ISO invariance is one of the most talked-about topics in photography today, yet most people don’t really understand what it is. That isn’t a surprise; ISO invariance can be very technical and counter-intuitive, and it doesn’t fit well with many photographers’ general understanding of ISO. However, ISO invariance is an important topic, especially since many cameras today are close to being ISO invariant. If you want to maximize your camera’s dynamic range and avoid using “useless” ISO values, this topic is directly relevant to your photos. So, what is ISO invariance, and how can you use it to your advantage in your own photography?
When light rays coming from a bright source(s) of light (such as the sun or artificial light) directly reach the front element of a camera lens, they can reflect and bounce off different lens elements, diaphragm and even off the sensor, potentially degrading image quality and creating unwanted objects in images. Better known as lens “flare”, the effect can impact images in a number of ways: it can drastically reduce image contrast by introducing haze in different colors, it can add circular or semi-circular halos or “ghosts” and even odd-shaped semi-transparent objects of various color intensities. Flare is not always undesirable in photography though – sometimes in is used creatively to add artistic elements to images. In fact, lens flare is often deliberately added to movies and computer games to add a sense of realism and boost the visual experience of the viewer.
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.
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.
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:
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.
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:
In essence, street photography is a type of candid photography done in a public place, be it a street, a restaurant or even public transport. It is similar in approach to photojournalism and mostly involves people (and/or animals) in a populated environment (which provides the context of a story told), such as a city. However, street photographers often focus on everyday lives of strangers rather than some kind of important event photojournalists are more interested in. Usually, street photographers try as much as possible to stay unnoticed when photographing. The goal of street photography is to capture scenes unaffected by the author of the work so as to show a natural story and subject. Story and subject are possibly the most important aspects of a good street shot. Henri Cartier-Bresson, arguably the best street photographer of all times, “the father of photojournalism”, had said: “Above all, I craved to seize the whole essence, in the confines of one single photograph, of some situation that was in the process of unrolling itself before my eyes.”
You will often hear some photographers say “shoot wide open”. What does it mean? Generally, the words “wide open” refer to the size of the lens aperture – it literally means to “shoot at maximum aperture”. For example, if you are shooting with a lens like the Nikon 50mm f/1.4, its maximum aperture is f/1.4. If I said something like “The center of the frame looks sharp wide open”, you can rephrase this sentence to “The center of the frame looks sharp at the maximum aperture of f/1.4”.