Myths About Slide Scanning and Negative Scanning: Scanned vs. digital camera images

Myth: Scanned slides look like high-end digital images from a DSLR.

We are spoiled by the quality of even inexpensive digital cameras. Digital cameras are wonderful. Even inexpensive digital cameras create excellent images. Digital single lens reflex (DSLR) cameras can produce fantastic images. Let's examine the differences between scanned images and digital images from modern cameras.

Pixel Count

If the only way that we compared scanned images with camera images was the number of pixels, scanned images look pretty similar to images from a DSLR.

35mm Film size	36mm x 24mm (1.4 in x 0.95 in)
Pixels scanned at 2000 PPI	~2800 x 1890 5.2M Pixels
Pixels scanned at 4000 PPI	~5600 x 3780 21.1M Pixels
Low-end DSLR (e.g., Canon Rebel XT, Nikon D40)	6-8MP
High-end DSLR (e.g. Canon 1Ds, Nikon D300)	12 - 21MP

At 2000 PPI, a scanned 35mm slide or negative produces about a 5-6MP image, comparable to low-end DSLRs. At 4000 PPI, a scan gives about 21MP, comparable with high-end DSLRs. However, all pixels are not created equally, as we will see in the discussions below.

Film is an Analog Medium

Scanned images differ from digital camera images in one fundamental way; scanned images are based on an analog medium. The images on film are composed of millions of tiny pigment elements that we call grain. Film grain is readily apparent in enlarged prints. A high-resolution scan of film will resolve (or nearly resolve) these film grains. Hence scanned images will (if enlarged enough) show grain. Digital camera images do not have grain.

Beyond the issue of grain, scanned images are fundamentally limited by the quality of the film and shot.

Defects

Scratches, dirt, fingerprints, and fading.

Nearly every piece of film that we see has at least some of this damage. Image correction technologies and techniques such as ICE and color restoration can repair much of this damage, but in many cases, not all such damage can be easily and efficiently removed. Scanned images will retain some of this "character" of the original film. This makes the scanned images distinctly different than images from a digital camera - which generally will not have such defects (though you can have dirt on the image sensor or lens).

Sharpness

Much of the film that we see at Pixmonix was shot on inexpensive point and shoot cameras by amateur photographers. This means that many images are not "tack sharp". The cameras used to create the images do not have top-quality lenses and shutters. Additionally, very few of us regularly take the steps required to produce a truely sharp image: using a tripod, careful exposure determination for shutter speed, etc. For these reasons, scans of the film are fundamentally limited by this less-than-sharp image.

Digital cameras, on the other hand, make it easier to get sharp, well exposed shots. They adjust focus and exposure automatically - usually with vey good results. They have automatic flashes. Many modern cameras have built in vibration reduction systems (i.e. anti-shake) to sharpen images without use of a tripod. All of these factors together mean that the average shot on a digital camera is sharper and better exposed than shots on standard film cameras.

Noise

Noise in digital images is the presence of pixels that are off in color or luminence (brightness). Often noise manifests itself as color speckles in areas of solid color such as the sky where there should be none. For example, instead of a blue sky, you notice faint pink, purple and other color speckles. Noise in scanners and digital camera sensors is caused by a variety of complicated electrical and electronic interactions. It tends to become more apparent as the scanner or image sensor is pushed to the edge of it's capabilities. For example, scanning at the highest available resolution will result in more apparent noise than scanning at a lower resolution. Using a digital camera at a high ISO equivalent (e.g., making it more sensitive to light) results in higher noise in the resulting images. One can think of noise in digital images as analogous to the subtle background hiss you may hear from your audio system.

Noise is in some ways the digital equivalent of film grain for film cameras. However, our eyes tend to find the grain appearance from film more favorably than noise in scanned images. Digital cameras tend to have very low noise levels. In particular, digital SLRs have large image sensors that have very low noise levels. Scanned pixels tend to be noisier than pixels from good quality digital cameras. In particular, underexposed images on negative film tend to show a lot of noise. Slide film tends to be finer grained and hence shows less noise and grain. However bright areas on slides can show more noise.

We have techniques and technologies that help to reduce noise when scanning. For example, we can apply a noise filter to the scanned images. This is a good way to reduce noise, but can have the unwanted side effect of reducing the finest detail in the image and tends to make areas of uniform color too "painterly". Hence we use these tools with a very light hand.

All of these factors mean that scanned images will not look like images from a digital camera. Scanned images are wonderful things. They can be used in many ways. They bring back the great memories of times past. But they are not the same as images from a DSLR.

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