How to determine the lens is centered on the ground glass?

[l2oBiN]

I have a 90mm angulon 6.8 which should just cover 4x5. The problem is that I do not know when my front standard is centered ( ie how much rise/fall) so that I could set the lens to the optimal projection of the image circle on the ground glass. Could anyone help me determine this? (I don't have a centering mark on the front standard).?

[Mark Sawyer]

Run the front standard back as close to the ground glass as you can get it. You should be able to see the back element through the ground glass; if not, remove the ground glass. Measure from the edges of the rear element to the top and bottom of the gg (or its frame). That'll tell you how far off it is in that position. It may be centered or you may have to move it up half of the difference between the two distances once you have the bellows out. Then mark that centered position on the front standard for future reference.

Hope this makes sense...

[l2oBiN]

Makes sense Mark. Will give it a shot! Thnx

[Barry Kirsten]

Another method is to make a mark on a wall the exact height from the floor as the centre of your ground glass. Looking through the GG you will be able to see if the lens has to be raised/lowered to make the mark coincide with the GG centre. The camera has to be absolutely vertical, of course. As you're using a wide angle lens, about 10' between wall and camera should be plenty, although a longer difference will give better accuracy.

Barry.

[ic-racer]

For quick centering in the field, match up all 4 crescents you see through the ground glass cutouts so that they are all about the same size.

[l2oBiN]

Thnx for the suggestions. The wall method might be difficult as it requires a number of accurate measurements. I have noticed there might be another way. I just compressed the bellows to the maximum and they self centered the front rise/fall (which were unlocked), this offcourse would require that the bellows themselves were centered and not warped. Ic-racer, could you please explain your duggestion a bit more? I am having trouble understanding what the crescents are??




For quick centering in the field, match up all 4 crescents you see through the ground glass cutouts so that they are all about the same size.

[Mark Sawyer]

I just compressed the bellows to the maximum and they self centered the front rise/fall (which were unlocked)...

Just be aware that on some cameras they are centered, and on some they may be a little high or low. That's why you should do a quick measurement from the lens rim to the edge of the frame to see things are equal.

[Struan Gray]

Place a mirror across the front of the lens, with the mirrored face inwards. Then shine a bright, focussed light onto an off-center spot on the ground glass (a laser pointer works well too). Focus until an in-focus second light spot appears opposite the first: halfway between the two is the optic axis. Move the lens until the axis is at the center of the screen.

This assumes that your angular movements are accurately nulled. By checking that the spots are evenly distributed either side of a constant point as you move the first one around the ground glass you can check for that too.

[GPS]

Measure where is the centre of your gg. Put a mark (scotch etc.) on the side of your back standard. Measure where is the centre (axis) of your lens, make a mark on its side too. Compare the two marks.

[Frank Petronio]

Don't forget to mark the rear element with a cross-hatch once you find the center, all the best photographers do this. If you can't afford the $129 Rodenstock Centering Etching Device, you can usually use the corner of a small mill bastard file held at a 45-degree angle. Use a steady hand and just etch the rear element enough that you can feel the indent. Then you'll be able to see the cross hatch on your GG and always be optimal.

[Leonard Evens]

The easiest way to check vertical centering is to set up the camera on what should be level " ground". For example, set it up next to a large body of water. Make sure the camera is perfectly level by checking that the front and rear standards are perfectly vertical. Then look at the image of the horizon, which should be parallel to the top and bottom of the frame and halfway between them.

To check horizontal centering is a bit more complicated, but you can do it roughly as follows. Find a building on level ground with facade a rectangle. Set up the camera so that it is pointing directly at the facade. Check that the camera is perfectly level. Also, check that the sides of the frame are perfectly vertical using a level. Check that the top and bottom of the building on the gg are parallel to the top and bottom of the frame and the vertical sides are parallel to the sides of the frame. Position the camera so that its center is equidistant on the ground glass to its sides. That center should now be equidistant from the sides of frame,

[ic-racer]

Ic-racer, could you please explain your duggestion a bit more? I am having trouble understanding what the crescents are??

Do you have the cutouts on the corners of the ground glass? Aim the camera at an illuminated field and rack the lens out so you see a crescent of light when you look through the cutout of the ground glass and look right at the rear lens element. Adjust the lens up and down or side to side so that all 4 crescents of light are about the same size.

