Checking for vignetting, g/g has no cut corner

[MacGregor Anderson]

I'm using a new camera that does not have cut corners in the ground glass. I remember reading recently that an alternative way to check for vignetting is to look through the lens. I've tried this several times but no matter how bizarre the movements I put on my 150 schneider I can see the ground glass at full open aperature. This is a cheapo lens (the one that came with the Toyo CF kit) and I can't imagine it has that much image circle.

Am I doing this right? If I can see all four corners of the glass one at a time by twisting my head up and down right and left...have I done my check? Or am I trying to find one spot where I can see all four corners at once?

Thanks,

Mac

[MacGregor Anderson]

sorry, just found the lens and it's a Rodenstock f 6.3 for a copal 0. And when I say "cheapo" I just mean it isn't some incredible piece of glass that would cover 8x10, it's just a regular entry level 4x5 lens. I'm not blaming equipment here, just trying to learn technique.

[N Dhananjay]

Since you do not have cut GG cornerx, you could try removing the back and sighting down through where the corner would be approximately. If you are looking through the lens towards the GG, you want to be able to see the GG through a round aperture (and not an American football/catseye shape aperture). Cheers, DJ

[MacGregor Anderson]

DJ, I can't really imagine taking out the ground glass with a screwdriver ever time I want to take a picture.

I'm more interested in this looking through the lens approach. I've used a camera for a year with cut corners, and I've seen what you speak of.

My question is, do I hold my head staring at the lens in one place, or do I peak around, like I would from the ground glass end with cut corners?

Right now, based on probably faulty recollection of something I read here recently, I'm standing in front of the camera with my face maybe three inches from the lens, maybe two or even one, haven't measured, and I'm closing one eye and trying to see the corner of the g/g. When I find one, I move my mug around and try to find another. If I can find all four, I am figuring I've got no vignetting. But then it strikes me that maybe I'm looking from outside the image circle and that's why I can always find the corners one at a time.

Is there a technique to this, or do I just need to stop down to F45 or smaller and hope I'm lucky?

Mac

[james mickelson]

If you can't see it on the ground glass, it ain't there. Seriously, I have never experienced vignetting when it wasn't apparent on the ground glass to begin with. Just look.

[MacGregor Anderson]

Why is there no edit feature on this board? Why must I post, review, and send off stupid responses?

I now understand your suggestion of removing the back, not the ground glass. Intersting idea. Still sounds a little cumbersome and approximate, but much better than unscrewing the glass. Sorry about the misunderstanding.

Mac

[JimL]

I just want to confirm about the "football" shape that's being described here. When you look at the iris from an angle to the lens axis, the "round" aperture will look slightly elliptical. Is it correct that the "football" shape is referring to any part of the internal lens barrel or filter flange intruding into the view through the aperture?

[N Dhananjay]

Dear MacGregor, apologies for my lack of clarity. I did not mean removing the GG from the back. Many cameras will have a facility to remove the entire back from the rear standard. But I just realized that many cameras do not (press cameras like the Graphics etc) do not. I am unfamiliar with your camera and do not know how much of a nuisance it would be to remove the back.

To clarify a couple of things, yes, you can move your head around. You are essentially simulating off-axis rays of light and seeing whether they will get to the GG.

The football shape is referring to the round aperture looking elliptical - technically, since you are looking at a circle from an off-axis orientation, it will look elliptical. But I am referring to the more obvious shape being caused by mechanical vignetting by the lens barrel or filter flange. Normally, this translates into an exposure variation across the film and as you stop down, the elliptical shape becomes more circular and evens out the exposure variation. But as the elliptical shape shows, the lens barrel is intruding into the image forming light. If you tried slightly more extreme movements as you were looking at this shape, you will see the shape change from being circular to increasingly elliptical until the elliptical shape will wink out (i.e., vignette).

Mechanical vignetting will be very obvious on the GG as James pointed out. Most lenses throw large circles of illumination but the performance at the edges is pretty bad. In modern lenses, the lenses are mechanically vignetted well outisde this range of poor performance and this should be very obvious on the GG, especially at brighter viewing apertures. Older lenses were less likely to be mechanically vignetted and I wonder if this way of checking for vignetting evolved as a way of knowing when you were entering the poor performance outer region of the lens circle. Many folks have specuated that the original function of cut corners on the GG was actually to prevent 'bellows suck' (i.e., prevent problems of low pressure inside the bellows if it was rapidly expanded). The lack of corners did mean that you could not check the GG and could have small vignetting there. The corners just provided a way to check for vignetting quickly - sort of the problem also being part of the solution. Cheers, DJ

[WeeGee]

"Why is there no edit feature on this board? Why must I post, review, and send off stupid responses?"

Didn't you send Tuan the $850 membership fee? That allows you to edit.
As for stupid responses... that is your responsibility.

[Eric Wagner]

I look through my lenses at normal viewing distance, say 12" or so, rather than up close like you have been doing. I would think that if you used the maximum amount of rising front, put the back in vertical position, and then looked through the lens at the lower (most distant) corners you would see what we are talking about, unless maybe your lens has a lot more coverage than we think.

DJ makes a good point about mechanical vignetting vs. the area of useable coverage. My 50+ year-old Wide Field Ektar is sharp right to the very edge of the image circle. But with a Fuji A I have learned that the image corners will be sharp at f22 if the ground glass corners still look good when viewed through the lens at f16, and the corners will be sharp at f45 if the ground glass corners still look good when viewed through the lens at f22.

Are you going to keep us in suspense about which new camera you are checking out now that you have returned the Wisner?