Mark,
A likely excuse for his appearance. I had a '52 MGTD. While re-connecting the battery, I momentarily forgot that this British beastie had a positive ground system. Double grounded the battery. Blew up in my face. That's the excuse for my appearance.
True or not - they're good stories.
What you see is an aerial image, so when the projected image on the ground glass is too dim to visualize you can determine the exact field of coverage by viewing through the corners with a loupe.
Wilhelm (Sarasota)
"I understand that one reason for the clip is to relieve air pressure inside the bellows. "
This is one of those rare things about which Ansel Adams was dead wrong. That isn't the purpose of the clipped corners. A view camera isn't an air-tight object, there are plenty of places from which air can escape without needing cut corners in the ground glass. If clips were needed in order for air to escape then no one would use an unclipped ground glass but plenty of people do.
The corners are there for the second reason you gave. As to what you look for, with the lens wide open when you look through one of the corners you'll likely see something shaped kind of like a football, i.e. oblong. If you keep looking as you stop down you'll see the football shape gradually get wider until eventually it ceases to look like a football at all and instead is either circular or shaped like an aperture. At that point you should have sufficient coverage to prevent vignetting. If there is mechanical vignetting (e.g. from a lens shade or filter ring) you'll see that too by looking through the corners.
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
How many times does a person use a specific lens before he just plain knows whether he's getting vignetting? For me a few uses tells all.
Yes, jj, but with so many variables (swing and tilt front and back, plus some shift,add some rise, add a red filter, plus a polarizer, maybe a lenshood...) and keeping track in your mind of half-a-dozen lenses... I admit I've been unpleasantly surprised a couple of times.
"True or not - they're good stories."
Here's a true one: when I was a kid, someone stuffed straw in the gas tank spout of my mom's station wagon. She decided to get it out with the vacuum cleaner. No human injuries involved, but the old Hoover was never the same after jumping six feet in the air with flames shooting out the sides...
if you can't see the entire shape of the iris, this means that you are outside from the coverage of your lens... what we call image circle !!!
fall off is progressive, as is sharpness... this is an other history !
For example, if you are using a 58 xl with 6x7 without mvts, looking at the slide, you can't see any fall off, if you use it with 4x5, then the fall off is obvious while you still in the image center and you still can see the iris (looking in the clip corner). In this case you can use a center filter in order to correct the fall off, but the image circle remains the same !
Considering sharpness, the center of the image circle is allways sharper than the border, so you can use a lens with movements, and be able to see the entire iris (looking in the clip corner), but the corners of the image won't be necessery sharp !
Off center sharpness, image fall off, Image circle... it all depends on the lens design !
I've test an apo ronar that was sharp till the end of the IC, but the Image circle was quite small,
Some lens have a big image circle, but the quality in the extreme of the image circle is poor... compromise...
the 110 xl is my favorite lens because it have a big image circle, without compromising on image quality, the best of both worlds !
I don't know if my explanation is clear enought, soory english is just my second language !
"Clipped corners on the groundglass are NOT NECESSARY to check for vignetting, as this can be done by looking through the lens"
I don't think anyone suggested that clipped corners were the only way to check for vignetting. You can do it by looking through the lens as you say. But for me the clipped corners are more convenient because I don't have to get out from under the dark cloth, walk around to the front of the camera, peer through the lens, then get back under the dark cloth as is done when checking hrough the lens.
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
This is one of those rare things about which Ansel Adams was dead wrong. That isn't the purpose of the clipped corners.
With due respect, what you're saying is the revisionist perspective. Adams said, "...the corners of the groundglass may be cut off to permit the movement of air when the bellows is expanded or collapsed." Implicit in this, I think, is the situation where one is making gross movements of the bellows, such as open and closing a folding camera, or a large movement on a monorail when changing lenses or such. Yes, there are lots of ways for air to move in and out of the camera slowly when focusing and adjusting, but not when you work the bellows like an accordion.
Occam's Razor tells us that the logical reason to add a feature, is to accomplish a purpose that could not be accomplished with what was already there. Cut corners were not put there (originally) to check for vignetting, because there were other ways to do that. They were not put there to allow air to move in and out when focussing, because it wasn't needed. The corners were originally clipped so a photographer could change from a long lens to a very short lens, and then unlock the camera and squish the front and back together -- and not damage the bellows in the process.
If I may mention smaller formats: another camera feature that has fallen victim to revisionist history is the mirror lockup feature on some 35mm cameras. The accepted belief is that MLU is there so we can reduce vibration when taking photos with very long lenses, high magnifications, etc. But the truth is that MLU was invented as a way to mount extremely short (i.e., "wide angle") lenses, in the era before the availability of retrofocus designs. A non-retrofocus lens, if too short, will be so far back into the camera body that the mirror would hit the rear of the lens. So they came up with MLU to get around the problem; you lock the mirror out of the way, then mount your WA lens. Framing was accomplished either by guess and by golly, or by using an accessory viewfinder that mounted on top of the camera. It wasn't long, however, until retrofocus designs became the standard for short focal lengths, making the original purpose for MLU a moot issue. Sometime in the interim, a few people had noticed that they could get much sharper images with their long lenses if they locked the mirror up first. Now almost everyone believes mirror lockup was always there for the reason they use it. Thus was history revised.
...don't think anyone suggested that clipped corners were the only way to check for vignetting. ...the clipped corners are more convenient...
And I never said, that anyone suggested that. I merely pointed out that the addition of clipped corners did not add the functionality of checking for vignetting, since that was already there, and that from a manufacturer's perspective there had to be another reason. Yes, using the clipped corners is MUCH more convenient in practically every case.
The point made (I hope) is that devices and ideas often end up being used for purposes entirely different than what was originally envisioned. History is written by the victors. I guess this is the case in every arena of human activity, not just wars. And so it goes.
Originally Posted by Steve Feldman
I imagine you're long since gone from this forum and I've just begun, (again). After you've focused, composed, you'll stop down then return to the ground glass and look thru the clipped corners. If the lens opening is dark it's totally outside the image circle and vignetted. If you observe the other observations, football etc., I suppose you can evaluate those comments.
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