Just as I have suspected, the Sony A7 and A7R cameras are not immune to the Red Dot Flare issue, thanks to the short flange distance. The effect of the red dot flare can be significantly reduced if the rear lens element has non-reflective coating applied to it. In the case of the two below, the Zeiss 35mm f/2.8 ZA handles flare a little better due to its optical design, but the red dots are still all over the place. Both shot at f/16, pointing directly at the sun.
A while ago, I posted a detailed article about a very defined pattern of red dots / artifacts that I saw on the Fuji X-series cameras when shooting against the sun. This was the first time I encountered such a problem, so without fully researching the issue and understanding the real cause, I wrongfully blamed the Fuji X-trans system for creating those patterns (my sincere apologies to all the Fuji fans!). A couple of our readers pointed me to some other links on the Internet that show a similar issue on different camera systems from Sony, Panasonic, Olympus and a number of others. The pattern indeed seemed to be quite similar between those and what I saw on Fuji cameras. I then decided to take my Olympus OM-D E-M5 camera for a side-by-side comparison and see if I could reproduce the issue on it as well. Now that I have done enough research to understand the root cause of this problem, I will not only explain the red dot phenomenon in detail, but also show image samples from two different mirrorless systems to illustrate the point.
The red dot patterns can be quite frustrating to see in images. Although this particular phenomenon only happens when the light source is very intense and the lens aperture is small, one would still probably wonder what causes it to happen and how one could minimize or even eliminate it. Before I talk about those things, let me first demonstrate that the red dot flare issue is not related to a particular camera or a lens. When shot in the same conditions, pretty much every modern mirrorless camera will show this and even our DSLRs are potentially prone to the same problems, as discussed below. Take a look at the following image taken by the Fuji X-Pro1 camera and the Fujinon XF 14mm f/2.8 lens at f/22:
Vignetting, also known as “light fall-off” (sometimes spelled “light falloff”) is common in optics and photography, which in simple terms means darkening of image corners when compared to the center. Vignetting is either caused by optics, or is purposefully added in post-processing in order to draw the viewer’s eye away from the distractions in the corner, towards the center of the image. Depending on the type and cause of vignetting, it can be gradual or abrupt. There are a number of causes of optical vignetting – it can naturally occur in all lenses, or can be caused or increased/intensified due to use of external tools such as filters, filter holders and lens hoods. In this article, I will talk about each type of vignetting and also discuss ways to reduce or increase the amount of vignetting in photographs using post-processing software like Lightroom and Photoshop.
1) Types of Vignetting
As I have already pointed out in the introduction of this article, there are different types of vignetting that one might encounter when taking pictures or viewing images. Some types of vignetting are naturally caused by the optical design of lenses, others can occur when using third party accessories such as filters and extended hoods and some are artificially added by the photographer in post-production. Let’s take a look at each type in detail.
1.1) Optical Vignetting
Optical vignetting naturally occurs in all lenses. Depending on the optical design and construction of the lens, it can be quite strong on some lenses, while being barely noticeable on others. Still, vignetting occurs on most modern lenses, especially on prime / fixed lenses with very large apertures. There are two causes for this. First, at the widest apertures, the light than enters the lens is partially blocked by the lens barrel, as indicated by the below diagram:
How would you like the future, if a lens like the Nikkor 800mm f/5.6 VR weighed a kilo / couple of pounds and cost 10 times less? Or perhaps a wide angle lens as big as a pancake that delivers the same quality images as your favorite 24mm f/1.4 prime? Sounds like a dream, doesn’t it? Well, we might not be that far away from this dream, since the researchers from the University of British Columbia and the University of Siegen might change the way modern optics work. Their current research on using a single lens element in a lens and correcting lens aberrations looks promising – a method called “deconvolution”, which is based on analysis and reconstruction of the image via software. Instead of using physical elements within a lens to correct for lens aberrations such as distortion, spherical aberration, chromatic aberration and coma, the idea is to use a lens with a single (or more) lens elements and correct such aberrations via computational photography techniques and software algorithms that are applied after the image is captured. This obviously results in lenses with very few lens elements, making them both lighter and cheaper to manufacture.
