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The hoods supplied with some lenses seem to be larger than necessary. That is, they step up the diameter and then make it longer.

The most egregious example I know is Canon EF 17-40L f/4 (or EF-S 10-22, which uses the same hood, as far as I remember):

Wiki

Sigma 150-600 made a special effort to raise its hood on stumps:

Why? Geometrically, it seems to be pointless: only the angle should matter, so a shorter, smaller diameter hood would do just the same job, while being much more compact (and could still be mounted in reverse for storage). The EW-83E was such a menace when I had to use it; it could only be packed away separately. The same with this Sigma...

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  • It's not a duplicate, but have you seen this question: photo.stackexchange.com/q/399/9161? The answer showing just a 3D rendering might be helpful. Aug 19 at 15:59
  • @Saaru, that question (or the answers to it) presents purely geometric reasons (for the hood shape). If that were all the truth, there would indeed be no reason to make the hood larger.
    – Zeus
    Aug 20 at 1:14
  • Sorry, re-reading your question content I understand that your question is only about the step up in diameter, not the hood petal shape itself. However, the why seems just to follow from the frustration of its size while packing, so is your underlying question perhaps "Is there a way to compact a lenshood" (or something along those lines)? In that case you might be interested in lenshoods.net Aug 20 at 8:06
  • @Saaru, thanks, yes, I'm aware of some practical solutions (like paper or rubber hoods). Here I'm rather wondering why. If my understanding is wrong, my custom compact hood may create unexpected vignetting or perhaps some other effects...
    – Zeus
    Aug 20 at 8:24
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Geometrically, it seems to be pointless: only the angle should matter,

Which angle? This is a zoom lens. So we have a number of different angles starting from different entrance pupils. If they all manage to run through the same kind of opening at some distance before the filter threads, that's where the hood needs to end.

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  • For zoom lenses, the hood is normally optimised for the widest angle: it can't obscure the image in any case. Whatever the view angle (even considering the entrance pupil), it can be satisfied by moving the hood edge closer along the line of view, i.e. making it closer and shorter. I don't see where is the fault here...
    – Zeus
    Aug 20 at 0:27
  • @Zeus The fault here is that most zoom lenses are designed to have the location of the entrance pupil retract in a non-linear manner as the zoom length increases. That makes the cones associated with visibility not share a common tip vertice in relation to the front lens.
    – user98068
    Aug 20 at 7:22
  • OK, I see how it could be a problem (solvable by a wider hood). I'm not yet convinced though it is the pratcical problem they are trying to solve... But resolving that would require someone with the actual lens design experience...
    – Zeus
    Aug 20 at 8:05
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One possible reason is that you should still be able to maneuver filters when the hood is on (a specific case being polarizing filters used often on standard/wide angle lenses).

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  • I doubt it. This wouldn't explain the sheer 17-40L size, and generally, it is not an issue with wide lenses (and thus small hoods). The newer 16-35 f/4L has a much more compact hood. For long lenses, maybe (it is a bit difficult to rotate the filter on the 70-200 f/2.8, which has a relatively compact hood), but I would certainly trade it off for compactness. A tight hood is better than no hood if I can't pack it...
    – Zeus
    Aug 20 at 1:05
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An appropriately shorter hood of smaller diameter will not vignette the image, but it also will not protect the front element from off axis stray light as well as a taller/wider hood will... stray light contamination comes from outside of the lens' field of view and causes varying degrees of veiling flare/loss of contrast.

A deeper hood will limit the stray light that can reach the objective element to being more forward. If it is deeper it also needs to be wider (or notched) in order to avoid vignetting.

And for critical work I have a bellows hood that has an adjustable mask. I set the mask to 2:3 format for my DSLRs and extend it so that it blocks absolutely all light except that coming from w/in the recorded image area.

It's like this one for large format cameras (same principle). enter image description here

Edit: additional diagrams to clarify

  • top left base diagram
  • top right shows the requirement for a source nearer/more forward
  • bottom left shows a larger diameter objective element (they can be
    larger/smaller in the same diameter barrel) enter image description here

Different element diameters in the same mount/barrel diameter; the hood can be shorter for the f/2.8 lens and provide the same protection (diameter dictated by the lens barrel).

enter image description here

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  • There is (probably) a grain of truth in the first sentence. That is, we may need to consider the size of the front element. The angle from the edge of the hood to the farthest edge of the element will be different for different hoods of the equivalent 'central' geometry. But if this is the reason, the answer should articulate it more clearly (and ideally with some proofs)...
    – Zeus
    Aug 20 at 1:24
  • @Zeus, Not only the angle to the farthest edge of the objective element (larger diameter), but also the highest point (high curvature objective element). I guess the part that may not be intuitive is that the majority of light contamination (flare/loss of contrast/etc) typically comes from light not w/in the lens' recorded FOV. Aug 20 at 2:50
  • @Zeus diagrams added Aug 20 at 13:29
  • Thanks! I'm inclined to think you are right (in the sense: this is the primary reason), although movement of the FOV origin for zoom lenses mentioned by user98068 could also matter. Perhaps, the 17-40 was rather bad for stray light which required such an oversized hood...
    – Zeus
    Aug 22 at 6:16
  • @Zeus, the lens' field of view is approximately twice as wide at 17mm as it is at 40mm; so the diameter and height of the hood has to be designed for the 17mm position (so it doesn't cause vignetting). Aug 22 at 12:46

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