Make a center filter using photoshopgreenspun.com : LUSENET : Large format photography : One Thread
Another fancy idea. I wonder if there is a tool in photoshop allow you to draw a gray circle then fade into a white backgroud, and then print it on transparency to achieve same CF effect.
-- Aaron Rocky (firstname.lastname@example.org), February 22, 2001
Yes it is possible, but I can`t explain because of my bad english! But if you go to a professional lab and digital service they can do it for you or explain to you how it is to do if they are really professionels! I only know 2 version thad works, if you version also works has to be testet, but it is not the fastest and easiest!
-- Armin Seeholzer (email@example.com), February 22, 2001.
you can find a brief description of one possibility under http://www.luminous-landscape.com/panorami.htm However, Radial Gradients in Photoshop will not allow an exact simulation of the cos4-falloff
-- Thilo Schmid (firstname.lastname@example.org), February 22, 2001.
Create a gray circle and then fade the edges using the gaussian blur filter. By changing the pixel blur you can adjust the amount of "fade" you achieve.
-- William Levitt (email@example.com), February 22, 2001.
If the purpose is to overcome light falloff, what about shooting a grey card, then scanning and then changing the brightness of the image in Photoshop? If you convert the image to a negative (from whatever the input was), wouldn't you would end up with a perfect correction pattern for that lens?
-- Dave Mueller (firstname.lastname@example.org), February 22, 2001.
I thought of this too. Looking at a center filter, there is no visible image structure: no grain at all, even under a microscope. Almost as if Schneider used black glass and melted it in. I have no idea how they did it. Making one will have the usual grain of the printer. This will absorb just fine, but will also scatter some light. There is also the problem of any thickness variations of the film, it is not an optically ground plane parallel. I suspect you will lose contrast from the scattering, and sharpness from the poor optical surfaces. An easy first experiment: try a shot through a grey transparency, and see what it does to the image. I'll be interested in the results. If it works, then work on the radial gradient tool.
-- Jim Galvin (email@example.com), February 22, 2001.
I've tried this: made a target of several radial gradients of different degrees of gradation, copied onto 8x10" film to produce a 3x3" image that I could use in a filter holder, and couldn't get one that was satisfactory. The contrast was usually too great or the gradation on film uneven. Maybe I just need a better printer or need to experiment more, but in any case, it will take a number of attempts. I also tried defocusing slightly to smooth out the printer matrix and any banding, but that seemed to make things more uneven. If anyone does it successfully, I'd be very interested to know.
I might try just taking an image of an evenly illuminated surface with the lens I want to correct, producing a negative with the right amount of falloff, then copying the negative in the right scale to produce a 3x3" filter on film.
I think the way glass center filters are made is by grinding a neutral density filter to produce an even amount of falloff correction then bonding it to a piece of glass ground in the opposite way to produce an optically neutral filter.
-- David Goldfarb (firstname.lastname@example.org), February 22, 2001.
David's idea is even better. The light transparency adapter for flatbed scanner is quite even. Hold it against your lens front so you can custom make CF for different lens. Exciting!
-- Aaron Rocky (email@example.com), February 22, 2001.
Some experiments last night: I have negatives from zone calibration with large areas of grey card. With a single lamp across the room, I put the negatives in front of 6x30 binoculars, to get an aperature and magnification in the range of a photo. The effects I saw were also visible with the naked eye. With a tri-X neg, the lamp is sharp, but a diffuse glow obscures shadows. The glow is much less with a Portra neg, but you are stuck with an orange filter. This is scattering or diffraction. With film from a Linotronic printer, I get multiple images, diffraction from the halftone pattern. I tried printing grey from an Epson 1270 on Tektronix Phaser transparency film, a very clear film that doesn't hurt the image that I could see. The ink is still wet and running this morning. I examined my Schneider IIIa CF, took it out of its cell. I can see no seam at the edge, if this is two pieces of glass cemented together I can't see it. I see the reflection from the back is dim in the middle, bright at the edge. I think the filter is a coating. I don't know why it is 1/4 inch thick. The only hope I can see is to put the homemade filter in the film holder in contact with the film. But if you use front motions or shifts, you have to put it in off center. I use the Schneider CF on a 90/6.8 Grandagon and also with a stepup ring on a 65/8 SA, it works fine, as opposed to older threads saying never use a stepup ring, and use only on the lens it was made for.
