 USD, by far the cheapest, and weighs 825 grams. The Nikon is designed to be lightweight the previous ƒ4 weighed a lot more, and there is a price premium for that.
SO just using your example, using available lenses, your wrong on price, incredibly wrong in both cases on price, weight is split the Canona ins heavier the Nikon is 100 grams lighter, but you pay for that. The previous Nikkor version the Nikon 300mm f/4D ED-IF AF-S Nikkor was pretty much the same as the Canon, $1300 and 1440 grams, 600 grams (more than a pound) more than the Pentax 200 2.8. SO even if we use your completely silly logic, you still haven't got it right.
All 300 ƒ4s are more expensive, close to twice as much. Two of the three examined are quite a bit heavier. Yes you can buy a lighter Nikkor, for 3 times the price.
You're pretty much grasping at straws here as far as I can tell, a quick check of the information on these lenses, and I doubt you would have posted such nonsense. Really? "forced into buying a faster lens than you need." That never happens. No one forces you to do anything. APS_c at least of your example in every comparison, saves you a lot of money, and in most cases, a lot of weight as well. I could also mention that the 300 2.8 lens you champion as "forcing you to buy" an ƒ2 APS-c lens is $5000. Very few of us are going to buy that lens anyway so for those of us who might, it could be a consideration. I makes me wonder though, do you own that lens? Would you have to buy an ƒ2 lens to match in APS-c what you have in FF? Or was this just the usual theoretical gobbly gook, and not even relevant to yourself?
Just curious.