Means to determine EXACT fl of a simple lens

[bglick]

rob, good issues, I should have explained further. The ultimate goal is to have a matched pair of lenses, less than .1 mm total variance in fl. The lens design is such that, varying the distance of the eye element (singlet) from the film element (doublet), you alter the effective fl of the optical train. The amount of change in fl vs. the seperation is ... .2% fl change per 0.5mm change in separation. The barrel will be designed with fine threads to allow for small changes in seperation. So, as Dan picked up on, the goal is getting matched pairs of effective fl lenses, not neccessarily determining the exact fl of each optical train, but I could not think of a better way to accomplish this goal, then matching effective fl's and tweaking them for pairing.... this further explanation may open up some possibilities. I think there is a nugget floating in the concept of setting up a dual test system, whereas each lens projects on the same gg, side by side. A 2" high straight line is projected on the gg, then tweak one of the trains till the lines are the exact same height...but not sure of the ability to measure lines on the gg. It may be easier to simply read the distance between two standards.

Also, this procuedure is not being used for a one time run. I plan to pair about 400 optical trains, for each run, ending up with 200 matched pairs, with effective fl's within the tolerances mentioned above. So the system will be used for each production run, hence why I am willing to spend some money to get it right, as sending all the elements to a lab would be very costly, not to mention, I want to be assured the work is done right, and if that is possible with a home brew system, I am willing to do what it takes.

BTW, the inter eye magnification tolerances were learned from a study performed 20 years ago regarding magnfication deviation in each eye when viewing the same object. It showed the % of test subjectst that were bothered (headaches, strain, etc.) at different magnification variances. So I am confident I am chasing the right number, I just need a simplified method to get there, and it seems it is possible without ultra expensive equipment.

So before I get too burried in the math to accomplish this, I am curious now that I have explained it in more detail, if the proposed techniques are still considered optimum.

TYIA

[rob_5998]

But like binoculars, if you have one adjuster which adjusts both barrels focal length simultaneously and one fine adjuster which adjusts only one barrels focal length to match them, then providing the adjustments cover the lens and barrel manufacturers tolerances then there should be no problem and little testing required. i.e. you should only need to test a sample of the lens/barrels supplied for manufacturers tolerances. If you don't want to incorporate a fine adjuster then you have a lot more tesing and precision assembly to do.

I think a LF camera would be OK for a small amount of testing but not for large quantities. One issue would be rigidity of the standards and parallelism of the standards. That might be fine for one off tests but for bulk tests it could be time consuming to be constantly checking the checking equipment.

[bglick]

rob, sorry, I forgot to address the diopter adjustment issue. I can NOT use a diopter adjustment on one lens similar to binos. The reason is, it would create a source of mis matched magnfication by people who do not suffer the problem. This can occur when the diopter is set off of zero, yet the user can still focus the system as the DOF is great enough to allow this to happen. So it can introduce a problem that otherwise would not have existed. instead, I keep both lenses at near equal magnification, and if a person has astigmatism, they wear their spectacles to correct this problem when using the viewer. I have built enough eye releif into the lens system so spectacles users will have no problems getting their eyes positioned at the optimum 30mm distance from the singlet.

Agreed on the flexibility of the standards for measuring...but its the concept I am after, as a custom made jig can be built to correct the shortcomings of using a view camera.

> To focus exactly on infinity place a mirror over the front lens element. Then place a point source light just off center of the GG and focus the reflection on the GG.

As for this description..... front lens element = the eye element (singlet) right? The source point light would be what type of light? I assume it can be positioned behind the gg, (not between the gg and doublet) projecting through a drilled hole in the gg, near the lens center. Then move the gg up and back till the light on the gg becomes sharp around the edges. Do I have this right? My thinking is, drill the hole in the gg just off center axis, then the focused light on the gg will be VERY near the light source, both being as close to center axis.

IF this is right, I am thinking this system is ideal, as with a solid optical bench, this system can be easily measured, i.e. I am now spreading the range of measurement out, which will reduce the margin of error vs. measuring the focus distance of each lens on the gg.

> Then focus on an object to give 1:1 magnification. You can measure the object height and projected image height to confirm 1:1 magnification.

I grasp the concept here, but want to nail down the details. Is this process treated identical to using a view camera trying to focus an object at 1:1 ? So I would move the lens to and from the subject, till the object is 1:1 on the gg. To keep it in focus on the gg, I adjust the spacing between the gg and the singlet lens. Do I have this right? The measurement is from the subject plane to the frosty side of the gg. Right?

TYIA

[robc]

"front lens element = the eye element (singlet) right?"

yes, so that the light passes right through all elements of lens and then back again. The mirror should be as closeto lens element as possible and a first surface mirror would be optimum.
The singlet will be on the subject side of the test setup.

