# Thread: How to align camera and flat object ?

1. ## How to align camera and flat object ?

The mirror method seems the simplest and most practical, but there is one potential problem. Your pointer towards the lens, no matter how it is arranged has to be exactly flat on the subject plane. That means it should have a large base of contact. Otherise slight errors could be magnified. Thus carpenters don't rely on T-squares when checking such matters for large surfaces. They use various simple tricks from geometry.

Here are some other things you might do to get around that problem.

If it is feasible, put a large rectangle centered in the field of view on the subject plane. Then measure the distances to the corners from the center of the lens. These should be very close to equal. (If you want me to, I can calculate how much the difference in lengths translates to in terms of a displacement from parellism.) A related method which could be used to check the miror or laser pointer method would be to measure the distance from the center of the subject to the lens along what you've established as a right angle, measure the distance in the plane to some reference point, and then measure the distance from that reference point to the center of the lens. The Pythagorean Theorem tells us that the square of the diagonal measurement should be equal to the sum of the squares of the other two measurements. Carpenters use this method by choosing dimensions so the triangle should be a 3. 4. 5 right triangle.

If the subject plane is vertical, once you have the rear standard leveled, you can also check the lengths of diagonals of the image of such a rectangle in the subject plane. If these are essentially indistinguishable using a scale in mm, and the rectangle comes close to filling the frame, it would seem to me that you would be close enough for all practical purposes. If the subject plane is too far away, you could tape a tape measure or other ruler to it in the center and in the corners and measure the lengths on the gg of equal lengths in the subject. These should be very close to equal.

Another related thing to check is that your standards are as close to parallel as possible. There is often some play in the default positions of the standards, and they can be slightly out of parallel. The easiest way to do this is to turn the tripod head so the camera is pointing vertically downward, using a level on the rail as a guide to see if it is plumb. Then check to see how close the standards are to horizontal with a level. Check on the gg for the rear level and on the front of the lens barrel for the front standard. For wide angle lenses, small displacements from parallelism of the standards can tilt the exact subject plane enough to disturb focusing in the corners or edges.

2. ## How to align camera and flat object ?

As one who is also a maker of sawdust, I'd lean toward Leonard's measurement approach. Use a plumb bob to ensure both the target and the standards are vertical, and then careful corner-to-corner measurements to check alignment. The key issue, I think, is not to assume the wall is plumb, nor that the floor is square to the wall.

3. ## How to align camera and flat object ?

I'm a woodworker also, but I think the suggestions posted here 'pale' in comparison as to how these problems have already been addressed by the animation stand, reference the history of the Acme animation stand, and their multi-plane animation stand which was used to produce 'Snow White'.

In terms of all of the calculations and measurements that have been suggested by the folks above, there's always the potential to be off trying to figure this out for the first time, where you have folks who produced tools like the Acme animation stand, who were professionals at this from way back.

You may have trouble finding a good animation stand, and it might not be a cost effective proposition, but in terms of what your proposing, particularly taking cameras on and off, which implies repeatability, the animation stand would be the most precise tool for the job.

It'll be your first time trying this, as opposed to getting a tool produced by folks who have several decades to perfect a tool to get this right, good luck, I'm going to the beach.

4. ## How to align camera and flat object ?

QT,

Do you have a mirror and a flashlight?

Put the mirror at your test target and the flashlight to the view finder and adjust the camera to set-up a 'hall of mirrors'. If you have a grid or micro-prism, you can set exact focus.

For a view camera have a mirror cut to fit in place of the film holder. Scribe an X from corner to corner thru the backing of the mirror. Remove the lens board, insert the mirror and hold the flashlight to the gg. and set-up the 'hall of mirrors'. Lock down the back and install the lens and set-up the 'hall of mirrors' again. If you have a grid on the gg. you can set exact focus.

You can get 12" mirror tiles at Home Base, Home Depot, Costco, ect. You DO NOT need front surface mirrors for this.

Fast, easy and cheap.

5. ## How to align camera and flat object ?

I like the sound of mike Gudzinowicz method since it works for any camera where you compose through the lens. But will it work for a range finder? Possibly not due to paralax error.
One point though, with a view camera, if the lens is not perpendicular to the gg then all bets are off. So before lining up the resolution target align your lens / gg accurately. I'd be interested to hear how this can be done simply. Possibly place a mirror on lens barrel and laser on GG with lens open to check reflection is same point as laser exit. Probably need to focus so that returning point is sharp which should be at exact focal length of lens. For fixed back cameras then I guess you have to trust that lens is perpendicular to back.

