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My Flickr - Photographs - Animation - Main Site Saturday 16-May-2009 | |||
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Please visit my new site Depth of Field adapter tutorials, articles and parts. |
I will leave this site up for reference. | ||
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35mm Adapters Self Build |
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Introduction Small sensor cameras such as the HV20 or FX1 have relatively small imaging sensors (around 8mm). And subsequently have a large depth of field, meaning that the range of distance, from the lens, that an object would appear in focus is very large. For example if you had a dialog between two actors both would have apparent sharpness at all times, unless you are using various techniques to narrow your DOF. On the other hand a camera using a 35mm frame has a much narrower DOF due to the fact that DOF is inversely proportional to sensor size. Using the same dialog example, with a 35mm frame, you would be able to selectively focus between the two actors, allowing you to draw the attention of the viewer, as well as lose distractions from the background. Looking at this example quantitively we can see that with a 35mm imager using a 50mm lens at f4 with the subject at 5 feet, the apparent DOF will be 0.6ft, small enough to selectively focus between the two actors as well as isolate them from the background. A similar example with a camera like the FX1 with the lens set at 9mm, giving an equivalent field of view, would give a DOF of 21ft. With this setup everything in the shot would be in focus. To have greater controll over selective focus we need to make our consumer camera have the properties of a 35mm frame. We do this with a 35mm Adapter. |
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A DOF adapter lets us fix manual 35mm photography lenses to our video camera while still keeping the characteristics that they would have on a 35mm frame. You cannot just mount a 35mm frame lens on a HD camcorder, as it have no effect on apparent depth of field. A 35mm adapter has optics inside which keep the characteristics the same on a camcorder and this is why they are neccessary. Put very simply using a 35mm adapter is like filming through the viewfinder of an SLR. The image from the 35mm lens is projected onto a diffusive ground glass element, which acts as a rear projection screen. The camera is then used to film this screen giving us a final image with the look of 35mm film. (See diagram). The GG can be made of plastic or glass, some very new screens are being made of bundles parrallel optical fibres. Unfortunately it is not quite as simple as this, due to the angle at which some of the light from the SLR lens hits the edges of the GG element the image will be slightly darker at the edges than the centre, this is called a vignette, or hotspotting. A condenser lens is used to solve this problem. It is placed either in front or behind the GG. |
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Also some cameras have a minimum focusing distance that is too long to allow them to fill the frame with the GG. In this case an achromat is needed. The achromatic lens allows the camera to zoom into the GG easily, as well as helping to minimise some problems caused by the lens working in extreme macro; such as barrel distortion or chromatic aberration. Another complication is that the ground glass usually has a visible pattern, giving the image a grain, which becomes even more evident when the SLR lens is stopped down. Many people use static adapters with the canon EES or EEA screens due to their small grain size, but dust and dirt can easily be atracted to the screen and ruin the image. There are two different solutions to these problems. Either you can make a round GG and rotate it at speed. Or you can vibrate a rectangular 35mm GG. |
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Types of mounting There are two ways of vibrating a 35mm GG; the most common is to mount the GG on a platform supported by thin flexable legs. A small vibration motor is also mounted on the platform causes the entire surface to oscilate. In my experience the pancake style motors are the most effective and i would particularly reccomend this one from solarbotic. The other main method used for vibrating the GG element is the leaf spring technique. This is an uncommon DIY method since it is hard to machine without expensive materials and tools; but you can purchase them ready made from www.jetsetmodels.info. The focusing screen is suspended between 3 plastic leaf springs which compress as the motor oscilates the GG. In this picture I show a simple "leg" style GG holder and a spinning GG with a small DC motor. The top picture shows a cardboard mock-up produced before the finalised plastic version. I made my GG holder using CADCAM, it is made out of 2mm thick laser cut plastic, and is supported by carbon fibre pins. I would recomend using a matte black acrylic for the GG holder to minimise internal reflections in your adapter. Also other plastics i used tended to bend when cut into thin sections; due to the heat of the laser. You will probably be able to find a local business which offers laser cutting services, but if you would preffer to buy one ready made, you can contact jetsetmodels or Greg Tay. For the legs i have used carbon fibre rods. These can be found on ebay, they are used for building RC vehicles and should cost around £3 for a large pack. Rods with diameters of less than 1mm should be used, 0.5mm is ideal. They are very flexible and strong and you don't have to be particularly careful to avoid breaking them. The thinner the better. For the part of the holder which houses the ground glass, i have used a clamping technique to hold the glass in the holder firmly. I have cut out two identical peices and sanwidged an oversized GG between them. To replace the GG i just loosen the bolts and slide in a different screen. only the lower peice of the GG housing is glued to the carbon fibre legs, this allows the top peice to come loose for changing GGs. The nuts are glued to the plastic so that they do not spin when the bolts are loosened. I shortened the bolts especially for this project. I have used JB weld to glue all the peices together. To set the focal flange correctly I fixed the holder into the pentax extension tube, and used a jig (cut using the laser cutter) which was basicallly a box with the same height as the pentax k focal flange. The GG is ballanced on the box and the holder is positioned and fixed around it so the finall GG placement is correct and the GG is parrallel to the lens mount. In the picture of the mount on the right, i show it before i have remove the excess carbon fibre. |
