The focal length of your lens (50mm, or 24-55) has nothing to do with your lens cap. The thread diameter of a lens cap is the front thread diameter of your lens, a specific thread diameter NOT dependent on the lens focal length. One way to get that diameter is to look on the inner side of a previous lens cap, or a thread diameter on a filter that fits. Sorry, it's just not that simple. Now you can get a ruler measure in mm (or centimeters marked off in tenths) and measure across the distance of the lens threads on the front of the lens, but that is a relatively crude measurement and would have to be refined by fitting a lens filter or cap on the actual front of the lens itself. They run the gamut of dimension and are rarely subject to the logic you are attempting to use.

In fact, using screw on caps, pinch on caps, and filter rings are so vastly different, that whole lens filter systems are sold so that you can put a filter holder on a variety of different lenses with an adaptor, and then use square filters at two inches or 2.5 inches and slide them on in front of any lens you use the system on.

Take your camera to a camera store that has a bunch of different size lens caps or screw on filters and get that dimension. Some lenses have it printed near the filter threads, but that's not dependable any more.

O.K., I have believed the size lens cover a lens uses is the same size as the lens (i.e. a 50 mm lens would have a 50 mm lens cover, for example). Today I was going to order some additional front (and rear) caps, blithely using my 50 mm cap for a 50 mm lens theory. Then I checked one of the covers already on a 24 - 50 mm lens. The size marked inside that lens cap is 62 mm. That kinda' blew my whole program. So, I have a 50 mm lens and 52 mm lens, each of which needs a lens cap........what size do I order?????? Yeah, yeah, I know I could measure the lens diameter, but there must be some sort of rule of thumb for this sort of thing. Obviously, the rear caps depend on the camera and its mounting.

Neal

O.K., I have believed the size lens cover a lens uses is the same size as the lens (i.e. a 50 mm lens would have a 50 mm lens cover, for example). Today I was going to order some additional front (and rear) caps, blithely using my 50 mm cap for a 50 mm lens theory. Then I checked one of the covers already on a 24 - 50 mm lens. The size marked inside that lens cap is 62 mm. That kinda' blew my whole program. So, I have a 50 mm lens and 52 mm lens, each of which needs a lens cap........what size do I order?????? Yeah, yeah, I know I could measure the lens diameter, but there must be some sort of rule of thumb for this sort of thing. Obviously, the rear caps depend on the camera and its mounting.

Neal

Also slip-on lens caps, they are called caps not covers (that is something different), are larger than the lens' threaded size (or diameter). Example I too ran into some confusion on ordering a replacement lens cap. I needed a new slip-on cap for my 120mm f/8 Schneider Super-Angulon View Camera Lens. The lens has a 82mm thread but since the factory caps are slip-on, I had to get a 85mm cap. At first I was confused by the lens data of "Filter Size 82mm", "Cap Size 85mm". I had to think about it.

Also note, the only common type of filters you might use for digital photography are Polarizers (CPL, PL), and various types of ND (Neutral Density)(split, graduated, etc.). Colored (Contrast) filters are (generally) for Black & White film photography. Does anyone use color compensating or color correcting filters today? That is what White Balance is for.

O.K., I have believed the size lens cover a lens uses is the same size as the lens (i.e. a 50 mm lens would have a 50 mm lens cover, for example). Today I was going to order some additional front (and rear) caps, blithely using my 50 mm cap for a 50 mm lens theory. Then I checked one of the covers already on a 24 - 50 mm lens. The size marked inside that lens cap is 62 mm. That kinda' blew my whole program. So, I have a 50 mm lens and 52 mm lens, each of which needs a lens cap........what size do I order?????? Yeah, yeah, I know I could measure the lens diameter, but there must be some sort of rule of thumb for this sort of thing. Obviously, the rear caps depend on the camera and its mounting.

Neal

Bob, see....., you have just proved your own theory!!!
You have no idea how big the lens cap on a 1000mm lens is, so you just guessed!!!
Now some poor little newbie is gonna read what you wrote and parrot that all over the Internet. That's how this missimfomation starts.
Do 1000mm lenses even have lens caps or filter treads?!?!

SS