A question about hoods, vignetting and flare.

[robert_clark]

I was wondering about the best way to sheild a lens from extraneous

light. There are specially designed hoods for most lenses, but are

they as 'tight' as they could possibly be?

 

James Ravillious (a well-known Devon photographer) used to use tape

to decrease the opening at the end of his lens hoods as much as

possible just to that point where vignetting would start - he

controlled this by using a focus screen in place of film, looking

through the open back of the camera.

 

The CV 40/2 SLR lens is sold with a lens hood that looks lik a

slightly domed lens cap with a rectangle cut out. I presume this is

to keep the lens plus hood as compact as possible. My question is why

aren't all lenses sold with such neat lens caps? Especially on

rangefinders when there are problems with lens caps getting in the

way of the viewfinder.

 

What, in principle, stops us from simply cutting the appropriate

sized rectangle out of any old lens cap and using that as a compact

and efficient hood?

 

There may, I realise, be problems with very wide-angle lenses and

with those that have large, fast, front optics, but for pancake

lenses, 35mm and narrower, what would be the problem?

 

One last question: if when trying this out (looking through the lens

at full aperture, either with an SLR, or with the Ravillious method)

no vignetting can be seen, does it mean vignetting will not be there?

[walterh]

Robert for several types of photography dedicated flexible bellows acting as lens-hoods are used. For example in large format photography these are common and are called compendium. Some have indicators for certain lenses and one can add indicators for close focus or far focus. A final test for vignetting would be similar to your described method. Because large format cameras usually do have a focus screen this is the preferred way to check for vignetting. However in dim light and small apertures it may be hard to see and testing on film and experience come in. The example I show clips on the lensboard an can be extended and moved in several directions to optimize the effect. Make your own hood for your lens and have fun.<div></div>

[robert_clark]

Walter, I suppose I was thinking of a tight, efficient and compact lens hood (as tight and compact as possible), hence the lens cap idea. These bellows hoods are far from compact.

[john_perkins2]

Where do you get a focus screen to do this? I really like this idea, but not to decrease

viginetting, to crank it up.

[walterh]

"What, in principle, stops us from simply cutting the appropriate sized rectangle.. "

 

Yes Robert it is far from compact. I just thought to encourage you to go ahead since the principle seems to work. It was my impression that your question seemed to target if the principle works. :-)

Have fun in your project and let us know the exact sizes you found out for certain lenses so we all can copy it.

[john_shriver]

Making the hood deeper allows the source of "offending" light to be closer to the axis of the lens. So, the lens-cap style hood doesn't really offer much protection, since it never shades the center of the lens. One's goal is to not have the sun (or other offending light source) strike the front of the lens, while not blocking any image forming light.

 

You can make a one foot deep lens hood for any focal length with a field of view less than 180 degrees. But it might be very wide by then for a wide angle lens. Who wants a lens hood 10 feet across? But there is a point of diminishing returns, but many hoods aren't nearly that long.

 

Of course, the front of any ideal lens hood is rectangular. But it's only uselul on a lens with a non-rotating mount, which leaves out a lot of the LTM lenses.

 

 

As for a cheap and simple focusing screen, Scotch Magic Tape works fine. Just run it across the film rails. Won't necessarily be accurate enough to focus or collimate with, but will be fine for viewing vignetting due to the hood.

[ronald_moravec1]

You can use a peice of Kodak diffusion material in the film plane.

 

One can`t use just a cap with a cut out because you are also want to keep side light from hitting the front element.

 

The ideal design is a shade as long as possible and then shaped to a similar proportion shape as the film format. This would be a pain for hand held photogtaphy if you expect to be stealthy and worse for rangefinder work.

[walterh]

BTW I remember a long time ago (40+ years?) there was a Bessamatic lenshood for a 50mm lens that had a snap on cover with a rectangle cut out for use of the same lens hood on the 135mm lens.

[bill_schneider1]

Here's a diagram that might help explain graphically why longer lens hoods provide better shading.

 

The lens hood that I chose to illustrate is actually more wide angle than needed, but still offers better flare properties than a lens cap modified with a rectangular opening sized to fit the projected image.<div></div>

[dan flanders]

I have expouunded on this issue to the extent that many surfers of this forum are bored to distraction, but it is an important question and this party seems to be addressing the essential factors.<p>

Logic leads to the conclusion that the light column admitted by the lens to the film plane should have the shape in cross section to that of the format. Leica (and others) have addressed the problem with some of their lenses but seem to have ignored in other instances. Usually the diagonal of the format determines the diameter of the light cone, thus the design of circular hoods -- but it does not recognize that light above, below, and to the sides of the actual format enter the lens and bounce around doing all manner of mischief. <p> Ravilious recognized this problem and his solutions were in the vein of the ideas under discussion and were a major factor in the quality of his efforts. <p> Early versions of Leicas hood for the 50 Elmar also addressed this issue and the hood had a cutout mask at the front in the shape of the format. Rotating lenses would then require that the hood be constantly adjusted for orientation as the lens was focused, and eventually the design was discontinued. <p> From the personal viewpoint I have found that the Leica 12526 rectangular hood is satisfactory for both the 35 and 50 Summicrons, however admittedly if it is satisfactory for the 35 it is not ideal for the 50.<p>

Nevertheless, it should be recognized that circular hoods do an unsatisfactory job of addressing the flare issue in the shorter focal length lenses. If they are deep enough to provide protection outside the format area, there will be light fall-off at the corners.

[roger_hicks1]

Dear Robert,

 

It's all a matter of convenience.

 

Round hoods are worst, but easiest & cheapest to manufacture and it doesn't matter if the front of the lens rotates.

 

Screw in hoods are easy to manufacture but it's much easier to make them round because of difficulties in aligning them precisely, i.e. they screw in a few degrees more or less. This is why clamp-on, bayonet or built-in sliding hoods are better for rectangular hoods.

 

Deep hoods are best but bulky. They can also cut unacceptably into the viewfinder area with an RF camera.

 

Bellows hoods matched to the format are ideal but slow, bulky and expensive.

 

Do not be distracted unduly by the idea of the sun falling directly on the lens, as this is often easy to shade with your hand or with a piece of card held in a Wiggly Worm (Rogert White) or other holder that slots into the accessory shoe at one end and carries a shade at the other (slow again). Your design will effect a great improvememt in veiling flare, the insidious all-over flare that is worst in a white-painted 'cove' studio or on an overcast day. Veiling flare reduces overall contrast and is at least as unwanted as the kind of flare that results in direct imaging of the diaphragm.

 

So yes, your idea will work a lot better than no hood, but not as well as the best hoods.

 

Cheers,

 

Roger (www.rogerandfrances.com -- and thanks, you've given me an idea for another free module for the Photo School on that site).