# Does a lens hood reduce the light?



## mreco99 (Nov 10, 2011)

Does a lens hood reduce the amount of light that is getting to your sensor, ie losing stops?
Because the lens hood on the 70-200mk2 is big.

thanks


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## dr croubie (Nov 10, 2011)

It reduces the light getting to the sensor, but not the light you want in the image. The only light it blocks is light from the sides that wash out the photo.
So in some certain cases, putting a hood on means you have to use a slower shutter speed for the same exposure. But that's not the hood blocking out light that's a problem underexposing, it's without the hood letting in bad light that is overexposing.


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## mreco99 (Nov 10, 2011)

for sure i know they help reduce flare, just thought that might reduce actual usable light, but i guess as the hood is outside the viewable field of the lens therefore there is no effectual light loss.


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## mreco99 (Nov 10, 2011)

dr croubie said:


> It reduces the light getting to the sensor, but not the light you want in the image. The only light it blocks is light from the sides that wash out the photo.
> So in some certain cases, putting a hood on means you have to use a slower shutter speed for the same exposure. But that's not the hood blocking out light that's a problem underexposing, it's without the hood letting in bad light that is overexposing.



oh ok, so it DOES actually reduce the light (whether its good or bad light is another thing) and there will be a need to adjust aperture or shutter to expose correctly?


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## Eisbaer (Nov 10, 2011)

mreco99 said:


> oh ok, so it DOES actually reduce the light (whether its good or bad light is another thing)...


Yes, that's why you're using it.



mreco99 said:


> ...and there will be a need to adjust aperture or shutter to expose correctly?


No, because you're camera meters through the lens.

Best regards
Eisbaer


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## neuroanatomist (Nov 10, 2011)

Just to clarify a couple of points...

First, hoods block light at angles of incidence too shallow for that light to become part of the _image_ formed by the lens. Without the hood, light at those shallow angles can enter the lens and reflect off lens element surfaces, decreasing contrast and saturation of the image. It might add slightly (probably insignificantly) to the total light in the image, but it would be added as diffuse, non-image-forming light.

Second, you should be using the _correct_ hood for your lens. Canon's hoods are computer-designed to be matched to the lens. There are some caveats. Hoods for zoom lenses are effective mainly/only at the wide end (except the 24-70mm f/2.8L with its reverse-zoom design). Canon makes several EW-83 hoods with different single-letter suffixes, each design for a different lens. Although they will all mount on another lens that takes an EW-83 hood, only the correct hood will block maximal off-angle light without physical vignetting. One side note is that the hoods are designed for the image circle (although the petals of petal-shaped hoods take the rectangular frame into account). So, hoods for EF-S lenses are designed for the smaller image circle those lenses project, and hoods for EF lenses provide the best flare protection when used with a FF sensor, and are less optimal when an EF lens is used on a crop body. In those cases, you can improve flare protection by using a hood not designed for the lens, e.g. the EW-83J for the EF-S 17-55mm can be used on an EF 24-105mm f/4L IS when that lens is used on a crop body, and it will provide better flare protection than the appropriate EW-83H (but using the 17-55's hood with the 24-105 on a FF will cause vignetting).


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## torger (Nov 10, 2011)

I've noted that the lens hood is not enough in some situations, one may need something to block the light (a "gobo") to further reduce stray light, especially in long exposures. Without the gobo one can get bright low contrast areas in the image.

Even lens hoods on primes may not shadow the front element completely for light sources outside the frame. A typical example is if shooting from a tripod at night and there's a lamp outside the frame but still in front of the camera, then one may need to put a gobo there (I usually stand at the side and use my own body to cast a shadow on the front lens element).

In a side-to-backlight situation, it can be worthwhile to check if the lens hood is providing adequate shadow and if not see if you can put a shadow on the front element with something that is not visible in the frame, which you indeed often can unless you're using a fisheye lens. Then you can get better contrast and/or avoid a lens flare.


