Lens caps are one of the annoying necessities of photographic life. Lenses are expensive, and their front and back elements easily soiled and difficult to clean. They are also scratched with relative ease. Dust that settles onto the back of a lens can easily move to the inside of the camera, and eventually ends up on the camera sensor, producing dust spots and requiring a delicate cleaning operation. For these reasons, all lenses are supplied with front and rear caps to cover these sensitive areas. Mounting a lens on a DSLR requires the removal of the rear lens cap. Operation of the camera requires also the removal of the front lens cap. With film cameras, forgetting to remove the front lens cap was one of the four traditional ways of failing to take a shot. Thank you for asking: the others were (1) forgetting to load a film into the camera, (2) forgetting to advance the film after a shot, and (3) putting a finger in front of the lens. On a DSLR or a digital camera with live preview on an LCD screen, the most common accidents are instead: (1) forgetting to turn on the camera, and (2) forgetting to recharge or change the battery.
While point-and-shoot cameras often have the front lens cap tethered to the camera by a string, or built into the camera and automatically operated, this is not practical with a DSLR. Thus, the front and rear caps often end up in a pocket, where they can collect abundant dust that later is transferred to the lens and into the camera.
Some lenses and accessories need special caps. Among these are the oversize front caps for super telephoto lenses, which usually are made of leather or similar materials, and sized to cover the front of the lens with its lens shade reversed for storage. This is because these heavy lenses usually are stored with the front element pointing downward, and all the weight of the lens rests on its front rim. Normal plastic caps would break under the weight, and for this reason there are no plastic caps larger than about 100 mm. Ultra-wide-angle and fisheye lenses often need special front caps as well. Some teleconverters (including all current Nikon ones) have protruding front elements, and their front caps are different from those used as body caps, in spite of having similar bayonet mounts.
Nikkor lenses for SLR and DSLR cameras use at least three different models of rear caps. Third-party lenses with Nikon F bayonet also use their own brand rear caps. The above picture shows three Nikon rear caps, and a Sigma one (top right). The Nikon cap at the bottom left is an older one, rather large but without particular drawbacks. The one at the bottom right is the current one that generally comes with more expensive lenses. It is small, sturdy, and generally sits well at the back of lenses. Cheaper Nikkor lenses (and lately some more expensive ones as well) come with a translucent white cap (top left) which is possibly the worst in current camera industry. Basically, it is a cylindrical plastic cup without a bayonet of any kind, which fits at the back of a lens by pressing around the rim of its bayonet flange. It goes on and off either by turning it, or simply by pushing or pulling it. The plastic is soft, and bayonet flanges often scratch away pieces of the cap interior, which tend to end up on the rear lens element. This wear makes the cap wider and looser, so after a while it tends to come off by itself.
The white Nikon rear cap should be regarded as just a part of the lens packaging. By all means, you should use a better rear cap for daily use and transportation of a lens. The price to Nikon of an ordinary, more useful rear cap is probably on the order of US 0.10$, as opposed to perhaps 0.05$ for the white one (retail prices are of course much higher because of packaging, distribution, and the natural desire of sellers to rip off customers). I fail to see why Nikon would choose to save 0.05$ on a lens that costs several hundred or thousand dollars to the end user, and to provide an obviously useless accessory that prompts the user to buy a separate cap (perhaps from a third-party manufacturer instead of Nikon).
Sigma rear caps are another peculiarity of the photographic industry. Sigma has chosen to provide rear caps that fit well only on Sigma lenses. They are too loose to sit properly on Nikkor lenses, and do not mount at all on Nikkor lenses, unless properly oriented (by aligning an almost invisible black dot with the mounting dot of the lens). A proprietary cap is simply nonsense because, in normal use, the cap that comes off a lens being taken out of the bag immediately goes onto the next lens that returns to the bag. For this reason, I bought several extra Nikon rear caps (they are less ludicrously expensive in Japan than in the West) and got rid of all Sigma rear caps and white Nikon caps. I do not know whether the rear caps that come with lenses from other third-party manufacturers have similar problems, but I have heard no complaints comparable to those often expressed about Sigma caps.
