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Old Apr 25, 2005, 9:59 AM   #1
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My Canon powershot S20 finally had the biscuit. The shutter broke, and the display has turned red. I guess I dropped it one to many times. The camera served me well. My major complaint about digital cameras I've worked with is that the photos always appear dark...or need lots of correction (which I do in Adobe photoshop).

I'm going to assume that the size or qualityof the lens has a lot to do with the quality of the final shot. My question is, "what does ISO have to do with photo quality?" I've noticed that cameras with bigger lenses range from 50 to 1600 ISO, and smaller lenses, like my PSS20 range from 50-400. Steve's glossary compares ISO to ASA films, but I don't really get the comparison.

Can someone explain ISO works and whether a higher ISO setting will eliminate the need to correct light conditions in photoshop?
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Old Apr 25, 2005, 10:31 AM   #2
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This should help. http://www.cameratown.com/guides/iso.cfm
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Old Apr 25, 2005, 10:39 AM   #3
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Iso works in a digital camera just like in a film camera. The higher the ISO, the more sensitive the film is to light, meaning you need less light to get correct exposure, which can translate to higher shutter speeds for indoor or action photos to freeze motion, or smaller aperatures to gain more DOF. In film, the trade off is higher grain for the greater speed. In digital terms the tradeoff is more noise. Because most consumer digicams have small sensors, image quality really suffers at high ISO's, more so than film. Nowadays 400 film is quite sharp and 800 film is more than acceptable. You can reduce noise with software, but this softens the picture.

To answer your question about correcting exposure, yes higher ISO's will give you more flexibility to shoot in tougher conditions. You will still probably need to do minor edits in postwork, as digital cameras have less exposure latitude than film, and as I said before, higher ISO result in higher noise.
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Old Apr 25, 2005, 10:40 AM   #4
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If you are taking pictures outdoors in good light and they are coming out too dark it is probably a result of the drop before the one that finally did the camera in. Just about any decent quality camera you buy will give good exposures. The same is true with flash shots within the flash range.

Dark photos are either a result of a malfunction in the camera or someone overriding program mode in limited light thinking there is some magic setting that will overcome not having enough light. The lens will already be full open in program in those conditions and many will have boosted the ISO to 200 and some to 400. About the only thing you could do to help would be to increase the ISO more if it goes higher. That increases the sensitivity of the sensor so it can get a picture in lower light. The tradeoff is higher noise and lower quality.

The ISO you can select isn't as significant as how well the camera does at a higher ISO. Most cameras go to ISO 400 anymore, but many produce images that are worse than marginal when you go that high. Others do quite nicely at ISO 400. Fuji has a new model prosumer camera with a little sensor that goes to ISO800, but I have no idea whether it is a practical setting.

A good alternative for static or relatively static targets is stabilization. Nikon and Minolta are claiming 3 f-stops. That means you can handhold at ISO 100 where you would need ISO 800 without stabilization. I think 3 full f-stops might be a little generous, but it is over 2. Combined with a good burst mode you can usually grab null moments in subject motion as well. I find stabilization to be a better solution than high ISO with the small sensors in consumer cameras. And you can still boost the ISO in stabilized cameras. The only company making small stabilized cameras is Panasonic and they are screwing it up royally IMO. Hopefully a company with engineers who are also enthusiasts will make a useable small stabilized camera. But there isn't a functional pocket camera with stabilization to replace your S20 at the present time. If you don't mind going larger there are quite a few to choose from, and the Panasonics are quite good.

The "bigger lenses" cameras you seem to be referring to that will take good pictures at ISO 800 and even 1600 are DSLRs. The large lenses are a result of having large sensors. The large sensors with their lower density is what allows the camera to take good pictures at higher ISO. In non-DSLR cameras I don't think there is a direct relationship between lens size and high ISO capability. That is more a function of the sensor and internal processing. Sometimes the large lens is to give better f-stop range like the Panasonic FZ20 that maintains f2.8 to 12X. That gives you better light capability in itself, but I think the lens is pretty large for a non-DSLR and ISO 400 is maximum and not great.

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Old Apr 25, 2005, 11:33 AM   #5
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backshed wrote:
Can someone explain ISO works and whether a higher ISO setting will eliminate the need to correct light conditions in photoshop?
The exposure you get (i.e., how light or dark your image is), really doesn't have a lot to do with ISO speed (provided you are metering the scene in the same way). ISO speed is more useful for getting faster shutter speeds in less than optimal lighting to reduce the possibility of motion blur.