This is a routine thing to do when using a lens that barely covers one's format to ensure you don't have one or two dark corners.

[E. von Hoegh]

Don't forget to mark the rear element with a cross-hatch once you find the center, all the best photographers do this. If you can't afford the $129 Rodenstock Centering Etching Device, you can usually use the corner of a small mill bastard file held at a 45-degree angle. Use a steady hand and just etch the rear element enough that you can feel the indent. Then you'll be able to see the cross hatch on your GG and always be optimal.

No no no. The BEST way is to bore a 1/8" hole through the vertices of the lens elements; then you can push a specially straightened diamond tipped piece of 1/8" unobtanium through the holes and align the lens with the center of the GG, making a reference pip with the diamond. Then, mark the front standard.:cool:

[l2oBiN]

Thank you everyone for your continued suggestions.


Do you have the cutouts on the corners of the ground glass? Aim the camera at an illuminated field and rack the lens out so you see a crescent of light when you look through the cutout of the ground glass and look right at the rear lens element. Adjust the lens up and down or side to side so that all 4 crescents of light are about the same size.

This is a routine thing to do when using a lens that barely covers one's format to ensure you don't have one or two dark corners.

I have the cutout corners. I racked out the lens as far as I can ( using a wista 45dx and an angulon 6.8) and n I look through the corners u can see the real lens element..? Not sure about the crescents you are talking about.. I can see the back element regardless of how much I rise or fall the front standard..

[kev curry]

If you use your camera and lens to make photographs why cant you let what you see projected on the gg determine where the lens is positioned?

[l2oBiN]

If you use your camera and lens to make photographs why cant you let what you see projected on the gg determine where the lens is positioned?

keV, could you explain how you could do this to accurately determine the projected image centre of the lens?

[kev curry]

You miss my point... I should have asked why you think that its important to accurately centre a lens in relation to the gg?

For me centering a lens on the gg is totally irrelevant.

When making photographs in the field I concern myself with composition. The composition alone decides where the lens -within its image circle limits- is positioned. If the composition that I'm trying to create need a little shift or rise or fall or tilt or a combination of movements thats what I do.

Why would I be concerned with a need to accurately centering the lens?
If a lens has a small image circle with limited movement capability I rely on what I see on the gg as the best informer of its limitations.

Forgive me if I'm wrong but I 'm wondering if your getting yourself bogged down with things that are largely superfluous in practice?

[l2oBiN]

You miss my point... I should have asked why you think that its important to accurately centre a lens in relation to the gg?

For me centering a lens on the gg is totally irrelevant.

When making photographs in the field I concern myself with composition. The composition alone decides where the lens -within its image circle limits- is positioned. If the composition that I'm trying to create need a little shift or rise or fall or tilt or a combination of movements thats what I do.

Why would I be concerned with accurately centering the lens?


If you have a lens that just barely covers the ground glass, centering the projected image becomes very important.

[kev curry]

Why not use the image projected on the gg to provide the ultimate answer?

If you have limited IC the gg will tell you when you have the lens positioned correctly.

Would that not be the best approach?

[l2oBiN]

Why not use the image projected on the gg to provide the ultimate answer?

If you have limited IC the gg will tell you when you have the lens positioned correctly.

Would that not be the best approach?

keV, the Schneider 90 angulon 6.8 projects am image circle of 154mm (almost exact on thd full 4x5 frame) when stopped down according to Schneider. Although lenses can project image circles beyond their stated values, I believe the quality of the image throw beyond is of mediocre value. Focusing a 90mm lens in itself is a challenge at 6.8 let alone judging the relative image quality on the ground glass of different cirners, and from there I would need tostop it down or take a picture and examine whether the corners are good. It just does not seem practical to me.. Perhaps I am missing something?

[Brian C. Miller]

Kev, I looked up the lens data (http://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/angulon/data/6,8-90mm.html), and it's not just a limited IC, the lens was made for MF. The lens will never cover 4x5 wide open. The best focusing guess with this lens is to get the same amount of vignetting in all four corners, and then stop it down. Therefore, l2oBiN just wants to quickly center the lens, and fahgettaboudit.

