High resolution film transparency digitalization using macro lens and stitching - Revision history

Torger: /* Post-processing */

2012-02-20T19:09:40Z

‎Post-processing

Torger: /* Color management */

2012-02-20T19:07:01Z

‎Color management

Torger: /* Pre-processing */

2012-02-20T19:06:05Z

‎Pre-processing

Torger: /* Shooting tips */

2012-02-20T19:04:24Z

‎Shooting tips

Torger: /* Introduction */

2012-02-20T18:42:11Z

‎Introduction

Torger: /* Hugin stitching tips */

2012-02-05T15:54:07Z

‎Hugin stitching tips

Torger: /* Pre-processing */

2012-02-01T21:06:17Z

‎Pre-processing

Torger: /* Hugin stitching tips */

2012-02-01T21:02:24Z

‎Hugin stitching tips

Torger: /* Hugin stitching tips */

2012-01-31T20:10:33Z

‎Hugin stitching tips

Torger: /* Hugin stitching tips */

2012-01-31T20:07:29Z

‎Hugin stitching tips

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 more affordable ones. If one wants to, one can make the dust healing on
 a separate layer to avoid modifying the original.
 ==HDR==

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 Light tables usually have cold cathode lamps which has good but not
-perfect color rendering index (CRI), I consider it good enough for
+perfect color rendering index (CRI), I consider it good enough though
-professional use though (drum scanners do have halogen lamps that have
+(drum scanners do have halogen lamps that have CRI 100 which is
-CRI 100 which is required for 100% perfect results). The light from the
+required for 100% perfect results). The light from the light table is
-light table is similar to daylight spectrum (will look blueish in an
+similar to daylight spectrum (will look blueish in an indoor setting),
-indoor setting), but still without calibration the colors in your shots
+but still without calibration the colors in your shots will not be
-will not be correct. Either you play around with white balance etc
+correct. Either you play around with white balance etc until you get
-until you get a pleasing look, but to get a good consistent result you
+a pleasing look, but to get a good consistent result you really need
-really need to calibrate your system.
+to calibrate your system.
 For this you need to get an IT-8 transparency and get the associated
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 model or light table lamps (lamps can vary a bit) you need to make a
 new profile.
 ==Hugin stitching tips==

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 correction for that was unnecessary. Hugin can also correct for
 vignetting (not a part of layout optimization, so it won't disturb), so
-it is a matter of taste were you correct it. Corrected in pre-
+it is a matter of taste where you correct it. Corrected in pre-
 processing you save some time in stitching, then you can skip the
 exposure-tab alltogether.

