Lens Shades

**Jerry Fusselman** · 16-Jun-2005, 23:02

How to check the corners for vignetting: Close down your lens to the taking aperture. How much of the circle (or oval or smashed pentagonish shape depending on several factors) is obscured by the hood when looking in the corners cut out from the ground glass? The ideal is if about 1/10 or 1/6 of the circle of the taking aperture viewed in the extreme corner is obscured by the hood. This will cause very slight vignetting in the corner, but it would be noticable rarely, if ever. The advantage of a hood that clips a little bit of the light at the corners is that you get maximum flare protection at the expense of a nonnoticable decrease in light in the extreme corners.

**Paul_5410** · 18-Jun-2005, 11:44

Jerry, I'm trying but I am afraid my eyes are having a bit of a strain seeing anything! Perhaps I'll try a faster lens than the Artar which is f9.5 I'll try my 210 Fujinon W it is f5.6 maybe I'll have better luck?

There must be some "Rule of thumb" to go with this problem besides seeing corners clipped after processing. Of course that is pretty darn certain then!!! Thanks everyone, Paul

**Jerry Fusselman** · 18-Jun-2005, 14:05

Paul, sorry, no, there is no processing of film involved with following my directions. Lens speed does not matter at all either. I will try to explain it better.

You know how when you look at a lens from the front (with light coming through) and vary the f/stop, you see a round or pentagonish shape changing size? Well, the same thing happens from behind. You can vary the f/stop and watch how the aperture changes shape. If you look from behind with your eye centered on the lens axis, it looks quite circular, but if you look from an open corner of the ground glass as you vary the aperture, it often has more of an oblong shape---especially with wide-angle lenses. Indeed, you can estimate the off-axis light loss from a wide angle lens by how much your circular aperture has been smashed into a more oblong shape. The ratio of areas between the oblong shape you see and a hypothetical circle circumscribing it is an upper bound to the ratio of light that will reach the film. (The other factor is the distance the light must travel.)

Anyway, when you add a lens hood, it can make the aperture appear smaller: It is obvious when you add the hood while looking from behind. How much light is lost in the corner because of the hood is measurable by seeing the area of the aperture that is lost while looking from behind. If the entire aperture image while viewing from the corner behind is obscured by the hood, then the corner will indeed be black. But if you can see some of the apeture, the corner will not be totally black.

I have talked as if the only way you can tell is by repeatedly installing and uninstalling the hood, but there is an easier way. You can learn how after 5 minute's practice; you should be able to tell, say, that at f/22 the current hood blocks 10% of the light and at f/32 it blocks 0% of the light in the corner. (When you vary the aperture, the part that the hood blocks does not move, so it is easy to estimate that part.) If 10% of the aperture is blocked, then 90% reaches the film, which is great.

10% obscured in the corner translates to 1/6-of-a-stop light loss in the corner; 16% obscured translates to 1/4-of-a-stop light loss. In situations where flare is a real possibility, I would adjust my hood or shade (and aperture) to this kind of light loss in the corners.

I have a short pdf written by a 6x6 manufacturer that describes this process better; if anyone would like it, please drop me an email.

**Jerry Fusselman** · 18-Jun-2005, 21:42

The pdf I mentioned is available from http://www.vanwalree.com/optics/lenshood.html It was once downloadable from www.hasselblad.com

I also recommend reading the lenshood article at vanwalree.com. His figure 5 shows what you would see in a case where you can tell that the lens hood causes a 40% reduction in the light reaching the corner.

**Paul_5410** · 19-Jun-2005, 07:26

Hello Jerry,
I went to the webpage and that is the best article I have seen regarding our topic. This morning I am going to line up my lens shades and keep trying until I get this down to root memory!

Thanks Jerry I think I will have a very good understanding after today! Back to reading now. . .Paul

**Jerry Fusselman** · 19-Jun-2005, 07:55

Paul,

You are welcome. Your expression "keep trying until I get this down to root memory" really made me laugh, for that's what I did when I read that article.

**Paul_5410** · 19-Jun-2005, 09:45

Hello Jerry,
I really recommend the pdf file to all because it also helps me with the use of my compendium shade on my view cameras. After reading the article and reviewing the diagrams on the end it help me to finally see what you have been explaining. So with success I think I'll go shoot some film today. . . Thanks to everyone. . .Paul