There are relatively cheap rear caps for Nikon lenses manufactured in the Far East. I have no general information on these. I do have a few rear lens caps and body caps that unashamedly sport the "Nikon" logo but are in fact fakes (they differ in subtle details from original ones, and have no part number inside). The fake rear lens caps generally fit well on my lenses, but one fake Nikon camera cap in my possession is way too tight and cannot be turned all the way to lock it.
Micro 4/3 rear lens caps are a whole different story. Genuine Olympus and Panasonic Micro 4/3 rear caps fit very well on all my Micro 4/3 lenses. The rear caps of Sigma and Samyang Micro 4/3 lenses are also of good quality and interchangeable on all my Micro 4/3 lenses. Until not so long ago, the only third-party Micro 4/3 rear caps easily available on eBay displayed a "MICRO" label at their rear, shown above (or sometimes "LUMIX" or "MICRO 4/3"). They sit too loosely on any of my lenses to be usable, except for an old Panasonic consumer zoom with plastic rear bayonet. These problematic caps sometimes fit properly on Micro 4/3 lens adapters with aluminium bayonets, which tend to have thicker tabs than standard chrome-plated bayonets. Other caps, like the "MFT" shown above, are sometimes better made than "MICRO" ones, but need to be tested.
A few months ago, I discovered on eBay fake Olympus rear caps almost identical to the genuine articles, at least on their outside (above picture). Can you tell which is genuine, and which is a fake?
Neither can I be sure, without looking inside. In the knockoffs, the interior is rougher and has no writing (the Olympus model number and "China" is present in the genuine ones). Unlike the "MICRO" caps, these Olympus knockoffs are a perfect fit on my Micro 4/3 lenses, and indistinguishable in use from the genuine article. I warmly recommend them, in spite of being more expensive than the "MICRO" ones.
Front lens caps are another source of frustration. Most modern lenses use clip-on caps with two buttons that must be pressed to mount or release the cap. This reduces the risk of the cap falling off during transportation and handling of a lens. Most of these caps have the buttons on the rim of the cap (above picture, left and centre columns). This makes it impossible to put on or take off the cap while the lens shade is mounted onto the lens (often, also when the lens shade is reversed onto the lens for storage). Old Nikon caps with plastic or small metal buttons fall into this category (above picture, centre column). Current Nikon caps (above picture, right column) solve this problem with a butterfly-shaped arrangement of the buttons, which can be pressed by pinching them near the centre of the cap as well as along its rim. Modern Tamron lens caps employ a similar design, while Sigma ones still have only rim buttons. For this reason, years ago I replaced all my Sigma front caps with Nikon ones, except for those 82 mm in diameter and wider. Nikon apparently does not manufacture butterfly caps of these sizes, but I have been able to find 82 mm and 86 mm no-name ones.
Olympus equips some of its current Micro 4/3 lenses with center-pinch-only caps that do not have buttons on their periphery (so far the only example I am aware of), and others of its lenses with buttons only on their periphery. Some have the buttons at the top and bottom, others on the sides. This sounds like a decision-by-committee, which ends up following all possible avenues except the most obvious one. These caps have a metal outer shell (left) or a metal insert (right), which can dent or separate from the cap if you happen to drop it. I prefer all-plastic caps.
Third-party butterfly front lens caps are now broadly available on eBay, with or without fake "Nikon", Canon" etc. branding. I have also seen front lens caps with a slightly more honest "Designed for Nikon" logo, and also these ones seem to be of good quality. I prefer those without branding. They are just as good as original front lens caps (they appear to be copies of Nikon caps), and have edge- as well as center-pinch buttons. Almost all of them now come with the "added value" of a small hole, or a small plastic ring, at the edge of the cap and a string probably supposed to tie the cap, through this hole or ring, to the lens or camera. The string is most often unusable for this purpose, because on most lenses there is simply no place available to tie a string without interfering with barrel extension and retraction, or the rotation of zoom and focus rings. Attaching the cap to the camera body or the camera strap is of course pure nonsense with a camera with interchangeable lenses. I just throw these strings away. The hole in the cap can be plugged with a small dab of epoxy or black silicone if it bothers you, but it makes no practical difference in terms of dust protection.