Camera lenses should always be protected when not in use.  Buying a lens cap is much less expensive than buying a new lens or camera because you've scratched it! Unfortunately... it's very easy to lose the lens cap that comes with most cameras and/or lenses.  Finding a replacement isn't always easy.  There are two main types of lens caps:

• Front lens caps: These caps go on the front of the lens (the part that is pointed at your subject).
• Rear lens caps: These caps go on the rear of your lens when your lens is removed from your camera body.  You only need a rear cap if you have a camera that takes interchangeable lenses.

Front Lens Caps

Let's talk first about front lens caps.  There are basically three main types of front lens caps.  They include:

• Snap-on lens caps: These caps fit inside the filter ring of your camera's lens.
• Push-on lens caps: These caps fit over the outer rim of your camera's lens.
• Proprietary caps: These caps are typically designed by the manufacturer of the camera and/or lens to specifically fit only one or two models

The most common of these caps are the Snap-on lens caps.  If you have a lens that can take filters... this is the cap for you! The standard unit of measurement for all lens caps is in millimeters.  If you already know the size of your filters... then you are all set.  The lens cap should be the same size (if your camera takes 52mm filters, you will need a 52mm snap-on lens cap).  If you don't know the size of your filter threads, you can find out by looking on your lens for the symbol Ø with a number next to it. This number is the size in millimeters of your filter threads, and hence the size cap you need.  This marking will typically appear either on the rim of the front of your lens, or on the side of your lenses barrel.  Standard sizes for snap-on lens caps include: 27mm, 28mm, 30mm, 34mm, 37mm, 39mm, 40.5mm, 43mm, 46mm, 49mm, 52mm, 55mm, 58mm, 62mm, 67mm, 72mm, 77mm, 82mm and 86mm. 

Push-on lens caps are also common, but not as effective as snap-on lens caps.  They don't affix to the lens as securely, so they are easily lost.  However, if your lens does not have filter threads this may be your only option.  If the rim of your lens barrel is extended from the glass of your lens... you may be able to slide a push-on lens cap over the rim.  To find out what size you need, get a metric ruler.  Measure the outer diameter of the rim on your lens barrel.  This measurement, in millimeters, is the size push-on lens cap you will need.

Proprietary lens caps are the least common type of caps, but are becoming slightly more frequent on digital cameras.  These caps are unique to your camera due to special tabs, extra area of coverage or a non-standard size.  Unfortunately, these caps can usually only be replaced by the manufacturer (or a dealer for that manufacturer).  The best way to find a replacement is to contact a dealer with your camera's model number and ask if they can get a replacement cap for you.

Once you find the right front lens cap for your camera... make sure you prevent yourself from losing it again!  Devices called cap keepers can be affixed to your cap, and then attached to your lens.  This way, the cap dangles from a string attached to your lens when you remove it.  Then, you no longer have to worry about walking away and forgetting where you put the cap!  These devices are usually less expensive then the cap itself... so they are an excellent investment

Rear Lens Caps

Now... let's discuss Rear Lens Caps.  These caps are specific to the type/manufacturer of camera you use.   The most common types of cameras that use interchangeable cameras are 35mm Rangefinder, 35mm SLR, Digital SLR and Medium Format cameras.  There are three main types of rear lens caps:

• Bayonet Mount: These caps attach by twisting and locking on to the Bayonet mount of your camera lens
• Screw-On: These caps attach by screwing on to a lens that has a threaded mount
• Push-on: These caps slide over the mount of your lens

Bayonet Mount are by far the most common type of rear lens caps.  To find out what kind of rear lens cap you need.... you should find out what kind of camera you're using.  Here's a list of different types of rear lens caps, and (for the most part) the camera lenses they will work on:

• Bronica ETR rear lens cap   -             Bronica ETR lenses
  
• Bronica SQ rear lens cap    -              Bronica SQ lenses
  
• Canon EOS rear lens cap  -               Canon EOS lenses
  
• Canon FD rear lens cap    -               Canon FD or FL lenses
  
• Hasselblad rear lens cap  -                 Hasselblad lenses
  
• Konica rear lens cap -                        Konica 35mm SLR lenses
  
• Leica M rear lens cap -                      Leica M-series lenses
  
• Leica R rear lens cap -                       Leica R - series lenses
  
• Minolta Maxxum rear lens cap -          Minolta Maxxum lenses
  
• Minolta MD rear lens cap    -              Minolta MD and ML lenses
  
• Nikon rear lens cap -                          Nikon manual focus AND Nikon Auto focus lenses
  
• Olympus rear lens cap -                      Olympus manual focus lenses
  
• Pentax K-mount -                               Pentax Auto focus and Manual focus bayonet-mount lenses (also used by Ricoh, Sears, Vivitar, etc..)
  
• Yashica / Contax rear lens cap            Most Yashica and Contax lenses for 35mm SLR camera systemss

There are basically just two main type of screw-mount lens caps (others exist but aren't very common).  They include:

• Leica Screw Rear Lens cap (M39) -   Leica rangefinder lenses, or any other lens (usually rangefinder) that has a 39mm screw mount
• Pentax Screw Mount (M42)               Pentax Screw Mount lenses, or any other lens with a 42mm screw mount

Finally, you can usually find push-on caps for just about any of the mounts listed above.  However... these caps are not as secure as the actual bayonet or screw-mount caps.  They tend to get looser with use, and eventually don't protect the lens very well.  They are also easy to lose.  However, some lenses only take push-on caps... such as the following:

Mamiya TLR Long Rear Lens cap -    135mm, 180mm and 250mm sized Mamiya TLR lenses

In this case, the caps are designed specifically for the Mamiya TLR mount... and the caps do a good job at protecting your lenses. Ultimately, it is always worth your time to find the right cap for your camera lens.  It will protect it from scratches, as well as dust and dirt.  Properly protecting your camera equipment will allow you to get many years of use from your camera system.