Walter, how are you measuring? I ask because the idea behind eyepiece diopters is that the user gets a prescription from his opthalmalogist/optometrist and then buys the eyepiece diopter that fits the prescription. If the lens doesn't match the prescription the poor buyer's in trouble.
One other possibility. Are your +2 and +3 marked +2 and +3 or are they in boxes marked +2 and +3?
thanks john, i do i see your point. i'm all for experimenting! but without a little guidance, i wouldn't know things like i should place the stop a certain distance before a landscape lens, but in contact with the front of a portrait lens etc. and i only have the vaguest idea why someone would stick a biconcave lens in between two positive lenses. how do i determine the proper distance between elements, etc...different building techniques other than the ones used in Primitive Photography.
Originally Posted by John MacManus
if i'm going to experiment, what do i experiment with? what's a good design to try, and how to i go about picking some interesting lens elements for it? these sorts of things.
however that french website looks interesting, just knowing what types of elements to try...
Originally Posted by Dan Fromm
They are definitely marked +2 and +3 on the rim of the diopter itself, and not on the box. No mixup here.
Regarding my measurement methods, the fact that I can use them both on my Shen Hao which has a maximum bellows extension of 330mm speaks more of my ability to measure than my assertions to that effect
You're right that it wouldn't make sense for Nikon to use some other weird convention though. So I don't really know what's going on.
The universe is a haunted house. -Coil
Walter, thanks for the reply.
You're right, it is impossible to focus a 500 mm meniscus with 330 mm of extension.
This prompts an evil thought. Is y'r Nikon's finder 0 diopters with the eyepiece lens removed? I ask because y'r 2 diopter lens measures around 320 mm and y'r 3 at around 200. ~ 3 diopters and 5 respectively, total.
Wild guess: diopter for eyeglasses and close up lenses are are usually specced for light from infinity. Viewfinder diopter values are usually set to match your prescription (i.e. if you have +1 glasses you buy the "+1" diopter for your camera's eyepiece) but they are used to view a virtual image that is at a much closer distance. The shorter working distance requires a shorter focal length to get the screen in focus.
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That was a bit garbled (or brief). My point is that viewfinder diopters are sold for a different purpose than close-up lens diopters. Perhaps an example will elucidate:
Say for the sake of argument you have a simple eyepiece lens 10 cm away from a viewing screen, that's a +10 diopter if you want the virtual image of the screen to be at infinity. If you wear +1 glasses and want to use the viewfinder without them on you will need an eyepiece lens with a power of +11 - the same power as the old eyepiece lens and your glasses combined. Thus, for this hypothetical camera, a new eyepiece lens marked "+1", sold for people with +1 glasses, will actually be a +11 diopter.
In real viewfinders there is usually a condensor lens (or Fresnel) right at the screen, the eyepiece lens tends to be closer to the screen than 10 cm, and the virtual image tends not to be at infinity (more like 40-50 cm). That makes the lenses in the eyepiece diopter less high magnification than my +10/+11 example, but a "+1" eyepiece diopter will not in general have the same focal length as a "+1" close-up lens.
For example, I recently got a "+1" lens for my Hasselblad's waist level viewer (old eyes :-). It has a focal length around 200 mm if I remember rightly, certainly not 1000 mm. I can check tonight, and compare it to the "0" lens it replaced.
Viewfinders are usually at -1 without correcting diopters, so to get to +2 you need to add a +3 diopter correction lens - 333mm. To +3, you would need a 250mm +4 correction lens.
-- Ole Tjugen, Luddite Elitist
I think the best option for experiments where you hope for salvageable outcome would be single lenses and symmetrical-to-nearly-symmetrical doublets. Next, maybe asymmetrical doublets like a Wide Angle Rapid Rectilinear or similar by other names I can't remember at the moment...aplanat?
We've been warned by many that even the seemingly simple Cooke triplet and it's grandchildren have very critical spacing...because it accomplishes alot through careful design and assembly. Through careless assembly (found optics, guessed spacing), what was accomplished won't be. If you have the parts, experimenting is cheap.
I'm pretty sure the idea behind additional elements was to accomplish additional aberration correction. At some point someone realized spacing, curvature and index of refraction all can be combined to correct to a large extent most of the 7 most common aberrations troubling lens design. There is a patent that discusses some classic designs and assigns a numeric value to the amount of correction accomplished in each design by the three techiques mentioned above. Brian Caldwell, optics and software designer has mentioned in some cases exotic glass can be done away with by altering spacing.
Cool, but beyond most home experiments.
4-element lenses like Tessars are a variation of a triplet, with one element 'split' into two to allow specific corrections.
