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Author Topic: Does a lens hood reduce the light?  (Read 9589 times)

TexPhoto

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Re: Does a lens hood reduce the light?
« Reply #30 on: November 11, 2011, 05:04:39 PM »
OK guys, you are taking one thing I said (out of many), changing it dramatically (dropping my if...), then saying you disagree.  OK you win, that thing I never said, I disagree with it too.

And you got me on the flare/flair thing, (though creamy white Canon lenses do have both) but that's not a corollary, it's a contraposition.  And it's not what i said.  Arguing against something that I did not say is hardly straightforward.

So mreco99 use the damn hood.

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Re: Does a lens hood reduce the light?
« Reply #30 on: November 11, 2011, 05:04:39 PM »

Jettatore

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Re: Does a lens hood reduce the light?
« Reply #31 on: November 20, 2011, 01:46:51 AM »
This should be pretty easy to test if you have a lens + a hood.  I would expect it to reduce light even if only by a miniscule amount, but easy enough to find out if you have the equipment...

epsiloneri

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Re: Does a lens hood reduce the light?
« Reply #32 on: November 20, 2011, 04:39:19 AM »
'Hoods protect mostly/only at the wide end of zooms' is a statement of fact...not advice against using a hood.

I think that statement is the reason for the confusion. As earlier pointed out, for most zoom lens designs, the hood is much closer to optimal at the wide end. Normally, however, it is just as effective at the long end as at the wide end (the obvious interesting exception being the EF 24-70/2.8L). It's just that at the long end, the hood could have been even more effective. But, it is just as effective at the long end as at the wide end.

The purpose of a hood is to reduce unwanted illumination of the front lens. How effective the lens hood is in achieving this depends only on the hood, not on the focal length (unless the hood changes with focal length as in the EF 24-70/2.8L). The difference between wide and long is that longer lenses can have more extensive hoods without vignetting the field of view.

This just highlights that lens hoods cannot be constructed as effective for wide angle as for long lenses. Which is kind of evident, if you look at actual lens hoods provided with such lenses.

epsiloneri

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Re: Does a lens hood reduce the light?
« Reply #33 on: November 20, 2011, 04:45:24 AM »
This should be pretty easy to test if you have a lens + a hood.  I would expect it to reduce light even if only by a miniscule amount, but easy enough to find out if you have the equipment...

No, a hood should not reduce the light from the intended scene at all (only if the hood is mismatched to the lens this can happen). Think of a hood as a glass-less window in a wall. As long as the window does not block your line of sight, it doesn't matter how big the window is, and whatever you look at will not change its brightness compared to the case without wall at all. The wall will, however, block disturbing lights from outside the scene.

Jettatore

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Re: Does a lens hood reduce the light?
« Reply #34 on: November 20, 2011, 08:49:57 AM »
OK, that makes sense, I can buy that.

neuroanatomist

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Re: Does a lens hood reduce the light?
« Reply #35 on: November 22, 2011, 09:46:41 AM »
The difference between wide and long is that longer lenses can have more extensive hoods without vignetting the field of view.

This just highlights that lens hoods cannot be constructed as effective for wide angle as for long lenses. Which is kind of evident, if you look at actual lens hoods provided with such lenses.

There is a physical difference in the hoods, yes.  But I don't think your statements are correct once you take into account the angle of view.  Optically, a hood for a 24mm lens will be designed to come just shy of physical vignetting for a 24mm AoV (the aperture is taken into account in the final design, i.e. the 24mm f/4 hood design for the 24-105mm lens has the petal edges squared off, whereas the 24mm f/1.4L II's hood does not, for some additional light protection at the expense of convenience - otherwise, the two 24mm hood designs are similar).  The hood for a telephoto lens is 'more extensive' becuase the lens has a smaller AoV, but a hood for a 200mm lens will still be designed to come just shy of physical vignetting for a 200mm AoV - it's just longer because the AoV is narrower but the hood still has to attach to the front of the lens, just basic geometry.

'Hoods protect mostly/only at the wide end of zooms' is a statement of fact...not advice against using a hood.

