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Understanding ISO

Understanding ISO. Just because your camera CAN shoot at HIGH ISO, doesn’t mean you should. Both are $1,700 cameras, one production year apart. JPEG Compression doesn’t really affect small images. Which image has greater compression? Left or Right?. Now can you tell?.

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Understanding ISO

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  1. Understanding ISO Just because your camera CAN shoot at HIGH ISO, doesn’t mean you should.

  2. Both are $1,700 cameras, one production year apart.

  3. JPEG Compression doesn’t really affect small images • Which image has greater compression? Left or Right?

  4. Now can you tell? • The image on the left is 11k, the right is 450k

  5. JPEG Generational Loss • This image shows the (accentuated) difference between a jpg-saved image and the original. Note especially the changes apparent on sharp edges. • Every time an image is saved in a "lossy" format, such as JPEG, some changes (errors) are inevitably introduced into the image. This example shows the "difference" between the original, and a JPEG-compressed copy. • This image was made thus: The original and the compressed image were put on separate levels, with the difference filtering chosen for the top level. The image was flattened, auto level adjusted and inverted to accentuate the difference, which is not as apparent in the images themselves.

  6. mosquito noise • A distortion that appears near crisp edges of objects in MPEG and other video frames that are compressed with the discrete cosine transform (DCT). It occurs at decompression when the decoding engine has to approximate the discarded data by inverting the transform model. The mosquito noise appears as random aliasing in these areas and requires sophisticated detection circuits to eliminate it. As TVs get larger, mosquito noise and other artifacts become more noticeable.

  7. Posterization • The effect may be created deliberately, or happen accidentally. • As an artistic effect, posterization may be created deliberately using most photo-editing programs. • Unwanted posterization, also known as banding, may occur when the color depth, sometimes called bit depth, is insufficient to accurately sample a continuous gradation of color tone. As a result, a continuous gradient appears as a series of discrete steps or bands of color — hence the name. When discussing fixed pixel displays, such as LCD and plasma televisions, this effect is referred to as false contouring.[1] The result may be compounded further by an optical illusion, called the Mach band illusion, in which each band appears to have an intensity gradient in the direction opposing the overall gradient. This problem may be resolved, in part, with dithering.

  8. Checkerboarding JPEG Artifacts • Square-shaped noise distortion known as "blockiness" or "checkerboarding"

  9. JPEGs lose quality every time they are opened, edited and saved. • If a JPEG image is opened, edited, and saved again it results in additional image degradation. • It is very important to minimize the number of editing sessions between the initial and final version of a JPEG image. • If you must perform editing functions in several sessions or in several different programs, you should use an image format that is not lossy (TIFF, BMP, PNG) for the intermediate editing sessions before saving the final version. Repeated saving within the same editing session won't introduce additional damage. • It is only when the image is closed, re-opened, edited and saved again.

  10. Resources • http://graphicssoft.about.com/od/formatsjpeg/a/jpegmythsfacts.htm - JPEG Myths and Facts

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