Olympus EE-1 red dot sight
The Olympus EE-1 is very similar to a holographic red-dot gun-sight, and Olympus calls it in fact "dot sight", which is close. The following discussion deals mainly with using the EE-1 with an Olympus E-M1 camera and 300 mm f/4 Pro lens, but is applicable to other lenses and cameras.
A dot-sight contains a holographic reticle illuminated by an LED or laser diode. The hologram displays a circle-and-cross pattern image projected at infinity. The apparent size and distance of the target pattern remains constant, regardless of the distance between eye and dot-sight. Except for the reticle, there is nothing else between eye and subject, so a dot-sight only provides a 1:1 magnification.
The fact that a dot-sight projects the target pattern at infinity means that, if you are short-sighted, you must wear prescription glasses or contact lenses to use this type of sight. The EE-1 has no provision for mounting correction diopters at its rear port. Elderly photographers may be at a disadvantage, since the capability of the eye to refocus at different distances decreases with age, and you may be unable to refocus your eyes onto the LED rear screen of the camera without removing or changing glasses, or must tilt your head if using bifocal/multi-focal glasses. A possible solution is using the camera's viewfinder, with its built-in diopter correction, to view camera settings and recorded images instead of the rear LCD screen.
The EE-1 is lightweight (73 g with battery), and designed to mount on the flash shoe of the camera. It folds down to about twice the size of the mini electronic flash included with all high-end Olympus cameras, and uses a cheap CR2032 battery that lasts quite a long time. Unlike the dedicated mini flash, it does not draw power from the camera, which means it can be used on any camera, or even off-camera.
The plastic foot of the sight is a tight fit within the flash shoe of the E-M1 (noticeably tighter than any flash unit I have used on this camera). However, always tighten the locking collar on the shoe of the red-dot sight, because even a small amount of side-to-side twisting will misalign the sight.
The internal LED is turned on with a knob on the left side of the casing, which also allows the LED intensity to be adjusted. The optimal LED intensity depends on ambient illumination. The EE-1 casing pops open by sliding a latch on its rear. The instructions say that closing the sight turns off the LED. I have been unable to confirm this, since the LED remains on when the case is even slightly open. If there is a hidden switch, it does not engage until the casing is completely closed and the LED is invisible.
When open for use, the EE-1 looks very fragile, with almost all parts in sight made from thin plastic or sheet metal, and definitely not as sturdy as the red-dot sights of handguns (some of which look so heavy and sharp-cornered that they might take down a target just by throwing the dot-sight at it). Hopefully, the EE-1 is stronger than it looks. There is actually a good reason for the thin parts, explained below.
The EE-1 is described as weather-proof, which I assume applies to the battery compartment and electrical parts. There is no weather protection whatsoever of the holographic reticle plate and casing mechanics when the sight is open, and no weather seal around the parts that close against each other. The top of the casing is only there as a sun-shade against stray light and as a finger guard to prevent touching the reticle.
Centering the red-dot sight requires the camera to be tripod-mounted, or at a minimum resting on a steady support (it is impossible to do this hand-held). The camera must be pointed with precision at an easily identifiable landmark. The landmark must be at the center of the frame. Then, without moving the camera, adjust the sight with its two rear X (left) and Y (right) alignment knobs, until the circle-and-cross is exactly on the target. The knobs have click-stop detents to make it less likely that they will be turned accidentally while handling the camera, but the detents are quite soft (any harder, and you risk misaligning the sight), and accidents are possible.
Avoid touching the sight afterwards, just to make sure you do not disturb its alignment. With a 300 mm lens, there is little margin for sloppiness. If you are in a hurry, you can begin a session by mounting the sight on the camera and trusting that the alignment used in the previous session is still good enough. If you have time, however, it is not a bad idea to confirm this with a test shot.
