In our reviews we usually give figures for the vignetting of the lens and thanks to a reader with programming skills I got the opportunity to extend this and show you some fancy graphs how the vignetting changes when stopping the lens down. You will get the fancy graphs, but there are some caveats we need to talk about first.
Measuring Light Falloff
So what we ususally do in our reviews is this: we take shots of the overcast sky with a white tissue over the lens and then we adjust the exposure in Lightroom for the corners to match the brightness (in terms of the RGB values) of the center of the frame.
Generally with this method we saw a good match with the vignetting figures e.g. Lenstip derived with their Imatest software or opticallimits is publishing, so we didn’t question this method. Now when assembling these graphs we made some discoveries that overcomplicated things significantly.
The first issue is that the relationship between the RGB values and the vignetting – measured in intensity or “EV” – is non linear. We tried to overcome this with a so called fit-function that maps the EV values to the RGB values. For that we took pictures of an evenly lit surface with varying exposure (EV) and compared those changes in exposure to the corresponding RGB values.
Then we discovered another issue, depending on the overall brightness in the picture the tonal curve changes. So this fit-function was only usable for a certain (and too small) dynamic range. The following graph shows the evaluation of 9 pictures which are all 1/3 EV apart.
The follow up issue was that even when trying to stay within this range the EV values in these graphs gave out values very different from those we derived with our previous method or could find elsewhere.
I will use the Sony FE 85mm 1.4 GM as an example. At f/1.4 I measure 2.2 EV vignetting in the extreme corner (I use the Raw file with Adobe Standard processing). Lenstip measured a mean value across all 4 corners of 2.26 EV (Lenstip is using OOC jpegs), these values are very much comparable.
When using the aforementiond fit-function it gives out a value of 1.7 EV (see graph above) which is too big of a departure from the other values for me to publish it.
So what I will show you instead is a graph which I will call “brightness”. This is a rather unscientific name that Lenstip is also using for a similar graph in some of their reviews, e.g. that of the Sony FE 85mm 1.4 GM.
This brightness graph is merely based on the RGB values and not to be mistaken with the light intensity graphs you find e.g. in Zeiss or Leica datasheets.
How does it work? The script is simply getting the red values along the diagonal and normalizing them, so an RGB value of 0 becomes a brightness value of 0 and an RGB value of 255 becomes a brightness value of 1. All the lines for different apertures are adjusted, so that the maximum value (to be found in the center of the frame) is always equal to 1.
At this point it shall also be noted: Zeiss and Leica are giving vignetting values for the lens alone, when we measure such things (or try to) we always measure the system which consists of lens, sensor, in camera processing and post processing (like raw development). We try to keep these effects as small as possible, still what we measure will always be higher than the values that manufacturers give for the lens alone.
What our graphs cannot do:
- You cannot calculate the actual vignetting in EV by using these graphs
- You cannot compare these “brightness” graphs to the light intensity graphs in Zeiss/Leica lens datasheets
- When comparing the graphs of different lenses here on this blog you cannot evaluate small numerical differences of the vignetting figures
What our graphs can do:
- You can easily see if a lens has high or low vignetting
- You can see if and how the vignetting changes on stopping the lens down
- You can see if a lens shows very odd vignetting behaviour
In this section I will show you a few graphs to see what kind of information we can get from them.
This graph of the Sigma 35mm 1.4 Art (old DSLR version) is a very typical one. We see high vignetting at f/1.4 that improves a lot on stopping down one or two stops, but as soon as you reach f/4.0 stopping down further does not make much of a difference anymore.
The TTArtisan 28mm 5.6 is a rather slow and very compact lens. You can see that there is noticeable vignetting at f/5.6 that barely improves on stopping down.
The TTArtisan 50mm 0.95 M is one of those lenses where the vignetting generally improves on stopping down, but the corners are already at the maximum possible illumination at f/2.8 and do not improve further.
The MS-Optics 50mm 1.0 ISM falls in the “odd” category. We see very high vignetting at wider apertures but we also see that at f/16 the corners turn pitch black as I have also shown in my review.
As outlined in this article there are some notable restrictions that come with these graphs, but I still think they are able to tell us some useful things when evaluating and comparing different lenses. If you have any further questions you are as always welcome to ask them in the comment section, I will try to answer them.
Also: keep the things explained in this article in mind when using these graphs in internet discussions.
Big thanks to Clemens who did the programming work here. And maybe take a moment to check out some of his very appealing landscape work here.