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 macro lenses usually do.
-At full-frame f/8 the depth of field is about 0.6mm. This is small, and
+At full-frame f/8 the depth of field is about 0.6mm. This is small
-one of the major challenges doing this is to keep the film within the
+(much larger than in a flatbed scanner though), and one of the major
-DOF.
+challenges doing this is to keep the film within the DOF.
 If you have mounted slides, film flatness is usually not a problem if
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 and film which is necessary to avoid newton rings.
-If you have unmounted slides, then precisely cut out a frame in
+For 35mm film film curl is rarely a problem even if not mounted in a
-cardboard, tape the slide to one side (not all sides, almost impossible
+slide frame. Then I precisely cut out a frame in cardboard and tape
-to make perfect, the slide film will curl, at least if 6x6 medium
+the film to it, it it will then be flat. This cardboard frame will
-format). This cardboard frame is to give some space to the light table,
+also give some space to the light table, and avoid light leaks on the
-and avoid light leaks on the side of the film. All light must come from
+side of the film. All light must come from the back of the film or
-the back of the film or else you will get contrast problems.
+else you will get contrast problems.
-Curl is a problem with larger slides (medium format), but one can
+For medium format film curl becomes a real problem. Taping to a frame
-fortunately make use of the fact that only a smaller part of the film
+may work for 645, but already at 6x6 it becomes truly difficult to
-is shot at a time. You can thus make an overlay from a smooth flat hard
+have it flat. One option is to cut out two frames in a plastic material
-material with a 30x45 cutout or so which you lay on top that press down
+(fibre free) which have about 40x30mm opening and sandwich the film
-the film, and you move the light opening to fit where you currently
+between those and move around. By only exposing a window of the film
-shoot. Put a lens shade or similar on top of the cutout to weigh it
+at a time the film is held flat.
-down and to avoid stray light from the sides. Instead of a cutout you
-can have two plastic cards or similar put on top of the film leaving a
-gap for the camera.
-If you are going to digitize many unmounted slides, make a quick and
+Another option is wet mounting, I have not tried that myself but may be
-effective reusable frame mount, since one otherwise may end up spending
+worth investigating. Then you sandwich the film between glass fluid and
-most time mounting the film.
+a thin transparent film.
 When film is mounted as flat as possible, focusing is the next
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 rail, lock it down and the check the corners with zoomed in live view
 and adjust with the levelling head if necessary.
 You will notice that at f/2.8, which is what you'll probably focus in,
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 For APS-C and shooting 35mm when the film area covered is smaller and
-film curl usually less of a problem you may attempt f/5.6. Actually
+film curl or lack of perpendicularity is usually less of a problem you
-you'd probably want to, since the small pixel cameras start to loose
+may attempt f/5.6. Actually you'd probably want to, since the small
-significant resolution due to diffraction if used at f/8.
+pixel cameras start to lose significant resolution due to diffraction
 Then there is the dust problem. A transparency shot this way will never
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 dust spots as anchors for control points :-).
-Use a dust blower, a soft brush and possibly burn candles close to the
+Use a dust blower, a soft brush and possibly burn candles or run a
-light table to cause an upward air flow to keep away as much dust as
+vacuum cleaner close to the light table to cause an upward air flow
-possible.
+to keep away as much dust as possible. Make sure humidity is not too
 Watch out for vibrations. Mirror up (typically shooting in live view
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 Shoot RAW in manual mode and adjust exposure as close to clipping as
 possible, but do not clip! Rather use some safety margin than clip.
 ==Pre-processing==

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 range. The best is still drum scanning but these are becoming even
 harder to come by and of course extremely expensive to buy new.
 So what about using a macro lens, a digital 35mm camera and a light
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 usually crop the borders some).
-I recommend a 16+ megapixel APS-C camera for 35mm transparencies and a
+I recommend a 12+ megapixel APS-C camera for 35mm transparencies and a
 + megapixel full-frame camera for medium format. At 1:1 macro
 distance (near limit for most macro lenses) the lens covers the same
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 may be a better solution.
-Why mess around with stitching at all? To get professional level
+Why mess around with stitching at all? To get higher resolution so you
-resolution so you can make large high quality prints! This tutorial is
+can make large high quality prints! However, how much resolution you
-not about getting "ok" results (then just use a flatbed, or shoot the
+need will depend on the situation, so in some cases you can shoot the
-whole frame at once without stitching), this is about getting very high
+whole frame at once. Fine-grained film has a native resolution of about
-quality, and really extract the detail that is on fine-grained film.
+ppi, which corresponds to about 21 megapixels on 36x24mm, this is
 ==Equipment==

