The "petal" lens hood

[Doug Kerr]

Some have wondered what is the principle behind a "petal " lens hood (shade), such as we see here:

Why is this shape advantageous?

The basic job of a lens hood is to block light coming from outside the field of view of the camera. Such light does not contribute to the image, and ideally would be harmlessly absorbed by baffles and such in the lens, but this prophylaxis is incomplete, and some of such light can contribute to "lens flare" of various sorts. Thus the lens hood, which could potentially completely eliminate almost all of this unwanted light.

Assume for the moment a fixed focal length lens (a fixed field of view of the camera), with a rectangular image frame. Conceptually, the proper lens hood would be "hopper shaped", with a rectangular mouth. We often see such in professional motion picture setups.

They are often called "matte boxes", since another function is to support masks ("mattes") in front of the camera.​

Suppose Canon were to supply such a hood for use with a certain fixed focal length lens, assuming its use on cameras of a certain format size. What would be wrong with that?

• The photographer's girlfriend would say:

"Jason, why do you have to have that monstrosity on the camera? This is Joel and Chloe's rehearsal dinner, for God's sake—you can't set a thing like that on the table"

• Canon management would say, "Wow! That has so much plastic in it that it will cost USD 0.11 each to make. That will wreck my bonus. Cut back!"

So what do we do? We imagine a slightly-tapered cylinder around the initial design, with its diameter such that it just embraces the tallest part of the mouth of the "hopper". Then we cut away all of the hood that lies outside that cylinder.

The result is the familiar "petal form".

So, how does it protect against unwanted light from the left and right, or from the "corners"?

Not very well.

But Canon can make it for USD 0.08 each.

Best regards,

Doug

 

[Bart_van_der_Wolf]

The result is the familiar "petal form".

So, how does it protect against unwanted light from the left and right, or from the "corners"?

Not very well.

Hi Doug,

Actually, it does it pretty well (except for the longer top and bottom edge of a rectangular image), and better than some other compromises. Depth, focus distance, and aperture, are other variables. Another practical consideration is the 'requirement' that the lens-hood should allow to be mounted in reverse for compact storage in a bag or another protective container/case.

A lens-hood on a zoom lens is more of a compromise, although some lens designs let the front element move deeper into the barrel with fixed hood for longer focal lengths, or move outwards for wider angle lenses.

Here is some more background info.

Cheers,
Bart

 

[Doug Kerr]

Hi, Bart,

Actually, it does it pretty well (except for the longer top and bottom edge of a rectangular image), and better than some other compromises. Depth, focus distance, and aperture, are other variables. Another practical consideration is the 'requirement' that the lens-hood should allow to be mounted in reverse for compact storage in a bag or another protective container/case.

Indeed.

A lens-hood on a zoom lens is more of a compromise, although some lens designs let the front element move deeper into the barrel with fixed hood for longer focal lengths, or move outwards for wider angle lenses.

Indeed, and I (intentionally) did not treat that in my essay.

Here is some more background info.

Thanks for that excellent reference. Paul's stuff is always very good.

Best regards,

Doug

 

[nicolas claris]

Interesting though a bit too "technique" for me…
As a side note and quite off topic, lens hoods also protect the front element from dust, rain and even shocks…
I always have them mounted whether they are "tubes" or "petal" shape.

 

[Bart_van_der_Wolf]

Interesting though a bit too "technique" for me…
As a side note and quite off topic, lens hoods also protect the front element from dust, rain and even shocks…
I always have them mounted whether they are "tubes" or "petal" shape.

Hi Nicolas,

That's right, they are also a natural barrier for objects that try to damage the front element of a lens. It also helps a bit against water droplets/spray coming from above or the sides.

The main use is to avoid stray (non-image forming) light from hitting the front element of the lens, or even enter the image forming elements of the lens. Light that enters the lenses at oblique angles is more likely to cause internal reflections which will reduce image contrast. That will quickly reduce the dynamic range of a system to a fraction (!) of its capabilities. A relatively cheap piece of plastic / rubber / metal can turn a mediocre looking image into something more sparkling and vivid.

