A perfect lens hood can make a big difference. Maybe.

Started Mar 3, 2011 | Discussions thread
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RussellInCincinnati Veteran Member • Posts: 3,201
A perfect lens hood can make a big difference. Maybe.

A perfect lens hood can make a big difference. Even when you are not pointing the lens into a bright light source. Er, Maybe. If, when...etc etc.

Have written previously about my assumption that deep lens hoods are important. This post started with thinking about the comments of a couple of other posters, that lens hoods never seem to make a picture better, unless of course the hood shades the sunlight that would otherwise fall directly on the lens. Of course you always learn the most from people who don't agree with you, and am glad that such posts made me think.

The key new thought was, what if the reason that most of us rarely see the benefit of a lens hood, is that most of us are not really using very good lens hoods? Put another way, why do million dollar video and movie cameras always come with huge, perfect "barn door" lens hoods?

So here's a small experiment. Take a simple, 6-element 4-group, non-multicoated 150mm planar-type (normal) lens that throws out a much bigger image circle than APS-C. With no lens hood, lots of irrelevant light is bouncing around out the back of the lens. That sounds odd, but then all 35mm full-frame lenses (and probably all optics UhOh07 owns) indeed throw out a much bigger image circle than APS-C.

Take a picture outdoors in sunlight, but by all means point the lens away from the sun. The first picture is take with a "perfect lens hood". The perfect lens hood is 3 inches long collapsible rubber, with the unusual tweak that the front of the hood is carefully masked off with black tape. The round hood is masked down and squared off to the minimum rectangular aperture that does not darken the corners of the Nex sensor. This took about 5 minutes to do. It is particularly easy to do when one person is looking at the viewfinder, while the other person is moving the photographic masking tape around until it's just barely not darkening the image edges (with the lens at minimum aperture and focused at infinity).

The next picture is taken with no lens hood at all. However even with no hood, the sun is coming from behind the lens, and direct sunlight is definitely not hitting the front of the lens.

Develop both pictures identically, with absolutely no contrast adjustment as we would normally do to sweeten most images.

Perfect lens hood:

No lens hood photo taken a few seconds later:

Finally, let's do the normal "contrast expansion" of the second, no-lens-hood photo. This is a common contrast adjustment as have done with almost all images. Will typical contrast optimizations (that Sony might do with in-camera JPEGs) "hide" the loss of contrast that we see in the "no lens hood" photo? Yes.

Discussion

  1. A lens designed for APS-C often already has internal rectangular baffles to cut down on the stray light that this post is documenting. If you see such a rectangular baffle upon peering down into the back of a given designed-for-APS-C lens, one would think there would not be so much benefit to a perfect lens hood.

  2. Have chosen for this experiment a 150mm enlarging lens that throws out a huge 5 inch image circle. This is a much huger image circle than even a full frame 35mm lens outputs. So the lens in this experiment needs a hood way more than most.

  3. This lens is not multicoated. One could speculate that a multicoated lens would cast less stray light. As a counter-example, have found just this week, after much experimentation, that contrary to my own prior beliefs (and posts) about multicoated lens always being better, am now finding that a non-multicoated simple lens with a "perfect lens hood" performs about the same as a multicoated one of similar design. This is a topic that pmong was asking about, wondering if controlling stray light is important with macro extension tubes.

  4. The theory behind why the contrast loss from unfocused stray light is so correctable, when the stray light is overall farely neutural in color, is fun to ponder. The stray light "error" is to a great extent a simple identical boosting of the light levels hitting every single pixel. It theoretically ought to be not too difficult to simplistically "subtract out" the stray light boost during post processing, restoring the original blackness of the dark parts of the picture.

  5. Pursuant to the above observation, a lens hood might be most needed when the stray light is not neutral in color. Say, when you're taking a picture of a sunlit scene with lots of blue sky overhead. In this case, the stray light will be real bluish, giving a blue cast to the darker parts of your photo that would be much more complex to correct.

  6. Good luck getting a perfect lens hood with a zoom lens! This is a sort of handling advantage of primes (interesting because such advantages are few and far between). When you swap your primes lenses, you are automagically also swapping perfect lens hoods. Each focal length can have a hood designed to perfectly mask out stray light. Of course you can make a perfect lens hood easily for the WIDE end of your zoom, but as you zoom to longer focal lengths the hood aperture "becomes too big" for that longer focal length.

  7. The reason this post belongs in the Nex forum, is because more people use more bigger-than-the-Nex-needs lenses, than probably get used on any other camera.

  8. With my normal workflow, I would not have noticed the difference between the 2 images compared here. Because the workflow involves "contrast expansion" as a matter of course, as shown in the 3rd sample image. With an ordinary contrast expansion, the no-lens-hood photo looks fine to me. In a way this is the most important result of the post (unfortunately even this is not real important), for technically advanced photographers/photofinishers who want to understand absolutely everything about everything.

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