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what is the proper way to verify infinity focus
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PostPosted: Wed Aug 12, 2009 4:37 am    Post subject: what is the proper way to verify infinity focus Reply with quote
All,

What is the proper way to check this? Do you simply turn the focus ring until it stops at the infinity mark, takes some shots and then judge it? What is the basis you would use to judge it?

Thanks for your time.
Cheers


PostPosted: Wed Aug 12, 2009 6:15 am    Post subject: Reply with quote
For my money I'd trust a properly adjusted collimator; I am still hoping to find a small one, like that made by Meopta, but that's a long shot. I used to go to a firm which had an Angénieux one which was about the size of a small child's bed. Very convenient for setting infinity on a camera with front cell focussing, or checking if the film plane agrees with the film plane. There again I am a film user so it might be a moot point.

I suppose you can try using the maximum aperture and use a subject far enough to be considered infinity: in our field, infinity is meant to be 1000x focal length, but being a pedantic person I tend to use the moon. Then enlarge as far as possible to the point of pixel-peeping; that should be ok.


PostPosted: Wed Aug 12, 2009 7:39 am    Post subject: Reply with quote
"Bringing Infinity Indoors" by Rick Oleson:

http://members.tripod.com/rick_oleson/index-123.html


PostPosted: Wed Aug 12, 2009 10:28 am    Post subject: Reply with quote
you could use a Hartmann mask.

It's basically a light shield with two holes in it. Cover front of lens with mask, point at bright star, and focus until the star images merge into one. Works better with Live View and a tripod.


PostPosted: Wed Aug 12, 2009 10:33 am    Post subject: Reply with quote
http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM#HM
http://www.billyard.ca/Hartmann.html


PostPosted: Wed Aug 12, 2009 11:15 am    Post subject: Reply with quote
Great thread, thanks a lot for sharing!.

My 5 cents: DIY Collimators:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/18226/page/0/view/collapsed/sb/6/o/all/fpart/1

http://vincechanblog.ca/laser/collimator3.pdf

http://www.instructables.com/id/Home-Made-Collimator/

Regards,

Jes.


PostPosted: Wed Aug 12, 2009 12:30 pm    Post subject: Reply with quote
Attila wrote:
http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM#HM
http://www.billyard.ca/Hartmann.html


Thanks much Attila.

The second link, http://www.billyard.ca/Hartmann.html
Is excellent for designing Hartmann masks for cameras, folders in particular.

You can use a piece of ground glass (easy to make) taped to the film plane of a folder then use a Hartmann mask over the lens.

Use the moon or a bright light at an infinity source (over 100 meters)

You must also use a magnifying glass or a telescope eyepiece to see the image properly on the ground glass. When it merges that's infinity.

Mark the focus ring on the lens!

Jules


PostPosted: Wed Aug 12, 2009 12:56 pm    Post subject: Reply with quote
Thank you guys for all valuable information ! I think this is a very important subject , I met with many lens what is not reach exactly infinity at the right place.


PostPosted: Wed Aug 12, 2009 1:33 pm    Post subject: Reply with quote
Thank you very much for all the information/links - so much to learn.

I suspect that one of my lens does not focus properly at infinity, so it sounds to me I can use a Hartman mask to help adjusting the lens to work with my specific camera.

Here is a followup question. In the http://www.billyard.ca/Hartmann.html for 'primary aperture' value, if I have a 55mm lens(max f1.4); do I enter 55mm or I should measure the actually opening formed by the blades at f1.4?

For '# of holes in mask' questions, why would one needs more than 2 holes?

For my purpose, am I asking the right question or taking the correct approach? Sorry for these perhaps dump question.

Thanks you for taking the time,
Cheers


PostPosted: Wed Aug 12, 2009 2:45 pm    Post subject: Reply with quote
Attila wrote:
Thank you guys for all valuable information ! I think this is a very important subject , I met with many lens what is not reach exactly infinity at the right place.


Well this is fact of live as lenses are designed such that they can operate at different temperature.

Lenses, especially those with thick lens element or fluorite element (i.e. tele lenses), temperature will change thickness of its elements and alter the actual focal length of the compound lens with a tiny value in accordance with the temperature. As focal length of the lens changed, the focal distance of infinity also changed and has to be compensated by twist of focusing ring.

If lens are made with exactlly infinity at one temperature and not allow to varies passing it, the lens may have risk that it cannot focus to infinity at some temperature range.


PostPosted: Wed Aug 12, 2009 2:49 pm    Post subject: Reply with quote
Hmm... not quite the same kind of collimator. I'll try to describe how that works.

At the far end is a very strong small light source, normally an over-run tungsten lamp, which back-lights a graticule plate. This plate is marked with fine lines so that you can see if it is in focus or not easily.

Moving on, at the other end, is an optical lens system which projects the grati cule image, like a slide projector, but is set to give parallel rays. In other words, the image of the graticule is set to be at infinity.

With a camera pointing at the lens, and the lens is set to focus at infinity, the graticule image would fall on the film plane, and if it is a single-lens reflex, on the focussing screen when the mirror is down. If the image is not correctly focussed on the film plane, it means the infinity setting of the camera lens is not exact.