My Toyo has a little red dot on the front standard to indicate when it is at the midpoint. If I didn't have that, here's what I would do:
Open the camera, and remove the GG back, exposing the Graflok tabs, and place the camera on a level surface.
Run the front standard all the way out, or at lease enough to give the bellows some room to move.
Place a ruler and a bubble level on the bottom of openings, and move the front standard until the bubble is centered. Use tape on the front standard to mark the position.
Hold the ruller up against the top of the openings, and repeat.
You will now have two tape marks on your front standard, and you can measure between the two for the actual middle.

[Frank Petronio]

Dude, I've used several 90/6.8 Angulons over the years and you're being way to critical. They are great lenses for 4x5 field work and I wouldn't think twice about using a little bit of movement, you can pull 1/2" if it's not of a brick wall or something with lines and details (skies and foilage are forgiving). Just eyeball it and go take some pictures. If you're worried about the edges being sharp, then shoot at f/32. In most cases it actually helps you to have a little softness and vignetting at the edges.

If it really is bothering you, throw a pencil mark down and do a test shot of a smooth even surface or the classic brick wall. Then move up half-an-inch and test, move down the same. Then you'll know what to expect for vignetting and softness and you can decide how much you can tolerate.

But really, for all this outdoor landscape type stuff you're being way to fussy. It's the world, it's a little f-ed up to begin with. Shoot more pictures and think less.

[GPS]

...

If it really is bothering you, throw a pencil mark down and do a test shot of a smooth even surface or the classic brick wall. Then move up half-an-inch and test, move down the same. Then you'll know what to expect for vignetting and softness and you can decide how much you can tolerate.

But really, for all this outdoor landscape type stuff you're being way to fussy. It's the world, it's a little f-ed up to begin with. Shoot more pictures and think less.

There is nothing fussy about being able to know where the centre of the front standard/lens is. That's why most of the cameras have centre marks on them.
But trying to find that centre point by making a series of pictures and comparing visually their light fall off is probably the most dudy way to accomplish that.

[ic-racer]

keV, the Schneider 90 angulon 6.8 projects am image circle of 154mm (almost exact on thd full 4x5 frame)

The 'rack out' part is for 'over-coverage' lenses,to make the crescents smaller and easier to match the size.
For your lens just focus normal and check all 4 corners. The 'crescents' may look more like 'bi-convex' shapes. In either case, just keep fiddling until all 4 are the same size and shape.

You really will need to learn this with that limited coverage lens. For example if you tilt the front or back, you will need to check the 4 corners through the ground glass and re-center the lens with rise or fall.

[MIke Sherck]

Ic-racer, could you please explain your duggestion a bit more? I am having trouble understanding what the crescents are??

Most ground glasses have the corners cut off; when the camera is all set up in the field for a shot and you're looking at the ground glass, look closely through the cut-offs at each corner of the ground glass. You should be able to see a little bit of the lens's rear element glass through each one; a crescent of light and lens. When the portion of the rear lens element you can see through each corner is about the same, the lens should be centered.

I used this method for years, with a 210mm lens which *just* covered 8x10. How well it works depends on the amount of care you use. For example, the angle at which you look through the ground-glass cutouts makes a tremendous amount of difference with what you can see -- try to keep approximately the same angle for each corner (easier said than done!) Lots of little bugs like that.

I had almost as good a result by just standing off to one side of the camera, once it was set up for the shot, and "eyeballing" the lens to see if it looked centered on the rear standard. With a bit of practice you can get remarkably accurate at that.

Good luck!

Mike

[Kevin Crisp]

When I've been concerned about this, I move the front standard close to the back and look at the bellows. If you're high or low it will be obvious from looking at how the bellows bunches up. Obviously a badly misshapen bellows would be an issue but I've never had a camera where it wouldn't work fine. If you want to be totally on and quick about it, mark the front standard after making a measurement with the lens all the way back and in the middle of the rectangle as viewed from the back without the gg in place.

[D. Bryant]

Perhaps I am missing something?

Ah yeah!
A) Go out in the field and make photographs. If they look right they are right. Unless you are totally inept the lens doesn't have to be centered dead on to make good photos.

B) If solution A doesn't work for you get a 90mm Super Angulon. It has tons of movement. In fact I recommend solution B. Use a 90 mm Angulon on an 8x10 to make circular images and you will be happy.

So get out there and shoot.

Don Bryant