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Gound Glass There are a lot of different options for what GG you can use in a DIY adapter but i am going to concentrate on one technique. I feel that Grinding your own focusing screen using glass and grinding compounds; such as aluminium oxide is one of the best options for DIY adapters. The screens produced are very resiliant and can be washed by hand with soap and water; unlike the canon screens that are ruined if touched. They are very sharp, and you can choose different grades of aluminium oxide to get different levels of diffusion. For vibrating adapters I would recomend the use of a microscope slide and for spinner adapters i would use the glass from large UV filters (72mm minimum). A microscope slide will be able to produce 2 36x25mm GGs. Both Square and round screens are ground in the same way. To grind your screen you need a larger peice of glass to grind against, such as a sheet from a picture frame. Put a pinch of aluminium oxide on the large sheet and make it into a paste by adding a drop of water. Place the glass to be ground on top and move it around the surface of the glass in large figures of 8. If there is too much resistance add a drop of water, but very little powder and water is needed to properly grind a screen. To protect the opposite side of the glass from being scratched cover it with masking tape. After 5-10 minutes of grinding, wash the screen thoroughly, repeat for another 5 minutes if necessary. Remove the tape and clean the GG, drying it with paper towels to avoid streaking while air drying. It will take a few tries before you get a good screen, and sometimes it is very difficult to tell by eye wheteher they have been ground properly. There are many different grades of aluminium oxide available. Commonly used are 5, 9, 15 and 25 micron, meaning that the individual particles of aluminium oxide are X microns across. The higher the number the higher the difusion produced by the GG. A 5 micron screen will be the sharpest, but more prone to hotspotting. A 25 micron screen will appear slightly softer, and loose more light, but will be less likely to hotspot. Some people think that the screens with more diffusion have a more "filmic image". |
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Condensers Condensers are used to reduce the hotspotting effect caused by light at the edges of the frame hitting the ground glass at a large angle of incidence. Daniel has a comprehensive condenser tutorial on his website. I purchased quite a few lenses from surplusshed, and found that there are quite a lot of differences between lenses. Some are marked and scuffed so it does pay to test out a few different types. You want to avoid marking you condenser lens; since it is very close to your focus screen blemishes will affect the image to a larger degree than marks on the achromat. You should mount your condenser in the EOS tubes using a lens cloth to position it with the flatter side facing the GG (inbetween the achro and GG). You can place it between the SLR lens and the GG but at the moment it seems to work fine for me in this configuration. I will perform some tests to check which setup produces the better image and report back later. I am selling condensers, for use with canon EOS tubes and to be used in conjunction with my ground glass elements. They are perfectly sized to fit in the canon EOS tubes and are checked to ensure there is no heavey damage to the glass. There is more information here: http://www.hv20.com/showthread.php?t=26297
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Electronics At the moment my adapter just uses a simple battery -> rheostat -> vibration motor setup. I have also added a resistor and LED in parrallel to show when the adapter is switched on. The rheostat is used to tune the motor speed for optimum GG movement. "In physics, resonance is the tendency of a system to oscillate at maximum amplitude at certain frequencies, known as the system's resonance frequencies (or resonant frequencies). At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system stores vibrational energy. " You want your rheostat to be set so that the frequency at which the motor oscilates is aproximately equivalent to the resonant frequency of you GG holder. You can do this very easily by slowly reducing the resistance untill the motor is causing the largest possible oscillations of the GG holder. This is your resonant point and you should periodically recheck this as it can change as your battery loses charge. My next planned improvement to my rig is to replace the disposable batteries with an internal rechargable design. With a port to charge the adapter using a simple DC transformer. |
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Here is a quick video testing the first version of my adapter with a 5 micron GG: http://stevenbrace.co.uk/dof/hd%20deit%20dof.mpg http://www.youtube.com/watch?v=p8IIPRhYdUg&fmt=22
Contact me if you need to know anything, email is at the bottom of the page, just replace the (AT). |
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| Under Construction - more to come | |||
| © Steven Brace Contact me : steven (AT) stevenbrace.co.uk | |||