The beauty of symmetrical lenses is alot of bad stuff is cancelled by an identical lens (oh, you have to have them centered and collimated decently, not impossible). The beauty of a small f-number is is makes alot of stuff go away also, until it's so small diffraction problems begin to be visible. So a small aperture symmetrical lens gives you alot for you money or scrounging.
The position of a stop or aperture, if centered is theoretically ideal for 1:1 macro. Thinking about whether moving it forward or backward of center for optimizing at infinity gives me mechanical dyslexia. Maybe someone will answer that here. Some people have told me that cell spacing isn't as critical with simple lenses and long f.l. lenses, but a gut feeling tells me that centering/collimation is more critical on longer f.l. No substantiation there, just gut feeling.
I still have no clue about coverage for a given design, other than the following:
From Green, Primitive Photography, probably taken from an old classic British lens I forgot the designer of, is try to use lens elements of diameter 1/7-1/5 times overall f.l. with spacing at similar proportion.
Coverage using the rule of thumb f.l. = diagonal of negative will probably result in illumination but aberration as you move outward from center. Some of the Greene lenses I thought had 'usable' coverage on the order of 20-30 degrees.
If you want sharper images over the full negative, aim for a much longer f.l. (2x 'normal' +/-) and be prepared to baffle some of the excessive coverage/illumination capability inside the camera to alleviate flare.
If you're shooting b/w and don't have achromat cells, try a contrast filter ranging from heavy yellow to orange to red to narrow the spectrum and reduce achromaticity. It lends itself well to most subject matter with some exposure/development effort, better than shooting everything thru a blue filte any way.
Back to symmetry...since choices are poor in surplus catalogs with regard to mimicking a careful commercial design, two of 'whatever' will reward you. Oh, just don't use two negative diopter lenses. To actually see what you are focussing the net diopter value has to be positive.
Now, to dispense with the obsession for perfection, which some say is over-rated, and bowing to evaluative esthetics, see John Siskin's article on homemade lenses. He goes right ahead with a number of triplet experiments - you just don't re-create a Cooke Triplet haphazardly.
I think it was a 2002 View Camera magazine article and it is on the web somewhere, maybe John's webs site. Oops, I just corrected spelling of his name
http://www.siskinphoto.com/magazine1a.html click on image for copy of article.
Better make room for every little piece of glass you start hoarding , disaassembling every malfunctioning or lousy lens you come across, buying old camcorders, etc. You don't find pairs of identical elements this way, but you find some cheap cool stuff.
Another fun thing to do, with no idea as to quality of results, is to take a negative diopter lens and put it behind a positive lens to lengthen the f.l. and coveage. A short lens (ie. 40-50 mm) has a large diopter value so you have alot of options with negative lenses before the net diopter sum goes negative.
Long lenses have small diopter values (1 m = 1 diopter) so you have hardly any off-the-shelf negative options.
I pulled a Tessar out of an Olympus AF 35mm camera, 38 mm I think, and put (-) elements behind it but haven't exposed any film yet, just looked at ground glass (it's cheap and I'm easily amused).
I use the Gullstrand's Equation calculator on the Hyperphysics (Georgia) site to e.s.t.i.m.a.t.e. what happens as you vary spacing.
I just finished my Abominatar lens for a camera obscura, theoretical f.l. 8 m (8000 mm), but I have no idea where to measure it from. I have to mount it because it's really shaky trying go hold it and focus a light bulb across the room. I suspect coverage will be narrow because it's very long for it's diameter (roughly 2m long, 40 + 70 mm glass diameter)
Last edited by Murray@uptowngallery; 11-12-2007 at 04:58 AM. Click to view previous post history.
yikes! and thanks! what a thoughtful, in-depth response. (not to belittle the other, thoughtful responses...)
first, i should be clear that i am NOT trying to replicate anything of super high quality. i don't plan on building a Super Angulon from cardboard and a magnifying glass. i'm looking for 'interpretive' lenses here, with personality. but not so much personality that i'm forced into abstract photography against my will.
so i guess i need to get experimenting! mounting these is the part that takes all the time though, which is why was searching around for either designs or rules of thumb, so i didn't waste time putting four negative elements and a goldfish into a tube and finding out it didn't work.
i've downloaded the siskin article and read it once. thanks! i'll refer back to your post as i muck around with this. i've also bought a (really bad, wavy) magnifying glass, and 'borrowed' my the front elements from my son's cheap telescope to play with.
anyone have ideas for a faster (and/or more secure) mounting technique than the one described in Primitive Photography?
I've found that using tin cans works well (seriously). I cut a slot for waterhouse stops and can mount the elements securely with a slight variation on the foam core idea that Greene presents in his book - most cans have ridges that work to keep the elements from shifting. I've used coffee cans as well as the smaller ones from canned vegetables - all very scientific of course