[jogesh12345]

Hey Mantone.

I always think of the three main manual settings - shutter speed, aperture, and ISO - as having two main functions each. One is to let more or less light in, or in the case of ISO, to be more sensitive to light which kind of amounts to a similar thing. And then we have the second function... which I'll talk about in a sec, but let's just say for now that second function is more artistic, it's more about you getting the effect you want.

So firstly, let's think about that light. Every photo has a point at which it can be declared a good exposure. Like everything, this is subjective. I prefer dark shots, and people say my work is under-exposed, but let's assume we all appreciate something approaching a nice light, but not too bright, shot. When you're on you're Auto settings the camera will give you this. Every one's happy.

If you're on manual settings, then you have to adjust those settings yourself to give you that nice exposure. I know you're asking about aperture or shutter priority, not manual, but bear with me. If your image is too dark you have three choices - slow the shutter speed (the shutter is open longer, it lets more light in, your image is brighter), open the aperture wider (the hole is bigger, it lets more light in, your image is brighter), or set the ISO to a more sensitive level (for any given amount of light the sensor picks up more of that light, hence the image is brighter). So you choose one of those options and hey ho, a nicely exposed image.

But now let's consider the second functions of aperture and shutter speed (more on ISO later). In addition to letting more or less light in they will also do this:

Shutter Speed - will allow you to blur images, or make images sharp. What I mean by this is that a slow shutter speed, whilst letting more light in, will also mean your images are more likely to be blurred. A high shutter speed, whilst letting less light in, will mean any movement in your picture is frozen. Now, which is best? In generally people like sharp images, so they tend to go for high shutter speed so the images are more likely to be sharp. If you're holding the camera by hand then a high shutter speed is a must to stop your natural hand shake ruining the picture. However, if you want to capture movement, say a dancer and you want the image blurred for artistic reasons, then a slow shutter speed might be just the job.

Aperture's second function is depth of field (how much of a picture is in focus, depthwise) - a big aperture let's more light in but also means that the depth of field is reduced. A small aperture means less light gets in, but the depth of field is good throughout. Which is best? If you're taking a portrait and want the background nicely blurred use a big aperture (small depth of field). If you're taking a landscape and you want as much of the image in focus as possible (i.e a largedepth of field ) then you'll need a small aperture. (As an aside, big aperture have small numbers e.g. f/1.8, small apertures have big numbers f/11. It's just the way it is!).

So you can see you have a light decision to make (do I need more or less light) and an artistic decision - what depth of field do I want, do I want the image blurred or sharp? Or a mixture of both.

So, let's go back to your question - priority mode and shutter speed mode.

Set your camera on Shutter Speed Priority and you're basically saying, I want control over the shutter speed so I can get the artistic effect I want (blurred movement, or sharpness) and I'll let the camera do whatever it needs to do with Aperture and ISO to get the right amount of light. i.e. you're in control of the artistic bit, the cameras in control of the nice exposure.

Set you're camera to Aperture priority and you're saying I want to choose my own depth of field, but I'm happy for the camera to do whatever it needs to with shutter speed and ISO to take care of the overall exposure.

It's a halfway house between auto and manual.

Now if you want to control the depth of field and the blurred / sharpness effect then you need to go full manual...

Regarding ISO, what you might find your camera doing in these priority modes is pushing the ISO up - i.e. making it more sensitive. This a safe bet for the camera - can effectively let more light in (or be more sensitive to light) without changing shutter or aperture very much. The downside, is that the higher the ISO the more grain/noise that you get in your image. Get too high an ISO and even if the picture is perfectly exposed the grain and noise can spoil it. More on that another day!
Cheers
Derek

Brief description of Aperture and Shutter Speed. Agree with you.