Cheers,
Bart

 

[Tom dinning]

Hoodwinked again. (Excuse the pun). So why do Canon charge $30 if you buy one separately.
I'm like Jason's girlfriend I so many ways. The little assistance the lens hood gives doesn't seem to warrant their clumsy and dorkish appearance along with the extra space they take up, even when 'folded back' for storage. I have found they used to make great mug holders in my old cars when glued to the dash board but even now that is obsolete. Never the less it does require some forethought about my relationship with the sun and direct light from bright sources, but that seems to come more naturally than carrying something that can look like a fog horn or missile launcher. I have a couple dozen Nikon hoods for sale if anyone is interested.

 

[Bart_van_der_Wolf]

Hoodwinked again. (Excuse the pun). So why do Canon charge $30 if you buy one separately.

Hi Tom,

I suppose because they are a commercial company. I ordered one from China the other day, US$ 4, including transport by air, and it was delivered within a few days.

I'm like Jason's girlfriend I so many ways. The little assistance the lens hood gives doesn't seem to warrant their clumsy and dorkish appearance along with the extra space they take up, even when 'folded back' for storage.

I'm surprised that you care about appearance. It must have something to do with your childhood, do you want to talk about it? ;-)

On a more serious note, different strokes for different folks. Some of us shoot images that require immaculate technique to differentiate our work from low cost competitors. Some of us even shoot subjects that require no artistic quality at all, e.g. reproductions. Some of us have great artistic vision but also technical skills that help prevent distraction from the artistic content that might be caused by substandard capture technique or post-processing.

Lens-hoods also help to protect the front element of a lens against impact, rain, and smears. Actually many of the images in this thread were shot in the rain, and the lens-hood prevented blur, and water entering my lens from the front of the zoom construction.

Your suggestion that all that a good image requires is an artistic photographer, seems a bit more provocative than realistic, especially in a thread about Imaging Technology.

Cheers,
Bart

 

[Doug Kerr]

The very interesting tutorial by Paul van Walree that Bart was kind enough to reference has helped me to grasp the various issues in the matter of lens hoods.

I realize now that some of the outlooks on which my first essay were based were incorrect.

I don't necessarily agree with Paul's observations on different configurations of lens hood.

I am working on some figures to illustrate the points to follow, but I am now using a new illustration application and I'm not yet fluent with it, so it is taking a while. Thus, for the moment, I must do this wholly by narrative.​

Simplistically and ideally, we seek to achieve (to the extent practical) both of these requirements:

A. To just barely prevent the hood from (significantly) exacerbating vignetting.

This means that its mouth should just barely admit all rays that will be admitted through the entrance pupil and form an image within our image frame).

I say "just barely" because if it does more that will clearly work against requirement B, below, so I am just getting a little out in front.​

B. To block all rays that would not be admitted through the entrance pupil of the lens and form an image within our image frame but would enter the lens.

The reason is that these by definition do not contribute in any way to image formation, and all light that enters the lens can contribute to flare, thus any light that does not contribute to the image and might enter the lens should be blocked. Of course, some of these rays would be "more dangerous than others", and real design work will take that into account, but here our simple approach is to "keep 'em all out".​

To do this is not as simple as it seems, because of the geometric optics involved.

Suppose that we have a "matte box" type lens hood with its mouth at a certain distance in front of the lens. Suppose that its mouth dimensions are such that, in combination with its location, it "just barely" admits any ray that would pass through the entrance pupil of the lens and form an image within our image frame. It fully meets our requirement A.

Then it must block all rays that would not pass through the entrance pupil of the lens and form an image within our image frame, but would still enter the lens, right? It also fully meets requirement B, right?

Not so. (I wish I had my illustrations ready.) There will be lots of rays that will pass through this mask that do not pass through the entrance pupil and form an image within our image frame but which nevertheless enter the lens.

Hmm.

So we see that we have not "fully" met requirement B.

Now lets use a larger matte box, one whose mouth is at a greater distance from the lens, and whose mouth dimensions are such that again, it "just barely" admits any ray that would pass through the entrance pupil of the lens and form an image within our image frame.

We know that this will still admit rays that would not pass through the entrance pupil of the lens and form an image within our image frame, but would still enter the lens.

But fewer than in the first case. It would probably do a better job of protecting against lens flare. We still have not "fully" met requirement B, but we have "come closer".

Now a petal-style lens hood (let's assume for a fixed focal-length lens, optimized for the format size in use) can certainly meet our objective A. How does it do for requirement B?