I think that statement is the reason for the confusion. As earlier pointed out, for most zoom lens designs, the hood is much closer to optimal at the wide end. Normally, however, it is just as effective at the long end as at the wide end (the obvious interesting exception being the EF 24-70/2.8L). It's just that at the long end, the hood could have been even more effective. But, it is just as effective at the long end as at the wide end.

Again, I must disagree.  'Just shy of optical vignetting' is where a hood provides optimal protection.  Otherwise, the hood for a 400mm lens would not be deeper than the hood for a 200mm lens, the hood for a 200mm lens would not be deeper than the hood for a 70mm lens, etc.  If a hood designed for a shorter focal length was 'just as effective' at a longer focal length, then hoods for longer lenses would be shorter, as there'd be no need to make them longer.  But they are longer.  As you noted, a hood designed for 200mm would vignette on a 70mm lens, so zooms require a compromise.  But logically, if a hood of a given dimension provides optimal protection from flare (i.e. it's just shy of physical vignetting), then logically, a shorter hood will provide less than equivalent protection, i.e. it will be less effective at the long end.  Honestly, I'm not sure how that can logically be disputed.

But as Jettatore suggests, it's easy enough to test empirically.  The attached images used a pair of 150 W-equivalent lights, placed on either side of the frame, and those were the only lights in the room.  I shot with the 5DII in M mode at f/8, ISO 100, and metered shutter speeds were the same for each focal length.  I adjusted the position of the lights so they were 5° outside of the frame at each focal length, and took a shot with and without the hood for the two indicated lenses. 

As you can see, the hood reduces flare at the wide end of the lens, and has essentially no effect at the long end - and that's true for a wide-to-tele zoom and a telephoto zoom.  Obviously, the native flare is less at the long end of each lens - but if you compare -/+ hood at a given focal length, you see a difference at the short end and not at the long end.  That is entirely consistent with the hood for the zoom lens providing less effective protection at the long end, not being 'just as effective'. 

No, a hood should not reduce the light from the intended scene at all (only if the hood is mismatched to the lens this can happen).

Also not completely true.  While I expect that it's practically true for typical scenes, I can see situations in which that would not be the case.  In the test shots below, the metered exposures were the same with and without the hood. But if you look at the images at the short end, where the hood is actually protecting from flare, you can see that the same metered exposure gives an image that looks a little brighter without the hood.  That's because the reduction in flare yields an increase in contrast, in part because some of the light is blocked - light that is defocused in the iamge, but nonetheless adds to the brightness (not in a good way).  I did try the empirical test, and if I moved the light sources closer to the lens, but still kept them outside of the frame, I could contrive a situation in which the metered exposure dropped by up to 2/3-stop with the hood in place.  I could see a real-world situation, such as shooting at night with streetlights/spotlights, where this effect could occur in practice.  Still, it would be better to use the hood, since the light that's affecting the exposure is not focused, but rather merely decreasing contrast in the image.
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epsiloneri

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Re: Does a lens hood reduce the light?
« Reply #36 on: November 22, 2011, 01:45:45 PM »
I'm amused you disagree on these things I consider obvious... I'm sure it's just because we speak slightly different languages, looking at things from a slightly different perspective, but in actuality we probably mean the same thing.

The difference between wide and long is that longer lenses can have more extensive hoods without vignetting the field of view.

This just highlights that lens hoods cannot be constructed as effective for wide angle as for long lenses. Which is kind of evident, if you look at actual lens hoods provided with such lenses.

There is a physical difference in the hoods, yes.  But I don't think your statements are correct once you take into account the angle of view.

When I talk about the "effectiveness" of a hood, I mean the effectiveness in absolute terms - i.e., how much of the front lens is shielded. This "absolute" effectiveness is independent of the focal length of the lens - obvious, because the front lens does not know what the focal length of the lens is! If you re-read what I wrote with this in mind, I'm sure it will make more sense.