A dot-sight is not meant to be used with the eye placed close to its rear port. It does work also this way, but not very well because it cuts off much of your peripheral vision and limits your field of view to a "tunnel" through the sight. Peripheral vision, and especially unobstructed vision of the area immediately surrounding the sight, should be unobstructed, because this is what makes a dot-sight superior to a camera viewfinder. The dot-sight should instead be kept at roughly 5 to 15 cm away from the eye, where the casing around the sight is unobtrusive (this is when you appreciate the fact that the EE-1 is so lightly built). Doing this with the EE-1 on the camera's flash shoe, however, forces the photographer to hold the camera away from the body, which makes the camera and a telephoto lens awkward to hold, and considerably less steady.
My first attempt to solve this problem was to turn the lens collar of the 300 mm upside down. Then, I attached the EE-1 to the tripod shoe with an Arca-compatible clamp and extra flash shoe. It does not need to be a large clamp like the one in the above picture. The flash shoe must have a 1/4-20 screw socket at the bottom, which simplifies attaching it to the clamp with a 1/4-20 bolt, and should be a "low-profile" shoe (lower than the one shown in the figure) to keep the dot-sight as close as possible to the lens axis. A further requirement is that it fits snugly the EE-1 foot (many of the flash shoes on eBay are quite sloppily made) and has a hole for the locking pin of the EE-1 collar. With the right parts, the result is at least as steady as the camera's flash shoe. This mount makes camera handling more comfortable, with the elbows resting against the rib cage, one hand gripping the camera and the other under the lens, and the back of the camera against the face in the usual long-lens handling posture. Parallax is, however, a bit more than I would like.
An alternative way of holding the camera is by keeping the camera strap above the left shoulder and under the right one, and push the camera away from the body until the strap tightens. In this way, the strap contributes to steadying the camera. This position increases the distance of the eye from the dot-sight, but may be vulnerable to arm fatigue.
The placement of the dot-sight with respect to the camera, except for the eye-to-sight distance, is not very critical. The sight can be mounted at the side of the camera instead of the top, for example. However, the distance of the sight from the lens axis should be kept at little as possible. While a difference of a few mm is not really important, parallax is one thing that the sight cannot correct. Parallax is more important at short camera-to-subject distances, and at large distances between the sight and the optical axis of the lens. Parallax can be compensated for only if the sight is aligned against a target placed at the same camera-to-subject distance as the intended subject. In this case, parallax at the calibration distance - and only at this distance - is zero.
An alternative is to calibrate the sight against a target at infinity (or at a high distance), measure the distance between lens axis and sight, which is about 90 mm with the EE-1 on-camera (i.e., about the width of four fingers), then point the circle-and-cross 90 mm above the center of the intended field of view. You will have to evaluate the 90 mm by eye, and with really large and distant subjects you can skip this step and simply center the circle-and-cross onto the subject. 90 mm off-target hardly makes a difference in the picture of a car or a buffalo, for instance.
Another thing that the EE-1 cannot do is display the contour of the camera's field of view. This might be technically possible, although it would require a reticle with multiple rectangles for lenses of different focal lengths with or without teleconverter. An interchangeable reticle might be a nice addition in a future EE-2, or even an optically zoomable illuminated rectangle controlled by an adjustment knob with detents at the most useful focal lengths.
The obvious advantage of the red-dot sight is that the eye's field of vision is much larger than the lens', so you have no difficulty training the circle-and-cross pattern onto the subject if it wanders out of the camera's field of view. The disadvantage is that you cannot see the viewfinder and its displayed settings (no, the EE-1 does not provide a heads-up display of the camera live view, which would be a formidable achievement for a device this lightweight), so you have to trust C-AF to do its job, and you must shoot in rapid sequence to be able to choose the best shots at the end of the day. Some planning can still be done to try and get more consistent results. For example, if you see a bird flying in from the left, it takes only a couple of seconds to move the AF area to the right of the frame center. This makes it more likely that the camera will focus onto the head (and hopefully, the eye) rather than the tail or the tip of a wing.