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## awinphoto (Nov 10, 2011)

Short answer, no, long answer, it blocks the incident light that could cause flare (causing low contrast and washed out scenes.) It will reduce the light you dont want in your scene assuming you dont want that light in the scene.


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## EYEONE (Nov 10, 2011)

To get to the heart of your question, the hood will not make your image darker or cause you or your camera to have to adjust the settings to expose correctly.

If you have the hood use it and if you don't have the hood go buy it and then use it. Hoods are good.


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## Rocky (Nov 10, 2011)

I always use lens hood (almost 100%) if I have time to put it on. The hood will not cut down USEFUL light and will not need additional aperture opening. It will cut down unwanted flare. It will help protect the lens from dust, water and foreign objects, especislly for people like me that do not use filter to protect the lenses.


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## TexPhoto (Nov 10, 2011)

As long as you have the right lens hood, it will only improve the photo.

It is possible to put the wrong a hood onto a lens that will vignette the photo, but as long as you use Canon's or a good knock off, you'll be fine. And you'd see that in the viewfinder anyway.

Lens hoods also provide some measure of protection in drops and bumps. I highly recommend them.


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## NotABunny (Nov 10, 2011)

I've tested my 70-200 F4 IS with and without its hood (Hoya HD protector on), in a bright summer day, sun high, close to the sun; UV should not be a concern at that altitude, but could have an effect.

Bottom line: there was no difference in the look of the images. (Did shoot at 200 mm though, so what Neuroanatomist said might the the reason.)

Bottom Bottom line: test it.


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## neuroanatomist (Nov 10, 2011)

NotABunny said:


> I've tested my 70-200 F4 IS with and without its hood...Bottom line: there was no difference in the look of the images. (Did shoot at 200 mm though, so what Neuroanatomist said might the the reason.)



That's exactly the reason, IMO. If the hood provided adequate flare protection at 200mm, it would vignette heavily with the much wider FoV at 70mm. So, hoods protect mostly/only at the wide end of zooms.


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## Meh (Nov 10, 2011)

neuroanatomist said:


> NotABunny said:
> 
> 
> > I've tested my 70-200 F4 IS with and without its hood...Bottom line: there was no difference in the look of the images. (Did shoot at 200 mm though, so what Neuroanatomist said might the the reason.)
> ...



Adequate or maximum?


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## dr croubie (Nov 10, 2011)

As a pretty extreme example, consider the following two images.
I've got a Zodiak 35mm Fisheye for Kiev88 (Medium Format), covers a 6x6cm image circle so it is *huge* (see photo top of this page here.
Just for fun, I mounted it on my 7D with a tilt adapter and took it down the river. Because of its huge front element, and no hood, it takes in enormous amounts of light from all angles and bounces them around inside. This reduces contrast and washes out the colours. With my camera on auto WB and auto exposure, this is what happens to the photo (camera-jpg). First one is with the sun in the top left of shot, second one is rotated right maybe 70-90 degrees.
If I'd had a hood on, or used a lens with a more sensible front-element size, it would have looked like a normal photo. Whether that results in having to use a longer shutter, what does it matter, really? If you are, the longer shutter is what you want, that's the photo with the normal amount of light, with the normal shutter speed. If you're not using a hood, you (might be) using a faster-than-normal shutter, not the other way around.


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## TexPhoto (Nov 10, 2011)

Even if the hood were designed to perfectly fit the edges of the image, and zoom as the lens does (a few do this, Canon 24-70 2.8), it would not protect 100% fom flair. 

Hoods provide some protection from flair, not 100%, but it is a whole lot better than none. A gobo can still be useful.

The hood of a 70-200 is shading the front element exactly the same amount at 70 vs 200. So it makes no sence to say hoods protect mostly/only at the wide end of zooms.


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## EYEONE (Nov 10, 2011)

TexPhoto said:


> The hood of a 70-200 is shading the front element exactly the same amount at 70 vs 200. So it makes no sence to say hoods protect mostly/only at the wide end of zooms.



