When is a compendium shade better than my hat?

[John Hennessy]

I used a linhof shade on my TKs for years till it fell apart for the 27th time. The shade is very poorly made and over priced. The first one I bought was in pieces in the box from B&H. So I got a Lee shade which is very sturdy.

But to work with 4x5 the Lee shade must be the one that attaches to the filter holder that in turn attaches to the lens. That way you can rise the shade when you rise the front standard. If the shade is attached directly to the lens, you can't do that.

My hat is reserved for protecting my tonsure.

[Paul Butzi]

As others have pointed out, if your lens has mondo coverage (typically the case) then using a properly adjusted compendium really cuts down on bellows flare.

For quite a while, I had a Lee compendium style hood. As time went on, I found myself using it less and less (probably because I am lazy) and using the Lee involved screwing an adaptor ring into the lens, then mounting and adjusting the shade, and then reversing the process afterward. When I was in the habit of making only a few exposures a day it wasn't a problem but when I switched to making 50 exposures in a morning, the compendium was the first thing to get left in the bag.

Recently I bought a couple of different metal shades, and a couple of folding rubber shades. So far, I've been using them far more than I used the Lee.

The circular fixed shade is not as effective as a properly adjusted compendium but it's certainly better than the compendium that gets left in the pack instead of put on the lens.

[Paul Metcalf]

Ok, all of my years of thinking I understood ray tracing have gone out the window. The light that bounces around inside the camera has to come from outside the camera, and preferably through the lens (vice through holes in the bellows or other light leaks). In order for the lens to cover the film appropriately, it needs to collect light from the front element at an angle of coverage at least as much (but I think usually more) than the rear element. In order to keep the shading device (whether a compendium shade, hat or your knuckles) from vignetting the projected image (onto the film), it needs to be out of the field of view of the front element (which is out of the field of view of the rear element). So, light that would be bouncing around inside the bellows would not be blocked by any such shading device otherwise it would show up in the picture. The shading is to prevent unwanted off angle light sources (light flashes or the sun) from "blooming" on the front element which will surely cause flare on uncoated and some coated front elements (the other reason for coating is to allow better light transmission via reduced losses through the numerous glass/air interfaces within the lens elements). If you want to prevent light from bouncing around inside the camera, you have to restrict the light path within the camera (View Camera has a recent article on testing film in which the author used an internal lens shade to restrict the light path directly at the film). If someone is concerned with this bouncing light within the camera I would think that one or more neutral density filters placed in front of the front or rear element (and protected from off angle light) would help in really bright situations by reducing the flux of light into the camera (if longer shutter speeds were acceptable). If I'm confused, I'd better sign up for another physics/optics course.

Paul

[Michael Mutmansky]

Paul,

You'd better sign up for a physics and geometry course! Just kidding, but I know what you are thinking, I think.

The issue is that the film sheet (be it 4x5 or 8x10, or whatever) typically only uses a small portion of the image circle that the lens projects. This image circle (you will see it called IC at times) is entirely made up of image forming light.

So, you are correct that the the hood will be blocking image forming light from entering the camera, and that is exactly what you want it to do, because ideally, the only image forming light that will get into the camera will be going directly to the film.

Any image forming light that enters the camera but does not go to the film will result in a projected image on the inside of the camera mechanism; the bellows, the back frame, etc. While these are all painted black, they will still reflect a little light to the film and cause fogging.

Removing this unnecessary image forming light from the camera will greatly improve the contrast of the image in many cases, especially where the sun may be in the angle of coverage of the lens, but not visible on the ground glass, and also when there is snow or sand in the foreground.

---Michael

[QT Luong]

Paul, I don't think that using ND filters to "reduce the flux of light" would do any good, since after you increase the exposure time, exactly the same amount of light will be transmitted through the lens, otherwise you would not expose your film correctly. Photography 101 :-)

[Michael Mutmansky]

QTL is correct, I neglected to address this. The rate of flux of light will be reduced by a ND filter, but the length of time for an exposure will increase proportionally, which will result in the same amount of light entering the camera.

The absolute best way to eliminate image forming light that is not being used by the film is to use an external hood. Once it's inside a camera, it can cause problems, but if you shunt it before it gets inside the camera, you will never have a problem. You just have to be careful that you don't vignette the film with too agressive light blocking.

---Michael

[John Hennessy]

<<In order to keep the shading device (whether a compendium shade, hat or your knuckles) from vignetting the projected image (onto the film), it needs to be out of the field of view of the front element (which is out of the field of view of the rear element). So, light that would be bouncing around inside the bellows would not be blocked by any such shading device otherwise it would show up in the picture.>>

If Paul Metcalf is saying that the purpose of a shade is NOT to keep extraneous image-forming light off the inside of the bellows, but merely to keep non image-forming light off the surface of the front element, then I don't think that is correct. But without a blackboard and plenty of chaulk these sort of discusions can get me confused.

Of all the light striking the front element only a subset is image forming (i.e., can potentially travel straight from some point in the intended image to the film); with a LF camera only subset of that light actually reaches the film (maybe as little as 1/5th if the lens has a large image circle). The extra image light (that that does not the film in its path) is going to bounce around inside the camera. It seems to me the purpose of a shade is to block ALL light that will not fall on the film out of the camera in the first place.

[Paul Metcalf]

Well, alrighty then. I stand corrected. And I can now use the money I saved by not signing up for a Physics class at the local community college and invest in a compendium shade, compendium shade company stock (have you noticed how ebay gets a surge in areas that have like forum discussions?), and a new hat.

;-}

[Christopher Condit]

Thanks for yet more good information. I now understand that the *point* of a compendium shade is to vignette out excess, perfectly good image light that will not be directly striking the film. And since no screw-on conical shade is designed to do that, the compendium is markedly superior. I'll have to sell some of my hats and buy one. My A's baseball cap has seemed disposable lately...

[Walter Glover]

I believe the term is non-image forming light — that is light that is not a part of the actual recorded image. To this end I have both Sinar and Linhof devices which facilitate shielding ALL non-essential light from entering the lens like window blinds.

I have never had sufficient hands to use a hat or slide to shield the lens.

One hand holds the cable release.

The other hand holds the darkslide to shade the opening of the film holder.

So where's the hat-holdin' hand?