Calculating DIY Lens Shade Dimensions

[jcoldslabs]

I was thinking of making some cheap cardboard lens shades (tubes) that I can tape to my various LF lenses for use in the field rather than buy a compendium. Is there a formula for how wide and long a lens shade can be before it encroaches on the image corners? There must be a sweet spot of length and diameter versus maximum shading.

As an example, I've got a Rodenstock Sironar-N 210mm f/5.6. How large a diameter should the shade be and how far forward can it extend? I could always stop down and eyeball it, but I was wondering if there is a tried-and-true method for this calculation.

Thanks for any tips.

Jonathan

[poliweb]

If I wanted to make a lens shade I would start by making a cone that had the same angle at the apex as the angle of coverage of the lens. A quick search suggests that this is 72 degrees for the Sironar-N. Then I would have to figure out how to cut off the apex so that I could attach it to the lens. Ideally, it should be attached so that the apex would have been at the nodal point (optical center) of the lens - though we may be splitting hairs at this point.

Bear in mind that if you do this you are only shading the lens from light that falls outside the image circle. If you really want to minimize glare you would want to have shade that was rectangular and was positioned to cut out all the light that wasn't falling on the film for any given shot.

Richard

[Joanna Carter]

If you are going to DIY, then look at something more like "barn door" shades, similar to those used for lighting.

Tubes will tend to vignette a lot sooner than a rectangular shade.

[jcoldslabs]

I could just as easily rig up a rectangular (or square) cardboard shade, if that makes more sense, barn-door style or fixed, rather than a tube.

The easiest thing would be to settle on the outside dimensions (maybe 4x4 or 3x3) and point the camera at the sky stopped down. Then I would just have to move the shade forward to see when vignetting happens and make its final length a fraction shorter than that.

J.

[Mark Woods]

You really only need, in most cases, what is called an "eye brow" over the lens. There are pictures of AA with one he made to shade his lenses. A wire with a clip and some cardboard.

[cowanw]

See here
http://www.largeformatphotography.in...ght=lens+shade
and here
http://www.apug.org/forums/archive/i...p/t-33564.html
for discussions

[Jim Jones]

It should be easier to make a series of rectangular cardboard shades of various lengths that attach to the front standard of the camera rather than to the lens. They can be either collapsing or nested for storage. Another variation is one that slips over the shutter, with a cutout in the bottom for a cable release and sync cord. This is a rear view.

[E. von Hoegh]

I could just as easily rig up a rectangular (or square) cardboard shade, if that makes more sense, barn-door style or fixed, rather than a tube.

The easiest thing would be to settle on the outside dimensions (maybe 4x4 or 3x3) and point the camera at the sky stopped down. Then I would just have to move the shade forward to see when vignetting happens and make its final length a fraction shorter than that.

J.

The aspect ratio of the frame/front opening should match that of the film.

[Bob Salomon]

And how would they adjust for lens and/or back movements to eliminate cutting off image area? A compendium shade for a view camera follows the movements of the lens and can be shifted up and down to work with back movements. That way it can give maximum protection from stray light.

It is very different then a lens hood or a bellows hood for a non-adjustable camera and that is what you are trying to make.

[genotypewriter]

And how would they adjust for lens and/or back movements to eliminate cutting off image area? A compendium shade for a view camera follows the movements of the lens and can be shifted up and down to work with back movements. That way it can give maximum protection from stray light.

It is very different then a lens hood or a bellows hood for a non-adjustable camera and that is what you are trying to make.

For a "dynamic" camera you need a "variable" shade...

If going for a static/fixed-shape design, it's always going to be a compromise.

Simple as that