Lori’s 60th birthday celebration picture with bourbon and cigar.

Lori has been a bit distressed about marking her 60th birthday: she’s reached middle age.  (She plans to live at least until 120.)  I wanted to commemorate it with a photo that emphasizes the spirit of how I would like her to approach it: with equanimity or indifference.  She’s too busy enjoying life to pay much attention to arbitrary chronological milestones.

I was a bit surprised when she agreed to the concept as I described it. A cigar and glass of bourbon were always part of the narrative but the venue changed from field of daisies to woods due to uncooperative weather.

A moderately wide aperture reduced depth of focus. A low ISO and 1/250th shutter speed enabled darker tones in the woods so that some lighting would help bring the subject out a bit more.

Lighting was via two studio strobes.  One was camera left in a pan reflector for a slightly diffused overall subject lighting.  Another strobe in a grid high camera right behind Lori brought up a little separation on her shadow side. Both were at very low power because of the wider aperture.

We are pleased with the results.  At least photographically.  We got many good poses to pick from.  Unfortunately the cheap cigar made Lori ill and it took her the rest of the day to recover from all the puffing to bring the shot to life.

So, aside from poisoning my wife on her birthday, I would say things went pretty well.  Lori may differ.

I’m consolidating and getting rid of decades worth of photographic gear, partly to simplify, partly to raise some cash to cover the cost of my new system.  (More on that later.) My best option is to sell on eBay but it requires a bit of homework to make it worthwhile.  I have to do the research to find the value of each item and then prepare photos and descriptions of each one for sale.

Preparing the photos for sales is relatively easy and a little care makes the results more appealing than most other eBay posts.  It seems a bit odd when people put fine photographic equipment up for sale with crappy pictures–it affects credibility.

After a little tinkering, I settled on a lighting setup I can use for a variety of product-style shots that look good for the eBay listings.

Lighting for product photos of items being placed for sale on eBay.

I use two LED light sources.  Key light is through a diffusion panel camera left and a background light below the table is reflected off a white fabric panel behind the table with the subject.  Items are placed on a piece of white tile board allowing for a plain background letting the eye concentrate on the actual items for sale.  An aluminum foil reflector, camera right is used for fill.

The setup is in a corner of our basement and allows for the tedious but efficient processing of the photos required for the listings.

Two lights (LED at left and one below table reflected off white panel behind) is used to light items to be sold online.

The results from this particular setup shows how the items for sale are featured without any other distractions and the lighting shows off the items in enough detail to illustrate the condition.

The image shows the effect of the simple 2-light setup to light photography equipment for sale.

Here’s a sample of some of the images made with this simple but versatile setup:

In addition to shooting some decent quality images for the sales, I had a few other things going for me:

• Most of the items are higher quality and performance with well-known specifications and track records.
• I’ve taken very good care of all of them; they’re in uniformly excellent condition.
• I’ve researched selling prices for each one and assigned reasonable prices near the upper end of used market selling prices.
• I include bonus items when it makes sense (e.g. lens plates, memory cards) and I offer free shipping to keep the transactions simple. (I build average shipping cost into the asking price.)
• I strive for accuracy on the descriptions including being meticulous about any flaws like scuff marks, missing peripheral pieces, etc.

So, how is it going so far?  I posted the first 5 items (4 lenses and a camera body from the sample images above) and sold 4 out of 5 within 24 hours and the fifth in less than a week.  I’m maxed out on my eBay selling limit for a month but have the next batch queued for listing in the next cycle.  I’m divesting my unused equipment and buyers are picking up some excellent quality items at reasonable prices. So far so good.

Twelve focus stacked wide aperture images create an in-focus blossom with an out-of-focus background.

Combining focus-stacking with wide-aperture (shallow depth-of-focus) images can create some nice effects.  I wanted this alstroemeria blossom to be completely in focus but the background blurred.  By using a wide aperture (f/2.8) and 12 images varying the focal point, the stacking operation enabled nice crisp focus on the entire blossom but the out-of-focus background remained soft, almost watercolor-like.  It’s a combo you won’t be able to get with a single image.

• Vignetting 1: Overview
• Vignetting 2: Parametric Correction
• Vignetting 3: Correction Images
• Vignetting 4: Photoshop Divide Layer
• Vignetting 5: A Pathological Example
• Vignetting 6: Other Options and Summary (this post)

For a little perspective on what we’ve been dealing with, we can posterize high-contrast versions of our flat fields to better appreciate the topology of vignetting in different circumstances:

A little processing can enhance the features of flat field images to visualize their contours more easily. Contrast was boosted and the images posterized to get this effect.