In photography, there are two types of distortions: optical and perspective. Both result in some kind of deformation of images – some lightly and others very noticeably. While optical distortion is caused by the optical design of lenses (and is therefore often called “lens distortion”), perspective distortion is caused by the position of the camera relative to the subject or by the position of the subject within the image frame. And it is certainly important to distinguish between these types of distortions and identify them, since you will see them all quite a bit in photography. The goal of this article is to explain each distortion type in detail, with illustrations and image samples.
1) Optical Distortion
In photography, distortion is generally referred to an optical aberration that deforms and bends physically straight lines and makes them appear curvy in images, which is why such distortion is also commonly referred to as “curvilinear” (more on this below). Optical distortion occurs as a result of optical design, when special lens elements are used to reduce spherical and other aberrations. In short, optical distortion is a lens error.
There are three known types of optical distortion – barrel, pincushion and mustache / moustache (also known as wavy and complex). Let’s examine each in more detail, but before we do that, let’s take a look at a lens with zero distortion:
When my article on field curvature was published a while ago, where I talked about how one could do a quick analysis of lens MTF data and determine if it exhibits any field curvature, some of our readers expressed interest in understanding how to read MTF charts. Since we talk quite a bit about lens performance and MTF data here at Photography Life, I decided to write a detailed article on the subject and do my best to thoroughly explain everything related to MTF curves, charts and all the verbiage that comes with them.
Field Curvature, also known as “curvature of field” or “Petzval field curvature”, is a common optical problem that causes a flat object to appear sharp only in a certain part(s) of the frame, instead of being uniformly sharp across the frame. This happens due to the curved nature of optical elements, which project the image in a curved manner, rather than flat. And since all digital camera sensors are flat, they cannot capture the entire image in perfect focus, as shown in the below illustration:
In a simple field curvature scenario like above, the light rays are perfectly focused in the center of the frame, at Image Plane A (where the sensor is). Since the image is curved, sharpness starts to drop as you move away from the center, resulting in less resolution in the mid-frame and much less resolution in the corners. The circular “dome-like” image in three dimensional form is shown to the right of the illustration. If the corner of the image is brought into focus, which would move the image plane closer (Image Plane B), the corners would appear sharp, while mid-frame would stay less sharp and the center would appear the softest. The effect of field curvature can be very pronounced, especially with older lenses.
Chromatic Aberration, also known as “color fringing” or “purple fringing”, is a common optical problem that occurs when a lens is either unable to bring all wavelengths of color to the same focal plane, and/or when wavelengths of color are focused at different positions in the focal plane. Chromatic aberration is caused by lens dispersion, with different colors of light travelling at different speeds while passing through a lens. As a result, the image can look blurred or noticeable colored edges (red, green, blue, yellow, purple, magenta) can appear around objects, especially in high-contrast situations.
A perfect lens would focus all wavelengths into a single focal point, where the best focus with the “circle of least confusion” is located, as shown below:
Focus Shift is an optical problem that occurs due to Spherical Aberration, when an object is brought into focus at maximum aperture and captured with the lens stopped down. Focus shift can lead to blurry images and focus errors, when working with subjects at close distances and using fast aperture lenses. With the lens aperture fully open or “wide open”, incoming rays of light converge at different focal points due to spherical aberration along the optical axis, as shown in the top illustration below:
Spherical Aberration is an optical problem that occurs when all incoming light rays end up focusing at different points after passing through a spherical surface. Light rays passing through a lens near its horizontal axis are refracted less than rays closer to the edge or “periphery” of the lens and as a result, end up in different spots across the optical axis. In other words, the parallel light rays of incoming light do not converge at the same point after passing through the lens. Because of this, Spherical Aberration can affect resolution and clarity, making it hard to obtain sharp images. Here is an illustration that shows Spherical Aberration:
As shown above, light rays refract or change their angle when passing through the lens. The ones closer to the top and the bottom of the illustration end up converging at a shorter distance along the optical axis (black/red dotted line), while the ones closer to the optical axis converge at a longer distance, creating different focal points along the same axis. The point of best focus with the “circle of least confusion” is illustrated as the thick green line. Spherical Aberration is not just caused by lens design, but also by the quality of the lens material. Lenses made of poor quality material and large bubbles can drastically impact light refraction.