Recall that in open sights, the rear sight is simply a plate or a post with a notch cut in it. The notch is usually a V-shape, U-shape or a square shape. The front sight is merely a post or a bead. The main problem with them is that open sights hide parts of the target when aiming at it and it is slower to acquire a target with them.
An aperture sight is similar to an open sight, except that the rear sight is a ring with a hole cut through the middle of it. The front sight on an aperture sight is similar or identical to an open sight (i.e. it may be a ramp, post or a bead) or it may have a ring too. The user peeps through the rear sight ring and aligns the front sight to the target. Due to the way that the human eye focuses on the front sight and the target alone, the rear sight ring appears as a blurred or a ghost-like ring to the eye. Hence, such sights are also called peep sights or ghost ring sights.
Click image to enlarge
In the above picture, we have an AR-15 A2 rifle (the civilian version of an M16). The two sights are marked in the above picture. The front sight on the left is simply a post type sight. Notice that the rear sight is part of the carrying handle Here's a view of the carrying handle from the other side.
Note the two knobs at the back of the weapon. The lower knob is used to adjust the range of the weapon (by controlling the elevation) and the top knob controls the windage. There is also one more interesting feature about this sight, as the picture below shows
In the above image, we have the actual rear sight piece alone. Note that there are actually two rings here. One has a smaller aperture than the other. The user can switch between one or the other by pressing the piece with a finger. The bigger hole allows the user to pick up targets easily and the smaller hole allows targeting with greater precision. The bigger hole is sometimes marked 0-2 to indicate it should be used for targets between 0 to 200 meters away.
This is what a sight picture through one of these sights would look like:
Because of the way that the eye focuses, the rear sight ring becomes an out of focus, ghostly ring and the front sight is sharp and focussed on the target.
The above image is of a sight picture from a Heckler and Koch MP-5 submachine gun. In this case, the front sight is also a ring with a post in the middle.
These sights share many of the properties of the open type iron sights. Like all iron sights, they are very simple to use. These sights may be fixed or adjustable type to adjust for range. They are relatively small and unobtrusive (though not as small as open sights) and don't affect the balance of the weapon much. Rain and fog don't affect these sights at all. Like the open sights, some of these sights may have tritium filled glass tubes, so that they can easily be picked up in the dark. They are also fairly cheap to manufacture.
Like all other iron sights, they have the disadvantage that they have no magnification or night vision enhancement and therefore depend on the user's eyesight alone.
These sights have one big advantage over open sights though. With open sights, there are three points that the human eye has to focus on simultaneously: the rear sight, the front sight and the target. Since these three points are at different distances from the eye, it is hard to focus on these simultaneously. In fact, only younger people with good eyesight can do this properly. Older people and people with far-sightedness find it much harder to do this correctly and hence it makes it harder to aim an open sight. With an aperture sight, there are only two focus points to handle: the front sight and the target. The eye simply centers around the rear sight ring automatically and it appears as a ghostly ring. This makes it much easier to aim the aperture sight quickly and easily.
This is why aperture sights became popular for several military rifles in the last few decades. They are the best all-round type of iron sight by far. They have been used with weapons like the M1 Garand rifle, the M16 family, some Lee-Enfield rifles e.g. the well known Enfield No. 4 SMLE rifle from World War II etc.
Aperture sights first started to gain popularity in the late 19th century for rifles. The early aperture sights came in two types, the tang type and ladder type. The difference between the two really has to do with where the rear sight is placed and adjusted. On a ladder type, the rear sight is on the barrel and can be folded down when not in use. The rear sight can be slid up and down the ladder to adjust for range. The tang type is mounted behind the action and is therefore closer to the user's eye. Tang sights could also be folded down and the aperture moved up and down for range. They could also be adjusted for windage. Many sights also had vernier scales or micrometers (screw gauge) attached, so that they could be adjusted more precisely. Such sights were often made by third party specialist manufacturers, such as Marble Arms Co., Lyman Gun Sight Co. etc., and often installed separately by the owner of the rifle. Sights like these were common in the wild west and used in historically significant rifles such as the Sharps rifle, Winchester model 1894 lever action rifle etc., which we've already dealt with in some previous posts.
The two images above are a ladder and a tang sight. The first one has a vernier scale attached to the side and the second one has screw gauges. The second one can be adjusted both horizontally and vertically to adjust for for windage and elevation. Both may be folded when not in use.
To illustrate the difference between open sights and aperture sights, take a look at the two images below:
These are two images of the same target taken from two different rifles at different distances. The first image is through an open sight of an AK-47 at 50 yards and the second is through an aperture sight of an AR-15 at 25 yards.
Aperture sights remained very popular for both military and hunting applications for a very long time, until reasonably priced good telescope sights became available. They are still available on many weapons to use as backup sights, in case the high tech sights fail.