Resolution: In essence, resolution is a prime
determinant of how large you can create a print from the scanned image. Much
like the megapixels in a digital camera, the larger the file produced the
larger the potential enlarge-ability. Of course, other matters come into play,
including the quality of the scanner lens and sensor, but given all other
things are equal (which is rare) the higher the resolution you choose the
bigger the file produced, thus the greater degree of enlargement.
Scanner
resolution is expressed as pixels (or dots) per inch. The important thing to
watch for is that the resolution is expressed as Optical Resolution. Some
scanners are misleadingly labeled as having high resolution when actually what
is being stated is the potential interpolated, or resampled resolution
capability. This means the image is crunched, if you will, to extrapolate a
higher resolution than the actual optical resolution.
This is an image size dialog box from a scanner 35mm slide that was cropped slightly in scanning. The scan was made at 3200 dpi. The dialog box on top shows the scan when it has been converted to printing resolution, resulting in an 11x16 image size. Note that this can be edged up a bit using resampling at this stage, a topic covered in the workflow sections in later posts.
Simply put,
the higher the optical resolution the larger the image file. When selecting a
scanner make sure it has a minimum of 4800 dpi; 6400 dpi scanners are becoming
more affordable and are a good choice if you work mostly from 35mm film. But keep
in mind that when scanning at 6400 dpi every sharpness and other flaw in the
original will become quite apparent, like looking at yourself in a magnifying
mirror in the morning. In addition, scanning at 6400 dpi can result in some
very large image files, perhaps beyond your needs. It also can point out flaws
in the lens that you used to make the photo that might not be apparent even
when viewing the film through an 8X loupe.
Here's a scan from a 6x7 medium format negative. Note the larger image print size that can be achieved. This scan was initially made at 3200 dpi.
Bit depth: Again, as with digital cameras, you
can choose the bit depth of the image, which means how many bits of information
are captured within the three color channels of the sensor. You often have a
choice of bit depths, such as 8-, 24-, 36- and in some scanners 48-bit.
Think of
the difference between 16-bit RAW file and 8-bit JPEG images made in your
digital camera. The 16-bit file simply has more image information available,
which means you get a lot more potential out of the scan. But just as with
resolution, the higher the bit depth the larger the image file, so consider the
tradeoffs. For the most part, 36-bit will do the job, and for some end uses
24-bit and even 8-bit (for black and white images for web) may do just fine.
Dynamic Range: This is one of the main scanner
specs to check. Dynamic range is a term you might be familiar with in the
context of your digital camera. At the camera’s lowest ISO settings that range
might be 10-11 f-stops, meaning that tonal values within that span can be
recorded; the dynamic range of any camera inevitably falls as you raise ISO. In
scanners, the measure is not f-stops but in something called Optical Density
and is within a scale of 0.0 (white) to 4.0 (black).
Watch for how
the scanner’s specifications (read capabilities) are expressed in Optical
Density terms. A scanner that’s Optical Density falls between 2.0 and 3.0 may
work for some prints and “quick” tasks, but generally there will be a loss of
highlight and deeper shadow information—in other words, it is quite flat. Scanners
in the 3.2 to 4.0 range are more suitable for the kind of work you might be
interested in, with the higher number offering a better dynamic range
capability. In other words, the higher Optical Density rating will bring out
the most tonal (thus color richness as well) qualities of the image.
Flatbed or Dedicated Film Scanner?
There are
two main types of scanners for photographers and those working with
photographic images. One is a flatbed that can handle most film formats (35mm
up to 8x10) and prints up to the platen size, the most common being 8.5 x 11”.
Flatbeds can also be used to scan tintypes, daguerreotypes and glass plate
negatives. A flatbed can be versatile, but if you have only 35mmn prints and
negatives a better choice would be a dedicated film format scanner, commonly for
35mm but also available for medium format (120) film. 120 film scanners can
cover all the 120 formats, including 6x6, 6x9 and even 6x17 negatives and
slides. (Larger format dedicated scanners are strictly for professionals and
will not be covered in this project.) Note that flatbeds can usually do a good
job with 120 format and a reasonable job with film as well. How well depends on
the scanner’s specs and build.
Thus, the
scanner you choose is highly dependent on the type of images you want to scan.
If you are a family photographer with lots of old snapshots and prints and only
a few boxes of 35mm film, go for the flatbed. If your collection is primarily
composed of 35mm film then go with a dedicated film scanner.
Quick Guide: Scanner Specs for Film
and Prints
Here is a
quick rundown of the scanner specs I believe are best for your work. Of course,
budget and the volume of work you plan to scan should guide you, but scanning
via a low-spec scanner is generally a waste of your time.
Optical Resolution: 4800 dpi; better: 6400 dpi
Color Bit Depth: maximum, 48-bits per pixel, 36-bit
is usually fine
Grayscale Bit Depth: 16-bits per pixel
Optical Density: 3.4 minimum; best: 4.0
Light Source: LED
Platen Size (for prints): 8x10”; better: 8.5x11”
Film Holders and Masks: Dependent upon your formats, 35mm to
4x5” film
Bundled Software: Covered in the next posting
Next
posting: Scanner Software