To make sure that you can see the actual image on the film plane, there is also a prism arrangement between the graticule and the lens system in the collimator. While it allows the graticule image to come out of the lens system, it allows you to see, through a side-mounted eyepiece, the actual image projected on the film plane. So, there is no need to put a sheet of ground glass on the camera back and squint like crazy; get the camera loaded with a roll of dud film, and the setting would be as exact as can be.

I hope my description makes some sense.


PostPosted: Wed Aug 12, 2009 3:04 pm    Post subject: Reply with quote
Seele wrote:
Hmm... not quite the same kind of collimator. I'll try to describe how that works.

At the far end is a very strong small light source, normally an over-run tungsten lamp, which back-lights a graticule plate. This plate is marked with fine lines so that you can see if it is in focus or not easily.

Moving on, at the other end, is an optical lens system which projects the grati cule image, like a slide projector, but is set to give parallel rays. In other words, the image of the graticule is set to be at infinity.

With a camera pointing at the lens, and the lens is set to focus at infinity, the graticule image would fall on the film plane, and if it is a single-lens reflex, on the focussing screen when the mirror is down. If the image is not correctly focussed on the film plane, it means the infinity setting of the camera lens is not exact.

To make sure that you can see the actual image on the film plane, there is also a prism arrangement between the graticule and the lens system in the collimator. While it allows the graticule image to come out of the lens system, it allows you to see, through a side-mounted eyepiece, the actual image projected on the film plane. So, there is no need to put a sheet of ground glass on the camera back and squint like crazy; get the camera loaded with a roll of dud film, and the setting would be as exact as can be.

I hope my description makes some sense.


Yes, I have seen such collimator used by professional optician.

It needs a point source, over-run tungsten lamp alone is not sufficiently small, and has to work together with aperture orifice, and pair of plano-convex lenses opposite to each other to make emerging rays parallel. Monochromatic light source is even better.

I actually use laser source and with a diffraction pattern to generate the parallel ray grid for collimation of my astronomical telescopes.


PostPosted: Wed Aug 12, 2009 3:32 pm    Post subject: Reply with quote
From my experience, general photography and astro-photography are two different beasts; so don't take too seriously on astro-photography focusing methods.

I also involved in astro-photography and had tried all sort of focusing methods like Hartman mask, Stilleto, Rochi screen, knife edge, parafocal eyepiece etc for astronomical telescopes. All of these methods have to work with point source at infinity, i.e. the most convenience are stars. However, astronomical telescopes are optimised for infinity and seldom has optical elements, lens or mirror, of more than three. Many good astronomical telescopes are diffraction limited at infinity which make focusing with these methods relatively easier; besides it don't need flat field. On contrary, photographic lenses have to compromise for flat field, infinity as well as close focus ranges; very often it has to compromise performance at infinity and there will be residual aberration like comet aberration. With these residual aberration, the image will no longer be point source and optimal focus is not easy to detect.

Actually, the Hartman method is the worst method, even with astronomical telescope it is usually used for coarse focus only. Actually, in astrophotograhy I use maximum intensity (point source) method. A medium bright star (magnitude 4 to 7) is used for focusing; the image of star is taken with exposure not saturating the CCD sensor and the optimal focus is reached with the star has maximum brightness (i.e. all energy is concentrated to a point). In fact, I use autofocus gear which take CCD measurement and make 0.0003" step increment for detecting optimal focus; please note average value of several measurements is used for merit comparison in order to negate transient atmospheric tuberance.

For photograhic lenses, it would be sufficient to tape a focusing screen at the film gate and use a 10x loupe to validate the optimal focusing position as it will be seldom to enlarge more than 10x.


PostPosted: Wed Aug 12, 2009 5:05 pm    Post subject: Reply with quote
amoebahydra wrote:

For photograhic lenses, it would be sufficient to tape a focusing screen at the film gate and use a 10x loupe to validate the optimal focusing position as it will be seldom to enlarge more than 10x.


This might be the case where it is convenient to do so, but the variation between the position of the ground glass surface and the actual location of film plane can indeed exceed depth of focus of the lens.

Besides, it would be tricky if you want to make sure the focussing screen position agrees with the film plane position in a reflex camera. I have seen a photograph of the instrument used at Agilux for fine-tuning the Agiflex camera, which had a ground glass screen set into the end of a high-power magnifier positioned at the film plane, and another high-power magnifier for examining the focussing screen; it seems to me like a rather labour intensive way to do it.

In the case of a front-cell focussing camera, such as a folding rollfilm camera, the accuracy of setting the focussing ring in relation to the actual front cell can be done very easily under a collimator; no chance for hit or miss as the whole operation can be done when actually observing the projected image on the film plane itself.

Regarding light source; I have used a couple of different collimators and the light bulbs could very well be point source but definitely over-run; you need a lot of light to project an image on the film plane and then expect it to bounce back and then reflected into the eyepiece off the glue joint between two prisms.
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