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 ==Hugin stitching tips==
-TBD. Theoretically, the stitching part should be straightforward. Unfortunately various micro errors can make it a bit difficult.
+Theoretically, the stitching part should be straightforward. Unfortunately various micro errors can make it a bit difficult.
 * different roll on the images after moving the film on the light table
 * very slight yaw/pitch errors due to non-perfect perpendicularity
 * film curl variations after moving
-* lens distortions
+* lens distortions which does not 100% match lens distortion model in Hugin
-General guide so far is however
+Just like your shots never will be 100% dust free, there will not be a 100% perfect stitch. It will be good, but not 100% perfect.
-* Add auto control points with Hugin CPFind
+However, after cropping away the film border the result should be very good. The amount of picture area you lose due to
 * Add one vertical or horizontal guide on only one image to indicate orientation
-** Adding on several images seems to disturb optimization quite often
+* Goto the optimizer tab, here we work custom so select "the Custom parameters below"
-* Optimize custom parameters
+** The general idea is to optimize the large movements first to give Hugin a good starting point and then add on more minor parameters and re-optimize
-** First X and Y on all but one
+** First opitmize X and Y on all but one
-** Then add roll
+** Open GL preview and check the layout - perhaps we need to add some more control points? Blue skies may need help.
-** Then add barrel
+*** It's easier to see the layout when a rough optimization has been made first, that's why we saved this until now.
-** Clean any poor control points, re-optimize
+*** Pictures with very small overlap (i e corner overlap between top-left and bottom-right picture) may be better to not have and CPs between them, so it gives Hugin more freedom to adjust where fit matters
-** Accept non-perfect rotation and overdone barrel correction as long as it reduces stitching error
+*** Generally Hugin CP find has done a good job so no more points are necessary
-** If desperate, you may try include yaw/pitch correction, with a hack - specify a very small FOV, 1 degree, then yaw/pitch corrections will probably be milder.
+** Then add roll to all, and re-optimize
 * Open up GL preview
 ** Choose rectilinear projection
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 ** Fit/autocrop the view
 * If lens does not vignette much, exposure tab can be skipped
-* Stitch to 16 bit tiff and rotate barrel correct the output if necessary
+* Goto the stiching tab
-To handle the micro errors in the best possible way in this type of photo setup, Hugin should probably support rubber sheeting to correct the small residual errors. PTStitcher has a "morph-to-fit" feature which may be useful (not tested by me at the time of writing).
+If you seem to have an impossible case, try to reshoot and be very careful with film flatness and perpendicularity. You can also try to have a bit more overlap and shoot more pictures, say 8 fullframe pictures of a 6x6 medium format slide instead of 6 (which is the minimal possible). Large overlaps usually give better stitches, but on the other hand consumes more shooting time.
 ==Post-processing==

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 * chromatic abberation
-* barrel/pincushion distortion
+* (barrel/pincushion distortion)
 * (vignetting)
 * color reproduction (apply icc profile, see color management)
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 time.
-Barrel/pincushion correction is for normal panorama projects better to
+Barrel/pincushion correction is normally better to let Hugin do. My
-let Hugin do. However, my experience for this special macro application
+experience is however that in this type of stitching Hugin can overdo its
-is that the Hugin optimizer does not behave that well as it does for
+barrel correction, so one may need to compensate on the final output.
-normal panoramas, here often overdoing lens corrections. Therefore I
+Correcting barrel distortion in pre-processing often leads to worse
-recommend correcting this in pre-processing as precise as possible.
+stitching results though (when Hugin does not have the barrel parameter
-Shoot something straight near the edges of the lens and adjust
+to play with), so I choose to leave barrel distortion uncorrected.
-correction parameters the line becomes straight.
 The lens I have used here has extremely low vignetting, so vignetting
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 the stitched tiff again in your favourite RAW converter or photo edit
 software.
 ==Color management==