"My thinking is, drill the hole in the gg just off center axis, then the focused light on the gg will be VERY near the light source, both being as close to center axis."

yes. Any light source which is small enough and probably needs to be shielded to cut out extraneous light. An led would probably do it or one of those laser pointers or possibly a micro torch. There are some GG's available which are clear in the center but it would probably be better to drill a hole to stop any chance of refraction of light from GG.

"So I would move the lens to and from the subject, till the object is 1:1 on the gg."

at 1:1 magnification the subject is exactly twice the focal length from the lens and the lens is exactly twice the focal length from the GG. (this is why the lens extension from infinity gives you the fl but it only works at exactly 1:1 magnification) This means you have to be able to adjust both the subject distance and lens to GG distance. A change in one is likely to require a change in the other until equilibrium at twice the focal length is achieved. Since you will know the approx fl to start with it should just be case of tweaking the positions to get it spot on. The larger the subject height then the less measurement error there should be (I think). You will have to determine the max subject height you can use for the focal length of the lens. Depends on image circle size.

Once you have determined the max subject height you can use, and providing you always use that subject then you can place height marks on the GG so that you can adjust the focus and subject distance until it fits the marks thereby negating the need to actually measure the height of each test. i.e. you use the GG itself as the ruler. The marks should obviously be the height of the subject apart. This will require that the subject position remains vertically and horizontally static in relation to the GG for all tests but should simplify the process.

[Donald Qualls]

Wheee! Lots of text up there. All of it depends on knowing when an image is "in focus" and when it's not.

There's a simple way to find that out that doesn't depend on how tired you eyes are.

Make a mask to go in front of the lens, with 3 or more holes -- the simplest is three holes radially distributed, but the holes can even be randomly located. Now, instead of a "scene", focus a point light source. You'll *very* easily be able to see when the three-or-more spots of light cast by the holes in the mask coalesce into a single point; that's when you're in focus.

Should greatly simplify all the other bits...

[robc]

the more I think about this the less I think it will work for the level of accuracy you require. I think the problem will be measuring the height of the image on the GG. The lens extension should be no problem with some kind of micrometer but the projected image height change will be negligible with plus or minus 0.1mm lens extension so getting exact 1:1 magnification is unlikely.

[bglick]

> at 1:1 magnification the subject is exactly twice the focal length from the lens and the lens is exactly twice the focal length from the GG. (this is why the lens extension from infinity gives you the fl but it only works at exactly 1:1 magnification)

rob, here is where I think my simple compound lens doesn't fit this scenario.... I fully understand what you describe above for camera lenses, I have tested this many times. However, for this lens, the distance from the eye releif point (which is 30 mm from the Singlet) to the subject is 94.5 mm, yet the effective fl of the combined lenses is 45mm. Now, if this lens was a single element such as one doublet, then thethe subject to lens distance, focussed at infinity would be equal to the lens fl. But that is NOT the case here, and is why I question how this lens would fit the same mold as lenses, where the lens fl matches the focus distance at infinity. Make sense?

Now, the next issue is the gg to singlet distance. I appreciate your concept here, it's what we use in a view camera. Change the gg to lens distance, and the focus on the gg changes drastically.

However, that does NOT occur with these type lenses. If you look through a loupe, and move your eye (gg) to and from the loupe, the focus will not change, only the field of view changes. I need to get my head around these issues first, as there seems to be too many differences between the way my lens performs vs. a normal camera lens? make sense? Any thoughts?

Donald....your method would work with a lens that acts more like a camera lens. But I think robs methods provides an easier means of measurement.....

[robc]

There is a simpler way but you will still have the problem of measuring image height accurately.
As has already been stated, you don't actually need to know the focal length. Therefore you can forget infinity focus steps.

All you need to do is to place lens barrel in camera at a fixed and repeatable distance from a subject with the singlet towards the subject. An image will be projected onto the GG and the height can be measured. Place next lens into camera without moving anything and with singlet same distance from subject. Image height should be the same if focal length is the same. If not adjust lens and retest second lens making sure singlet to subject distance remains the same as first test and that distance between camera standards does not change beween tests.

I think this should work without knowing focal length and is much simpler to test but you will still have the problem of being able to detect image size difference for a 0.1mm change in fl.

[robc]

methinks that if you have the means to capture image from gg and enlarge by projection then you will be able to measure a very small change in image size. A digital back for the camera would be ideal but I guess thats out of the question.

[bglick]

> A digital back for the camera would be ideal but I guess thats out of the question.

rob, not sure why you lost confidence in this suggestion, I think its the breakthrough I was looking for! Brilliant! Use a digital camera with about 150 pixels per mm. This should measure projected image to .01mm. A variance of .01 mm in subject height for a 20mm tall subject represents .05% difference in magnfication represents a difference in fl of .02mm.... this assumes a lens of about 45mm effective fl. So, therefore, I can nail each lens to within .02mm fl, even though the fl value itself is insignficant. Now, I need to massage these numbers a bit to develop a fast way to covert the difference in image height to to % of variance in effective fl. Make sense? Thanks for your continued persistence....unless I forgot something, I think this is a winner!