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

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?

Not F-stops but <b>stops</b>.

 

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.

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

Even though stops originally applied only to apertures.

 

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

(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)

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?

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.

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?

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.

Balvab;

 

"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.

[[Without IS, the "average" person can hold a camera steady enough to get a reasonably sharp image at 1/Focal length shutter speed.]]

 

This should probably read "1/field of view" to encompass both full-frame and reduced-frame sensors in DSLRs.

All I know is that at a recent event, my Image stabilized 24-105/4L wanted 1/8, F4 at ISO 3200.

 

1/8th shutter speed simply wasn't going to cut it when photographing *bellydancers*. (You can shoot potted daisies all you want. *I* want to photograph pretty women with bare bellies and many parts in motion!.)

 

My 50/1.8 and 85/1.8 got a workout that night. I regretted not having the 35/2 in my bag. I am now considering a 24/1.4.

Jim,

 

This is not a "IS vs Non-IS" thread.

Your mileage will vary using IS. I saw a post here recently comparing shooting a rifle accurately and good hand-holding

technique in photography and I think that's a good analogy. So, if you practice you can get better. Some people may get

acceptably sharp shots hand-holding long telephoto zooms (i.e. 70-200mm f/2.8L) at shutter speeds that were unthinkable

before IS (VR, etc) came along. Ideally, it's still not as good as using a tripod and it's not the same as very fast lenses but

I think it's great because I'm just not a big fan of lugging a tripod unless it's absolutely necessary.

F Stops relate to the light allowed into the sensor, the aperture opens or closes one Fstop at a time, (some 1/2 and

some 1/3 at a time) with IS or VR you can gain several Fstops of light over a non is lens. I can now shoot my 100-

400IS at F5.6 and slow the shutter down to 1/30 w acceptable sharpness, unheard of b4 IS. B4 IS the rule of thumb

was if you have a 400mm lens you better shoot at 1/400 or faster, and have enough light to do so...

Beau, as somebody who carries a firearm for a living I think you are on the right track with your analogy but I'd fine

tune it by comparing shooting at low shutter speeds (hand-held) to shooting with a pistol at a distant target. I use the

same technique for both. I hold my breath and push the shutter or squeeze the trigger.

 

Back on topic.

I'm happy with my 24-105 IS and I have some great handheld 10th/15th sec shots ( running water and the like) that I

could have never got with my 24-70, that aside I have some great moving/low light shots that I have taken with my 24-

70 that I wouldn't get with my 24-105.

 

IS is great but not a 'solve all' solution.

I think it is just a quicker and easier shorthand for what happens. Would you rather say "IS gets you two stops" or "IS lets

you shoot at a shutter speed that is four times as long as the shutter speed you would otherwise use."?

 

Get my drift? ":-)

 

Dan

This may help: The old "rule of thumb" was, if you were reasonably normal and could hold steady without shaking,

you could shoot at a <b><i>minimum shutter speed</i></b> of 1/focal length of the lens for acceptable sharpness.<p>

 

Thus, a shutter speed of 1/50 for a 50mm lens, 1/100 for a 105mm or so, 1/500 for a 500mm lens, etc. That made

it tough when shooting with a long lens and hand-holding the camera, if your subject wasn't in full sunlight or

you were shooting with slow [lower ISO] film.</b><p>

 

With the IS or VR, they're claiming you can shoot at from 1 to 3 stops <b>slower</b>:<p>

 

With a 500mm lens, instead of 1/500, they're claiming you could now hand-hold and shoot at a shutter speed of

between 1/250 to 1/60 [the range from 1 to 3 stops: 1/250 = 1 stop slower; 1/125 = 2 stops slower; 1/60 = 3

stops slower].<p>

 

As the above posts indicate, the doubling or halving of shutter speeds and apertures is referred to as a

difference in <b><i>stops</i></b>.

IS lenses are not technically measured in stops. It may seem that way because of the fact that using an IS lens will allow you to use a slower shutter speed if needed when shooting hand held. Aperture, shutter speed and iso are all measured in stops. Stops are units of measurement that determines how much light is let in to create the desired exposure. Check out the book entitled "Understanding Exposure" by Bryan Peterson.

"This may help: The old "rule of thumb" was, if you were reasonably normal and could hold steady without shaking, you could shoot at a minimum shutter speed of 1/focal length of the lens for acceptable sharpness." From S. G. Bono, above.

 

Quite true but there was a catch or exception that most people never thought about. It had to do with the length and weight of the lens being used. I believe that you can handhold a Canon 80-200/4.5-5.6 at a much slower speed than you can a Canon 80-200/2.8 L. The L lens is several times longer and heavier than the consumer lens and is much more difficult to hold steady than the small and plastic consumer lens.

Thank you everyone for your responses, they were all elucidating and certainly educational. Until now, I had no inkling that "stop" measured ISO and shutter speed also, which are typically explained as "sensitivity to light" and "fractions of one second" respectively. I have used the 1/focal length technique for some time now with my non-IS lenses, and am looking to purchase an IS lens in the future, if only to allow me some modicum of confidence for sharp captures, good hand-held skills notwithstanding.

Think of "1 stop" as either twice as much light or half as much light (depending on which way you are adjusting things) as the setting before.

 

1/125 of a second is a stop slower than 1/250 second (Tv)

f/2.8 is a stop slower than f/2 (Av)

ISO 100 is a stop slower than ISO 200 <--- in this case actual light "sensitivity" is changed but for a given Av and Tv you are effectively doubling or halving the light required for a correct exposure.

 

IS allows you to hand hold a given lens at a much slower shutter speed without blur caused by camera motion. This ability is measured in "stops" of shutter speed, as others have mentioned.

 

Lens based IS also gives you a stabilized image in the viewfinder, which is helpful even when shooting at higher shutter speeds.

 

Neither lens based or body based IS will help freeze action, so just keep that in mind when relying on IS to enable a hand held shot at slower than accustomed to shutter speeds.

>>L. The L lens is several times longer and heavier than the consumer lens and is much more difficult to hold steady than the small and plastic consumer lens.

 

That's not necessarily correct. A small lens bobs about more as your muscles twitch; a heavy lens can be self-stablilising by virtue of its own inertia.