John Ryberg wrote:
. . . Most sources say to use a daylight ( 5K - 6K) bulb or led.
Nothing is ever mentioned about the brightness or lumens needed to accurately reproduce the slide. Daylight bulbs I have seen range from 500 to 2000 lumens. If to dim the colors can't be seen and if to bright the colors will appear washed out.
Is there a correct brightness for this or is it just personal preference and trial-and-error?
John,
Your digital camera can easily give you the proper white balance regardless of whether you use a daylight or other light source. However, some LEDs and fluorescents do not give a complete light spectrum.
You might find bulbs which have a color temperature of daylight (in the neighborhood of 5000 K) but they may be weak in certain wavelengths. Sometimes this may work fine -- particularly if working with black and white -- but it may cause some unexplainable color shifts at odd times. You want a full-spectrum light source. Incandescent bulbs seem to be good, with proper white balance from the camera, so do electronic flash units.
The color of your light source can also be affected by how it is diffused. Opal glass or frosted plastic may have a filtering effect that slightly tints the color of the light. Same goes with bouncing the light source off of a "white" paper or painted surface. In camera white balance should correct this.
The intensity of the light affects your exposure and your working conditions. With electronic flash you control the exposure with the lens aperture and occasionally by changing the flash location or using neutral density filters. With LEDs or incandescent lights you can change your shutter speed as well.
The actual number of lumens is not critical. You can get the same results with 500 lumens or 2000 lumens-- depending on how far the light source is located from the slide you are copying, if the brighter source is farther away. I'd go with the less expensive ones. Higher lumens frequently means a hotter temperature. Try to avoid heat build up. It becomes uncomfortable for the photographer and can cause slides to slightly change shape as they warm up.
Since the original should be rigidly mounted, and not moving relative to the camera and lens, very long exposures are possible -- but using very dim bulbs makes it harder to focus, and when the exposure time gets over 20 seconds it getting into the realm of using a scanner. So, while a single candle is theoretically a possible light source, I'd save that option for use on a deserted island. On the other hand, very bright bulbs will give you eyestrain and a headache after staring into them for a few hours. I do not suggest using a slide projector as a light source for that reason.
Use the lowest ISO setting your camera has, to get the best image. You can adjust that, or adjust the distance of the light source to the original slide to tweek your exposure if you find an optimal lens aperture- which is usually f/8 --give or take a stop. With electronic flash exposures those may be your two exposure options. With continuous light you can change shutter speeds.
If you use electronic flash as your light source be sure to let the unit recycle completely between flashes, and give it a few extra seconds to cool down between flashes. You will also need need a secondary light source (a modeling light) to allow you to focus on the original. For best color, turn off the modeling light before using the flash for your exposure. If this affects autofocus, you may need to turn that off, too.