6. ## How to align camera and flat object ?

"One point though, with a view camera, if the lens is not perpendicular to the gg then all bets are off."

I think the method I described above for ensuring the standards are parallel suffices for most practical purposes. I don't think most view cameras are machined precisely enough nor have sufficiently fine controls to justify the use of more elaborate measures. QT wants to do this in order to make careeful MTF measurements. My guess is that he would be better off repeating his setup procedure, including making sure the standards are parallel, several times, and then averaging the measurements for each position in the field. That way he would use statistical theory to compensate for the inevitable random errors. Any systematic erros would show themselves in differences in the four corners. Of course that would assume that the lens is symmetric to within tolerances smaller than anything he might measure, but I think that is a safe assumption, at least with a modern lens which has not been abused. Lack of symmetry in the lens would likely produce other rather obvious degradation of the lens.

8. ## How to align camera and flat object ?

What I did was to swing a tape from the lens edge to the wall on a diagonal about 25 feet away then swing the tape off in the other diagonal and mark that. A string won't cut it because it stretches - wire or a steel tape will do the trick. Measure carefully to find the center between these two marks. This mark is also the center of your target. From there plumb bob down the wall to transfer to the floor, plumb bob down from lens to floor for your second mark. Snap your chalk line there and coat it with clear coat to keep it from getting erased. You now have a line that puts you in rough position for moving closer or farther when different focal lengths and formats. Just plumb your lens to the chalk line each time you begin for a close start. It makes no allowance for plumbness of the wall. I "measured" that by noting that the plumb line was parallel to the wall. A water level might be good for getting same height of camera and target but still no promise of planarity.

What I got from this was close but not quite there. I could stiil get good focus in one corner most of the time but not all corners. I could compare several frames and interpolate (swag) the compound results but nothing that would allow a definitive comparison between cameras. Not good enough to prove or publish but good enough that I could make a decision about which was better and which might not be so good.

So in my first post I suggested the laser with mirrors to attempt to increase the precision. There will be plenty of error in reading your plumb line, tape and levels. It would never show up if you were building a wall but it will in your test target film.

Leonard:

Would you do some math on the resolution of the numbers of the triangle for finding center and perpendicularity? The two legs were about 20-25 feet measuring out from the camera to the subject wall. What does that translate to given some small errors in measuring? Perhaps +_ 1/16th inch for a sloppy measurer?

9. ## How to align camera and flat object ?

it now occurs to me that if Mike Gudzinowicz method is used then, for view cameras, by first aligning the target as described and then placing the laser pointer on the ground glass center with the lens open, the reflection from the mirror on the mtf target should show on the gg. This should show the minutest deviation of the gg not being at 90 deg to the lens. It does of course require that the laser pointer has a flat barrel head which can be placed on the gg and that the laser is perpendicular to the laser barrel end. Now how do we test that?

I am only advocating this method since if a laser pointer is already owned then there is no additional cost or construction involved.

10. ## How to align camera and flat object ?

It seems that using a mirror on the test target is the best blend of affordability and accuracy.
There were some suggestions of using an animation stand, but I would think they are too small for testing LF lenses.

So that will ensure that camera is in the correct position and facing the center of the target.

However the film plane and the lens plane still need to be parralel to each other and to the target if you are going to be testing edge sharpness. This complicates the matter.

Assuming the plane of focus was perfectly flat, and you focused on the center point. If your target had many radial lines, then theoretically one of the lines will be in sharp focus and the others will be out of focus (to a varying degree). This should make it obvious which adjustments to make so that the entire target is in proper focus.

One note on lens testing though. Lenses that have a slight curvature of field will appear much worse on edge numbers than other lenses. However when photographing 3d objects, the curvature of the plane of focus may actually match the object better than a perfectly flat plane. Also, many photographs have a focal point in the center area of the image. Thus I have allways interpreted lens test by weighting the center numbers more than the edge numbers.

*disclaimer.. I am an amature photographer with about 5 years experience, and in no way an expert*

-Darin

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