Ground glasses - Why are they clipped?

Am new to LF, so I am toying around with a focusing back and thinking of replacing the ground glass for a

brighter image. There seems to be the lazy way and the DIY way. Both seems interesting and depending on time and

resources equally attractive. However, something has piqued my curiosity, that is why are some ground glasses

offer with their corners nipped off? Is there a functional rationale for that? I might then be able to get a

good nights sleep knowing the reason. :)

I believe its because the bellows pump air in and out when you pull them as you set up the camera. This lets the air move without putting pressure on the thin glass.

You can look in the corners to see if you can see the whole opening in the lens. You are pulling the bellows in front of the camera as far as possible to cut of unwanted straylight. If you pull it to far, you get vignettting on the film. Try it, and you will see it in the lens through the cutout corners.

Erlend is right at the point. The corners are exactly to check vignetting.

The only reason is airflow.

I think its both!

 

I couldn't close or open the bellows on my 12x20 Korona if the GG wasn't clipped. Its also mildly helpful to see the edges

of the lens.

Cool, so if i have an existing full unclipped ground glass, is it a good idead to give it a clip and by how much? Making a ground glass looks to be a great way to spend a sunday lol. How thick should i go? I reckon thinner is better but with that come fragility. Also whats the ideal approach, with or without a Fresnel lens in front?

Ian, for what reason wanna see the edges of the lens for?

Kenneth, if you have difficuties to see sharpness on the ground than you should defenetely use a fresnel, It's enough if you just clip a half of one sqeare cm so you got a little hole in all four corners. that would do the job.

If you can see the lens aperture from all four corners, then you know that your bellows is not getting in the way of the light path.

 

And if you can see the entire aperture -- not vignetted by the lens barrel or other reason, then you can tell that the lens covers the format. This comes in handy if you are using a wide angle with movements.

 

But it is no less important that you want air to be able to flow into and out of the camera!

Thanks Michael, but I never bothered to look as the bellow thing I have visualised from the outside as you clearly can se it if it hangs to much but, my guess it's in the case of low quality bellows which hangs to much which I don't have.

 

The control of the wide angle might be a good reason but than again only if you don't know what is your lens are good for.

 

The technical reason that manufacturers have in mind is the airflow which they had advertised to a long time ago when it come.

 

It's nice to hear that people find other use for it to.

If you've never peeked at the lens through the corner it might be 'tricky' for the first time to see what you're meant to see. I wrote this on my blog about the bessa 6x9, but I'm sure you can 'transfer' the technique to your LF camera

 

http://cjeastwd.blogspot.com/2008/04/bessa-useful-lens-limits.html

"The technical reason that manufacturers have in mind is the airflow which they had advertised to a long time ago when it

come."

 

Some cameras. Linhof, in those days, did not clip the corners as their backs had ventilation slots to relieve the air pressure

inside the bellows.

:-) Bob, It's just shows that Linhof are superior to others. Did it right from the beginning and don't had to advertise it.

Frank,

 

now they do clip the corners.

OK, there seem to be two possibilities here: the corners are clipped for airflow, OR the corners are clipped so

photographers can check for vignetting.

 

Let's establish a baseline, namely what motivates manufacturers: from a manufacturer's point of view, clipping

the corners on a groundglass involves more time and labor. Therefore, if it wasn't necessary, it wouldn't be

done. That is a given for this analysis.

 

Although none of us are idiots, a wise manufacturer would clearly understand that an idiot photographer might

attempt to collapse a bellows too quickly. Without a way for air to escape, the bellows could easily balloon

from the internal pressure (created by the idiot photographer) and be ruined. In fact, even a non-idiot might

accidentally collapse the bellows too fast. Thus a manufacturer, wishing to provide a quality product that will

be adequate to the task and have a long working life (an archaic concept, but one that used to be in play,) might

opt to clip the corners of the groundglass as a way to allow easy airflow without compromising the

light-tightness of the system. Here is a functional advantage to clipped corners, versus non-clipped.

 

Now the clipped corners are indeed also handy as a way to check for vignetting. However, it is equally possible

to check for vignetting by looking through the lens from the front of the camera. If you can find the corners of

the groundglass through an unobstructed aperture, there will be no vignetting, and the corners needn't be

clipped. New photographers can easily be trained to check for vignetting in this way, and if there is no

alternative they will adopt this method. Therefore a prudent manufacturer will look to his bottom line, and will

NOT add the extra production step(s) needed to clip the corners of the groundglass.

 

So, looking back to earlier days, when manufacturers made a decision to start clipping the corners of LF

groundglass screens, we attempt to discern their motivation. Logic dictates that the reason must have been

functional, and the only necessary function of clipped corners is for airflow. Clever photographers soon

realized that here was a way to check for vignetting without having to go around to the front of the camera.

Good for them, but there was no logical reason for the manufacturer to provide a solution where there was no

problem.

 

Occam's Razor tells us the simplest explanation is the most likely. Point to airflow.

Bob, it's probobly cheaper to manufacture it that way otherwise I don't se any reason why they would give up a rather smart sollution from the beginning.

 

Alan, it's exacly what I stated from the beginning, by the way I love William he was a very sharp fellow one of my absolute favorite.

Bob.

One question arised here. I have checked my Linhof III and can't locate the air slots. And my GG is cliped which is

probobly not the original or it is who knows. I have brought it in that way many years ago. It might be so that they

allready changed in that modell, than I got the Kardan Super Color ST which has the full size GG ,what's the

situation there?