Now, there are some exceptions to this. For example, flash photography (where the camera may be adjusting the flash output to compensate for your ISO speed settings as not to overexpose or underexpose your subject).

In this case (using a flash), a higher ISO speed can help to get a brighter background, since ambient light can contribute more to the exposure. Ditto for using a larger aperture. The camera will vary the flash output (length of the flash burst) to compensate for different aperture settings or ISO speeds.

But, you must be careful not to allow too much ambient light. Otherwise, you can get motion blur/ghosting from ambient light exposure if shutter speeds are not fast enough. This is a more complex issue, since the flash burst is short enough to freeze the action at longer shutter speeds if you don't allow too much ambient light.

Allof these settings would produce identical exposure (in the same lighting -- not when the camera is varying the flash output).

ISO 50, 1/100 second, f/2.8

ISO 100, 1/200 second f/2.8

ISO 200, 1/400 second f/2.8

Each time you double the ISO speed, you can double your shutter speed for any given aperture setting and lighting condition (because the sensor is twice as sensitive to light).

But, this won't change the exposure of the image (they would all look to be exposed the same way if the camera was metering the scene in the same way). That's because it would adjust the shutter speed and/or aperture to insure the same exposure when you changed ISO speed settings.

Here is a helpful online exposure calculator you may find useful. Note that Film Speed with this Calculator would be the same thing as ISO speed:


So, the question then becomes how do you control exposure (i.e, you want a brighter or darker scene).

Well, it depends. If you're shooting with a camera's autoexposure mode using matrix metering, you may have conditions where your subject is darker than the rest of the scene. For example, a backlit subject, orother conditions where your subject is much darker than other portions of the scene (i.e. lots of bright sky, etc.). You could use fill flash in this case to help if your subject is closer, and/or adjust the way the camera is exposing the image.

With the vast majority of cameras, you have what we call Exposure Compensation (a.k.a, EV Compensation). This allows you to vary the exposure from what the camera's metering thinks is correct for the scene.

Using a +EV setting will cause the camera to use a larger aperture and/or slower shutter speed than it normally would (resulting in a brighter exposure). Using a -EV setting will cause the camera to use a smaller aperture and/or faster shutter speed, resulting in a darker exposure.

If you're using Manual Exposure instead, you can simply adjust either the shutter speed or aperture so that the metering scaleindicates how much you're overexposing or underexposing the scene versus what the camera would normally use for the metering mode selected.

A camera has a limited dynamic range (ability to capture both light and dark areas of a scene). So, the metering system must make choices. In other words, it may decide to use an exposure that's appropriate for the majority of the scene, versus your intended subject.

Another way to vary the exposure is by using a different metering mode. Many models have several (Matrix or Multi-Segment metering which looks at the entire scene, Center Weighted Metering that places more emphasis on correctly exposing the center portion while still looking at the entire scene, and Spot Metering which meters for a small spot giving you very precise control.

Correctly exposing a scene requires practice and experience -- understanding your camera's metering behavior, knowing what to meter on, what metering mode to use, etc. For example, if you tried to use Spot Metering on a person wearing a dark outfit, it may overexpose your subject (and blow the highlights in surrounding portions of the image).

One of the most common complaints you see with an image is overexposed highlights. For example, you expose for shadow areas (so that they're not underexposed), and end up losing detail in brighter areas from overexposure.

So, given a choice between underexposed shadow areas, and overexposed highlights, underexposed shadow areas is almost always preferable. You can usually pull out some detail in shadow areas later during Post Processing, but you usually can't get back detail from the overexposed areas. It's simply lost. In this respect, shooting with digital is like shooting with color slide film. You'll want to expose for the highlights in most conditions.

Now, you can recover some detail in Post Processing shooting RAW (depending on the software you're using to do the RAW conversion). So, for high contrast scenes (big differences between light and dark areas), if your camera can handle it, RAW is a better way to go.

There are also techniques for blending multiple exposures together to increase dynamic range --usingone image exposed for the highlights, and another image exposed for shadows. I've even seen some users report doing this to some extent witha single RAW photo. They adjusted Exposure Compensation so that highlightswere properly exposed and saved it, then adjusted Exposure Compensation so that shadowswere properly exposed and saved it. Then, they blended the two saved photostogether. Usually, more than one photo of the same scene (exposed differently) is used for this technique.

You'll find one method of blending more thanone image together here:


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