In some cameras, the ground glass comes with open corners. Supposedly, the advantage is the ability to look in through the back at the lens diaphragm as a check against vignetting. Questions about this:

1. What am I looking for? To see the entire diaphragm or just any part of it?

2. Will this help me to prevent dark corners and not just frank vignetting?

3. Presumably, one has to be stopped down for this to work accurately?

4. Any other tricks to check for vignetting? Frankly, I just don't see it on the GG sometimes, even when checking carefully, and even when stopping down (incidentally, why do I see vignetting _more clearly_ when I stop down? I thought that increased the image circle?)

5. Is this enough questions for one post??

Thanks!

Nathan?

-- Nathan Congdon (ncongdon@jhmi.edu), February 02, 2000

Answers

You must see the entire diaphragm in order to make sure you are not vignetting. It doesn't matter what stop you are at, as long as you don't see part of the opening cut off by the non-glass parts of the lens. It is easier to avoid vignetting for this reason. You pull the diaphragm leaves farther from the edges and closer to the center of the glass thereby giving the light reflected from your subject a more unobstructed "shot" at the outer reaches of the projected circle of the lens.

Vignetting is one thing, dark corners from inherent wide-angle fall off is another and they are caused by different things and treated differently (stopping down vs. center filter). Stopping down lessens the effect of vignetting and also makes it clearer to see because in stopping down you lose some of the "hazy" look caused by wide-open viewing. I also look back at the ground glass from the front of the lens to check for vignetting. You must see the entire opening clearly and perfectly shaped projecting right up to each of four corners. This is also good for GG without cut-out corners.

-- Rob Tucher (rtphotodoc@juno.com), February 02, 2000.

1. You're typically looking at any change in the shape of the aperture as you move your head from the center of the ground glass to the corner. Try this with your lens wide open. Remove the ground glass. The lens aperture will look round when you look at it straight on through the center area of the ground glass. As you move your eye towards the corner you will see the circular shape flatten and take on the shape of an American football as the barrel of the lens cuts into the circular shape of the aperture. If you stop down a couple of stops, this should be minimized. The other thing you want to check for is if you use a lens hood. You can look through the corners to see if the lens shade is in the field of view. If you've used extreme movements you can check for vignetting caused by that as well.

2. This will allow you to check for mechanical vignetting. Corners also get darker than the center due to cosine falloff. This is typically a problem with wide angles and you need to decide if its a problem in your prints in which case you can use a center filter. However, this will not vary by other conditions i.e., it is fixed.

3. See 1 above. Definitely check at working aperture. Wide open, you will typically have some mechanical vignetting.

4. Stopping down makes the image circle more clearly defined. In other words, when your lens is wide open, performance trails off quite steadily towards the perimeter of the image circle. In other words, performance declines steadily as you leave the center. This large good-to-bad transition area makes it difficult to assess exactly when its become unusable. When you stop down, the evenness becomes better and thus it makes the transition area from good to bad smaller and easier to spot. As far as I know these are your best methods - check through the corners for mechanical vignetting. Compensate for cosine falloff (if you feel its a problem) with a center filter.

Good luck. DJ

-- N Dhananjay (ndhanu@umich.edu), February 02, 2000.