Bingo.

I bought a clean used 17-40 f/4 L recently and love it. It's not IS, but wide-angle is where that is the least needed.

For mostly landscape (including twilight/night and astro), portrait and 'gig' (musical performance) photography, my 6D has served me very well these last 2 years.

ETTL?

To be sure, you'd have to do the test in better-controlled lighting. Bur I'd be almost willing to bet that some of the contrast improvement is from the makeshift hood extension. Even a fixed hood designed for a zoom lens is only going to be optimal at the wide end. I.e., an optimal hood for the lens at its long end would be longer, but if you zoomed out, you'd get vignetting.

Given the generally good design of the 24-105, if you're not seeing flares in the viewfinder, glare probably isn't critically harming your image contrast. It might be reducing it a little, though. But the eye is good at not noticing such subtle things. You'd probably have to take side-by-side, with and without the proper hood to see any difference.

I'm not saying that at all. I'm just saying that when the sun is behind you, (I.e., more than 90 degrees away from where the camera is pointed) it's generally not shining directly on the optics anyway, unless you're using a lens with a very convex and protruding front element (generally an ultra-wide-angle or a fisheye), so a lens hood would not provide any additional protection against flare. It will still (at least theoretically) help against that component of veiling glare caused by light from diffuse sources outside the lens' field of view.

The sun is so far around the back that it doesn't matter a whole lot what you're using. I think I understand your original point now. You always shoot with the sun over your shoulder, nowhere near where it could ever shine on the front elements of the lens. In that case, why do you even bother with a shade at all?

How about some where the camera isn't pointed within about 45 degrees of straight away from the sun? In those, you're using the back of the camera (and maybe your head) as your lens hood.

A petal-type hood is theoretically the best solution because it minimizes the amount of area outside of the scene that can project 'flare' (images of the lens' stop superimposed, more or less badly focused, on small parts of the image) or 'veiling glare' (a diffuse wash of light over all or most of the image) onto the sensor (or film). It would, with a fixed focal length lens, be possible to engineer a hood that just barely is not in the optical path on all sides and therefore allows almost no flare or glare to get inside from sources outside the image area.

With a zoom lens, unless there's some kind of mechanism to extend and retract the hood, it has to be designed for the wide end of the zoom range, or there will be vignetting.

Even this is a help.


There are three ways that light can get to the sensor (or film) through the lens. (We're not counting light leaks in the camera body or lens here just light that actually gets through the optics.)

The first is image-forming-light. This is the light that enters the lens at the front, refracts* at each lens surface in turn, and arrives at the sensor, forming the image.

Then there are the other two-- glare and flare-- the light that 'cheats the system' and gets to the sensor without having been properly formed into the image. All this light is superimposed on the image, but does not contribute information to it and usually indeed detracts from it. It's a form of noise, in other words.

Flare manifests itself is when light from a small, bright source (like the sun), within or outside of the field of view, reflects off one of the optical surfaces, goes some way back subject-ward in the lens before bouncing off another surface back towards the sensor. This light is usually not well focused and even if it is, each pair of surfaces will form a differently focused image at a different place in the image

Veiing glare happens the same way, but with diffuse ambient light as the source. Instead of discrete images of the stop, if puts diffuse washes of light across larger portions of, or even the whole, image. This results in an overall loss of image contrast.

Multi-layer coatings hugely reduce the amount of light reflected at each surface, often down to less than 1% per surface (compared to typically >10% per surface for uncoated glass and around 5% for single coated glass surfaces). Many current lens designs would be unworkable without multicoatings. Even with all this, you want to keep as much light from coming into the lens from outside the field of view as you can.


* Or, occasionally reflects or diffracts as intended by the designer in 'mirror' or Canon 'DO' lenses. The majority of lenses only use refraction to form the image, though.

I have been using one for two years, now. I leave it to the others here, if they care to judge for themselves who is the ignorant one. BTW, I am an optical designer by trade. What is your knowledge of optics?

Not 100% useless, but definitely enough less effective than the proper hood that I wouldn't take the chance.



I'm sure that you can. Most modern lenses, including the two in question, have very good antireflection coatings and part of the design process involves suppressing those localized flare images that used to be the trademark, but that tends to leave the 'overall-wash' kind of flare that reduces contrast over the entire area of the image, especially if the light source is a big swath of sky, rather than a localized source like the sun. Once you've let a little extra non-image-carrying light into your picture's shadows you'll never get that contrast back.

That makes no sense at all. If you're using a hood intended for a wider angle lens (17-40) than the lens you're using it on(24-105), then there is a big area inside where the hood blocks light from, but outside the lens' field of view that extraneous light will enter the lens from. You lose, thank you for playing.

Then you're using a hood on the 24-105 that is letting enough extraneous light in that you might as well not use a hood on it, especially at the long end of its zoom range.

Can always use them for templates to make hoods in a less water-soluble medium.