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In fact, image quality is a much more complicated interplay of both sharpness and SNR - or “signal to noise ratio.” SNR is the digital equivalent to film grain. It is called “noise” in the digital world because, much like film grain, it is distracting. SNR is actually just as important as sharpness in terms of perceiving image quality. The lower the SNR, the more grain or noise will be visible in the image.
Fuji Provia, a transparency film (ISO 100) - is generally considered the finest grained film in the world. When scanned, it typically produces an SNR of 95, while grainier films, such as Fuji Velvia (ISO 50), scan at an SNR of 50.
In comparison, most pro digital cameras of today have SNR’s well over 200. The Nikon D100 for example, has an SNR of 308, some 3-6 times that of typical films. This means that a digital image is FAR less grainy than film, even up to ISO 1600.
In fact, the noise of a digital image is so superior to the grain of film, that digital images can often appear to be sharper, even when they are actually lower resolution. That is why digital images can score at or above film on an Image Quality Index Scale, even with smaller sensors -with some 15 Megapixels less of “pure” resolution.
The Normalized Image Quality Index is used to compare the Image Quality of various sensors, both film and digital. It is based on an ideal case of 1.0, where a 35mm frame such as Provia would be digitally enhanced using Adobe Photoshop to reduce film grain. However, this requires a very particluar kind of image to work. Only large areas without detail, such as a cloudless sky, can have grain removed without drastically reducing sharpness.
|
Image Sensor
|
MegaPixels
|
Sensor Size |
MTF
|
SNR
|
Image Quality
|
Normalized Image Quality
|
| 35mm Provia |
21.4
|
36x24
|
46.7
|
95
|
7,363
|
0.84
|
| Nikon D100 |
6.1
|
24x16
|
51.7
|
308
|
6,667
|
0.76
|
| 35mm Velvia |
21.4
|
36x24
|
46.7
|
53
|
6,450
|
0.73
|
| Canon D30 |
3.1
|
23x15
|
41.0
|
213
|
4,788
|
0.54
|