Sat 09-Jun-12 09:15 AM | edited Sat 09-Jun-12 09:21 AM by PRW
I have been reading this with great interest.
>>If LGP doesn't rule, then why do people spend extra for a 600/4 - and haul the extra 4-5 pounds around - if they can do the same with a 500/4 and just get closer? What makes a 600/4 better? It's the 6" of glass on the front, verse the 5" of the 500.
I always thought it was purely to do with reach. You can shoot from further away with the 600mm. I understand your thoughts on the light gathering power of the front element, but at the same f-stop the amount of light leaving the rear of the lens is the same for any lens, surely? My understanding has always been, a long telephoto needs such a large front element because, due to the long focal length, the front element needs to be the size it is, in order to gather the number of photons necessary to be f4. Therefore the 600/f4 needs a 6" front element in order to gather the SAME amount of light as the 500/f4 5" front element.
>>If you pop a TC on a lens you will NOT reduce noise because you did not increase your light gathering power.
Can you clarify something for me? I may have misunderstood. Are you saying that if I shot a 600/f4 at f5.6, and a 300/f2.8 with a 2x teleconverter wide open (f5.6), with the same shutter speed, ISO and camera body, the shot using the 600mm would be contain less noise? Forget the IQ hit from using the teleconverter. It has always been my understanding that the amount of light, the number of photons reaching the sensor, would be identical and therefore the noise levels the same.
I have always thought the only reason to carry a 600/f4 instead of say 600/f5.6, was the advantage of being able to select a higher shutter speed when shot wide open, better subject isolation, the more accurate focusing and brighter viewfinder image. Assuming both lenses were shot at f8, then I always thought there would be no difference in the images. Are you saying there would be?
>> For that matter, if only focal ratio mattered, then a 300/2.8 would be even better. It's a stop faster, cheaper and lighter. You're a 3 time winner. Why are we not birding with 300/2.8's?
Purely, because we would have to get a lot closer to the bird. If you can get close, it's all you need. Where am I going wrong in my thinking?