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confused about iso ratings

I'm in the market for a digital camera, and I'm confused about ISO ratings. I'm hoping somebody can clarify this for me:

Basically, it seems the higher the ISO rating, the more sensitive the "film." However, as I understand it, this is used for 2 very different purposes: For low lighting, a high ISO rating can mean that it pulls in more light and gets a better picture, usually in combination with longer shutter times. Whereas for high-action speeds, a high ISO rating can mean that it pulls in more light, allowing LOWER shutter times, so there is less blurring.

Am I right about this or wrong about this? I am super confused. This quote isn't helping either (in a review of the powershot SD900):

"Finally there is the excellent and more versatile Canon Powershot SD800 IS. With its wide-angle lens, the SD800 covers a wider-range of subjects. With its stabilization, the SD800 IS can shoot at lower ISO settings (with lower shutter-speeds) than the SD900 in the same condition. Effectively, this means that under less-than-ideal lighting, the SD900 loses some of its resolution advantage compared to the SD800."

All of this just blows my mind. Can somebody provide me with a simple explanation of how ISO works with digital cameras and how I would choose ISO when I am taking a photo?


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In reply to: confused about iso ratings

Another related question I had is, do these cameras allow one to vary the shutter speed along with varying the ISO? Or is there always a fixed shutter-speed-to-ISO ratio? That's another thing I don't get. Because it seems like you would want a different shutter speed for "high movement" shots than for dark shots.

And I was confused by CNet's review of the SD800 IS when they said the higher ISO's would be good for "high movement" shots ( - does that mean shots in which the subjects are moving a lot, or shots in which you want a lot of movement in the photo (i.e. blur)?

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In reply to: also...

It helps if you understand "exposure".

See if this helps:

The camera has to adjust the amount of light coming into the camera to end up with exact amount of light to create a perfect exposure.
The amount of light is always the same.
What changes is the ambient light surrounding the camera.

In Auto mode....

On a bright sunny day, there is too much light and the camera must choose a shutter speed and an aperture setting that will limit the amount of light getting into the camera.
The ISO will be set to its lowest setting (lets us say it is ISO-50).
The light meter built into the camera will measure the available light and based on that measurement, the aperture and shutter speeds settings are determined.

And here is what the camera has to choose from on this sunny day:

Shutter speed in fractions of a second ----1/500 - 1/250 - 1/125 - 1/60 - 1/30 - 1/15
Aperture in f-stops-----------------------------------2.8------4------5.8---------8-----11-----16

Any of those settings will give you a perfect exposure.

Whoever wrote the firmware for the camera will determine which setting the camera will use for this shot. Most likely they will choose a setting near the middle.

All digital camera have various mode settings.

If the camera is put in "sports" mode, the camera will choose a setting near the left end of the chart above. Because this offers a faster shutter speed.

If the camera has manual controls, you can select the exact shutter speed you want. I usually start with 1/250 of a second and then check to see if that eliminated any motion blurring in the trial photo.

You will notice that I stopped the aperture setting on the left side at 2.8
That is because that in this example the lens on the camera is rated at f2.8. That is the limit that this camera can handle.

How far the right end of the chart can be extended is dependent upon the darkest f-setting the camera can provide. Most small digital cameras stop at about f8. DSLR cameras can go much further.
The right end of the shutter speed chart can be extended into several seconds (exact amount is determined by your camera specifications).


Now....the sun goes behind a cloud, the light meter in the camera has to adjust the sample chart above.

Image that the shutter speed part of the chart remains the same and the aperture part moves to the right by two notches.

Notice that your maximum possible shutter speed went from 1/500th of a second to 1/125th of a second.

Well shucks....I wanted to catch that race car at 1/250th of a second.

Now imagine I double the ISO setting from 50 to 100.

That will move that f-stop part of the chart to the left by one notch. Now I can shoot at 1/250th of a second.


That is exposure in a nutshell.


Now imagine what happens when it gets dusky dark.
You will end up with a maximum shutter speed of about 1/15th of a second or slower.

You should not hand-hold a camera at a shutter speed slower than 1/60th of a second (1/30th if you have image stabilization).

You have two choices if you want a blur free photo .....
1. raise the ISO setting to 200 or 400.
2. put the camera on a tripod.


I do hope you followed all of explains why a camera needs flash when it gets dark.


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In reply to: confused about iso ratings

For best picture quality, the lowest ISO setting is the best setting.

The higher the ISO setting......increases the noise of the CCD sensor.
Which shows up in a photo as muddy, furry areas.
(i.e. a plain wall may look like it has fur or is textured.)
This is most prominent in the dark areas of the photo.

Most small cameras have a useful ISO setting of only 400.
Anything above that and the noise gets so bad that the photo is unacceptable to most people.

Camera makers are resorting to overstating the useful ISO setting.
Before they just didn't let you go beyond 400, now they have opened up those higher ISO settings....but they have not improved the noise level.

So when you see ISO settings of 800, 1000, 1200........take it with a grain of salt. They are pushing the limits of acceptable noise levels.

Fujifilm's new 5th generation sensor is the only low noise CCD sensor available for smaller cameras.

Image Stabilization...

You see statements that say that a camera with image stabilization give you an advantage in lower light.
Yes.....but....very little.
Normally you should not hand hold a camera when the shutter speed is less than 1/60th of a second.
With image stabilization, you can hand hold the camera down to 1/30th of a second.

That is not as helpful as it sounds.
Most of the time, in low light you find yourself shooting at 1/8th of a second and longer. Image stabilization has no advantage at any speed slower than 1/30th of a second.

Image stabilization is most useful for a camera with a 5X optical zoom or longer. Or for someone with a very unsteady hand.


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I own an 800IS

In reply to: confused about iso ratings

I have had it for about 2months now. I pack it with me everywhere. I filmed all the christmas activities, the snowboarding, the bike ride...I works great.

It takes a great med to high lighting situation picture, however when you want to take pics in low light you need the flash or a tripod. Even with image stabilization its almost impossible to hold it still enough for a real crisp pic...and your subject needs to be very still as well.

When you crank up the ISO to anything over 400, expect lots of noise so just dont do it. I really have no use for noisy pics so i just use the flash when my sample pics are dark in the non-flash manual setting. Its just how the camera is.

Mostly I use it for video and it takes a pretty nice video at 30fps and a 320x280 frame. You will notice the wide angle lens and since nothing else with this tiny a form factor exists, its an easy decision.

Buy it and enjoy!

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