ISO Help

[Monkeymicra]

right the ISO in film days used to be how quickly the film exposed the picture you took, so ISO400 would be good for motor sport as it was a "quicker" flim than iso 50, but ISO400 was more grainy.

so if im shooting Motor racing i should be using the fastest ISO (on my camera its 400 and the lowest is 50), and if its a staici shot of a building i sohuld use ISO50

but some times (in bad weather/dim light) it gets really grainy when shooting with ISO 400, how do i over come this.

also in this photo

http://i71.photobucket.com/albums/i130/UJCCPhotos/MX5%20Meet/Jan%202007/MX5HantsMeet-Jan07-800x600Photo59.jpg

its static but really grainy, but the EXIF data say's its ISO-0?? (the photo has had no PP just resize for web)

in digital cameras what does the ISO do?

im getting really confused, there is so much conflicting info on this its making my brain hurt

[bcap]

The ISO is just how sensitive your film (or sensor) is to light. The higher the number, the more sensitive.

Therefore, ISO 1600 is very sensitive to light and therefore you can use a faster shutter speed. ISO 100 is not very sensitive to light, therefore you need a slower shutter speed than you would at a higher ISO to get the same exposure.

[Jon, The Elder]

As of today (2007) very much of the terminology used to explain or define things, is based on the well established film business.
ISO is and has the same effect for digital as it does for film.

Remember, that The ISO ratings for digital are only approximations of what film represented.

Higher ISO ranges produced film "graininess". That visual effect is now known as "noise".

Everything in photography is a compromise of some sort.

[StewartR]

so if im shooting Motor racing i should be using the fastest ISO (on my camera its 400 and the lowest is 50), and if its a staici shot of a building i sohuld use ISO50Not necessarily. You get better quality with lower ISO, so you should use the lowest ISO that you can get away with, which is compatible with your requirements for aperture and shutter speed. Shooting fast moving objects like racing cars might require you to go to a higher ISO, but not necessarily - it depends how brightly lit they are.
but some times (in bad weather/dim light) it gets really grainy when shooting with ISO 400, how do i over come this.You might not like this, but the simple answer is to get a better camera. Seriously. One of the biggest determinants of noise is the pixel density on the sensor. Your Powershot has a tiny sensor with lots of pixels, so it's noisy. A DSLR like my 350D has a larger sensor and lower pixel density so it's less noisy. A full-frame DSLR like the 5D has an even larger sensor and less noise still. Another big factor is how new the camera design is. Other things being equal, newer designs manage to reduce noise than older designs just through having better electronics.

The other way to reduce noise is to make sure you don't under-expose your shots. Under-exposing and then upping the exposure in post-processing can generate huge amounts of noise.

[Monkeymicra]

cheers for the replys guys,

StewartR i know i need a better camera, i want a 350 or a 400, but i just simply cant afford to (i have the G/F looking over my shoulder to make sure i dont look for one and order it LOL)

my point is ive had less noise in shots if i shoot on auto, but i dont waht ot shoot in auto mode, i was just after a bit more understanding so i know why and when to change my setting and what effect it will have on the photo

[Pete]

It could also be a problem with underexposure. The top half of your shot is pretty bright and the bottom half is fairly dark, which may have tricked your camera's exposure meter to average out the light between the two. This leads to under exposure of the dark areas, which in turn would cause more noise.

If the camera has exposure lock feature, try pointing the camera downward (not having too much of the bright sky in frame), locking the exposure, and then reframing the composition before pressing the shutter. The dark areas should lighten up and you'll get less noise.

Either that or try to frame the shot so the background is darker (more trees behind or more light on the cars). Also, don't be afraid to use the flash to lighten up the dark areas).

Getting results in circumstances like this doesn't mean you have to get a new camera (although you're plenty good enough to warrant getting a new camera).

[StewartR]

It could also be a problem with underexposure. The top half of your shot is pretty bright and the bottom half is fairly dark, which may have tricked your camera's exposure meter to average out the light between the two. This leads to under exposure of the dark areas, which in turn would cause more noise.Sorry Pete, but I don't think this makes sense. But it's close.

I have a black-and-white cat (see my avatar). When I take a photo of him, the meter averages out the black and white bits and sets an exposure that would be appropriate for mid-grey. The white bits come out lighter than mid-grey and the black bits come out darker than mid-grey, but they are not under-exposed. They're meant to be darker than mid-grey.

However, if I were to spot-meter off a white bit, then the camera would still set an exposure that would be appropriate for mid-grey. So the white bits would come out mid-grey and the black bits would be very dark indeed, and this would be under-exposed. I would bump up the exposure when processing the image and that's where the noise would come from.

Monkeymicra, the issue with the noise in your photo might be that there are a lot of light bits in it - the bright sky and the yellow car. The camera's exposure meter would probably under-expose the scene - it doesn't know how bright they're meant to be, so will set the exposure to make them mid-grey - and then the noise comes from when the image is processed (either by you or in the camera) to lighten it.

[islandtime]

As of today (2007) very much of the terminology used to explain or define things, is based on the well established film business.
ISO is and has the same effect for digital as it does for film.

Very true. In the film days higher ISO film used more grains of silver to get the picture. The higher the ISO the granier the picture. This was due to the grains of silver. This holds true for digital. As the ISO increases so does the noise. Most digital sensors have to "guess" how to interpret underexposed areas in a photo and the result is noise.

Don't think of ISO as something that you set based on the speed of movement of the subject. On a sunny day you may shoot action at 100 ISO and a shutter speed of 1250. On an overcast day or in the evening you may have to bump the ISO upwards to get the same shutter speed. Of coarse with each level of ISO you get a coresponding level of noise. You can test this for yourself and see that the movement doesn't decide the ISO as much as available light does.

I usually shoot in 100 ISO and only raise the ISO when I can't get the shutter speed that I desire because of the lack of light on the subject.

Try the sites below for a great explaination of camera settings and film speed.

Flim Speed:
http://www.photonhead.com/simcam/filmspeed.php

http://www.camerasinteractive.com/virtual_camera/main.htm

http://www.photonhead.com/simcam/

[Wilt]

As indicated by islandtime, higher ISO film had grainer pictures. That is because the larger silver grains improved the light catching ability of the grains in the emulsion. They were simply easier to see!

In the case of digital, grain size is irrelevant, as the pixel size is fixed (unless you change the pixel matrix to be captured). When you have a low ISO, there is relatively low amplification of the signal from the sensor. So there is low amplification of the inherent noise in the circuit and your eye does not detect that noise. With high ISO, there is relative high amplication of the weaker signal from the sensor. So there is high amplification of the noise as well as the relatively weak image. So the signal-to-noise in a high ISO picture is reduced and your eye SEES the noise as well as the image.

Low ISO: Noise = 0.1, Signal = 10; assume aplification to be 10x; s/n ratio is 100:1 ('noise free')
High ISO: Noise = 0.1, Signal = 2; assume amplificiation to be 50x; s/n ratio is 100:5 or 20:1 ('noisy')