Far from exhausting the possibilities for vignetting correction in this series of posts, there are many other options to consider.  Here are a few:

Capture One Pro

If you are using Phase One’s excellent Capture One Pro to process your images, the Lens Correction Tab includes tools to not only correct images for lenses in their database but you can create and apply LCCs (Lens Cast Calibration) images.  It uses flat field images like we’ve been talking about, to enable you to selectively correct for Color Cast, Dust Removal, or Enable Uniform Light in any combination. It works really well. The tab also includes the more conventional parametric sliders for vignetting.

Hugin

Hugin is freely-available panorama stitching software that has a number of tools available for image corrections.  The fulla command-line tool in the package enables a couple of different ways to address vignetting.  One way is to use the –flatfield command line option to specify a flat-field image used for a division operation very similar to what we did using ImageMagick commands in an earlier post.

Another option offered in fulla is the –vignette command-line option used to specify 4 coefficients for a 6th-order polynomial describing the fall-off curve for radially-symmetric vignetting.  I’ve used it, it works quite well, but it’s probably not for anyone who recoiled when I mentioned 6th-order polynomial coefficients.

Video

Although possibly worthy of another entire post, it should be mentioned that vignetting can be corrected for video in a number of ways too.  Besides the usual parametric slider corrections, Adobe After Effects, for example, provides layer blending modes.  You can use the same techniques we’ve been discussing to create a correction layer above a clip and change the blending mode to Divide or Multiply, as we’ve previously demonstrated with Photoshop, to get accurate vignetting control.

Vignetting Perspective

Contrary to what a 6-part blog series on destroying vignetting might indicate, I don’t have a visceral hatred of vignetting. But vignetting is a consequence of the camera, lens, and settings you use; it’s not a characteristic of the subject you’re shooting. I generally don’t want it to be a noticeable component of an image unless there’s a specific reason for it. Maybe adding some judicious vignetting to an image can improve it but that added vignetting is likely to be of different character and magnitude than the system’s innate vignetting.  Vignetting often seems gimmicky and detracts from an image but at least there are techniques available to fully control it on your terms.

Bogus Alert

There are, like anything else, numerous explanations, tutorials, and remedies on the Internet that address vignetting.  Some are even accurate.  Much is (possibly well-intentioned) bad advice.  For example, there are discussions here and there about creating radial gradients and applying them in layers using Multiply or whatever.  These are often accompanied by some hand-waving about tuning with opacity adjustments etc. (to disguise the damage done to the image). Ignore all that.  Radial gradients are linear by nature and, as we’ve seen, real-world vignetting is not linear at all.  Making a linear adjustment to an exponential problem just shoves pixel values around, improving one thing, creating problems elsewhere.  Bogus, bogus, bogus.  If you want quick and easy, stick with parametric vignetting adjustments common in almost all image processing software.

Summary

There are a few high-level lessons I’ve learned about vignetting and vignette control.

• The best, most accurate, vignetting adjustment is enabled by your creation of good flat field images.  Those flat fields can then be used to either determine the optimal settings for parametric slider adjustments or can be converted to highly-accurate correction images themselves.
• For radially-symmetric vignetting (the most common case) optimal parametric settings are usually “good enough”, creating results that the eye is quite satisfied with.
• For most accurate results, or for the occasional oddball non-radially-symmetric case, flat field correction images are the way to go.  They can be used in a number of tools in different ways:  command-line manipulation in ImageMagick and layer blending modes in Photoshop being two we focused on.  (Under the covers they’re doing the same thing.)
• There’s still a lot more we haven’t covered in this series but we have covered some solid tools to address the most common problems.  There’s little practical need to get any deeper into this swamp than we already have.

• Vignetting 1: Overview
• Vignetting 2: Parametric Correction
• Vignetting 3: Correction Images
• Vignetting 4: Photoshop Divide Layer
• Vignetting 5: A Pathological Example (this post)
• Vignetting 6: Other Options and Summary

There are cases where vignetting is not radially symmetrical like the examples shown so far.  Those cases cannot be fixed with parametric vignetting adjustments such as those in Lightroom or Photoshop.  They are, however, handled nicely by the use of correction flats.

As an example, here’s an image shot with a Panasonic GH4 and Lumix 20mm lens at f/2 (which we’ve been using for our examples all along).  But in this case a Singh-Ray variable neutral density filter is on the lens as well.  The max. setting for the filter does strange things to the luminance distribution for any image.

