John_F Loc: Minneapolis, MN

Now there's a thought. One could get some special effects with color filters. What would blue sky look like through a red filter. If you went deep enough red could you get psuedo infra-red.

NewBEE161 Loc: Olney, Maryland

Hey SS,
Don't pick on us NewBees! I thought the numerical value was in inches! JUST KIDDING

amfoto1 Loc: San Jose, Calif. USA

I'm sure others have responded, but I'm too lazy to read them all...

Yeah, your "theory" is going up in smoke. It's way off.

In fact, the optical focal length of a lens has almost no direct relation, by itself, to the physical diameter of a lens, which in turn dictates the size of the front cap and filters it uses.

The maximum aperture in combination with the optical focal length, are deciding factors of lens diameter and, thus, the filter/cap size. For example, a 50mm f2.8 lens would need an aperture that can open to about 18mm diameter. A 100mm f2.8 lens would need a 36mm diameter opening, while a 300mm f2.8 lens needs 108mm diameter (over four inches)! A lens also needs mechanisms around that aperture opening, in order for it to be adjustable... so that 300mm lens likely needs to be 5 or 6 inches in diameter! Of course, the 50mm or 100mm focal length lenses can be a lot smaller in diameter.

Angle of view is another consideration. Wide angle lenses, in particular, need a larger diameter so that nothing "up front" will intrude on the image. Telephoto lenses, with a narrower view, don't typically need as large a diameter at the front. (For example, I've got 10-22mm, 24-70mm and 300mm lenses that all use the same 77mm size filters and lens caps.)

Now, most lenses are marked with their filter thread diameter... a number in millimeters asdjacent to a small circle with a diagonal line drawn through it. More often than not, the front lens caps use this same number in their designations.

For example, a Canon E-58 lens cap fits their lenses that have 58mm filter threads.

If the lens is not marked in that way, look it up on the manufacturer's website or at one of the big, online retailers (B&H Photo, Adorama, etc.) In the specifications, they will usually tell you both the filter thread size and the exact model of lens cap needed. Even third party manufacturers selling cheaper lens caps often use the same designations.

FYI and for example, Canon currently or recently has made five different 50mm (focal length) lenses:

- EF 50mm f1.8 II (recently discontinued) uses a 52mm filter and E-52 cap.
- EF 50mm f1.8 STM (superseded the above lens) uses a 49mm filter and E-49 cap.
- EF 50mm f2.5 Compact Macro uses a 52mm filter and E-52 cap.
- EF 50mm f1.4 USM uses a 58mm filter and an E-58U cap.
- EF 50mm f1.2L USM uses a 72mm filter and an E-72U cap.

Note: The "U" on the last two indicates the cap is labelled "Ultrasonic", referring to the USM focus drive type on those particular lenses. A plain E-58 cap will fit fine, too, just won't have "Ultrasonic" written on it. Also, newer Canon lenses come with an improved "pinch" type cap that's further indicated with a "II" suffix on the model number. Where available, these are also fully interchangeable with the same size, earlier caps. Finally, some lenses require special lens caps... such as very oversize lenses like the Canon EF 300mm f2.8L IS USM II (which has a 130mm diameter front element and no front filter threads)... or the Canon EF 14mm f2.8L II or EF 11-24mm f4L, both of which have a convex front element and a permanently installed lens hood that a special cap fits over.

It's another matter with the rear lens caps, which fit onto the bayonet mount of the lens. The rear lens cap is largely the same for all lenses in a particular manufacturer's system. For example, as far as I know, all Canon "E" rear lens caps fit all Canon EF/EF-S lenses. There are also corresponding body caps, for example Canon "EB" cap, to cover and protect the camera's internals when no lens is mounted.

Madman Loc: Gulf Coast, Florida USA

To my knowledge, all lenses are marked with the diameter information that you need. It could be anywhere on the lens, so you will have to look for it. You are looking for a number, either preceded by or followed by a symbol that looks like a 0 (zero) with a / (slash) through it. That is the diameter measured in millimeters (mm).

photoman022 Loc: Manchester CT USA

What speters said (on page 1)

forjava Loc: Half Moon Bay, CA

Hmmm... Here is a case study: I just looked at my 50mm f/1.2 lens and the cap is a LC-52 (52mm) cap, like my slower f/1.4s and 1.8s. I have the impression that designers have some leeway in choosing a front-lens diameter. For example, the 50mm 1.4G filter size is 58mm. I have read a bit of Nikkor lens history and noticed a case where Nikon issued a lens with a filter size, maybe 58mm, and redesigned it at the same speed to be 52mm, which is more convenient, among other advantages.

Calipers can help you measure filter size until you learn to eyeball it. Digital calipers are great. I recently used mine to measure distances on the mount of my screwless Nikkor H 50mm f/2. It turns out this mount has some peculiarities.

Nikkor and Series E lenses are the same size at the back. Use a LF-1 or newer LF-4 cap.

SharpShooter Loc: NorCal

Java, better take another look at your lens. Both the 50 and 85 AF f1.2's that I have used are 72mm. They are HUGE compared to the 1.4. Better take another peak at yours!!! LoL
SS