Originally posted by dipo 1 Can anyone tell me why so many shapes?
Similar to rectangular hoods, tulip shapes are intended to maximize shade with lower risk of vignette. The gotcha is that they are usually tuned to a particular lens when made for primes and are an exercise in compromise when used on zooms. Avoid generic tulip hoods.
Round hoods come two ways, generic and dedicated. Oftentimes the reason there is a dedicated version is that it is the only hood that will work on a particular lens (e.g. Tamron 70-210/3.5 (19AH). Generic round hoods (both metal and rubber) are usually available in a range of sizes broadly suitable for a general field of view; commonly, these include "tele", "normal", and "wide" with "ultra-wide" sometimes available. The better hoods will state in their description the intended focal length range and format. Hoods labeled "tele" for 24x36mm FF are often appropriate for focal lengths as short as 50mm on APS-C. Likewise "normal" hoods may be used and preferable as short as 28mm on APS-C. There is a type of generic rubber hood that may be used for multiple focal lengths based on how far it is unfolded.
There are ongoing discussions regarding use of hoods for front element physical protection. Some believe strongly that a rigid metal or plastic hood will transmit shock through to the lens body in much the same way as a metal filter ring or a direct blow to the lens. For those folk, the preference is often for flexible rubber on the theory that material will better absorb a blow. I have both types.
Steve