How to Use “Vlads Test Target” for Camera Scanning
- the film holder is absolutely parallel to the camera's sensor plane (and keeps film absolutely flat) and
- you need a lens which is capable of faithfully reproducing your image at 1:1 scale ( or around that value depending on the camera sensor size and film format)
The first requirements can be
achieved by carefully positioning and fine-tuning the camera so that when the
image to be captured is in focus, it neatly fills the camera frame, and the
center and all the corners are equally sharp. The "Vlads Test Target" (Google
it!) has been designed specifically to
allow visual control and the best alignment detection. The Vlads Test Target film
strips are so sharp that even the slightest loss of focus causes image edges in
the digital camera to become purple or green clearly signaling sub-optimal
scanning setup configuration.
The second requirement concerns the quality of the lens. The lens should ideally be a true macro lens. In this context that means the lens with very good flat-field performance. While other lenses may be serviceable, they may not be achieving the edge-to-edge sharpness and the freedom from distortion the really good macro lens provides. "Vlads Test Target" has been designed to facilitate comparative lens testing and allows for quantitative analysis using USAF 1951 approach. The transparent target is produced on the real highest resolving 35 mm film currently available on the market with ten (10) USAF 1951 Resolution Charts strategically placed across the frame so the resolving power of a lens can be easily quantified and lenses can be compared using techniques well developed by the US Air Force for its spy planes. For type 120 film the Siemens stars and USAF 1951 are combined for the best focusing experience.
Suggested Camera Scanning Workflow
Prepare
your scanning rig - whether it's camera on a copy stand or the bellows with the
vintage film holder from Nikon, Canon, Minolta, etc. I will assume that camera
and lens are able to slide forth and back in relation to the film holder.
Ideally, if you want to maintain constant magnification ratio, you should be using
micro-focusing rail to move camera and lens, rather than focus with the lens.
Keep
the film gate empty. Adjust backing lights for uniformity, mask out all stray
light, optionally tether your camera to the computer and connect your camera’s
HDMI output to TV. Set white balance and exposure for empty gate. Make sure your picture of empty gate imported
in Photoshop or similar application reads RGB (254,254,254) across the frame
and clipping does not occur. Details of that setup are beyond the scope of this
write-up.
Leave
camera in the manual mode so no settings will be changed by the camera itself.
Switch off lens autofocus if any. If any steps are required to make lens axis
be strictly perpendicular to the plane of film – do it now. There is a number of
ways to employ a mirror placed in the film gate to achieve the desired result –
that’s beyond the scope of this write-up.
Set
your lens to the desired magnification factor - move camera and rotate lens
until the image of the frame fills the viewfinder. Once the required scale is
achieved leave the lens alone. From now on you may need to set your F-stop, but
don't use lens to focus - the scale factor will immediately change.
Now
comes the critical part. Insert “Vlads test target” 35 mm or type 120 film in
the film holder and center the film frame in the holder - plan to be able to
place your originals exactly in the same position later on.
Ideally
your camera HDMI output should be connected to your big screen TV or hi-res
computer monitor. Review the image captured
by camera in the live view mode. Using micro-focusing rail, start focusing
on the pattern. Siemens stars are extremely sensitive elements of the design.
If focus yet to be achieved, the internal part of Siemens star looks like a
gray disk. Once camera gets close to the focused state the size of the disk
becomes smaller and smaller and more and more individual strokes will became
discernable. On your TV you will be able to see all Siemens Stars at once
with the decent resolution. You may notice so called moiré effect: this visual
perception occurs when a fine pattern on your subject (Siemens star or USAF pattern)
meshes with the pattern on the imaging chip of your camera, and you see a
third separate pattern. This is actually very good indicator that means that
tiniest parts of the image are in focus. Note that moving camera forth and back will make
all Siemens star to become more or less gray - ideally all of them should reach
the smallest gray disk state simultaneously. If that happens - bingo! - you are
done and all the image is in focus. Set the desired f-stop and fire the
shutter. Review the picture on the screen in your photo processing software and
evaluate Siemens stars and USAF patterns - they all should look very much alike
across the frame. If your camera has focus peaking - all the better - that will
help tremendously as indication of achieved focus will be much more pronounced.
Unfortunately,
it's almost never happens - alignment from the first sight - you are bound to
spend another 15 minutes tweaking the holder position, checking your camera
alignment with the rail axis, tightening all the clamps and so on. But with all
of that you have pattern which will be giving you cues on what is going on,
what direction the holder or camera is misaligned. It can be just one corner
and you know by moving camera on the rail forth and back which corner needs
correction.
Now, when focus is achieved you may start making test shots with the different F-stops. Take the highest resolution images, turn off in-camera sharpening or use RAW files. Review your images on the computer screen (this is where the tethering is of a great help). Now you will be able to take advantage of another feature of Vlads Test Target – USAF 1951 patterns. Take the following approach (that's how the pattern is intended to be used). Set certain magnification on your computer display -say 400%- and review centers and all four corners of each image and find the smallest element for which you can discern both horizontal and vertical bars in USAF 1951 patterns. The camera is said to “resolve” a chart element, if the horizontal and the vertical bars can still be recognized as three distinct bars und don’t blur into one another ( see more here). Write the Group and Element numbers down on a piece of paper along with the F-stop number. For each image you will have six values: F-stop, and Group/Element number for the center and four corners.
At the end you will have the table
with six columns and few rows (for the number of F stops tested). Looking at
this table alone you will be able to objectively call a winning F stop. Center
and corner will give you different readings - that's normal - and that allows
you to make informed decision on overall resolution you can achieve across the
frame. Basically, you may prefer more uniform if lower resolution across the
frame rather the one in which center is significantly sharper then corners.
Some lenses may exhibit the focus shift while changing F-stop value – in that
case you may need to close lens all the way to desired F-stop, potentially
increase the brightness of your light source to compensate for the loss of
light and repeat the focusing exercise. I know this does not sound like much
fun, but the whole exercise is about getting as much image details from the
film original as possible. So, the ultimate reward for your hard work will be
the ultimate scan quality.
Please share your pictures, your
techniques and your overall workflow with other camera-scanning enthusiasts. If
you have any inclination to discuss the subject, you may want to join the vibrant
community of digitizers at private Facebook groups: “Digitizing film with a
digital camera” and “Digital Film Scan Tools”.
All the best!
Sincerely,
Vlad
of “Vlads Test Target”
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Is there a measurement scale in the target, or do any of the items have an actual size? I'm trying to set the scale in PS, but I need to know the size of some element in the target.
ReplyDeleteThe outer size of the target is 24x36 inches in real life. Which exact measurement you need? Thanks!
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