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! :p ;)
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.info/lenses/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! :p ;)
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? :D

[Richard K.]

Oblate spheroid...that sounds like it hurts!

It DOES !! I had one once but had it removed! :D

[Paul Fitzgerald]

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

Old wives tale, total claptrap. The clipped corner are to allow dust to enter and contaminate your static charged film, plain and simple. :eek:

[Sheldon N]

This link has a good diagram as to why you want to see a full circle aperture from the corner of the glass... look at the images of the 50mm lens and it makes a little more sense.

http://www.vanwalree.com/optics/vignetting.html

[Vaughn]

Take the old Deardorff or whatever and with a fine woodworking chisel clean out the curved corners, refinish and then put in a full coverage ground glass. That way you won't be surprised by something intruding into the corner of the negative. Something you would have seen if you had the full ground glass but don't see when the corner is cut off.
When closing the view camera, do it gently and there is not problem. They aren't air tight.

Actually, it is possible to see the image all the way to the corner even with cut corners on the GG...I believe it is called an aerial image -- and it is brighter than that thrown on the GG.

I might have the advantage of being very near-sighted and can focus the aerial image easily without my glasses. I have been tempted to clip my full 8x10 GG, but don't wish to risk breaking it.

Vaughn

[Capocheny]

Actually, it is possible to see the image all the way to the corner even with cut corners on the GG...I believe it is called an aerial image -- and it is brighter than that thrown on the GG.

I might have the advantage of being very near-sighted and can focus the aerial image easily without my glasses. I have been tempted to clip my full 8x10 GG, but don't wish to risk breaking it.

Vaughn

Hi Vaughn,

If you take the glass to a glass shop... they'll just grind/sand the corners down to whatever size you require.

I had mine done at a cost of $5.00 and it just took them minutes to do.

Cheers

[Vaughn]

Thanks C -- but just the idea of removing the GG from my 8x10 camera makes me nervous. Which is totally silly since I have to replace the GG on our 4x5's when our students go "Opps!" on a semi-regular basis.

Perhaps if I have a replacement GG on hand, I would feel more comfortable letting someone handle my nice one!

Vaughn

[Kuzano]

Oblate spheroid...that sounds like it hurts!

I think it's a procedure known only to Proctologists, and their patients, and not discussed much outside those environs.

[aduncanson]

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).


Ole,

Did you mean cos^3 in the case of a modern wasp waist lens?

Thanks

[Ole Tjugen]

Yes of course. Thanks for pointing it out.

Modern "wasp-waist" wide angle lenses have a fall-off approaching cos^3(theta), while the older compact WA lenses (Angulon, Protar V/f:18 and the like) have fall-off approaching cos^4(theta).

The only way to reduce this fall-off is by making a retrofocus WA lens; and as far as I know there are none for LF.

[aduncanson]

The only way to reduce this fall-off is by making a retrofocus WA lens; and as far as I know there are none for LF.


Not to be difficult but just for your information, the Komura 90mm/6.3 Super W is said to be a retrofocus lens. I just focused mine at a light across the street (about 30m) and found the lensboard to ground glass distance to be about 130mm. By comparison, Schneider gives the "Flange Focal Distance" for the 90mm/5.6 Super-Angulon as 102.7 mm.

Long ago I bought a recessed lens board to use this lens on a Calumet CC-400 and decided that the recessed lens board was neither necessary nor worth the hassle.

[Ole Tjugen]

There's a slight difference between a flange focal distance being longer than the focal length, and a true retrofocus lens. But yes, the Komura 90mm f:6.3 has a long back focus. Not enough to get the other benefits of a retrofocus design, though.