@@ Line 244: / Line 244: @@
 ==Hugin stitching tips==
-Theoretically, stitching part would be straightforward. And if you have
+TBD. Theoretically, the stitching part should be straightforward. Unfortunately various micro errors can make it a bit difficult.
-a very high precision setup it can be. In practice though, you probably
-don't have. I don't. It is more important to strive to get every frame
-as sharp as possible over the whole frame, which may require slight
-adjustments after film has been moved. Some parameters Hugin stitcher
-may need to deal with:
 * different roll on the images after moving the film on the light table
 * very slight yaw/pitch errors due to non-perfect perpendicularity
 * film curl variations after moving
-* very slight changes in Z due to focus readjustment
+* lens distortions
-You should first try to only correct for X, Y, and roll, and hopefully you get a good result, but probably not always.
+General guide so far is however
-Here is the stitching workflow I use:
+* Add auto control points with Hugin CPFind
+* Add one vertical or horizontal guide on only one image to indicate orientation
-* Create new project
+** Adding on several images seems to disturb optimization quite often
-* Open the 16 bit tiff images (usually four)
+* Optimize custom parameters
-* Don't use the field of view from EXIF, instead pre-set to 1 degree.
+** First X and Y on all but one
-* Make control points to connect all images
+** Then add roll
-** I'm a bit old-school and always do them manually, about four per image is enough, just try to spread them out widely
+** Then add barrel
-** Well-placed manual control points often perform better than auto-generated
+** Clean any poor control points, re-optimize
-* Make horizontal (and possibly also vertical) guides on all images
+** Accept non-perfect rotation and overdone barrel correction as long as it reduces stitching error
-** You should have included film edge on all your frames and that is what you use as guide, the top-down film edges are generally more reliable as guides than the sides
+** If desperate, you may try include yaw/pitch correction, with a hack - specify a very small FOV, 1 degree, then yaw/pitch corrections will probably be milder.
-* Now the exciting part - the optimizer tab, we are going to do it all custom, Hugin often delivers best results if we optimize the parts that requiure largest corrections first and then add on parameters that need less correction for further optimization
-** First correct X and Y for all but one
-** Then add roll to all and optimize again
-*** Theoretically this would yield perfect result already here, and if you have sub 2 pixel max error you probably should be satisfied.
-** If not satisfied, then add Z for all but one and yaw and pitch for all, and optimize again
-** Then open up GL preview, goto move/drag and select drag mode "mosaic", and then drag the image into the center of the view, close and optimize again
-*** Typically the optimizer has put the image far from center, and moving it back into the middle again and reoptimizing can give better result.
-** You can try to add lens view as a final parameter but that usually does not improve things.
 * Open up GL preview
 ** Choose rectilinear projection
 ** Mosaic-drag so the center is in the center of the image
 ** Fit/autocrop the view
-* Skip the exposure tab if input was corrected for vignetting (or the lens had very low vignetting)
+* If lens does not vignette much, exposure tab can be skipped
-* Stitch to 16 bit tiff output, for further processing in your favourite raw converter or photo editor
+* Stitch to 16 bit tiff and rotate barrel correct the output if necessary
-−
-About field of view: at macro distance a macro lens's field of view is
-usually considerably smaller than its focal length indicates.
-Hugin uses field of view as a guide for yaw/pitch corrections and while
-these often are reqiured to compensate for imperfections in the setup,
-the Hugin optimizer in this special case often makes too large
-corrections if the field of view is not set to a small value. Even if
-the lens has 10 degrees of FOV, try set it as low as to 1 degree. Note
-that this is not normal Hugin operation, but this macro lens
-application seems to fall outside normal optimization space.
-−
-About vertical/horizontal guides: it is often hard to set up perfect
-horizontal/vertical guides, they usually conflict a little, so with
-these involved you often don't get down to subpixel fit. If you get a
-lot of trouble you can try having only horizontal (or vertical) guides,
-using the most reliable film edges. Beware that film edges can be slightly
-bent, due to micro-curling or imperfections in the film.
-Skipping the guides alltogether usually lead to slightly distorted
-results (non-rectangular frame).
-−
-Sometimes I get good result with only X/Y/roll correction, and sometimes
-I do the whole optimization workflow. A typical fitting result I get on
-my stiching is 0.5 pixel mean error and 1.1 pixel max, on a bit more
-troublesome cases I can get 1.4 pixel mean and 5.1 max. Even in such
-cases enblend will handle the situation excellently for all normal
-type of images. Sometimes one gets a very good fit earlier then
-expected in the optimization workflow, and then one should not
-optimize more, just drag the result into the center fit, crop and stitch.
-−
-Be sure to check that the image becomes rectangular. If yaw and pitch is
-involved it sometimes happens that the shape becomes distorted even if the
-fit is good. Try add both vertical and horizontal guides if this happens,
-or avoid using yaw/pitch optimization.
-−
-If optimization seems hopeless, it may be the case that the film was too
-curled or you had some other problem during shooting leading to distorted
-input.
-The exact setup and shooting technique may vary a bit from mine, which
+To handle the micro errors in the best possible way in this type of photo setup, Hugin should probably support rubber sheeting to correct the small residual errors. PTStitcher has a "morph-to-fit" feature which may be useful (not tested by me at the time of writing).
-means that there may be an optimization order that works better for
-your system. Feel free to experiment. The key concept here is applying
-a very narrow FOV and correct for yaw and pitch although one
-theoretically should not need to.
 ==Post-processing==