tomcat wrote:
I have a 70-200 f/2.8 and it is very noisy because the ISO climbs to >20,000 at distances of more than 50' away. That's one of the reasons I'm trying to replace it.
Huh? This makes little sense!
Distance has very little do with exposure! It is the brightness of the subject and how you're trying to render an image of the subject that determine what exposure factors you need to use. And, unless you are shooting in near darkness, with way too small aperture and way too fast shutter speed, there's no reason you should be "forced" to use such a high ISO.
When you say the "lens is noisy", I assume you mean you are finding yourself using very high ISOs and you are seeing digital "noise" in images.
BUT... there is no reason why focusing on something "more than 50' away" should force you to use high ISOs.
The three exposure factors are: ISO, shutter speed and lens aperture.
ISO is the sensitivity to light of the medium (in this case a digital image sensor).
Shutter speed is the amount of time that light is allowed to reach the medium.
Aperture is the size of opening that's allowing the light to pass through.
It's a combination of these three factors you need to use to make a "correct" exposure. The choices are determined by the brightness of the subject being photographed.... regardless of distance, for the large part.... and how you're trying to photograph it.
Think of it as a bucket full of water with a drain in the bottom. You can change the size of the drain and control how long it remains open. You can use a small opening that's held open for a long time OR a large opening for a short time.... and drain the same amount of water from the bucket either way.
With photography, you generally want to use shutter speed and aperture that allow you to use as low an ISO as possible, in order to minimize "noise" in images (this was true with film, too, BTW.... higher ISO films got "grainy").
But you need a shutter speed that's fast enough to be held steady to prevent "camera shake blur", unless you're using a tripod or other type of support. Aside from that, with stationary subjects most any shutter speed can work. But with moving subjects you either need a fast enough shutter speed to freeze the movement or a slow enough one to allow deliberate subject movement blur effects in your images.
And you need to choose an aperture that renders the depth of field you want to see in your images. A larger aperture renders shallower DoF effects, while a smaller one increases DoF. Maybe you want a shallow DoF in order to make a background blur down so that the subject stands out against it. Or, maybe you want things to be sharp from your toes to the horizon. Achieving different DoF effects varies depending upon lens focal length and distances. A wide angle lens (short focal length) gives greater apparent DoF than a telephoto (long focal length).
A very close subject at macro magnifications can make for very shallow DoF, too. A very distant background behind a subject may be more easily blurred down, too.
Over time and with practice you will learn how different focal lengths perform and how to control DoF and subject movement effects. There's a limit to how much you can do, of course. But there are "work arounds". For example, I wanted to make a macro shot of a flower with some other flowers recognizable in the background. If I used a typical macro lens to photograph the flower, the ones in the background became too strongly blurred and unrecognizable. So instead I used a 20mm wide angle lens with a short macro extension tube to make it able to focus closer, which I knew from experience that the shorter focal length in addition to a smaller lens aperture would reduce the strength of the background blur so that the more distance flowers wouldn't be blurred down to unrecognizable blobs of color. I had to use a slower shutter speed to achieve this at a fairly low ISO. Here's the the result:
Other times I wanted to do the exact opposite. For the following, 15 or 20 feet behind the flower was a fence that I didn't want to see in the image, so I used a 500mm lens and a fairly large aperture (for that lens), on a tripod for stability at a reasonable shutter speed. As you can see, doing this completely obliterated the background to help the subject stand out:
You can do similar things with shutter speeds and moving subjects. Of course you need to use a shutter speed fast enough for you to hand-hold without camera shake blur problems, unless you're using a tripod or similar to support it. If you want to freeze a subject's movement in the image, use a faster shutter speed, as I did for the following shot:
Or perhaps you'd like to let some of the subject's movement blur.... or pan with the subject to cause the background to show motion blur.... or a bit of both... and need to use a slower shutter speed:
It sounds to me as if you are really struggling to grasp how the three exposure factors work and you can use them in various ways to achieve a "correct" exposure. It's a lot to try to cover properly in forum posts... you're likely to get only fragments of info and coming from people with wildly different levels of experience, responses that try to help but may be right or wrong, or are described in a way that's easy to understand or not.
There have been whole books written on the subject. It sounds as if you have some fairly serious investment in camera gear, so I'd recommend you spend another $18 over at Amazon and order a copy of Bryan Peterson's "Understanding Exposure". Many people find that book very helpful. It will certainly be better than the snippets of knowledge you'll get from a discussion forum or Youtube videos. Alternatively, enroll in a local photography class.
Hope this helps!