If your S2 images aren’t as sharp as you think they should be, the reason may be that the CCD is not at the right distance from your camera's lens. The CCD must be at the same optical distance from the lens as the film plane to deliver sharp focus. Why isn’t the CCD placement automatically perfect when you snap the S2 onto your camera? Because not all cameras have the same lens to filmplane distance. Why? Because each camera manufacturer has a tolerance for deviation from perfect. The reason for this, in turn, is that each piece of the assembly (lens, body and film back) is itself previously assembled. When you’re making parts that bolt together, you do your best to make each piece the exact same size, something that can't be done perfectly. When you bolt all the stuff together and assemble the individual parts to make a complete camera, dimensions aren't precisely uniform from one camera to another. In addition, some of the camera bodies have adjustable lens-to-filmplane (and lens-to-groundglass) distances which go out of adjustment. So how come it’s not a problem when you shoot film? Because the percentage of enlargement from the full frame film capture is much lower than the percentage of enlargement from the same capture from the CCD area. If we use the 6x6 cm film size for an example, the film capture has 4 times the image area of the 3x3 cm CCD sensor we use to make the S2. There is another factor to consider, and that is that the focus isn’t derived from focussing the filmplane; it’s derived from focussing the groundglass, which can be a different distance from the lens than the filmplane. If this potential conundrum has you interested, read on.
The first order of business is to check your camera's focus. Generally, the person who installs your camera will perform this test. If, however, the focus validation was missed, it's easy to perform on your own. The standard procedure is to focus on a resolution-intensive target some 20 feet away, using a wide open aperture (we don’t want depth of field to play a part in the visual perception of sharp focus). Note where the groundglass image focuses best and note the mark on the distance scale of your lens, shoot, process, and save that image as a TIFF with the filename "Focus Zero". Focus, shoot, and save two more images, using the distance scale. Focus first a little closer than the best focus mark (save this one as "Focus Minus"), and another slightly farther than the best focus mark ("Focus Plus"). Compare the developed MEGA files for focus. Open all three files simultaneously in Photoshop for convenient critical evaluation.
If the Focus Zero file is the sharpest image, you are finished. Otherwise, some shimming is required.
Before you start, examine the images that accompany this topic; you can see what’s involved and decide if this shimming procedure is within your mechanical ability. If your skills don’t warrant this type of hands-on relationship with the innards of your camera, you can send the camera body, lens, and digital back to us at the factory. We’re saying this because we cannot assume the responsibility of replacing your chip if you completely destroy your camera using the instructions included in this topic. Carefully check out what’s involved here. It’s only 4 screws (Hasselblads have 8) and some thin metal plates to be added or subtracted. If you go slow and be careful, you can easily do it. If there is any doubt in your mind, send in your equipment and we’ll do it for you.
MegaVision’s S2 design allows for the movement of the CCD relative to the lens: you can change the lens-to-filmplane (siliconplane, actually) distance easily, so that sharp focus on the groundglass results in sharp focus on the CCD. Each S2 back is shimmed to the nominal manufacturer’s spec at the factory, but may require adjustment, for the reasons discussed above.
You’ll need a small phillips tipped screwdriver. If we take off the S2 mounting plate we want to be extra careful not to lose any of the screws (set them inside a large lenscap so they won’t roll away) and we want to perform the shim work on a clean, uncluttered surface with plenty of good light.
When you remove the screws and separate the interface plate from the S2 back you’ll find one or more paper thin plates of steel. These are the shims. Read through the preceding focus validation procedure. If the shim adjustment process seems too complicated, you may send your camera body, lens, and S2 back to the factory for adjustment. You should however, do the simple focus test first to make sure of your camera’s focus before you start.
If the closer focus image, Focus Plus (that means the lens moved away from the filmplane) is sharper than the best focus point, move the chip away from the lens using a thicker shim structure than the currently installed shim setup (find a shim in the lid of the S2 shipping case, or check with the factory, we’ll put one in the mail to you). We'll move the chip farther from the lens because the closer focus features an increased lens-to-filmplane distance. If the farther focus image, Focus Minus, looks sharper, use a smaller lens-to-filmplane distance by using thinner shim structure; try removing one of the thin shims. Since we're shimming only the S2 back, the focus for film will not be affected. However, you'll want to know if your camera needs adjustment or if the S2 back needs adjustment. We can easily make sure the S2 focusses at the same distance as the groundglass in your camera, but you might want to have your camera body checked if the focus needs more than minor movement with shims to be sharp.
We want to also mention that we’ve seen some wide angle lenses render soft images at wide open apertures. We believe this to be a result of older retrofocus designs diffracting through the protective glass surfaces of the CCD sensor. The newer, more modern wide angles look sharper; aspheric elements, internal focussing, extra-low dispersion glass, and aprochromatic designs certainly have a solid presence in digital photography. Since we’re using a smaller image area than film we can use all the technology we can get to make sharp pictures.