Abnormal light fall-off characteristics are evident in this image shot with a Singh-Ray variable neutral density filter.

The abnormal darkening of the sky at upper right is clearly evident.  The lower left of the field is very dark as well.  Upper left and lower right seem to be okay.

Shooting a flat field image with this equipment combination shows the odd asymmetric fall-off effect caused by the ND filter on this camera.

Flat field image shot with a Panasonic GH4, Lumix 20mm lens at f/2 and Singh-Ray variable neutral density filter at Max.

A graph of intensities along each of the four diagonals running from center to corners gives another view of this mess.

The Lumix 20mm lens at f/2 with a Singh-Ray ND filter at the max. setting shows very asymmetric luminance curves in the flat frame.

Note how the brightest areas are about ¾ of the way along the upper right and lower left diagonals in the graph, not the center like the more typical vignetting curves we’ve shown.

It’s obvious that the usual parametric slider vignette controls in Lightroom couldn’t possibly deal with this odd luminance profile.  However, our flat field correction approach, either using ImageMagick or Photoshop, can handle this with little trouble.  Using the Photoshop Divide blending layer approach we described previously, we get good correction for this pathological case.

A Photoshop Divide blending mode layer was used to correct the abnormal vignetting.

The moral of the story: pick the right tool for the job.  You may think another moral of the story is not to use Singh-Ray neutral density filters at Max. setting.  But once you know how to adjust for the side effects, it’s not an issue.  Variable neutral density filters are very useful when shooting video and we don’t want to restrict ourselves when we have a workable solution for odd fall-off effects in different equipment and shooting configurations.

• Vignetting 1: Overview
• Vignetting 2: Parametric Correction
• Vignetting 3: Correction Images
• Vignetting 4: Photoshop Divide Layer (this post)
• Vignetting 5: A Pathological Example
• Vignetting 6: Other Options and Summary

In the previous post we discussed converting a flat field image into a correction image which we then apply, using a divide operation, to remove vignetting.  The technique used several ImageMagick commands.  We can accomplish the same thing in Photoshop: it’s a simple sequence of steps which, under the covers, does the same thing we did with the ImageMagick approach.

For this description we’re using Photoshop CC 2015.  Other versions may have some differences.  We’re going to do the same two things we did in the ImageMagick procedure, except using Photoshop layers rather than command-line options:

1. Create a correction image by dividing each pixel value by a factor that scales the brightest pixels up to a value of 1 and all other pixels scaled up proportionally. (We’ll use a Levels adjustment to do this.)
2. Use the correction image to divide into the subject image to correct the brightness levels to remove vignetting. (We’ll use a Divide blending mode layer for this.)

Create A Correction Image

We’re going to create a correction image from our flat-field image that we can then use to remove vignetting on any image shot with the same camera/lens/filter/aperture combination.

1. Open the flat field image in Photoshop.
2. Add a Levels Adjustment Layer.
3. In the Levels control, hold down the alt key and move the highlight (white) triangle slider left toward the histogram mass of the flat field image.  Keep an eye on the image and stop just when the totally black image starts showing some brightness levels.  The slider will be just touching the right edge of the histogram’s non-zero values.
4. Flatten the image and save it as an uncompressed TIFF file.  This is your new correction image which you’ll use to correct vignetting in other images.

Add a Correction Layer

Correcting vignetting in a subject image is simple with a properly-prepared correction image available.  We’re just going to add the correction image as a layer above the subject (Background) image and change the blend mode.

1. Open the subject image in Photoshop.
2. Click File, then Place Embedded… and select the correction image you prepared previously.  Click Enter to accept the placed image. This adds the correction image as a layer above the subject Background image.
3. Change the layer blending mode of the correction image layer to Divide.  You’re done.  Save the image.

A Photoshop correction image layer uses the Divide blending mode to remove vignetting from the subject image.

The Results

Applying the correction Divide layer to our vignetted flat field test case yields excellent results, as expected after what we experienced doing the same thing with ImageMagick previously.

Using a Divide layer correction in Photoshop removes vignetting from the test flat field image.

Statistically, the results are nearly perfect.

After applying a Divide-layer correction to the flat field image, the histogram shows a perfect correction.

The histogram for the corrected flat field image looks perfect as well.

Example

An image of a brace and bit on a concrete floor is plain enough that vignetting, and its correction are readily apparent.

Before: Lumix 20mm at f/2 shows signficant vignetting.

