The well-known rule-of-thumb is a
very rough approximation which doesn't scale particularly well to different camera formats. It's usually quoted in relation to 35mm film cameras. It's a rule (well, more of a vague guideline) for
hand-holding, not for tripod use. If you were to translate it to digiscoping scales, where the effective 35mm equivalent focal length is typically 2-3 metres (i.e., 2000-3000mm), it tries to tell us that you need a shutter speed of better than a 2000th to hand-hold a digiscope. I say
tries to tell us this, as what it really tells us is that the rule makes no sense at all once you get to digiscoping focal lengths. If you were hand-holding a scope, you'd need
way faster than a 2000th - maybe a 10,000 or close.
You might be misunderstanding aperture priority here, Gramayr. It's not about how far your lens is stopped down, it's all about
you controlling how far your lens is stopped down. So, if you want a small aperture for depth of field, you use aperture priority and set it to something small (such as f/16) and let the camera decide what shutter speed to use to make the correct exposure. Similarly, if you want the fastest possible shutter speed, you still use aperture priority, but you set it wide open, so that the camera then uses a faster shutter speed.
You could do this the other way around too - i.e., set the shutter speed high using shutter speed priority, but it doesn't work as well. Consider:
- You set the camera to a 1000th of a second. The light is such that the camera opens up to f/4. (Which, we will assume for the same of example, is the widest your lens will go.) You get a great shot.
- Now the light improves a little. You are still at 1000th of course, so the camera stops down to get the exposure right and shots at f/5.6. That's OK but you could have got a better shot (with a long lens) at a 2000th and f/4 - even less camera movement.
- Then the light fades a bit. You are still at 1000th and the camera wants to open up to f/2.8, but you only have an f/4 lens, so you get an underexposed picture which is useless.
Now let's work the same example using aperture priority. You want the best possible shutter speed, so you set the lens wide open at f/4.
- You are at f/4. The light is such that the camera uses 100th of a second to get the exposure right. You get a great shot.
- Now the light improves a little. You are still at f/4, so the camera bumps the shutter speed up to a 2000th and you get an even better shot.
- Then the light fades a bit. You are still at f/4 so the camera drops the shutter speed to a 500th and while that isn't as good as the faster speeds you got in better light, it's a country mile better than the muddy and underexposed shot you'd have got doing it the other way.
What about auto? Well, we can't be sure about auto because we never really know what the camera will do, but we can guess that, like most cameras, it will assume that you are shooting "normal" scenes at "normal" focal lengths (family happy snap sort of stuff) and thus select mid range values to suit.
Let's work the example again.
- For the first shot (1000th at f/4 in the examples above) the camera selects a middle-of-the-range aperture, f/8, which makes the shutter speed a 250th. You get a lot of blur. (Or it might select f/5.6, which would be bettter as it gives you a 500th.).
- Now the light improves a little. Still at f/8 (because cameras like the mid-range apertures if you give them a choice), you get a 500th, which is much better, or maybe even f/5.6 and a 1000th which would be great considering you are on auto).
- Then the light fades a bit. The camera opens up a bit to f/5.6 and drops the shutter speed to a 250th again - you get blur.
Summary:
Aperture priority: you got the three best shots you could have got.
Shutter priority: you got two out of three, with one of them not quite as good as the aperture priority shotwould have been, but close.
Auto: you got one.
In reality, the ratio of good shots to bad shots using auto for digiscoping is quite a bit worse than this: you really struggle to get any good shots unless the light is absolutely fantastic - a seagull on an Australian beach in summer, for example.
I'm late for work! No time to proof read, hope this makes sense!