You have a point (kinda) but what they are saying is that the hood is designed for 70mm in that case. The hood for a 200mm prime could be much much deeper and keep even more stray light out of the lens. But if you put that hood on the 70-200 it would vignette at 70mm

For the 70-200 (with the proper hood) there is a lot more light entering the lens that is not part of the picture at 200mm than at 70mm.


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## mreco99 (Nov 10, 2011)

dr croubie shame your image examples wernt of exactly the same scene, would have beena little more obvious then, with the shutter speeds and stops.


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## neuroanatomist (Nov 10, 2011)

TexPhoto said:


> So it makes no sence to say hoods protect mostly/only at the wide end of zooms.



Angle of view matters. 

Look at the ET-74 hood for the 70-200mm f/4L IS (which is 3.15" long). Imagine the level of flare protection provided by that hood at 70mm and f/4. 







Now, look at the EW-83H hood for the 24-105mm f/4L IS (which is 1.7" long). 






Do you think the dedicated hood for the 24-105mm lens is going to provide the same level of flare protection at 70mm and f/4 on that lens? What would a shot at 24mm look like if you put a round, 3.15" long hood on the 24-105mm? Holding a toilet paper roll up to your eye might give you some idea...


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## Meh (Nov 10, 2011)

To be fair, I think @texphoto is pointing out that in a zoom lens such as a 70-200mm the lens hood limits the angle of incident light to the front element the same regardless of the internal positions of the zoom mechanism. This much is true. However, he's not correct in his statement (I'm paraphrasing) that the zoom position doesn't matter at all. Clearly, at 200mm the front element is being exposed to a far greater angle of incident light than will be in the field of view of the lens which is how flare is created


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## neuroanatomist (Nov 10, 2011)

Meh said:


> To be fair, I think @texphoto is pointing out that in a zoom lens such as a 70-200mm the lens hood limits the angle of incident light to the front element the same regardless of the internal positions of the zoom mechanism. This much is true. However, he's not correct in his statement (I'm paraphrasing) that the zoom position doesn't matter at all. Clearly, at 200mm the front element is being exposed to a far greater angle of incident light than will be in the field of view of the lens which is how flare is created



Makes sense.


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## dr croubie (Nov 10, 2011)

mreco99 said:


> dr croubie shame your image examples wernt of exactly the same scene, would have beena little more obvious then, with the shutter speeds and stops.



Well, the point was more to show that:
In the first shot, the sun is right in the top left of the frame. Washes out everything because of flare.
In the second shot, the camera is rotated maybe 90 degrees to the right. The sun is nowhere near the frame. But because it's a Medium Format Fisheye lens, there is still glass facing the sun. You can still see how the colours are washed out, it looks like an old film photo, there's no contrast, the green looks hopeless, it looks like there's a reddish-orange cast in the centre of the frame. Even with the camera set to AutoWB (which the 7D does very well, according to most accounts), it couldn't make this shot look realistic. A hood and/or gobo would have made this shot a whole lot better.

I don't have any others taken at the same time, maybe I'll get out and some direct comparisons when i've got time (coincidentally, I bought fleabay ripoff hoods from china for everything I don't have last week. Less than $4 each, you can't go wrong). The only lens I've currently got a hood for is the 70-300L, which I use all the time, except when I'm using a CPL.


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## TexPhoto (Nov 11, 2011)

Meh said:


> To be fair, I think @texphoto is pointing out that in a zoom lens such as a 70-200mm the lens hood limits the angle of incident light to the front element the same regardless of the internal positions of the zoom mechanism. This much is true. However, he's not correct in his statement (I'm paraphrasing) that the zoom position doesn't matter at all. Clearly, at 200mm the front element is being exposed to a far greater angle of incident light than will be in the field of view of the lens which is how flare is created



You guys are missing the point entirely. If you are shooting a 70-200 zoom, and you have the stock hood: you have 2 choices: Use the hood or don't. Which of these 2 choices will better control flair at 70mm? Using the Hood. Which will control flair better at 200mm? Using the hood. In fact, if the hood is preventing all flair at 70mm, I will guarantee there is no flair when zoomed to 200mm assuming no other changes.

