A few scanning tips
The ES-1 tube itself telescopes over nearly an inch of range (about 24mm of extension), but it is designed short, expecting to focus close for a 1:1 full frame film copy. To fill the smaller frame on the sensor of DX digital cameras, the idea of a 1:1 slide copy is instead 1:1.5 (about 0.67 size - Note that 1/1.5 = 0.67). This smaller image means the slide has to be farther in front of the lens than the ES-1 adapter can manage. The macro lens can focus anywhere, but the ES-1 can't move forward enough to get the full frame onto the little digital sensor. It needs nearly an inch more extension. This extension is in FRONT of the lens.
In my case of the Nikon 60mm macro lens and D70S DSLR, it needs 20mm extra extension added (between lens and ES-1) to be able to cover to full frame (to see a little of the black edges). You probably do want to shoot with mild cropping, to prevent having to do a manual crop operation on thousands of slides. And a little cropping is probably a very good thing for most slides anyway. But without any extra extension, the 60mm will crop to roughly about 70% of the width (half the area) at maximum ES-1 extension. That is excessive loss. It is fixed by adding a 20mm long tube with 52mm threads at both ends, male and female (which goes between front of lens and the ES-1 adapter).
I use a K5 tube from an old Nikon F extension tube set K, and it works great. The K set is the old non-AI mount - NOT compatible today, and is said to damage today's cameras if used as normal extension tubes - however this use only needs the central K5 threaded tube. The K5 tube from the K set is an aluminum tube 20mm long with 52mm threads at each end (so it fits the ES-1 and BR-5 directly). The K set also includes a threaded 10mm K4 tube, which is nearly enough for the 60mm (but not quite - some cropping). The Nikon Extension tube K set is long out of production, but used tubes may be available. Sometimes found on Ebay (search Nikon K extension) or www.keh.com (search Nikon K5 tube - cannot search KEH, so search Google and include keyword KEH). All you need is the K5 ring (which was also sold separately, in its day).
Does anyone know of any other similar 52mm threaded tubes that might be found? A simple tube about 20mm long, with 52mm male and female threads on the ends? Finding an extension is the problem. I wish I could tell you an easy answer, any answer.
Another possibility is knock the glass out of a few old battered 52mm filters, using only the threaded rim, and combine a few of them threaded together to make a longer threaded tube. KEH has 20 old 52mm filters for $19. Or a few of these could add to a too-short longer tube.
This is no doubt confusing, but what I can see is that on a DX body with Nikon 60mm macro lens, the 20mm K5 extension tube is sufficient to show an area very slightly larger than necessary at full extension of ES-1 (final image has thin black borders around the mounted slide, except at the bottom - the slide needs to be lifted very slightly - I put a couple of layers of thick paper at the bottom of the ES-1 slot). The ES-1 can be collapsed some to crop slightly. The 10mm K4 tube (alone) at full extension will crop the slide to about 89% percent width. No extension at all will crop to about 73% width, probably not acceptable. 20mm or 30mm seems about right. I am speaking of the Nikon 60mm macro lens on a 1.5 factor DSLR body.
The ES-1 telescopes to provide from 45 to 68 mm of that distance (so plan on about it providing 55mm to be in the center of its range, to have some adjustment range on either side). So you need a threaded spacer ring to make up the difference, to get within that range. Focusing distance makes the slide image be larger or smaller, and the ES-1 range locates the slide at that correct spot, where the image size is right, at 1:1 for FX or at 1.5:1 for DX. Looks like the 105mm macro on DX is about 190mm (7.5 inches), so the ES-1 will need around 135mm extension tube in front of lens (I have NOT seen or done this, but that is where my DX and 105mm focuses on a slide copy). This might become feasible on a FX body, but DX 105mm users would not use the ES-1, they would just find a way to mount the slide about 7.5 inches in front of the lens.
