Rule of Thumb for ISO Setting
Notice this is the beginners forum, so asking a beginners question (which I am)..

What is the rule of thumb, day or night for choosing the correct ISO? You don't always have a book or guide with you to refer to. I think there is something I'm not understanding. I know it's elementary, but I really don't know. My book says to raise to use fast shutter speed, or to turn flash off when shooting in a dark area. How dark? Is it a try different ISO's and shootand look type thing?..

Comments (11)

Depends on your subject and lens. the rule of thumb for shutter speed is don't try to hand hold anything slower than 1/focal length( 1/100 th for 100mm lens) but this depends on how steady you are and how much coffee you drink. so anything slower than that needs a tripod, higher iso, bigger aperture or an image stabilizing lens. if you are trying to capture action in low light and can't use flash then you have to just live with the high iso noise and boost the iso to give you the fastest possible shutter speed..

If the trade off is high shutter speed or high ISO noise. Noise can be reduced in post processing but not motion blur. What speed will freeze action depends on the action...

Comment #1

Nick, I'm using a Canon A720is, with it (so far) just set on Auto. I'm getting good photos, but I'ed like to use the camera to it's fullest capabilities. It's not expensive, but I think it's a nice P&S...

Comment #2


Here's this old git's rule of thumb based on year's of film:.

Winter - use ISO 400.

Autumn and Spring - use 200 and.

Summer - use 100..

Have a look at the recommendations for film and you'll see similar repeated. Try a search on Kodak film, f'instance..

Regards, David..

Comment #3

The simplest rule about iso is to keep it as low as you can for max image quality..

With that said, there are 3 leg that proper exposure rests on:shutter speed, fstop, iso..

Not counting iso you the user has to determine what you need to make your picture work. a fast or slow shutter speed; or small(numerically) or large (numerically) fstop. a fast or slow shutter speed is dependent on how fast the motion is that you want to stop. the faster the motion the faster the shutter speed needed to stop it. on fstop, it is how much of the scene do you want in focus. only the subject standing 20ft away and nothing else, or do you want everything that you can see on your lcd in focus?.

Once you decide on the shutter speed and fstop you will use, then the next question is: what iso is required to make that possible? remember on any and all conditions you still have to have the correct amount of light on the sensor to get the normally and correctly lit exposure..

Under certain dim conditions you may want a certain shutter and fstop combo but there may be just not enough light, so you raise the iso to make what you want possible. but you should note that as you raise iso you also lower the image quality. and you cannot just keep the iso low because if you do then you will end up with a too dim image..

Balancing the 3 legs of exposure(shutter speed, fstop, iso) is all about what you the user want to do and what is possible with the light you have to do it with. in other words a comprimise of the light available, your wants, and the camera's ability to take the picture under those conditions...

Comment #4

Thanks Dave and Nick. I think this will get me off on the right foot. I am taking a trip in a couple of months, and I want to take some in-city night shots, if possible without the flash so I won't be so obvious to the subjects. I know it will take some experimenting, and trying different things...

Comment #5

Robert740 wrote:.

Thanks Dave and Nick. I think this will get me off on the rightfoot. I am taking a trip in a couple of months, and I want to takesome in-city night shots, if possible without the flash so I won't beso obvious to the subjects. I know it will take some experimenting,and trying different things..

Taking pictures at night without a flash will require loooonnnnggg shutter speeds, which will require a tripod (or perching the camera on a convenient wall). Fine for 'cityscape by night' shots but you aren't going to be able to take people shots: people move too much (even when they are trying to stand still they are wobbling). if you want to take pix of people at night you'll need to use the flash: I can't see any way around that!.

For 'cityscape' type pictures you wouldn't want the flash anyway as it will have a range of only a few metres... you would get a bit of brightly lit pavement, then blackness. Much better to use a tripod and long shutter speed. In these cases there is no need to use a high ISO: if you have a tripod to hold the camera steady, you might as well use a low ISO (to get best image quality) - the reusltant even longer shutter speed won't matter with a tripod..

Best wishesMike..

Comment #6

The rule of thumb for digital photography is to use the lowest ISO speed possible..

So for your A720 you have the choice between ISO80, ISO100 and ISO200 with the emergency at ISO400..

This is not a camera to catch action in poor lighting. A full frame camera with very fast lens would be more appropriate (like the cheapest from them Canon 5D with a lens of f/1.4 to f/2.8). You can use quite successfully ISO3200 on that and with f/2.8 and 100 mm you can get speeds of 1/125 s in fairly low light (like in a sport arena).VictorBucuresti, Romania

Comment #7

Aaah, this is nice to know! When do you use higher ISO's and for what?..

Comment #8

I am new to photography too. I found this really cool site called You can ask questions and PROs (in that field) will respond to the question..

Also they have some cool photo contests..

I hope that helps...

Comment #9

Generally most Cameras do well with Auto ISO but if you have a point and shoot I think 400 would be your limit.look at this site to help you understand ISO, aperture & speed it out it will be an invaluable reference...

Comment #10


Here are some pictures taken at night:.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

Taken with flash and showing it was night (about 10 pm). A second version was taken with the thing on "P" and handheld (ISO 100 - also, second and f/2). Not so good as the camera tried to make it the usual 18% grey blandness..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

And finally, still handheld but under exposed by a stop to make the sky dark (1/30 second, f/2 and ISO 100)..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

Hope this is some use..

Regards, David..

Comment #11

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