ground glass corners how to use please

[brighamr]

Hi Guys

Can somebody please tell me what to do with the corners
I know its to do with coverage but how do I work it
I have a Sinar f2 and a Nikkor 75/4.5

What am I looking for

thanks

robin

[jss]

Someone may correct me here, but I think the basic operation is to peek through the corners at the aperture. If you can see all blades/edges of the aperture, you have completely covered the exposure area.

What I don't know, is how fall-off is related to this. Maybe someone else can chime in?

Hi Guys

Can somebody please tell me what to do with the corners
I know its to do with coverage but how do I work it
I have a Sinar f2 and a Nikkor 75/4.5

What am I looking for

thanks

robin

[David A. Goldfarb]

If the aperture looks like a circle, you shouldn't have falloff. If it looks like the two dimensional projection of an oblate spheroid (i.e., an American football), then you'll have falloff.

If you can see the lens shade through the aperture, then that is also a source of falloff or potentially vignetting.

You can also check that bellows sag isn't obstructing the light path from the lens.

[Geert]

Robin,

the other way'round is also possible, if your tripod is set up so you can see through the lens: with the lens stopped down at the desired aperture, look through the lens to the back of the camera to see if the corners of the ground glass are clear.

this is particularily handy if you use a camera with viewing hood and it's the way it is teached at most LF workshops in Belgium.

A Nikkor SW 75mm f4.5 has an image circle of 200mm at f16. That's 23mm of either rise/fall or shift on 4x5".

G

[Jeffrey Sipress]

I thought the purpose of cut corners was to allow air to travel in and out of the interior as bellows are drawn. I've never used them for any actual photographic purpose.

[PViapiano]

Oblate spheroid...that sounds like it hurts!

[Ole Tjugen]

With a 75mm lens on 4x5" film, you will have light fall-off. If it's a modern "wasp-waist" WA lens, light fall-off will follow cos^4; if an older it will follow cos^4 (or thereabout).

If you can see the whole aperture, you will not have more light fall-off than that. If part of the aperture is obstructed from view, either on the inside or the outside, you will have more light fall-off than that.

It is often easier to look through the lens and see if you can see all corners of the ground glass instead.

And yes, cut-off corners help letting air escape from the bellows.

[Geert]

I thought the purpose of cut corners was to allow air to travel in and out of the interior as bellows are drawn.

Jeffrey,

You're also right about that, but leather bellows are permeable up to a certain degree. Also, ther's plenty of gaps in an LF camera that allow air to pass but block the light (think about the lens board fitting).

I already made groundglass for victorian era cameras with only 1 (one) cut corner for exactly that purpose.

G

[Geert]

With a 75mm lens on 4x5" film, you will have light fall-off. If it's a modern "wasp-waist" WA lens, light fall-off will follow cos^4; if an older it will follow cos^4 (or thereabout).

If you can see the whole aperture, you will not have more light fall-off than that. If part of the aperture is obstructed from view, either on the inside or the outside, you will have more light fall-off than that.

It is often easier to look through the lens and see if you can see all corners of the ground glass instead.

And yes, cut-off corners help letting air escape from the bellows.

Hey Ole, you stole my reply!
Go to sleep now, it's 1:13 AM morning here and also where you live! Or are you going to watch the Olympics too at 3:30?

Seriously, I based my reply on these charts:
modern lenses
http://www.largeformatphotography.in...s/LF4x5in.html

classic lenses
http://www.largeformatphotography.info/lenseslist.html

Everyone suffering the slightest form of GAS should consult these before purchase.

G

[Ole Tjugen]

Hey Ole, you stole my reply!
Go to sleep now, it's 1:13 AM morning here and also where you live! Or are you going to watch the Olympics too at 3:30?...

I'm at work, working night shift trying to get an oil well placed optimally so we can get as much oil as possible out of the ground. What's your excuse?