Focus distance and Working Distance are very different things, especially for macro. Nikon specs the closest focus distance of lenses, which is the distance to the sensor plane (the film plane, so to speak). There is a focus plane mark (an O with a line through it, something like Θ) near the rear edge of the top LCD, which marks this sensor plane - and all focus distances refer to it. Whereas "Working Distance" is from the front of the lens to the subject, important for slide copies (but DX magnification complicates that, not at 1:1). Here is a good chart showing working distances of various macro lenses.
Again, the ES-1 adjusts to hold the slide at a working distance of from 45 to 68mm in front of the lens, unless you add extension there. And the entire point is that different macro lenses of different focal lengths have very different working distances. Internal focusing changes it too. Whereas the ES-1 has much less range.
I also have a Nikon 55mm macro lens (manual lens, old f3.5 modified to AI), which came with a 27.5mm M2 extension tube to do 1:1). On a DX digital body, it still needs additional forward extension to size the slide, and the 10mm K4 ring works well for it (extension in front of lens) on a Nikon D300 body. This requirement for extra front extension is due to the smaller DSLR sensor size. Some lesser rear extension should work too, but I am unable to guess how much.
To be sure it is clear, the B&H page for the ES-1 says it is for the 55mm macro lens and PK-13 extension tube (27.5mm). The PK-13 is a bayonet mount extension tube for use BEHIND the lens, to allow the 55mm lens to focus to 1:1. The 60mm AF macro lens already focuses to 1:1 and does not need or use the PK-13 for 1.1.
Here is Nikon's chart that comes with the ES-1 adapter says the 60mm lens alone (no extension tube) does 0.96 to 1.0 reproduction - on a FX full frame body. But note the DX DSLR can only use 0.67 reproduction for a slide copy. The DX problem is that forward extension will still be necessary to position the slide far enough out to capture all of its size.
All the rest about the ES-1 may be great, but Nikon is letting us down on this. For $60, seems Nikon should be providing such tube for DX with the ES-1. Or at least they should make one available. It is required to use the ES-1 with their DX digital cameras.
Or maybe the necessary extension tube is too difficult to find. Forgetting about the ES-1, maybe there are other innovative ways to make something at home. It will likely only be used for a few days anyway, until all the slides are copied. All that is really needed is the macro lens pointing perpendicular to the slide, and a diffused light source on the back side. I have seen one made from about one foot of 1x4" board, with the camera bolted to one end (1/4"-20 screw for tripod socket), and a plastic pill bottle with a notch across the top to hold the slide - attached at the other end on a small moveable board to adjust position. This slide holder seems the only tricky part.
Or any standard macro setup should work too, simply with a tripod or copy stand, and a DSLR and a good lens, pointing at the slide which is located appropriately close. Two parts of this seem harder than normal work: One is making a slide holder that is convenient, fast and reliable to use, but which might be as simple as laying the slide at a certain spot on a translucent slide sorting tray. The other (to copy slides) is getting the lens to focus at the one correct close distance and enlargement size.
Macro Photography is a big subject (Search Google), but standard methods to get the closeup enlargement are:
Extension tubes or close up lenses are very handy for photographing things we come across, like bugs or flowers, where we accept whatever enlargement size we get, but this slide copy problem is harder because it requires an exact size ratio, and an exact size area. It is difficult to predict the exact extension giving that exact situation. A zoom lens is not the best optically, but its variable focus length surely makes this job much easier.
The rule for bellows or extension tubes behind the lens is that an extension length equal to the lens focal length will give 1:1 ratio when the lens is focused to infinity. However, near 1:1, there will only almost zero focusing size range other than 1:1 (perhaps only 0.99 to 1.1), and 1.5 will not be possible. A shorter extension will give a greater range farther from 1:1. But this is trial and error, and there is not sufficient control possible for exact ratios. Again, a zoom lens will help this by offering many choices.
This degree of magnification needs everything to be motionless. Using the self-timer on the shutter would help, or a remote control or shutter cord, to prevent shaking the camera. Or the speed of a flash used as a source would greatly help that aspect.
There must be several other ways to use the macro lens and DSLR to quickly copy slides. But what makes the Nikon ES-1 desirable is that it provides an adequate slide holder, and it attaches to the lens so that any relative motion between slide and camera is zero - no shake.