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Old Jul 20, 2006, 2:25 AM   #1
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Can some one explain to me what the ISO setting has to do with minimizing the possibilitiy of blurry photoswhen shooting a fast moving object? I assumed this was achieved simply by using a fast shutter speed. How does the ISO come into play? Please give an example.

I do vaguely recall reading about this some where. That having more ISO range allows you to use a slower shutter speed...hence less blur due to camera shake? Whereas, if you had less ISO range you'd be forced to use a higher shutter speed? Or something along those lines? Please correct me if I've got it wrong!

Thanks in advance!
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Old Jul 20, 2006, 3:24 AM   #2
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Basically a camera collects light, and 3 variables apply. The Shutter speed, Aperture and ISO. The Shutter speed controls how long the sensor is exposed to light. The Aperture controls how big the opening is, and hence how much light. And the ISO is how sensitive the sensor (or film) is to light. So basically, if you cut the shutter speed in half, you can double the ISO and get the same amount of light since the sensor (or film) is now twice as sensitive.

Of course, nothing is free, the more sensitive the sensor is, the more likely it'll show noise.

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Old Jul 20, 2006, 6:34 AM   #3
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Higher ISO's increase the sensor's sensitivity to light. In any given situation, a higher ISO will allow faster shutter speeds to achieve correct exposure. In turn , this is less of a chance of blurring from camera shake. If you were just to simply increase shutter speeds, without changing iso's, eventually the image would be underexposed because of the lessened light reaching the sensor.
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Old Jul 20, 2006, 8:27 AM   #4
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Just to follow up on rjseeney's excellent answer:

aperture, ISO and shutter speed all work together to achieve proper exposure (i.e. your picture not looking too light or too dark).

ISO makes the sensor more sensitive to light

Aperture controls how much light is let in

Shutter speed controls how long the light is let in

ISO moves in crements like this: 100, 200, 400, 800, 1600, 3200 - each of these jumps can be considered a full stop

Aperture moves like this: 1.4, 2.0, 2.8, 4.0, 5.6, 8.0 etc....

Shutter speeds, like ISO always roughly double 1/60 1/125, 1/250, 1/500, 1/1000 etc

Now assume that you take a picture with aperture of 5.6, shutter of 1/500 and ISO of 400 and that picture is properly exposed.

Now, If I want the same exposure but a faster shutter speed - say 1/1000, I have to affect one of the other 2 variables or I will underexpose the pic. I can either change my aperture from 5.6 to 4.0 (lower number means more light is being let in) OR I can keep my aperture at 5.6 and increase the light sensitivity by upping my ISO to 800.

So, bottom line - for every one-stop move from any of the above I need to adjust one (or more) of the other 2 variables at least one stop to compensate.

Now, when shooting sports the most important factor is shutter speed - you want to stop action. As light gets darker (i.e. clouds, dusk, etc...) you need to adjust the other 2 variables to keep high shutter speeds. Quite often you'll end up very quickly with the widest aperture your lens allows so you can no longer adjust that variable. The last variable to adjust is ISO. And, if you're like me, you almost ALWAYS shoot at the widest aperture (it provides background blur).

Most advanced cameras have a mode: shutter priority that allows you to set ISO, set shutter speed and the camera will adjust aperture as needed for a proper exposure. Those cameras also have an aperture priority mode where you set ISO and aperture and the camera adjusts shutter speed to get proper exposure. And they'll have a full manuallmode where you set all 3 variables ignoring what the camera thinks the proper exposure should be. In all 3 of those modes the user is required to adjust the ISO. Sounds like this camera has a mode that will automatically adjust ISO to keep a proper exposure - I can't tell from that blurb whether the user sets aperture, shutter speed or both when using this auto-ISO feature.
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Old Jul 20, 2006, 8:28 AM   #5
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see the other thread where you asked the same question (I answered there):

http://stevesforums.com/forums/view_...amp;forum_id=9
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Old Jul 20, 2006, 11:12 AM   #6
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rey wrote:
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Basically a camera collects light, and 3 variables apply. The Shutter speed, Aperture and ISO. The Shutter speed controls how long the sensor is exposed to light. The Aperture controls how big the opening is, and hence how much light. And the ISO is how sensitive the sensor (or film) is to light. So basically, if you cut the shutter speed in half, you can double the ISO and get the same amount of light since the sensor (or film) is now twice as sensitive.

