Clean and place
Clean the slide
Make sure the slide and the scanning surface are clean and dust free. Even the tiniest spec of dirt or smudge will look pretty awful when enlarged. Hold the slide by its mounting, being careful not to touch the surface with your fingers. If you need to remove dust, do so with a blower brush (available from photographic stores) or another soft brush. Don't blow on it because you're likely to spray flecks of spittle on it's surface that'll be very difficult to remove without doing damage to the slide.
If the slide is really dirty, wipe the non-emulsion side (see Dictionary below) with a lint-free cloth slightly moistened with distilled water. Never use detergent. If all else fails, your local photo lab may be able to clean it for you.
Hint: You can tell the emulsion side of a slide by looking through it. If the scene is correctly oriented - not back to front - you're looking at the non-emulsion side. Alternatively, take the slide into strong light and look for ridges on the slide's surface - that's the emulsion side. Emulsion can be hard to spot so relying on the first method is usually safest.
Depending on your scanner, you'll need to place the slide in a grid, or in a particular position on the scanner's surface. It's important to pay attention to these instructions because the scanner software will 'expect' to find the slide in that particular place. Never cover the calibration space (see Dictionary below) on the grid (if there's one) and always let the scanner lamp warm up for the recommended time - the light colour changes as the lamp's temperature increases which can make a difference to the quality of the scan.
Make sure you place the slide right side up - the emulsion side should be facing the light source, which is usually - but not always - in the lid of a flat-bed scanner or the slide scanning attachment. If you get it wrong you can reverse the image later using image manipulation software, but you risk reducing the quality of the scan.
Most slides are mounted in plastic or cardboard, which means they sit just above the scanner's glass plate. If you can take them out of the mounting without damaging them, you may get a better result because the lens is usually focussed on the top of the glass plate and anything above it will be out of focus. If the slide is inclined to curl or crimp when out of its mount try pinning the corners down with a small, flat weight like a fridge magnet. If this doesn't work put it back in the mounting.
Select the right settings
Select 'colour transparency' or an equivalent as the source material in your scanner's software. In general, the only other setting you're likely to see for transparencies is 'colour negatives' which are quite different because they have magenta built into them that needs to be removed during the scan to give its true colour. In addition, negatives contain a reverse image, so a slide scanned on this setting will be pretty useless - unless you're looking for special effects.
To avoid scanning more of the slide than you want to reproduce, use the marquee tool to select the scan area. Most scanners look for the darkest and lightest points on a scan and adjust their settings accordingly, so if you scan lots of space around the slide you may cause the scanner to degrade the colour of the slide. You'll also create a bigger file than is necessary.
Sharpen things up
To help compensate for the scanners' tendency to produce slightly out of focus (soft) images, scanner software often uses a 'sharpen' effect by default for slides. However, if you have the choice, select 'unsharp mask' rather than 'sharpen' (see Dictionary below). It'll give a better overall result for photographs. If you overdo it the first time, you'll need to play around a bit with the settings until you find one that seems to work for most scans. Simple sharpening is usually most effective when applied by the scanner software, but you can also sharpen using image manipulation software after the scan is complete. In most cases the unsharp mask is best used after scanning and doing any other manipulations.
Some more advanced scanners allow you to control the image contrast by setting the white and black points in your slide before you scan. This usually involves clicking a pointer on the lightest and darkest parts of the slide. It's useful if the slide has a definite white and black section. Otherwise it's best to rely on the automated scan and to fix any lack of contrast etc later.
Detail and format
How much detail?
To decide what dpi (see Dictionary below) to scan at, it's helpful to know how large you'll want to print the resulting image. For example, a 35 mm slide is only 24 x 36 mm, so printing at 10 x 15 cm means you're blowing it up to 16 times its original size. If you scan at 600 dpi it'll be reduced to around 150 dpi - a quarter of the original resolution - when enlarged for printing (see below). If you're scanning for archival purposes, scan at the maximum optical dpi your scanner can manage. You can always reduce its size in future, but you can't add more detail once the scan is complete without degrading image quality.
Some software will automate the process by allowing you to set a print size before you scan, regardless of the size of the original. If you use this option, be careful the software doesn't use interpolated resolution, which can reduce image quality. Scanning at a dpi higher than the scanner's maximum optical resolution will result in interpolation.
Choose a format
Selecting a format in which to save your scans is not always easy. While there's no reason not to use the preferred format of your current image manipulation software, if you're thinking of the future it would be wise to make a backup copy in a non-proprietary format that's likely to be around for a while and is supported by a large number of software programs. Saving your scans as .tif or .bitmap may produce big files, but will ensure they have a long lifespan. Alternatively, the ubiquitous Jpeg format may suit if you're willing to accept some reduction in quality. The main thing is to remember that nothing lasts forever without care and it may be necessary to convert treasured scans to another format at some time in the future.
Calibration space: used to set the position of the scanning head on the scanning surface.
dpi - dots per inch: the resolution of the scanner. The more dots (pixels) per inch, the more detailed images the scanner can produce.
Emulsion: the film coating that records an image.
Unsharp mask: a tool for sharpening edges. Best suited to photographic images. Simple sharpening tools can cause photographs to look false.