Now if you want to argue that you should not use the hood you have because a theoretical 200mm only hood (that you do not have) would be better than the stock hood, then you are right from a theoretical standpoint, but wrong from a reducing flair/practical/not just being difficult standpoint. Now if anyone here owns a 200mm only hood for their 70-200, I will... eat a bug/admit defeat/ etc Whatever.


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## Meh (Nov 11, 2011)

TexPhoto said:


> You guys are missing the point entirely. If you are shooting a 70-200 zoom, and you have the stock hood: you have 2 choices: Use the hood or don't. Which of these 2 choices will better control flair at 70mm? Using the Hood. Which will control flair better at 200mm? Using the hood. In fact, if the hood is preventing all flair at 70mm, I will guarantee there is no flair when zoomed to 200mm assuming no other changes.
> 
> Now if you want to argue that you should not use the hood you have because a theoretical 200mm only hood (that you do not have) would be better than the stock hood, then you are right from a theoretical standpoint, but wrong from a reducing flair/practical/not just being difficult standpoint. Now if anyone here owns a 200mm only hood for their 70-200, I will... eat a bug/admit defeat/ etc Whatever.



Sorry but who was saying don't use the hood. Of course not using the hood is worse than using the hood despite it not providing maximum protection at focal lengths greater than the widest focal length.

The hood on 70-200mm zoom is designed to limit the incident light to to the FOV of a 70mm focal length. Zoom to anything greater than 70mm and the hood is still allowing the same light to fall upon the front of the lens. Therefore, at any focal length greater than 70mm light from outside the FOV is entering the lens and can cause flare.

The only point of contention is your assertion that the lens hood prevents all flare regardless of zoom position.


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## neuroanatomist (Nov 11, 2011)

TexPhoto said:


> You guys are missing the point entirely. If you are shooting a 70-200 zoom, and you have the stock hood: you have 2 choices: Use the hood or don't.



Not quite. There's a third choice, the one you raised - use a gobo. If you understand that the flare protection the hood provides to a zoom lens is maximal at the short end, and minimal at the long end...that tells you when you'd _want_ to use a gobo. If you (incorrectly) believe the hood is equally effective at all focal lengths (assuming you're not using the 24-70mm, that is), you're either risking more flare and less contrast at the longer end, or wasting your time mucking about with a gobo when the hood could do the job at the short end.


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## Rocky (Nov 11, 2011)

A hat with big rim will do wonder. with shorter lens, you just wear your head low and use the rim to shade the sun. If you can hold and shoot with one hand,, then the other hand can really manipuate the hat for shade.


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## TexPhoto (Nov 11, 2011)

Meh said:


> The only point of contention is your assertion that the lens hood prevents all flare regardless of zoom position.



I am not sure how to argue this because that is pretty much the opposite of what I have been saying.

Here's the thing, unless your shooting with a pinhole camera and the pinhole is size .00001" you cannot make a perfect hood. The lens people keep bring up here is the 70-200 f4, which has a round hood. And I have yet to see a petal hood for any lens that is not squared off on the top and the bottom. So none of these hoods are close to perfect at even the wide end of the zoom. They are a compromise in size/shape etc. 

In every post I've made here my advice has been use the hood because it will help, and if you need to, even add a gobo. (hand/hat gobos are cool too Rocky)

And the reason I've been stating this is because the original poster here seems on the fence of whether to use his hood or not. My advice: use the hood (and maybe a gobo). Lots of other peoples advice here: "So, hoods protect mostly/only at the wide end of zooms." Or some longer version of that. Which does not seem too encouraging to the OP.