-- Jim Galvin (firstname.lastname@example.org), February 23, 2001.
re: Centering filters- There is a standard lens formula (based if I remember correctly on the Square Root of 5-I could look it up), that predicts light fall off based on the angle off center-this is why if affects wide angle lenses more than normal focal length.
To make a centering filter on a computer/inkJet printter for use in the camera I don't think will work. The transparency film that is available has a surface for the ink to stick to that is about as blurry as the frosted back of a tansparency sleeve. Also there is the dot pattern--which even on a clear substrate would act as a diffuser. (Cinematographers have available a very nice diffusser that is made from black particles but is very expensive--softens without the glow)
What you can do on a computer is to make a mask for use in an enlarger. View Camera Mag describes methods to make masks for B&W printing. I've done this and they work-though they are time consuming. You have to saandwich the elements from the Light source to the lens as follows. On top the mask(transluent inkjet film), than 1/16th inch translucent plex then the neg, Lining up even the tightest masks is not at all difficult. A Centering filter made from a scanned camera neg (focused at infinity) of a blank surface should be fairly simple-though I'm betting you could just make a 1" circle in the middle of a 4x5 and then guassian blur the whole image. (Should probably bleed out to white at 3" diameter)
I own two centering filters. One designed for my Rodenstock 65mm and another Schneider IV? that I got with my used equipment. The 65mm centering filter works fine on the 90mm/f8 and the larger Schneider works on a 120mm lens, I tape them to the front of the lens.
I've learned that anything close will work just fine even for critical work and that Centering filters do make a significant improvement (architectural stuff/white walls).
-- William Nettles (email@example.com), February 25, 2001.
Hey, I'm new here so don't laugh!
Would it be possible to make a center spot filter on a glass negative in the old fashioned way by photographing a grad' grey circle on a black background?
-- Clive Kenyon (firstname.lastname@example.org), June 30, 2001.
Clive, your idea is not bad, but I think the layer of gelatin will act as a softening and diffusing filter and produce some interesting effects. Maybe it would be worth trying the other way around (clear center), this would perhaps produce an interesting portrait and still life filter! Just a thought... never tried. The best way of going around a center filter is by using a Photoshop technique after the scan, but a center filter, when needed, is a worthy investment. They not only produce an even illumination of the film, but they also protect the lens, from the stray light and parasite reflexions that occur inside the lens with very wide angles in bright light.
-- Paul Schilliger (email@example.com), June 30, 2001.
Paul, I take your point - but if you can project light through a negative to give sharp results on an enlarger, then why will it not work on a camera?
Answers on a postcard please.
-- Clive Kenyon (firstname.lastname@example.org), July 01, 2001.
Clive, industrial films have extremely even and accurate layers and even so, they would produce a diffusion effect if put in front of a taking lens. Older enlargers were equipped with sets of optical condensers to direct the light and minimize the diffusion effect and produce sharp images. The paper's contrast will also reduce the diffusion effect. I am not aware of any modern standard glass plate films available on the market, so making them yourself by painting a sheet of glass with photosensible emulsion would be far from the standards of a good optical filter, without underminding your skills! But as I said earlier, it could be worth a trial. Why don't you shoot a grey card on a B&W negative, use the neg as a neutral density filter and report here of your findings? (Was that too long?)
-- Paul Schilliger (email@example.com), July 02, 2001.
T-max 100 4x5 glass plates are available at around $12.00 each.
-- Sal Santamaura (firstname.lastname@example.org), July 02, 2001.
Well, that's me done folks. I supplied the idea - it's up to you now!
-- Clive Kenyon (email@example.com), July 02, 2001.