Why 'Petal' lens hoods?

Roy Mathers wrote in post #2615486
20drgoger - would you care to elaborate?

err

think how easy and ergonomic using a circular focus or zoom ring is

and then think how daft using a rectangular or square focus/zoom ring would be

or let's put circular rings on a rectangular lens, how un-ergonomic is that likely to be?!!11

Dave_G wrote in post #2616018
err

think how easy and ergonomic using a circular focus or zoom ring is

and then think how daft using a rectangular or square focus/zoom ring would be

or let's put circular rings on a rectangular lens, how un-ergonomic is that likely to be?!!11

Why do you need a manual focus and zoom ring in this digital age? Surely electronic focus and zoom with idiot buttons would be better.

Why do you need to attach a ring to the front of your camera. Many super telephotos and the Sigma 12-24 use internal filters.

I am not advocating rectangular lenses. I just said it would be possible to build one. Not practical, but possible.

I love the petal hoods. Ever since I saw the first one many months back I've loved them. My 28-135mm IS uses a petal hood and I live the way it looks compared to my other hoods which are just circular.

Oh, it's possible to build a lens with rectangular elements, but hardly practical.

First, there is the problem of those elements that must shift for focus and/or zoom. With circular elements, this is a relatively simple matter of helical tracking. With non-circular elements, this would become the far more complex problem of sliding elements while maintaining precision and stability. This is especially true of focus; zoom, not so much. (Ever use a spyglass?)

The mechanics of rectangular lenses would be complex beyond belief.

The biggest problems, however, would (I think) be ones of optical and thermal stability. If the radii from the optical axis are not equal for an element, then the slightest rotational or positional wobble would introduce coma like you wouldn't believe.

Similarly, variant radii would lead to differential thermal expansion problems that would make it virtually impossible to control spherical and chromatic aberrations, and would also produce coma.

The optics of rectangular lenses would also be complex beyond belief.

The digital age has nothing to do with it. Lenses are physical pieces of glass and/or crystal, and are most stable when all radii are equal. To make them acircular would increase lens complexity by at least one or two orders of magnitude with substantially no practical gain.

It ain't gonna happen.

Of course, the biggest single reason a rectangular lens is impractical is cost. If you don't think cost is a primary consideration in lens design, then you definitely do not design lenses for Canon or Nikon.

Feel free to design and build a practical rectangular camera lens and prove me wrong.

A rectangular 2:3 format lens would, however, not need a petal hood (the subject of this thread), because the rectangular shape would naturally produce a "petal" view over a fixed-length rectangular hood. I leave the basic geometry as an excersize for the reader.

There's a very technical explaination of the petal design:
http://www.vanwalree.c​om/optics/lenshood.htm​l

tellingthm wrote in post #2615835
you can try locking the hood on, and then turning it back slightly. depending on the lens the and crop factor of the camea, you'll notice immediately why hood are petal-shaped. and unless they're in the correct position, they'll show up in the image.

I was going to say that as well.
I recently took a load of shots with my Tamron 17-50f/2.8 and most of the shots at the wide end had a black area in two opposite corners. it took me a while to figure what it was as i have used the hood many times. It was that i hadn't clicked it on properly. so i guess being petal shaped does make a difference.

troutbreath wrote in post #2615156
Don't know about the hood, but I like that lens cap design.

I like that cap too. Nikon and Tamron caps are better then Sigma and Canon.

And Tokina too You can handle their caps with quite a deep hood still on the lens!

you can get the Tamron type of lens caps off ebay very cheaply.
I have one for my 24-105L and it works perfectly. Doesn't touch the lens glass at all.

Would anyone recommend purchasing a petal lens hood for my canon 30D
EF-S 60mm f/2.8 Macro USM lens? And if so would you be kind enough to let me know where i could find it?

Many thanks!

Tris