• At the top and bottom, to some degree.

• At the left and right, to a lesser degree.

• At the "corners" of the field of view, to a lesser degree yet.

Is the "matte box style" hood the optimum in terms of some metric (maybe overall lens flare for a described test protocol defining the incidence of light from different angles, versus total shade surface area and thus presumed material cost)?

If it were, how much does a "petal form hood" fall short on that metric? I would have no way to even guess. How much "lesser" is a Nissan Altima than a Nissan Maxima?

Just a thought.

Best regards,

Doug

 

[Doug Kerr]

This is further to the discussion of lens hoods.

Introduction

In this presentation, I will describe as "undesired rays" those rays that cannot contribute to the formation of an image of any object point but nevertheless can enter the lens.

That is not to suggest that all such rays have equal potential to contribute to lens flare.

I will describe as "useful rays" those rays from an object point at some location in the field of view that can contribute to the formation of the image of that point (in particular, that can pass through the "net" entrance pupil of the lens.

Our object in using a lens hood is to block as many of the unwanted rays as possible, consistent with out other requirements. One of those requirements i that the hood not block any useful rays for any object point in the field of view, so it does not exacerbate vignetting.

Illustrations

The following illustrations draw upon the approach used in the tutorial article on lens hoods by Paul van Walree (cited by Bart van der Wolf). I have in fact taken the liberty of adapting one of Paul's lens sketches.

We start here:

Figure 1. Zeiss 50 mm f/1.4 at full aperture with illustrative hood "A"
Object point assumed to be at infinity, 30° off-axis​

In the lens sketch, the red object is the base entrance pupil, the image of the actual aperture stop (itself shown in black) as seen from in front of the lens. The purple object is the image, as seen from in front of the lens, of the rim of the rearmost element (another potential aperture stop). The front rim of the lens (green) serves as a third potential aperture stop; it is in front of the lens, so we need not draw separately its virtual image as seen from there.

We assume an object point substantially off the optical axis (30°), perhaps beyond the actual field of view limit of this lens, but this is only a "blackboard" illustration. We assume the point to be at infinity to simplify things. Assume that this is the most off-axis we can handle, so this is perhaps in the corner of the field of view.

At the upper right we see the three aperture stops (one intended, the others accidental) as seen from the location of the assumed object point. The red circle is the "intended" aperture stop; the green and purpose circles represent the "accidental" aperture stops that are the rims of the front and rear element.

The overall "net" entrance pupil is the intersection of all these circles, shown there in pink. We see that for this object point, there is already severe vignetting.

Now, we return to the side view of the whole battle zone. The pink stripe represents the "shaft" of light from our object point that can travel through all these obstacles and form a point image on the focal plane.

But we don't show the rays of this shaft being refracted by the various lens elements. We show it traveling as if there were no lens elements, and as if the various entrance pupils exist in their virtual manifestations. That is, this is the picture as it would seem to be from in front of the lens.

The shaft is shown in pink, corresponding to the "net entrance pupil".

The chief ray is the ray that passes through the center of the (real) entrance pupil (though the center of the aperture stop, if we actually "look inside the lens").​

We show a hypothetical lens hood, type "A". At the plane of our object point, its mouth just touches the outside of the shaft of "useful rays". Thus this is the "tightest" the hood mouth can be (at this distance from the lens) and not further exacerbate the vignetting.

Now lets see how our "unwanted" rays do. Seen from the side in the plane of the object point of interest, these are any rays that lie outside the shaft of useful rays from our extreme object point but would still strike the front element of the lens.

What about rays inside the shaft of "useful" rays? Well, those might be "useful" rays from other object points, not so far off-axis, so we dare not exclude those.​

The range of the angles of arrival of the unwanted rays that are nor blocked by the hood is shown in red. We do not shown them that way once they get inside the hood, only because that would clutter up the drawing.

Now, we see the same thing for a "larger" hood design:

Figure 1. Zeiss 50 mm f/1.4 at full aperture with illustrative hood "B"
Object point assumed to be at infinity, 30° off-axis​

The story is just the same as before, except that now the range of unblocked undesirable rays is much smaller.

The implication

We might expect hood B to do a more thorough job of countering lens flare.

In a "petal" hood, at the corners of the field of view, it behaves more like hood A than hood B.

Best regards,

Doug