Of course, nothing is free, the more sensitive the sensor is, the more likely it'll show noise.
Thanks for the explaination Rey! Very clear and to the point. As I said I had no idea that 3 three variables were important to every single shot. I assumed you only had to worry about aperture and shutter speed. Now I know better.

John,

Last night when I was typing in my question I pressed Send but on my end it looked like nothing was going on. So I typed it in a second time, adding the quote, then sent that one. Hence the double post. Whoooooops!!! Sorry people! My bad.

JohnC....feel free to merge Rey's and John's post with my other version of this post or kill this one altogether.
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Old Jul 20, 2006, 11:26 AM   #7
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Thanks JohnG and rjseeney for the excellent explainations! Much appreciated!! I printed out your explainations to be kept for reference.

I have a Sony DSC-H1. Just got it a few weeks ago. Which does have shutter priority, aperture priority and manual modes. Will be experimenting in time. But I will keep in mind now that I do have to adjust the ISO if in manual mode. My camera does allow me to set the ISO to Auto when in either shutter or aperture priority modes.

And, I will ask JimC if he can't add the answer given by Rey, in my duplicate thread into this one. In short, I was about to post my question, pressed Send. But it looked like nothing was happening. So, I retyped the question adding the quote. Hence, it looks like I asked the question twice intentionally. Sorry guys!!

Cya around on the forum and thanks again for your help!!
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Old Jul 20, 2006, 11:40 AM   #8
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Moderator Note:

Two threads with similar questions and answers were merged at the Original Poster's request.

Jim C.

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Old Jul 20, 2006, 1:58 PM   #9
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As already pointed out, there are multiple variables that control the shutter speed you can use (and faster shutter speeds result in less blur from subject movement or camera shake).

Think of the aperture opening in a lens as a pupil in your eye. If you open up the aperture wider (smaller f/stop numbers), more light gets through, allowing you to expose the image faster for the same lighting and ISO speed (ISO speed represents how sensitive the sensor or film is to light).

Lenses are rated by their largest available apertures (smaller f/stop numbers), and for most (but not all) zoom lenses, you'll see two aperture ratings... the first one is for the widest aperture at the wide end of the lens (least apparent magnification), and the second is the widest aperture at the long end of the lens (most apparent magnification).

The largest available aperture (smallest f/stop number) will fall somewhere in between these two numbers at focal lengths in between the two extremes.

Some zoom lenses can maintain a constant aperture throughout their focal range (with f/2.8 being the most common). Of course, a brighter zoom lens is larger, heavier and more expensive. For indoor use without a flash, a lens with a constant f/2.8 aperture is preferred in a zoom (but, a brighter prime is even better, allowing faster shutter speeds and/or lower ISO speeds for the same conditions).

Aperture as expressed by f/stop is a ratio, and is determined by dividing the focal length of the lens by the size of the iris opening.

The aperture scale (in one stop increments) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, etc. With each one stop move to a smaller aperture (represented byhigher f/stop numbers), you will need shutter speeds twice as long for proper exposure.

So, a lens with a larger available aperture is desired to get fast enough shutter speeds to reduce motion blur (either from camera shake or subject movement) in many conditions.

Of course, the downside to using larger apertures is a shallower depth of field. This can be a pro or a con, depending on what you're shooting. But, for portraiture, you typically want a shallower depth of field (to help your subject stand out from distracting backgrounds). Again, just because a lens has larger available apertures, doesn't mean you need to shoot that way.

For more information on how aperture impacts Depth of Field, see this Depth of Field Calculator:

http://www.dofmaster.com/dofjs.html

The lenses can also use smaller apertures than the largest available (and a lens isn't going to be as sharp at wide open apertures anyway, so "stopping down" a bit can yield sharper photos, and you've got more room to stop down with a prime starting out with larger apertures versus a zoom that may start out at f/2.8 or smaller). But, you need to keep an eye on shutter speeds in less than optimum lighting, no matter what lens you use.

Here is a handy online exposure calculator that lets you see how larger apertures impact shutter speeds, in 1/3, 1/2 or 1 stop increments (you can change this via check boxes at the bottom). Film speed in this calculator is the same thing as ISO speed.

http://www.robert-barrett.com/photo/exposure_calculator.html

You may also want to get a book on basic photography and read it. The basic concepts of photography are going to be the same for film or digital.
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