ISO advice needed,

i need advice on when and why i should alter the iso, should it be higher when there is less light and vice versa.
in basic terms please..remember i'm new to this.


regards daph

The primary aim when taking most photo's is to achieve a sharp and properly exposed image.

The three variables you can adjust to achieve that are, aperture (the size of the hole in the lens), shutter speed (adjusts the amount of time that the sensor is exposed to the light you are allowing in by your aperture selection), and sensitivity (ISO).

The aperture setting not only changes the amount of light you allow through the lens, but also the depth of field, or DOF. That is, how much of your subject is in focus from front to back.

In low light conditions you can't always achieve this primary goal by adjusting only the aperture and shutter speed settings because there simply is not enough light to get a properly exposed image. You can then choose to increase the sensor's sensitivity to light by increasing the ISO setting.

Beware though, increasing the ISO too much will introduce noise into the image in much the same way as high ISO film exhibits a grain effect. Most image processing software has noise reduction capabilities so don't worry about that for now. You'll know what noise is when you see it, particularly in the darker shadowy areas of an image.

I hope that's not too basic for you.

Daph, in basic terms doubling the ISO, doubles the sensitivity to light.

So in a situation wheere you selected (for example) f5.6 with an ISO of 100, and founf you needed a shutter speed of 1/125sec for a correct exposure. Then if you kept the aperture identical but doubled the ISO, you would need to half the shutter speed to 1/250sec to achieve that same correct exposure.

A high ISO is often used in low light as we need that higher sensitivity. Downsides of high ISO are increased noised and decreased contrast.

Hope that helps.

EDIT: beat me to it, Peter.

Yes, increase ISO for less light - that will let you use a higher shutter speed and will avoid camera shake.

This is a rough rule of thumb (I hope) and assumes that you are using the standard kit lens, ie a zoom lens of 18-55mm.

For photos with no moving subject, e.g. landscapes, you want the shutter speed to be 1/30th of a second or faster. So if the speed drops below that, increase the ISO (your camera may have an auto-ISO setting which tells the camera to increase ISO automatically).

For photos with moving subjects, e.g. kids running around, you would want something like 1/250th or faster.

If you are using a longer lens, you want faster speeds too. In that case, you can take the lens focal length as a guide. I've got a 70-300mm zoom, at its maximum zoom that would obviously be 300mm. So you would need a 1/300th second exposure. There's no 1/300th setting on cameras, so going to the next setting would be 1/500th.

Hope that helps.

oh, multiple replies all at the same time!

Yup, I prefer noise to camera shake, unwanted motion blur or too small a DOF (unless I deliberately want a small DOF)

Generally speaking I use my cameras at ISO200 - a compromise that offers high quality with reasonable shutter speed/aperture combinations.

If the light falls to a level where camera shake is a real possibility, then I don't hesitate to increase the ISO value as much as I need to.

Or use a tripod.

hi daph, what camera are you using and have you got auto iso set.

Of her two posts so far, the first gives a big clue.

She's using a K-x with 18-55 * 50-200.

I have created a PDF file with all shutters ISO's and apertures based on the sunny 16 rule. It is pretty accurate, even on digital.
So if anyone wants a copy, just e-mail me.

thank you all for your advice, i shall make notes and start practising, thanks again,
daph

and i thought this site was suppose to be friendly.

Oh it is, and I was sure I put a smiley on there. No offence meant, just my sense of humour I'm afraid.

Oh it is, and I was sure I put a smiley on there. No offence meant, just my sense of humour I'm afraid.

[/quote]thanks peter