It is a common misconception that image quality is solely a function of image sharpness (resolution or MTF).

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 3200.

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.

In any case, one can see from the chart below that all pixels are not created equal. A pixel captured by a professional digital camera is superior to its film counterpart.

Image Quality – Why Digital Has Caught Up to Film

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 1D	4.2	28x19	38.5	284	5,996	0.68

Keep in mind also that this is solely an index for 35mm film.  Today's top cameras like the Nikon D2x or Canon 1ds Mark II completely now outperform 35mm film, matching more closely the image quality of Medium Format film. And digital backs like the Phase One P25 or the Leaf Aptus 22 - can closely match 4x5 large format transparency.