How dark are those corners in your photos, by the numbers? How much do you have to stop down a lens to lighten the corners? You can get the answers for yourself using a variety of image editors.
The only special equipment you probably need is a grey card or a subject with neutral tones and even illumination. If you want to explore how vignetting changes from close-focus to infinity, then you can photograph the clear blue sky for a target (unless your lens is a super-wide). I’ll show you in three different image editors how to measure RGB values.
Capture NX-D example to get RGB values (resolution chart photo)
The mouse cursor location is used for RGB value feedback in Capture NX-D. It is shown on the bottom edge of the window.
Zoner Photo Studio Pro shows RGB and cursor X,Y
The Editor in Zoner Photo Studio Pro displays both the cursor location and the RGB values while using the “magnifying glass” cursor, for instance.
Photoshop example using a grey card target
Shown above, you can use Photoshop to sample locations of interest from a photo of a grey card. Here, I selected a point near the center and a point in a corner. I used the “color sampler” to get the RGB values. Make sure you have correct white balance, so that the R,G, and B values match (or they're at least close) when using your grey card.
The selected central point in the example has an average of 158 for the RGB, while the corner point has an average of 79. You would probably assume that with values that are half as big, there would be a one-stop difference between the center and the corner. But life isn’t quite that simple. The RGB values are non-linear in response to brightness.
The use of a grey card makes viewing the lighting distribution across a photo much simpler. Since the RGB relative values should be pretty close to the same at any selected location with a neutral grey card, the overall evaluation of vignetting is just easier (R=G=B in daylight with proper white balance). My resolution charts also work well for this purpose.
The gray card analysis above was done using the Nikkor 18-140 f/3.5-5.6 wide open at f/5.6 and 140mm. This is the lens pretty much at its worst. This happens to be my worst lens for vignetting that I own. Stopping down quickly minimizes whatever vignetting there is, by the way.
So, how do we use these RGB values to get F-stop values? As I mentioned above, the RGB values don’t relate in a very straightforward way to F-stop values. One way to solve the problem would be to start by setting your camera on ‘manual’ exposure, and take a set of RAW-format pictures of a gray card. Start with a photo that’s about 3 stops over-exposed, and then change your exposure (aperture or shutter) by a third of a stop for another shot. Keep this up until your last shot is at least 3 stops under-exposed (a total of 19 shots). For Ansel Adams fans, this would be shots from Zone 8 through Zone 2. In your photo editor, you can then read the RGB values of each shot to note the progression. If any shots have a “255” RGB reading, then you’ve got a blown-out photo and you won’t be able to use it.
There are few lenses that have more than 3 stops of vignetting. With this exposure shot collection, however, you should be able to use these RGB values for more than just vignette analysis. Your library of 1/3 stop photos and their RGB values will let you later analyze any photo where you want to critically analyze brightness and contrast ranges in terms of F-stops.
For my own tests, I changed the shutter by third-stop values all the way from +3 stops through -5 stops (Zone VIII through Zone 0). I used the “ExifTool” program explained here to get the “Light Value” (or “EV”) in each shot, to get a list of decimal numbers for easy math with the “stops”. I then used a photo editor to get the RGB for each shot (it varies a little bit in each shot, so I noted a typical value, where R=G=B). Notice that the Zone VIII shot is close to the 255 maximum.
Stops Zone EV RGB
+3.0 VIII 2.7 250
+2.7 3.0 239
+2.3 3.3 234
+2.0 VII 3.6 225
+1.7 4.0 215
+1.3 4.3 200
+1.0 VI 4.6 180
+.7 5.0 160
+.3 5.3 140
0 V 5.7 125
-.3 5.9 105
-.7 6.3 85
-1.0 IV 6.6 70
-1.3 6.9 60
-1.6 7.3 44
-2.0 III 7.6 40
-2.3 7.9 37
-2.7 8.3 33
-3.0 II 8.6 28
-3.3 9.0 24
-3.7 9.3 22
-4.0 I 9.6 21
-4.3 10.0 19
-4.7 10.3 15
-5.0 0 10.6 13
Given the EV-RGB list above, let’s get back to the lens vignetting problem. The lens center measured about 158 (R), and the corner was about 79. This relates to roughly EV 5.0 and EV 6.4, for a difference of about 1.4 stops. I noticed that the DxOMark site rated this lens vignetting at “1.2 stops”. Pretty close.
Exposure Value versus RGB value
You can see how non-linear the RGB values are, compared to the EV. It’s well-known how the numeric separation is large in bright areas and small in dark areas.
I tried the experiment above on a couple of different computers and in three different image editors; the results were in close agreement on each. You could probably just use the results of my experiment directly for your own lens vignette analysis.