Please, help me out here if you know more about it.

Frank, The backs are still vented but the corners are clipped to make it simpler to check vignetting. But the vent is

simply a small slot - it is not an obvious vent.

 

In the 80s we use to demo how with a camera with cut corners how it was necessary to move the back around to see

what was happening to the composition in the corners with a Sinar and how that was not necessary with the Linhof as

Linhof did not cut the corners and Sinar did.

 

Sinar showed how their cut corners did let you check for vignetting and Linhof showed that by sliding the ground glass .

frame off vignetting was also easily checked but the whole frame could be used on the uncut gg.

 

Sinar did not ventilate the back and Linhof did.

 

Alan "Let's establish a baseline, namely what motivates manufacturers: from a manufacturer's point of view, clipping the

corners on a groundglass involves more time and labor. Therefore, if it wasn't necessary, it wouldn't be done. That is a

given for this analysis."

 

No, it is cheaper to cut the corners then to ventilate the back. Also all gg has to have the edges cut and polished so any

savings in clipped corners is meaningless.

Thanks Bob, now just one more question. Which one has the original GG the one I have on the Kardan ST or the

one I have on the Teknika III? Maybe non of them or both of them.

 

Ps: Now I know how come that you know that much about Linhoff! :-) I will get back to you if you ship to Sweden

that is. :-) I never checked your personal file here before not others either as I don't have that time, but I'm glad i did it

this time anyway.

I really don't see the point of this controversy. If your camera's bellows balloons out when compressed, or collapses when extended, then you

need

some way of releasing the pressure or vacuum inside the bellows. Corner notches on the GG will accomplish this, as will other design

features. Also, slightly opening the spring-loaded back will release the air as you extend or retract the bellows.

 

Minute adjustments to the

bellows when fine-focusing will not cause pressure problems, they usually occur when rapidly extending or collapsing the bellows when

initially setting up the camera, or when changing from a long lens to a much shorter one. It's not that big of a deal.

 

Bellows pressure aside,

corner notches are also a perfect way to

check for vignetting. Anyone can verify this....it only takes a second.

 

Some lenses have limited coverage for a given format and will vignette at certain apertures. Usually photographers will have some sort of

attachment on the front of the lens, it can be a screw-on filter or polarizer, a ND filter with a slotted holder or an

adjustable hood (like a collapsible Lee hood). Often these attachments are combined and can intrude into the lens' light path at certain

apertures or camera setups. Also, extreme movements can displace the image circle and cause vignetting. In all of these cases corner

notches are a convenient way to determine if the final setup will vignette.

 

Try this: Set up your shot as usual. Before inserting the film holder, peer through the GG's corner notch into the aperture of the lens at

maximum aperture and at your taking aperture. If your setup is vignetting, the most prominent area of darkening, or outright clipping of the

image on the groundglass, will be readily visible in the corners, and can be seen clearly by peering through the notched corners and through

the lens opening.

 

You should see a perfectly round aperture as you look through the lens. If you see an ellipse, then you know your setup will

vignette. The more pronounced the elliptical shape, the greater the degree of vignetting or darkening in the ccorners. Often you can see the

vignetting appear and disappear as you open and close the lens' diaghram. Peer through the lens at

maximum aperture, then close the down the diaghram to your taking aperture.... you will usually

see the lens opening change from an ellipse to a circle. This only takes a second and it can spare you some grief.

Sergio has nailed it perfectly! To see a complete aperture circle should mean there will be no vignetting, however, some lenses may cause light falloff at the corners that may require using a centre filter. You will have no real advance notice of this problem until the film is developed, even then it's only an apparent when using wider angle lenses. Having the corners of the ground glass cut may help with this falloff, as it allows a little more light in at the corners, but you must be quick to load a holder before it dissipates....

you guys maybe smarter than I'm but it's confusing a bit because I have clipped gg on my 4x5 Teknika III where it's really useless and a whole on the 4x5 Kardan where it could be useful if it's ment to check vignetting. Unclipped on my newer 8x10 Tachi and clipped on the old 8x10 Seneca.

 

By the way working with LF for decades never checked any of those you talk about and never get any vignetting either.

 

I got only vignetting on my Tomiyama 6x24 cm pano when I got the camera and wanted to fabricate lens shade to it, I were misscalculated the angle of the shade. Getting vignetting of filter is possible but again than you have chosen a worng filter to your WA lens.

 

Getting vignetting of bellows...? Well yes when I use my 760 mm Tuner-Reich on my Tachi but than my bellows are long and hangs but I can see this from the outside and help it out with a little thing I manufactured to help me to jump over it.

I forgot to say.

 

Choice of my lenses allways are with thoght of movements so they are "oversized" to be sure that vignetting and and

light fall which requires centerfilter eliminated.

for ex i use the 165 angulon on my pano but the 121 would be enough. But than I got to have centerfilter.

I never use any screw on filter but do have a bellow "compendium" shade which uses the sinar leaf filter close to the

lens on all of my LF cameras.

Kenneth,

 

The fresnel usually goes behind the GG, not in front. If you place the fresnel between the GG and the lens (in front of

the GG) the image that is focused on the GG will be slightly out of focus when you put in your film holder. I said

usually, I am sure there is some old obscure camera where the fresnel is placed in front of the GG without effecting

the image displacement. Also on eBay, Dagor77 is selling acid etched GG, but the corners are not clipped.

 

Paul