The digital camera is a very fast way to digitize thousands of slides, and a 12 to 24 megapixel camera with 1:1 macro lens is quite capable of getting the all the resolution that a normal 35 mm slide has to give. Even a 2 or 3 megapixel camera can create an copy very suitable for a HDTV or monitor. Best case is a slide copy allowing one film dimension to fill the frame (slides smaller than the camera sensor will not fill the frame at 1:1). The best film scanners were 4000 dpi, and it was debated then that 3000 dpi was plenty to resolve film detail. And the high quality camera macro lens is superb quality.
A wide range of many film sizes, sensors, lens ratios and crop factors can be computed. You specify the lens situation you have, which might limit the calculated size it can produce, or it may compute that your situation needs a reproduction ratio with less than the maximum magnification specified. The largest image size possible for the situation will be computed.
This calculator computes expected results. It does of course generally expect macro lens capability. The calculator shows the maximum image size result, and the necessary macro lens magnification setting to do it, and also that scan resolution.
If specified megapixels is a rounded approximate number, the pixel dimensions could be few pixels off, but it'll be close enough. Or, you can enter precise pixel dimensions. Increased crop factor of a smaller sensor increases the reproduction ratio to fit the smaller sensor dimensions. Larger Crop Factor also increases the necessary subject distance (to be a smaller magnification), but if we have same pixel dimensions, it does not change the resulting scan dpi.
Reproduction Ratio: Note that 1:2 and 2:1 are very different. This ratio convention is:
35 mm slides will work best using a 1:1 macro lens. The 1:1 is a fixed ratio, so if at 1:1, slides smaller than the sensor create images smaller than the sensor (same size as slide). So to copy full size slides, a 1.5x crop DSLR sensor will use the lens near its 1:1.5 ratio setting. The macro lens is very versatile, and can be refocused to any workable ratio for larger slides at a greater distance. Many current macro lenses can do 1:1, but several older ones only do 1:2 (half life-size images). And a few regular lenses can do 1:3 (and they call it macro, and 1:3 can copy medium film size).
The 1:2 lens offers less image size and resolution than 1:1, but it may still be sufficient in some cases (with more extreme cropping). Or for 35 mm slides, a 1.5 or 1.6 crop factor will partially compensate for a 1:2 lens (with 75% or 80% image size instead of 50%. A 2x crop factor would be 100% size at 1:2.) We can't move the slide closer because the lens won't focus closer than 1:2. Or adding a regular extension tube with extension length 50% of the focal length will convert a 1:2 lens to 1:1.
8 mm movie film is the smallest, and small film could use normal extension tubes (between body and lens). Tubes creating from 1:1 up to 5:1 (quite extreme) can be computed (magnification of tubes is more effective on shorter lenses). The same numeric ratios give the same result sizes for macro lenses or extension tubes. The useful result shown may need less ratio than the maximum you specify, if the tube choices allow it. A smaller sensor with larger crop factor can help with the smallest film.
For larger sheet film, regular lenses out to 1:8 at closest focus can be computed. Specifications for interchangeable lenses normally show the maximum reproduction ratio at closest focus (B&H product pages are good to show this).
Focus with Extension tubes: Much trickier, see extension tubes here.
Focus with Macro lenses: Extremely convenient, these focus easily like any lens, from infinity to closest macro range. At 1:1 reproduction ratio, the distance between lens and slide will be roughly twice the marked focal length (probably a little less). If setting a specific ratio like 1:1, the focus ring on the lens simply changes that reproduction ratio. To keep the ratio, focus at setup is best adjusted by just moving the slide slightly back and forth, to the place that it is in focus at this setting. Then the macro lens focus will work there, still 1:1.
Numbers: Example at the calculator if copying 36 x 24 mm slide film with 1:1 lens ...
If copying film size equal or larger than your sensor size, then you will be able to fill your frame with a 1:1 lens, at least with one dimension of it (both depends on aspect ratio of film matching your sensor). This calculated output image size assumes you will "fill the frame" with one dimension of the slide image. Not fully filling the frame width would be less dpi. Overfill (causing cropping) would be greater dpi. Dpi shown is "scan resolution" ("pixels per inch" of film, just like a scanner), and it also shows "pixels per mm" of film.
If copying film smaller than your sensor, then with a 1:1 lens, your largest copy will be 1:1, and that resulting image will be smaller than your frame (so subsequent cropping will be necessary). This "fill the frame" magnification is not possible for the tiniest film (since most macro lenses only go to 1:1). Filling the frame with film smaller than the sensor (small film or large sensor) requires exceeding what 1:1 magnification can do. Then (if crop factor is entered here) these small sizes instead are computed using a 1:1 lens at 1:1 (frame is not filled). This computation of copying smaller film does need an accurate crop factor (see determining crop factor). Otherwise, without crop factor, filling the specified frame is all that it can assume, but which is not likely possible for 1:1 and smaller film.
There is Magnification and there is Reproduction Ratio (like 1:1). These are reciprocals. Magnification is (size of image : real life object size), and Reproduction Ratio is the reciprocal (opposite, or upside down). Camera images are typically a size reduction, normally significantly smaller than 1:1 (like 1:100 or 1:1000). We may take a picture of something large on a few mm of sensor, which maybe is a reduced reproduction of 1:100, which is a magnification of 0.01x. But a magnification of greater than 1 is larger on the sensor than in real life. Which might be accomplished by adding extension tubes, but normally a macro lens does not focus larger than 1:1.
When we used film, we could hold the processed film in our hand, and see that image for a clear understanding about it's size. We can't see the actual sensor image in a digital camera, but it's exactly the same size concept, size at the sensor, created by the lens. The reproduction ratio 1:1 means the external field of view (in millimeters) is exactly equal to the sensor size.
A macro lens at 1:1 means that the subject image is full real life-size on the sensor. A full-frame camera copying a 35 mm slide would of course need a 1:1 macro lens, to make a full size copy of the slide. At 1:1, a 36x24 mm slide size exactly matches a full frame 36x24 mm sensor size.
An APS or DX size DSLR has a smaller sensor (smaller than the 35 mm slide), so cannot use as much as 1:1 (for this slide copy). The 1:1 lens magnification is the SAME subject size on any sensor (it's a lens property, NOT a sensor property), but the cropped sensor does seriously crop the full frame view of it. The 35 mm slide is larger than the cropped sensor size. A 1.5 crop sensor does need 1:1.5 reproduction (for a 35 mm slide), so the slide is held a little farther in front of the lens (to be seen smaller). Then at 1:1.5, a 36x24mm slide size matches a 24x16 mm sensor size.