Originally posted by Jinnax If it's dark, higher ISO allows for higher shutter speeds, but the price is the noise ("grain") shown in narual's ISO 51200 picture above.
If you have enough light, you keep the ISO between 100-400 and get no-noise pictures like old4570's ISO 200 picture above.
If you have never increased ISO beyond 400, there is no noise to discover. I recently had a good experience with my K50 and ISO1600 - I was very happy that the pictures looked more like old4570's than narual's, so I agreed with the reviews that said the K50 was excellent with noise (at high ISO).
I think you have the idea there.
When you take a photo you take care of a few things:
Shutter speed
High shutter speed allow to freeze action and ensure moving subject look sharp and still. On the contrary slow shutter speed allow to show any movement from the subject with potentially blury effect. Notice that because your hands are not perfectly still, if your shutter speed is too slow, it will blur the image randomly and un-esthetically. In such case a tridpod is required.
Here an example of very slow shutter speed, used to hide the cars on the road but only see their lights as long lines:
What is important to understand related tu shutter speed is that the slower the speed, the more light get in. 1/100s let enter twice the light than 1/200s.
Apperture
The apperture define the size of the hole that let enter the light through the length. The wider the apperture (the hole is bigger), the more light pass into the camera and also the less deph of field you get
, allowing you to isolate the subject from surrounding environement.This is ofen desired for portraiture. The narrower the apperture, the less light you let in, but the more deph of field you get. This is typically what you want for landscape to have everything in focus and sharp on the picture
. This is also what you want typically for macro as for such near subject the deph of field is almost always too small.
Portrait at wide apperture (here f/2.8). Notice that the background is quite blured
Landscape at smaller apperture to get everything sharp an in focus. Here f/8
Correct exposure
Now when you take a picture usually you want the blacks to be blacks and the white to be white and have all colors and variation in between.
If your photograph is over exposed, everything is light tones and potentially some areas are pure white. Except for few asthetics cases this isn't what you want. It simply doesn't look like in reality and is un-pleasant.
If your photograph is under exposed, everything is dark, potentially even some areas are pure black. It is difficult to see any details. Again no realistic and except for some few cases you want to avoid that.
So either yourself in manual mode or the camera in some automatic mode, ensure that the right amount of light get onto the film/sensor. Basically if the photo is too dark, using a wider exposure or setting a longer shutter time will correct the issue. On the contrary using a narrower explosure or shortter shutter time allow to darken the photo.Again the camera can manage it itself by changing the shutter speed, the apperture or both. Until now we are exactly like for film.
Sensitivity
What happen then if you are going out for a walk, at dawn or dusk, the sun is almost gone and you want to take a picture. The camera try to set a wide apperture but your lense is limited to no wider than f/4. So the camera only possibility (or yourself in manual mode) is to select a slow speed. Let say the camera choose a very slow speed 1/10s. You take the picture and it look blured because of your hand shake.
The only solution to keep a high shutter speed so there no blur from the hand shake, is to increase the shutter speed. But there no margin on the apperture that is at it widest setting. What to do? On film you select a roll with more sensitivity say iso 400, it allow to set the speed to 1/40s and the shake is no longer visible.
This is the sensitivity. For film you buy another roll with different sensitivity that can record the image with less light. But usually the result is not as pleasing. Not as natural, no as detailled: this is the noise.
On digital, you can dynamically select the iso to use (or the camera can do it for you).
On modern sensors (in Pentax that's starting from K5 so also K30, K50, K5-II, K3, K-S1, K-S2...) the performance is really really great compared to what you could have been used on film. Iso 1600-3200 iso on such camera give quality that is comparable something like 400 iso on film and anything in 100-800 iso range look almost perfect. If you choose such kind of camera and combine with a reasonnably fast lens then you'll be able to take great picture in many low light situations. In pubs, at concerts, nightscapes, cityscapes, interiors. Without having to use a flash or a tripod. The result may not be always perfect but they will be quite acceptable. In good light with iso 100-800, the result are going to be outstanding with lot of details, more than you typically seen on film.
On older sensors (ist familly, K10, K20, K7) depending of the exact camera and generation the iso performance is not as great. Starting at 800 iso the noise is quite visible. The camera also have less MP and less detail/sharpness (in particular the istD familly). Depending of the exact model the perform is either similar to film or a bit better for iso handling.
What to choose
All that being said, if money is not a huge issue, I'd advise to at least get a K5, K5-II or a K30, K50. Such body are under $300 used and give a lot. High iso and noise was one aspect. But there much more. The autocus is much better, the screen to display the pictures you have taken is bigger, the ergonomics are better... The camera is also resistant to dust and water ingress (light rain...) and also include a stabilization system to counter hand shake and allow to take picture at slower speeds.
It is only if you can't afford to spend that much money that it would make sense to use a quite older model counting all the benefit the new generation bring.