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Why are Image Stabilized (IS) Lens measured in f stops?

Bambam Ilusorio , Jul 01, 2008; 03:23 p.m.

Often when researching lenses, you come across articles touting Canon's IS feature (and Nikon's VR for that matter), giving you accordingly "several stops (from 1-3) advantage" over non-IS lens. What does this mean exactly? As I understand it plainly, IS or VR are special features of high-end lenses that help achieve sharp photos, so shouldn't the measurement be in millimeters, degrees, or something similar instead of f stops?


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Bryan Tan , Jul 01, 2008; 03:29 p.m.

Not F-stops but stops. One stop more is twice, while one stop less is half. There was a time long before I was born when the term "stop" made actual sense.

Rob Bernhard , Jul 01, 2008; 03:32 p.m.

Shutter speeds are measured, relative to each other, in stops.

Bryan Tan , Jul 01, 2008; 03:35 p.m.

Even though stops originally applied only to apertures.

I've heard ISO settings referred to as stops too.

Rob Bernhard , Jul 01, 2008; 03:37 p.m.

(I keep messing up the "confirm" vs "update" button. I apologize)

For example, 1/30th is "1 stop" slower than 1/60th. So if you need a shutter speed of 1/60 for an acceptably sharp image at a particular focal length, an IS lens with a 3-stop advantage should allow you to shoot at 1/8 and achieve the same level of sharpness.

(Of course, this assumes everything else being equal including your ability to remain stable, which varies from person to person. There's also the problem of what is or is not "acceptably sharp" which also varies)

David Lau , Jul 01, 2008; 03:38 p.m.

its just a measurement, eg IS gives you an extra 2 stops. you was probably shooting at 1/125 now you can go down to 1/30 with IS..... does that sound about right?

Giampi . , Jul 01, 2008; 03:38 p.m.

Canon has introduced optical Image Stabilization (IS) in their lenses a while back and continues to develop such technology, improving it and adding it to even more of their lenses.

What that (and similar systems like Nikon's VR) do is allow you to shoot a SLOWER shutter speeds than normally possible, without getting blur due to camera shake. The shutter speed difference between an IS lens and non stabilized one is measured in f/stops. For example: let's say you have a telephoto lens which would require a MINIMUM of 1/500s to avoid blur dur to shake. Now, let's say that the same lens with the IS system in it allows you to shoot at 1/80s without showing any camera shake. That would be a gain of f/stops from 1/500, 1/250, 1/125 and finally to 1/60. So, you'd have a gain of 3 f/stops for hand holding the lens. OF course, the IS system does NOTHING to stop motion blur due to subject movement.

Baivab Mitra , Jul 01, 2008; 03:43 p.m.

Adding to Giampi - assume you did shoot at 1/500. You would have used an aperture (say) f/2.8 ; now being able to shoot at 1/60, you can use aperture (say) f/? ... any guesses?

David W. Griffin , Jul 01, 2008; 04:10 p.m.

Without IS, the "average" person can hold a camera steady enough to get a reasonably sharp image at 1/Focal length shutter speed. So if you were shooting at 100mm (with a 35mm camera), you might shoot at 1/125th of a second, or maybe 1/250th to give you a little margin.

Now with image stabilization (IS, VR, whatever), you can do 2-3 stops better, so maybe you can get down to 1/30th of a second, maybe even 1/15th of a second, still getting a sharp image. That is what those stops mean.

Terry Smith , Jul 01, 2008; 04:42 p.m.


"Adding to Giampi - assume you did shoot at 1/500. You would have used an aperture (say) f/2.8 ; now being able to shoot at 1/60, you can use aperture (say) f/? ... any guesses?"

If your correct exposure was in fact 1/500 at 2.8 and the IS gives you a three stop advantage, you can use any combination of shutter speed and apeture that will pass the same amount of light to the film or sensor. So...

1/500 @ 2.8 equals 1/250 @ 4, 1/125 @ 5.6 and 1/60 @ 8, so you can put the camera in AV mode and do this. You can choose a lens opening to give the effect you want and let the IS deal with the rest. IS was primarily designed to be used when the lens is already wide open. Let's say you have a 300/2.8 IS L. At 2.8, the camera tells you that you need a speed of 1/60 to get the right exposure. Normally, you can not hand hold a lens that heavy at 1/60. That's when you turn the IS on and let it do it's magic, or as much as it is capable of. You may or may not get a sharp image given the light level and how many stops the IS can deliver.

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