After: in Photoshop, a correction frame is placed above the subject image using Divide blend mode.

The Multiply Alternative

The process described above corrects vignetting by lightening the parts of the image that suffered fall-off, reversing the darkening that becomes more prominent farther from the center of the image.   Another way to address the problem is to darken the center portion to bring the levels down proportional to the corner levels.  This is usually not the preferred choice but there may be circumstances where this alternative is preferable.  For example, raising levels toward the corners may blow out important highlights that you wish to preserve so the darkening approach may be called for.

Briefly, here is how to darken lighter portions rather than lighten darker portions to correct vignetting.  First, you will want to create your correction flat differently than the original approach.  Invert the flat field image first, then add a Levels correction layer, click Alt, and move the white slider left until you start to see non-black regions in the image showing.  (This time it will be the corners rather than the center because you inverted the image.)  Save the correction image as an uncompressed TIFF file.

To apply the correction using this inverted correction flat, load the subject image, then load the correction image as a layer above it.  Change the blending mode of the correction layer to Multiply.  That will cause the lighter central region to darken in line with the corner regions, removing the vignetting.  This darkens the overall appearance of the image so another adjustment may be required to bring the levels back up to where you want them.

Here’s a comparison of the Divide blending mode correction (raising darker tones) and the Multiply blending mode correction (lowering lighter tones):

After: in Photoshop, a correction frame is placed above the subject image using Divide blend mode.

After: using a Multiply blending mode with an inverted correction layer darkens lighter areas, preserving darker areas.

Although equally effective at removing vignetting, clearly the Divide blending mode layer technique yields a better overall tonal result, as it will in most cases.

• Vignetting 1: Overview
• Vignetting 2: Parametric Correction
• Vignetting 3: Correction Images (this post)
• Vignetting 4: Photoshop Divide Layer
• Vignetting 5: A Pathological Example
• Vignetting 6: Other Options and Summary

When we’ve gone to the trouble of making a flat field image for a given system configuration and aperture, there are several ways the flat frame can be used to correct vignetting in other images shot with the same configuration and aperture.

Using ImageMagick (version  6.9.2-5 Q16 x64 for this example) is one way to do our processing.  ImageMagick is a suite of image processing tools with the advantages of price, power, and precision. The price is right: it’s free.  It’s very powerful with a rich set of commands and options covering a broad range of capabilities.  It is very detailed in reporting information about an image and enables precision with its robust option set.  It lends itself well to automation scripting for large jobs.  APIs are available for a variety of programming and scripting languages.

The Concept

The goal is to exploit the flat field image to correct the vignetting that it manifests itself.  This will precisely control the adjustments of the pixel values.  Unlike the parametric control technique, this method can correct nearly any type of intensity anomalies including more complex fall-off patterns that are not radially symmetric.

To understand how and why this works, it is helpful to think of the pixel intensity values falling in a range of 0 to 1, where 0 is pure black and 1 is white, the greatest possible pixel value.  All other tonal values are represented by some fractional value between 0 and 1.  This is, in fact, how ImageMagick and Photoshop perform some of their calculations under the covers.

The following discussion is simplified by ignoring multiple channels.  We’ll look at a single channel, understanding that the same exact mechanism and process applies to all channels.

Imagine we’re correcting the vignetted flat field image.  We want a process that will convert the varying flat field pixel levels to a uniform single tonality across the entire image (or restated, we want to remove the vignetting or light fall-off).  We want to do this in a way that the lightest pixels in our flat field are left alone and the darker pixels are adjusted up to be that brightest level.  For typical vignetting, this means we want to leave the central brighter values alone while boosting the darker levels toward the edges and corners.

Every pixel value in a vignetted flat field is divided by a corresponding pixel value in a correction frame so that the resulting pixels are all at the same value. The trick is creating a correction frame that can do that.

For every pixel in the flat field image, we’ll have a corresponding pixel in the correction frame.  That corresponding correction pixel will have a value (also between 0 and 1.0)  such that when it is divided into the flat field pixel value, the resulting value is the target brightest level we want for the uniform corrected image result.

In the example above, the brightest pixel value is 0.9. A pixel value of 0.7 must be divided by 0.7777… (0.78) to bring it up to the target value of 0.9.  Each correction pixel value is calibrated to bring up the darker pixels in the flat field image in just the right amount.