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## neuroanatomist (Nov 11, 2011)

Let's recap. 

OP asked if a hood results in a loss of light that would impact the exposure setting, making reference to the large hood on the 70-200/2.8 II. 

The consensus was no, but it does reduce unwanted light (i.e. flare). 

Everyone agreed that hoods were good things, and recommended using them. 

NotABunny indicated trying a hood on a 70-200/4 IS, and at 200mm it made no difference in the shot, i.e. no reduction in flare.

_Now...why was that the case? For the reasons previously stated, i.e. zoom lens hoods are designed to not vignette at the long end of the zoom, and as a consequence, they do not protect from flare at the long end of the zoom._

Then TexPhoto made an assertion that it makes no sense to say that hoods protect from flare only/mostly at the wide end of a zoom lens. The most straightforward interpretation of that statement is that you're saying the corollary is true - i.e., if it's wrong to say hoods protect only at the wide end, then they must protect throughout the zoom range. That was the point of contention. 

Now, in retrospect, you may have meant that it makes no sense to say that, becuase it might be construed as a reason not to use a hood at all. Clearly, neither you nor anyone else in this rambling thread is saying that (I hope!).

To correct a few other minor points, it's flare, not flair. Flair is a distinctive elegance or style, something you almost always _want_ in your photography, vs. flare, which you usually want to eliminate (unless it's used for artistic effect, in which case maybe the flare is contributing to your flair). 

Also, the EW-83K hood (for the 24/1.4L II) is a petal-type hood with the ends of the petals rounded off.







But yes, most are squared off a bit, I expect as a design compromise for the convenience of being able to set the lens front-down on it's hood to facilitate changing lenses, etc.



TexPhoto said:


> My advice: use the hood (and maybe a gobo). Lots of other peoples advice here: "So, hoods protect mostly/only at the wide end of zooms." Or some longer version of that. Which does not seem too encouraging to the OP.



Again, 'use a hood' is everyone's advice, as far as I can tell. If someone stated that the Canon front lens caps are easier to knock loose because of the side-pinch design, would you construe that as advice against using a lens cap? 'Hoods protect mostly/only at the wide end of zooms' is a statement of fact...not advice against using a hood.


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## Rocky (Nov 11, 2011)

May I stress that the hood also help protect the lens from foreign objects like dust, water splash your own finger etc.


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## Meh (Nov 11, 2011)

TexPhoto said:


> Meh said:
> 
> 
> > The only point of contention is your assertion that the lens hood prevents all flare regardless of zoom position.
> ...



Sorry but I don't see how that is the opposite of what you're saying, a generous paraphrasing perhaps but certainly not opposite. Your exact words were "if the hood is preventing all flair at 70mm, I will guarantee there is no flair when zoomed to 200mm". Sounds very like zoom position doesn't matter.


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## TexPhoto (Nov 11, 2011)

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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## Jettatore (Nov 20, 2011)

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...


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## epsiloneri (Nov 20, 2011)

neuroanatomist said:


> '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.


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## epsiloneri (Nov 20, 2011)

Jettatore said:


> 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.


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## Jettatore (Nov 20, 2011)

OK, that makes sense, I can buy that.


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## neuroanatomist (Nov 22, 2011)

epsiloneri said:


> 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.



epsiloneri said:


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



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 5Â° 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'. 



epsiloneri said:


> 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 (Nov 22, 2011)

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.



neuroanatomist said:


> epsiloneri said:
> 
> 
> > The difference between wide and long is that longer lenses can have more extensive hoods without vignetting the field 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.



neuroanatomist said:


> epsiloneri said:
> 
> 
> > neuroanatomist said:
> ...



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.



neuroanatomist said:


> 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).



neuroanatomist said:


> epsiloneri said:
> 
> 
> > 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).
> ...



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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## neuroanatomist (Nov 22, 2011)

epsiloneri said:


> 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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