Welcome to lesson 1. The journey begins with vignetting, the involuntary darkening of the edges and corners of a photograph. Usually quite benign, this is much tougher to explain than to test for.
Vignetting has several possible causes.
Number one. Shine a light through a paper roll tube. If you do so straight through, you see light at the other end. If you tilt the lightat an angle, some rays will reach the exit, others only the sidewall of the tube. As rays of light enter the lens along its axis, they all get through to the sensor or film. But as they are progressively further out at steep angles, some do not reach the other side of the lens.
This is particularly true for fast lenses (f/2 or f/1.4, for example). When you use a small (slow) aperture such as f/11, only the rays passing right through the middle of the glass are used. But at wide (fast) apertures (f/2) even rays at the edges of the front lenses enter the optical system and those with steep angles “hit the side walls” rather than make it to the exit and onto the sensor.
For long lenses, this is rarely a problem. But short focal-length lenses (also called wide angles) face more problems: in order to collect all the peripheral (edge) rays at steep angles, the front elements of lenses need to be very large, heavy and expensive. So only very high-end lens designers bother with this.
The further away the object photographed is from the center of the image, the more likely it is that some of the rays originating from it are lost in the system and the darker is appears.
Number 2. Lens design has a strong impact on vignetting. The rear elements of rangefinder / mirrorless lenses are typically much closer to the film/sensor than those in SLR lenses. As a consequence, the fan of rays exiting the lens is much more spread out and vignetting is stronger. The closer the rays get to the edge, the more sensor surface they cover, thereby appearing darker.
Imagine a fan (the actual object used to cool down in hot weather). The further out you get from the center, the more “stretched out” the paper gets. Although light doesn’t get stretched physically, the ratio of rays making it to the sensor to rays entering the lens gets smaller as the fan is opened wider. When the rear of the lens is furter away from the sensor (as in DSLR cameras) the fan is less wide open and less vignetting occurs.
These two explanations are theoretical. The designer can calculate or measure the cumulative loss of light and present it in a graph such as this one.
This shows how much light reaches the sensor at various distances from the center of the sensor, for 2 apertures. For example, at the edge of the frame (18mm), roughly 40% of rays are transmitted at f/1.7 and 85% at f/5.6.
In real life, things get more complicated if the lens shade is poorly designed or if filters are added to the lens (imagine the paper roll getting longer and longer). So the measurements can be different to those measured on the lens alone, making automatic correction very difficult.
The corners of your image look gradually darker as you move away from the center. The effect tends to be stronger with wide-angle lenses (35mm below) and at wider apertures (because small aperture limit rays to those entering at the very center, which have more “breathing room” than those entering at the edge of the lens).
The photograph above shows an average amount of vignetting for a wide angle lens. It’s distinctly visible but easy to correct.
Note that this technical defect can be an artistic strength. Some of the world’s most highly regarded lenses exhibit a fair bit of vignetting : the Canon 85/1.2, the Leica Summilux-M 75/1.4, the Zeiss Distagon 1.4/35 ZM … We’ll go into that in a separate chapter.
A more serious issues can arise when you correct the vignetting. Sometimes, the darker parts have a colour cast that shows up when you brighten them. In the picture below I have grossly exagerated this by boosting saturation to 100%. In real-life, the colour this lens isn’t an issue. But for others the colour cast in the vigneted zones may be much stronger and can be more problematic.
Vignetting also has an impact on the out of focus zones of the image. Out of focus highlights get clipped as they get closer to the corners, ressembling cat’s eyes, which give the effect it’s name.
Vignetting is plainly obvious when you photograph the sky or any object with low detail and a constant luminosity.
To detect it, you can defocus the image so that all texture and details on your subject (sky, white wall, …) are blurred, leaving vignetting as the only visible information on the photograph.
You can also use stone walls or any regular pattern, although this is far less effective if you do not defocus. See below at f/2.8 (left) & f/8 (right).
There is no point in testing at all apertures. Vignetting will always be worse at maximum aperture, so you can test at that extreme and at one or two lower apertures such as f/4 and f/8 or f/5.6 and f/11. What you are looking for is the impact wide open (to determine how objectionnable it is) and the point at which it is no longer visible. Very often closing 2 stops cures vignetting to a point where only very specific applications will find it objectionable (photometry, for instance).
In most cases, a little bit of vignetting is not objectionable and can go unnoticed. It is considered benign in mild cases. As an example, vignetting is the only aberration that Zeiss designers let slip a little bit even for their photographic range topping OTUS lenses. But sometimes, vignetting can look ugly or detract from the readiblity of the image, see small black patches in the corners, below.
If you plan on using filters and lens shades, make sure to test with an without them on the lens to identify any possible impact.
If you are using thrid-party lens shades, test with and without the shade, as it may not have been designed to the same speicifcations as the native one and could induce hard to correct vignetting. If you find this to be the case, only use the lens shade when it is *really* needed and remove it in other cases.
For zoom lenses, you will need to repeat this at several focal lengths, but the strongest vignetting will almost always occur at the shorter focal length. So, if you are short on time, just test the lower end of the range (for instance, 24 and 35 mm in a 24-70mm zoom lens).
Most modern post-processing software (DxO, Phase One Capture 1, Adobe LightRooom …) allows you to use lens profiles. These are pieces of code built into your image editor that know the characteristics and aberrations of the lens and enable the software to correct for them automatically. This is the best way to correct for vignetting (though it will not take additional filters or shades into account).
Dedicated sliders can help you manipulate vignetting manually, but are based on a standard vignetting formula that may not match the exact pattern of your lens’s vignetting, which may lead to some unnatural correction that becomes far more obvious than the vignetting itself.
Note that some vignetting tools are designed to correct the defect (lens vignetting, under lens correction at left) whereas others are intended to create an artistic effect (Post-Crop Vignetting, under effects below it). Be sure to use the first.
In case of strong manual corrections, check for colour effects by boosting saturation of your white wall test shot. Do this for every camera you will be using the lens with, as the effect appears to be sensor-dependant.
In extreme case, cropping may be the best (or only) way to get rid of unpleasant vignetting.
This concludes lesson 1 on Vignetting, the simples in the series. Grab your favourite lenses and start testing them on a white wall somewhere to see if they all exhibit the same behaviour.
Next up : Chromatic aberration. A far more complex and very interesting subject.
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