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Today's e-tip:

ISO Speed -
When to change it

 
ISO Speed - when to change it
 
In the OLD days… there was film. When you went to buy a roll of film, you had several choices to make: What brand of film (Kodak, Fuji, Agfa, etc.); what kind of film (slide/negative) and what ISO (or in the really old days, what ASA).

Well, this last choice has to do with how sensitive the film was to light. The higher the number, the more sensitive the film was to light. But, as sensitivity went up, the quality went down – film got "grainier" and sharpness and color rendition suffered. High ISO speeds were great for shooting at night without a flash, and low ISO speeds were great for high-quality results. Commonly available film ISO speeds where between 25 (Kodachrome 25) and 800.

Film Containers
Fast-forward to digital
In order to keep the terms the same, the term "ISO equivalency" is now used for digital cameras as well. Every digital camera has a base ISO speed. This is the native ISO speed of the sensor – it’s usually between 64 and 200 – generally, it’s the "default" ISO speed for the camera. This is the speed at which the highest quality images will be taken. Any deviation from this speed causes a degradation in quality. Often, you won’t notice this degradation until you get to the higher ISO speeds.
File to Digital


Sensor and Pixel Size issues
On small sensor cameras (most point-and-shoot models), the speed at which you may see lower quality may be as low as ISO 200. On larger-sensor cameras (most DLSRs), this speed may be as high as ISO 1200. The size of the sensor and the size of the pixels on the sensor has a LOT to do with how fast quality goes down as ISO speed goes up. Newer sensors also have better quality at higher ISO speeds. Sensor speeds on some DSLRs now extend to 51,000 or higher.


ISO Speed and Exposure
In order to get an image that isn’t too dark or light, your camera determines what the exposure should be. The exposure is a combination of shutter speed, lens aperture and ISO speed.

Exposure

Your lens has an aperture that can restrict how much light reaches your sensor. The "wider" the aperture (smaller numbers), the more light is let in to reach the sensor. Lenses have a maximum aperture (f 2.8, f4, f5.6, etc.). Because you can’t open your lens aperture more than its maximum aperture, this is a limiting factor. At some point, you just can't let in any more light!

Your camera has a shutter - this is a mechanical device that opens for a pre-determined amount of time to let light in to reach the sensor. You shouldn’t use a slower shutter speed than is required in order not to blur your image. Slow shutter speeds let in more light, but can also cause blurry images because of camera shake. So, the shutter speed is another limiting factor.

What’s left is your ability to change the ISO speed. Using a faster ISO speed effectively allows you to take a picture with less light available. By making your sensor more sensitive to light, it allows you to use a smaller lens aperture and/or a faster shutter speed, which helps prevent camera-shake blurring. Higher ISO speeds can cause lower quality, but the quality will be much better than a blurry picture or one that is too dark! So you shouldn’t hesitate to use higher ISO speeds!


ISO Speed and Light
When shooting without a flash with limited light, you need to use a faster ISO speed. These conditions may include: a cloudy day, open shade, night-time, indoors, etc. If you don’t have a bright-sunny day, chances are you need to increase your ISO speed. This is generally done thru a dial, button or menu choice on your camera. HOW MUCH you need to increase your ISO speed depends on a number of factors: what focal length you’re shooting at (longer focal lengths require faster shutter speeds), what the maximum aperture of your lens is, how much depth of field you want and how much light there is. You want to select the lowest ISO speed that will allow you to shoot at an appropriate shutter speed and lens opening.

In short, the less light there is, the higher the ISO speed should be.


Auto ISO
Just about every digital camera has a default option called Auto ISO. This option attempts to adjust the ISO speed to provide you with the appropriate ISO speed for the light conditions. But, since most camera manufacturers are paranoid about you taking BAD pictures with their cameras (bad pictures may cause you to switch brands!), this option often sets the ISO speed much higher than it needs to be. With an excessively high ISO speed, you’re less likely to get camera-shake blurring, but will undoubtedly get lower quality images. Some cameras let you limit how high the Auto-ISO speed option can set the ISO speed. This can be a good thing – unless you need it higher!
Typical ISO menu
Auto ISO

Example of Camera-Shake
In this case, the shutter speed was too slow and
the whole image is blurred. To correct this, a faster
ISO speed should be selected so the camera's exposure
system will select a faster shutter speed.
Auto ISO



Manual ISO
One of the best changes we saw when switching from film to digital, was the ability to shoot every image at a different ISO speed (not that we actually do that). With film, a 36 exposure roll of film required that every one of the 36 images be taken at exactly the same ISO speed. With digital, we now had the ability to shoot some at ISO 100, others at ISO 800 and still others at ISO 3200. When going from bright sun to deep shade, we had a problem with film – not so with digital!

I manually set my ISO speed based on the lighting conditions. Here are some guidelines:
  • Bright sun: 100 to 200
  • Cloudy day or shade: 400
  • Night scene: anywhere between 1200 and 6400, depending on how much light there is.
If you shoot in Aperture Priority mode (A on Nikon, Av on Canon), just set your lens aperture and look though the viewfinder. If your camera shows a shutter speed that’s too low, increase the ISO speed until the shutter speed is above what you think it should be. There is a basic rule for determining what this shutter speed should be which is: The Slowest Shutter Speed you can hand-hold your camera = 1/effective focal length (the "Effective Focal Length" is the lens focal length multiplied by the sensor crop-factor).

Using this rule, when shooting at 18mm, the slowest shutter speed that you can expect to hand-hold your camera is 1/18 of a second (with a full-frame DSLR) or 1/24 of a second (with an APS-C sensor-size). But when shooting at 200mm, the slowest shutter speed that you can expect to hand-hold your camera is 1/200 of a second (with a full-frame DSLR) or 1/300 of a second (with an APS-C sensor). And that assumes that you can hold your camera VERY steady! Most people will require significantly faster shutter speeds to avoid blurry pictures. VR or IS can help, but you shouldn't expect it to do miracles!

If you shoot in a AUTO shooting mode, you probably can’t manually set your ISO speed. Most cameras automatically set the ISO speed to AUTO ISO in this mode. In PROGRAM exposure mode, you can usually set the ISO speed, as this mode usually gives you more flexibility than AUTO exposure.



In Summary...
You can improve the quality of your images by selecting the appropriate ISO speed for the lighting conditions. Always use the lowest ISO speed that the lighting requires, but don't go too low, as you'll end up with blurry images due to camera shake. Don't be afraid of using higher ISO speeds - although higher speeds can degrade  image quality, they NEVER degrade it to the point where it's worse than a blurry image!


Happy shooting!!
 

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