Sat 09-Jun-12 09:56 AM | edited Sat 09-Jun-12 10:34 AM by nrothschild
Light gathering power is all about shooting as long as you can, as fast as you can.
Stopping down is irrelevant and somewhere above I added that caveat.
If you are shooting a subject at distance, such that you cannot fill the frame, and the image is too noisy, you need a physically bigger lens. Nothing else will solve your problem. More pixels will not solve your problem. More glass will (at least help). That is all I am trying to say. Nothing more.
>> 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?
No, I am not saying that and I did not say anything remotely like that.
I said that if you take any given lens, and put a TC on it, the resulting images (shot wide open), and displayed at a given output size, will have the same amount of noise, with or without the TC. If you want the focal length that the TC gives you, with less noise, get a bigger lens.
No one ever put a TC on a lens to cut down noise. They buy bigger lenses to do that.
>> 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?
I already clarified the idea (several times) that if distance is not an issue, then Light Gathering Power is not an issue, per se, although LGP is a factor in every lens, no matter how small. It's just LGP in smaller quantities. If you can get as close as you want, then focal length is immaterial and ultimately you may need a macro lens more than a tele.
Or you can always just use a $200 50/1.8 and shoot as fast as you want for peanuts. You just need to get close. But that is quite out of the context of sports and wildlife shooters where the idea of LGP really matters and costs the big bucks.
Since I am not getting through, let me try a simple mathematical approach.
f/ = FL/A
f/ = focal ratio FL = focal length A = aperture (diameter of the glass)
If you don't have enough reach, and you have a noise problem, you must increase FL while maintaining f/ constant or even making it wider. To do that you must always increase A at least proportionally with FL. A is the Light Gathering Power of the lens.
Bigger A = bigger lens.
It is truly that simple. I should go back, delete everything I tried to say about LGP and replace it with this simple formula. To try to explain it conceptually, in terms of real world practice, is obviously beyond my pay grade.
Edit: If you add a TC, you increase FL without increasing A. You get a bigger image, but with the same noise. A TC can solve a resolution problem, but not a noise problem.
If you try to solve your noise problem with an ever denser sensor, your end result, in terms of the basic physics, is identical to trying to solve a noise problem by adding a TC without increasing A.
It is really that simple. I wish I knew how to get that across.