Image Stabilized Lens - what do stops mean?

Quick question, I was looking at the T3 and the kit lens (the new Canon EF-S 18-55mm f/3.5-5.6 IS II zoom lens) it says, "yields up to a full four stops of image-shake correction".

I don't quite understand what stops are. Are they talking about f stops? Assume I'm photographing a non-moving subject, what kind of a lens would yield the same 4-stop correction on a non-stabilized image If I'm at 55mm f/5.6?

Thanks.

In the case of image stabilization, four stops of correction would refer to shutter speed. Say for example you could handhold a non-stabilized lens and get a blur-free shot at a shutter speed of 1/125. A stabilized lens would allow you to get the same blur-free shot with as slow a shutter speed as 1/8.

the reference is to hand-holdable shutter speed.

Assuming 1/focal length as the normal minimum shutter speed for a handheld camera, with IS you should be able to handhold the camera at a shutter speed 4 stops slower. in this case, "stops" refer to doubling or halving shutter speeds.

In your example, the minimum shuter speed to hand-hold a standard 55mm lens should be about 1/60 sec. With IS, you should be able to go as slow as 1/4 sec at the same aperture (starting with 1/60, 4 stops would be 1/30 > 1/15 > 1/8 > 1/4).

Obvioulsy this is only for static subjects. IS cannot compensate for subject motion.

the above is true with regards to blur due to camera shake. Not everyone has the same steadiness. The "general" rule of thumb is you need a shutter speed of 1 over focal length to have blur free images. But that's not all that relevant. A 200mm f5.6 and 200mm 1.8 lens have the same focal length but I assure you the ability to hold them both steady is very different. And, of course if you give a setup to two different people they will have different degrees of success holding a given setup steady. I mention this only to point out that the actual shutter speed YOU can hand-hold a lens at without camera shake is dependent upon the weight of the lens, the focal length AND your own steadiness.

It's also worth noting that anti-shake does nothing to reduce blur due to subject motion - so a person moving, leaves blowing, feathers blowing, etc are all things that will show some blur at slow shutter speeds regardless of how steady the camera/lens is held.

While the responses so far are more or less true, it is best to think of "stops" as exposure value units, or EVs. The reason for this is that, while you want to avoid motion from shaking, you also want to avoid noise from high ISO and you may want to stop your aperture down to increase your depth of field. So 4 stops gives you the ability to lower ISO or shutter speed or stop down the f-stop in any combination that adds up to 4 EV over what would be possible without the IS in an otherwise unmoving scene. However, it is worth mentioning that "4 stops" may be more marketing hype than physical reality, and you may find that you really only get 2.5-3 EV from the technology. FWIW

Thanks for explaining, that makes a lot more sense now, thanks!!

I'm using this to try to understand how it all fits together: http://camerasim.com/camera-simulator.html

I'm trying to compare the kit lens with a Tamron 17-50mm f/2.8 as far as shutter speed goes.

If I'm shooting the kit lens at ISO 100, 18mm, f/3.5, 1/4 sec. With 4 stops then I can achieve an equivalent 1/60 exposure time (1/8 > 1/15 > 1/30 > 1/60).

Using the Tamron lens stepped down to f/2.8 I get a shutter speed of 1/6. In order to achieve the same results I have to bump up the ISO. So...

200: 1/13
400: 1/25
800: 1/50
1600: 1/100

So I would need roughly an ISO of 800 to get about the same results with the 2.8.

Now I move to the focal length to 55mm on the kit lens and take a shoot at f/6.3, ISO 100, 1/4 sec.

If I use the Tamron I get 1/20. Bumping up the ISO:

200: 1/40
400: 1/80

So to achieve the same results on the 2.8 I'd need an ISO of 400.

So either way an IS lens is the better way to go if you're shooting in a dim situation with a non moving or slow moving subject, correct?

Just for fun the simulator tells me that f/11 in brighter sunlight the 2.8 will yield the same speed as the IS kit lens.

Of course this is all provided I actually get the 4 stops of image reduction.

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Ok, now I want to shoot a sports scene. I want a shutter speed of 1/1,000 at the 50-55mm focal length. With the kit lens with the IS turned off (f/6.3) then I would need an iso of about 400 which would be 1/800.

At the other end, at 18mm focal length I would only need an ISO bump of 200 to get 1/1250 on the kit.

So the 2.8 doesn't make much of a difference on the wide end but starts to shine at the 50mm end.

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Soo... to me it kinda seems to me like the IS lens isn't as bad as I had thought. The 2.8 will have to bump the ISO to 400-800 to get what IS lens does. The IS lens would have to bump it's ISO to 200-400 to get what the 2.8 can.

I know I'm not talking about image quality or background blur, and probably an ISO of 400 or 800 isn't that big of a deal for a dslr camera. And it would depend if I really did get a full 4 stops of image stabilization ... but not bad for a kit lens...

(For some reason the post didn't include this)

Ok, now I want to shoot a sports scene. I want a shutter speed of 1/1,000 at the 50-55mm focal length. With the kit lens with the IS turned off (f/6.3) then I would need an iso of about 400 which would be 1/800.

At the other end, at 18mm focal length I would only need an ISO bump of 200 to get 1/1250 on the kit.

So the 2.8 doesn't make much of a difference on the wide end but starts to shine at the 50mm end.

Soo... to me it kinda seems to me like the IS lens isn't as bad as I had thought. The 2.8 will have to bump the ISO to 400-800 to get what IS lens does. The IS lens would have to bump it's ISO to 200-400 to get what the 2.8 can.

I know I'm not talking about image quality or background blur, and probably an ISO of 400 or 800 isn't that big of a deal for a dslr camera. And it would depend if I really did get a full 4 stops of image stabilization ... but not bad for a kit lens...

ugotkevin wrote:

I'm using this to try to understand how it all fits together: http://camerasim.com/camera-simulator.html

I'm trying to compare the kit lens with a Tamron 17-50mm f/2.8 as far as shutter speed goes.

If I'm shooting the kit lens at ISO 100, 18mm, f/3.5, 1/4 sec. With 4 stops then I can achieve an equivalent 1/60 exposure time (1/8 > 1/15 > 1/30 > 1/60).

I want to make sure that we're communicating here. The problem that needs to be addressed in an unmoving dark environment is that, when you hand-hold your camera, you will introduce shake into the photo that will make it blurry. What image stabilization does is compensate for your shakiness. The rule of thumb for these things is that most people, using good technique, can keep their shake from degrading the image quality as long as they shoot with a shutter speed that is at least as fast in seconds as the reciprocal of the full-frame focal length equivalent of the lens in mm. So, if you are shooting on a DX camera at 18mm, your full-frame equivalent will be about 28mm. So, as long as your shutter speed is at least 1/28 second, you will not see blurring from hand-holding the lens. Now, if you have image stabilization, you could lower that speed from 1/28sec to about 1/4 sec and still have no visible blur from shakiness. You can use that additional "headroom" to lower your ISO or to make your aperture smaller. The total amount of light that you need hasn't changed, it's just that you are less limited by hand shakiness in how you set your camera for the exposure you need. If you are shooting sports, you aren't limited by your hand shakiness, but by the speed of the action you are shooting, and image stabilization won't help you at all.