When you talk about how well a hood protects a lens, you are talking about relative effectiveness - i.e. what is the improvement in an image with a hood relative to without hood. And that certainly does depend on the focal length.

'Hoods protect mostly/only at the wide end of zooms' is a statement of fact...not advice against using a hood.
I think that statement is the reason for the confusion. As earlier pointed out, for most zoom lens designs, the hood is much closer to optimal at the wide end. Normally, however, it is just as effective at the long end as at the wide end (the obvious interesting exception being the EF 24-70/2.8L). It's just that at the long end, the hood could have been even more effective. But, it is just as effective at the long end as at the wide end.

Again, I must disagree.  'Just shy of optical vignetting' is where a hood provides optimal protection.  Otherwise, the hood for a 400mm lens would not be deeper than the hood for a 200mm lens, the hood for a 200mm lens would not be deeper than the hood for a 70mm lens, etc.

No, this does not follow at all. If you put a hood optimised for 24mm (i.e. 'just shy of vignetting' for a 24mm lens) on a 200mm lens, then yes, the hood would protect the 200mm lens just as much as the 24mm lens (recall what I wrote above). BUT: it would not be the optimal hood for a 200mm lens, because it would be quite far from vignetting. An optimal hood for 200mm would be much more restrictive and block all light outside a much smaller field of view (being "deeper"). In absolute terms, the 200mm hood would be much more effective than the 24mm hood.

That is entirely consistent with the hood for the zoom lens providing less effective protection at the long end, not being 'just as effective'. 

Yes, in your language! But look at the portion of the 24mm/70mm image that corresponds to field of view of the 105mm/200mm and you will see that the hood/no hood difference is about the same. Not exactly, because  internal reflections change as you zoom, but the incident light on the front lens should be the same for the wide and long ends of the zoom.

Again, this is just talking different languages - you mean that the difference hood/no hood for the obtained image is greater at the wide end. This is natural, because wider angle lenses are more sensitive to surrounding lights, and any fixed shielding makes a greater impact on wider-angle lenses. I was talking about how well the hood actually shields the lens in absolute terms, and this does not normally change with focal length (except for the EF 24-70/2.8L).

No, a hood should not reduce the light from the intended scene at all (only if the hood is mismatched to the lens this can happen).

Also not completely true.  While I expect that it's practically true for typical scenes, I can see situations in which that would not be the case.  In the test shots below, the metered exposures were the same with and without the hood. But if you look at the images at the short end, where the hood is actually protecting from flare, you can see that the same metered exposure gives an image that looks a little brighter without the hood.

Key here is that I used the word intended scene - of course the hood reduces light from sources outside the scene, that's exactly the purpose of the hood! The reason your image without the hood is brighter is that light is scattered on the lens from the bright sources outside the field of view. That light does not come from the intended scene. And the light from the intended scene is the same in both cases. Surely you agree on this?

Again refer to my simile with a large hole in a wall. As long as the wall does not interfere with your line of sight, whatever you see on the other side of the wall will not change its brightness depending on how big the hole is (assuming the object is not illuminated from your side of the wall etc). The wall is your hood, protecting you from light outside of whatever you want to see.


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Re: Does a lens hood reduce the light?
« Reply #36 on: November 22, 2011, 01:45:45 PM »

neuroanatomist

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Re: Does a lens hood reduce the light?
« Reply #37 on: November 22, 2011, 02:28:37 PM »
I'm sure it's just because we speak slightly different languages, looking at things from a slightly different perspective, but in actuality we probably mean the same thing.

When I talk about the "effectiveness" of a hood, I mean the effectiveness in absolute terms - i.e., how much of the front lens is shielded. This "absolute" effectiveness is independent of the focal length of the lens - obvious, because the front lens does not know what the focal length of the lens is! If you re-read what I wrote with this in mind, I'm sure it will make more sense.

That makes sense, semantically.  Practically, I don't care about the absolute effectiveness - I care about the effect on my images.  In this case, I think relative = relevant.
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Re: Does a lens hood reduce the light?
« Reply #37 on: November 22, 2011, 02:28:37 PM »