So where do the correction values in the correction frame come from?  They are calculated from the original flat field frame.  In the example with the brightest flat field value at 0.9, we’ll divide every pixel in the entire flat field frame by 0.9 to create the correction frame.  This makes sure that the brightest values are left alone during the correction division (dividing by 1.0 in the 0.9 case) and the darker pixels are adjusted upward proportionally.

So the strategy with using correction frames is deceptively simple: create the correction frame by dividing each pixel in the flat field frame by the highest pixel value in the flat field frame; then use the correction frame on any image by dividing the input image’s pixels by corresponding correction pixels.  This correction image can be used with any image shot with the same camera, lens, filters, and f-stop that was used to shoot the flat field used to derive the correction frame.

The methods we’ll be discussing will, by default, perform these operations on all three channels in the image automatically.

Thoroughly confused yet?  It really does make sense when you think through the whole thing a few times. Okay, here’s the nutshell version of what we’re going to do:

1. Create a correction image by dividing the flat field’s pixels by the lightest pixel value in the flat field.
2. Correct subject image vignetting by dividing the subject image by the correction image.

Create A Correction Image

The correction image contains all the pixel values such that when divided into the corresponding subject pixels, the darker pixels are corrected to the desired value.

Using our Lumix 20mm f/2 flat field image (named vig20mmf2.tif) as an example, we first find the brightest levels in the image.

identify -format %[fx:maxima] vig20mmf2.tif

The ImageMagick identify command can display detailed information about our image including the calculated maximum level (-format %[fx:maxima]) which in this case is 0.6604.

Now we can create the correction image.

convert vig20mmf2.tif -evaluate divide 0.6604 vig20mmf2_corr.tif

The ImageMagick convert command simply divides each pixel in our flat field image by 0.6604 (-evaluate divide 0.6604) and outputs our correction image, vig20mmf2_corr.tif.

Apply The Correction Image

This correction image can then be used by another convert command to correct vignetting for any subject image shot with the Lumix 20mm at f/2.  Let’s be clever and apply this correction to our original vignetted flat field image to see how well it corrects the vignetting.

convert vig20mmf2.tif vig20mmf2_corr.tif -compose dividesrc \
          -composite vig20mmf2_IM.tif

The convert command takes several command-line options:  the subject image name (the one to be corrected), the correction image name, “-compose dividesrc” to indicate the operation we’re performing, “-composite” to perform the divide operation, and finally our output corrected file name.  (Note: the backslash is not part of the command above, just an indication that the command continues on the next line.)

One little gotcha to be aware of: if your subject image and correction image have different orientations (one is landscape oriented and the other portrait oriented), the convert command won’t adjust, the operation will divide dissimilar images, and you won’t get the results you intend. In that case, add a -rotate option for your subject image to get them both on the same orientation before it proceeds.  You rotate the output image back in post if necessary.  Here’s an example of the convert command working on a vertically oriented image with a horizontal correction frame:

convert verticalimage.tif -rotate "90<" vig20mmf2_corr.tif \
           -compose dividesrc -composite corrected.tif

See ImageMagick’s documentation for the syntax of the -rotate command-line option.

The Results

The resulting vig20mmf2_IM.tif looks indistinguishable from our ideal uniform flat field image.  The graph couldn’t look any better either:

Applying a correction image to the Lumix 20mm at f/2 flat field image using ImageMagick shows an excellent resulting graph.

Finally, let’s look at the statistics for the corrected image.

That result is within a gnat’s eyelash of perfect.  We can keep the vig20mmf2_corr.tif image and apply it any time we wish to remove vignetting from a photo shot with the Panasonic GH4 with Lumix 20mm lens at f/2.

Obviously you would need to shoot flat field images and convert them to correction images for every camera/lens/filter/aperture combination that you wish to correct in this manner.

Not So Fast

As delighted as we are at how close our correction image comes to correcting our vignetted flat field image, we have to realize we created the correction image from the flat field image so of course it looks great.  When used against other real world images, however, all kinds of variables get fired off to thwart perfection.  The correction applied this way is a very accurate but not-always-perfect way to correct vignetting.  It’s a great start, and often perfectly sufficient, but there may be shooting circumstances where the correction under- or over-compensates for the particular subject shooting conditions.  Use discretion and adjust as needed.

Example

Before: sunset rainbow image with visible vignetting. Shot with Lumix 20mm lens at f/2.

After: image with vignetting removed by using a corrective image applied with ImageMagick.

Using a correction image made from a vignetted flat field image shot with the Lumix 20mm lens at f/2, ImageMagick has effectively removed the vignetting.