Are Lens Shades Really Necessary?

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

The idea behind the lens shade is very simple. An ideal lens shade should:
1) allow the passage of ALL image-forming rays to the lens
2) block ALL non-image-forming rays

Failure to satisfy condition 1 produces vignetting. However, even if the condition 1 is fulfilled, image-forming rays have potential to produce flare and gosthing due to internal reflections in the lens. Make it certain that if a strong light source like the sun is included in the field of view, the effectiveness of lens shade to prevent flare is zero!

Failure to satisfy the condition 2 means that non-image-forming rays can penetrate the lens. However, this does not necessarily imply the occurrence of flare or gosthing. It all depends on specific details of each lens.

L = F x D / d

where:

L = lens hood length
F = focal length
D - lens hood diameter
d = sensor diagonal

This formula, which I had presented in a previous post and repeat here for convenience, gives the length of the lens hood that satisfies both the conditions 1 and 2 above, by assuming the lens aperture is small (large f-number). That formula is very simple and serves as a good starting point to analyze real cases in which lens aperture cannot be ignoredl.

When the aperture of the lens is taken into account, it is easy to see that the hood's diameter must be increased, or length must be reduced to prevent vignetting. Theoretically, for a lens with finite aperture, the hood dimensions must be infinite for simultaneously having a total absence of vignetting and blocking of all non-image-forming rays. In practice, a lens hood is not infinite, of course, but to be really efficient, its size must be huge, much bigger than most commercial lens hoods. Commercial lens hoods are in general sub-sub-sub-optimal from an optical point of view. Nonetheless, as said before, they can be useful to protect the lens from bumps, rain, snow, etc. And most people like the appearance of a lens with a hood. They think the lens looks cool...

Below a situation where no lens shade in the world would help as the sun is in the field of view of the lens (a Sigma fisheye 16mm). By the way, would the picture be better without gosthing?

Analog lex · Joined: 10 Nov 2013 Posts: 95 Location: East Europe

I prefer to use lens flares as art effect. Tuzki with lens

meanwhile · Expire: 2016-11-28

calvin83 · Joined: 12 Apr 2009 Posts: 7555 Location: Hong Kong

anktonio · Expire: 2017-02-22

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

A Thought From A Lens Hood Addict

"Shooting naked will make you vulnerable to... disapproval and even wrath from fellow photographers who religiously practice shooting with hoods."
http://oldlenses.blogspot.com.br/2016/02/a-thought-from-lens-hood-addict.html

... naked, disapproval, wrath, religion, addiction, lens hood Laugh 1

padam · Joined: 09 Oct 2012 Posts: 175 Location: Hungary

If you use a rangefinder, you cannot be sure about how the flaring will affect your image, so it is safer to have a lens hood on (even if it adds a little finder blockage)

But yes, it is more important that it looks badass Wink