Filter size question

[James3shin]

I was looking at filters and noticed that lens size does not match up with filter size. For instance, I want to get a filter [or hood, still debating] for a Canon 50mm F1.8 and B&H suggested a 52mm filter. Are these filters threaded to screw onto a lens or are they applied like stickers?

 

[996GT2]

50mm is the focal length of the lens. The 52mm refers to the front diameter of the lens thread. Most filters are threaded onto the front of a lens, so in your case, you need a 52mm filter for the 52mm front thread diameter on your 50mm f/1.8.

Lenses with faster f/stops tend to have larger front openings, since the f/stop is the ratio of the lens's focal length divided by its maximum aperture opening. Therefore, a 200mm f/2.8 needs to have a maximum diameter opening of about 71.5mm, and as such uses a large 77mm filter thread. It is often possible to use larger filters with a step up ring on your 50mm lens; usually, using a 77mm filter on a lens with a 52mm filter thread won't cause any vignetting.

 

[ivan2]

52mm is the diameter of the lens, and the physical size of the filter. not the focal length.

 

[James3shin]

Thanks for the knowledge 996 and ivan!

 

[ElFenix]

Originally posted by: 996GT2
50mm is the focal length of the lens. The 52mm refers to the front diameter of the lens thread. Most filters are threaded onto the front of a lens, so in your case, you need a 52mm filter for the 52mm front thread diameter on your 50mm f/1.8.

Lenses with faster f/stops tend to have larger front openings, since the f/stop is the ratio of the lens's focal length divided by its maximum aperture opening. Therefore, a 200mm f/2.8 needs to have a maximum diameter opening of about 71.5mm, and as such uses a large 77mm filter thread. It is often possible to use larger filters with a step up ring on your 50mm lens; usually, using a 77mm filter on a lens with a 52mm filter thread won't cause any vignetting.

a good 77 mm filter will cost about as much as a 50 f/1.8, so i wouldn't go that route myself

 

[996GT2]

Originally posted by: ElFenix

Originally posted by: 996GT2
50mm is the focal length of the lens. The 52mm refers to the front diameter of the lens thread. Most filters are threaded onto the front of a lens, so in your case, you need a 52mm filter for the 52mm front thread diameter on your 50mm f/1.8.

Lenses with faster f/stops tend to have larger front openings, since the f/stop is the ratio of the lens's focal length divided by its maximum aperture opening. Therefore, a 200mm f/2.8 needs to have a maximum diameter opening of about 71.5mm, and as such uses a large 77mm filter thread. It is often possible to use larger filters with a step up ring on your 50mm lens; usually, using a 77mm filter on a lens with a 52mm filter thread won't cause any vignetting.

a good 77 mm filter will cost about as much as a 50 f/1.8, so i wouldn't go that route myself

I know, but I'm just saying, in case he ever decides to buy a lens using 77mm filters, they are still usable on his 50mm f/1.8 II using a step up ring.