How do you decide which ISO speed to use?

platypus

platypus

platypus

Caporegime
Joined
25 Jul 2003
Posts
40,527
Location
FR+UK
I have a vague idea about ISO settings, and that a higher number should be used for lower lighting, due to the sensor taking in lower frequency light...

..but other then that I'm pretty clueless (like most things in photography). How do you guys decide which speed to use, and when you're out snapping pictures, do you take a bunch of photos at a range of ISO speeds?
 
I always aim to take things at the lowest possible ISO as its less grainy and better quality.

I only up the ISO if in low light conditions or situations where I want the Shutter speed cranked up as high as possible.

Best advice I can give is, just experiment till you are happy with the control.
 
Lower iso as possible, generally during daylight hours I leave my d40 set to 200, but at night, especially at smaller apartures, going higher is the only way.
 
depends on what your shooting

if you need to freeze action , dont have time to fiddle with the iso settings and know you could struggle with light then start off with a high iso to guarantee a fast shutter
 
Longer focal lengths(smaller apatures) generally allow less light to the sensor, meaning you have to use a longer shutter speed to expose the image perfectly. However if you're shooting a moving target you must use a short shutter speed, but if at long focal lengths it will underexpose. Thus using a higher Iso will counter act that.
 
platypus said:
Sounds good. If only I knew what you meant :p.

Explain for the idiots round here :)?
Click to expand...

Well if you are shooting with a 50mm lens then to get sharp photos hand held you need to be shooting at at least 1/50th second. So if the shutter speed drops below this, then up the ISO to get the shutter higher and avoid blury photos.

Obviously if your shooting fast moving objects then this rule won't work so well. And also a 50mm lens is actually 80mm on DSLR so you may want to use at least 1/80th second.

Obviously this is the same for other focal lengths, so for a 100mm lens you want at least 1/100th second and a 28mm lens at least 1/28th etc
 
Last edited:
balls said:
Longer focal lengths(smaller apatures) generally allow less light to the sensor, meaning you have to use a longer shutter speed to expose the image perfectly. However if you're shooting a moving target you must use a short shutter speed, but if at long focal lengths it will underexpose. Thus using a higher Iso will counter act that.
Click to expand...

sorry, some of that is wrong


focal lengths and aperture are totally different things

you are correct that at a small aperture, say f22 you need a slower shutter speed to get the correct exposure, the slower the shutter the more prone to shake and motion blur you are

longer focal lengths do not change anything except the depth of field



i think what SDK was trying to say is if you are using a 300mm lens then your shutter should be 1/300 or faster to eliminate camera shake, or for a 100mm lens you should be at 1/100+ this is a general rule to reduce shake, obviously this doesnt matter if you have image stablilization
 
SDK^ said:
If the shutter speed doesn't match the focal length then increase ISO.
Click to expand...
icon14.gif
 
I always find the water / bucket / pipe / tap analogy to work well in describing how a camera works and how you can work out whats best for you...

Bucket - sensor
Bucket size - ISO
Time to fill bucket - shutter speed
F-stop (aperture) - pipe thickness how quick/slow water flows to bucket
Tap - shutter

These can all be set in a variety of way to achieve the same shutter speed, but results will differ. A higher ISO means the bucket will be full quicker than the same settings on a lower ISO.

http://www.digital-photography-in-focus.com/exposure.html
 
You must log in or register to reply here.
Back
Top Bottom