Digital Camera Lenses

3. Image Quality

Fields of View, Lens aberrations,  and Focal length & Shutter Speeds 

Fields of view

As we said on page one a standard lens is called so because it gives a natural field of view that is not subject to perspective distortion because it's focal length matches the sensor or film format being used. The exit angle of the image matches the entry angle (fig 1). When a lens with an field of view greater or smaller than this is used the entry and exit angles are no longer the same, and this causes perspective proportions to become distorted. 

With a wide field of view the image has to be squeezed into a smaller space (fig 2). The result is that scale and distance proportions between foreground and background are increased, objects appearing smaller and distance greater the further away they are. The wider the angle the more exaggerated this becomes until a fish eye lens is reached. If you have ever seen an image from one of these lenses you will know how distorted the perspective can become, and how surreal the image that results. 

Using a narrower field of view - a telephoto - results in the image having to be stretched to fit the space (fig 3), and perspective is again distorted, but in the opposite way. Scale and distance proportions between foreground and background diminish so that objects far away appear larger than they are and nearly as big as those close by, and it appears there is virtually no distance between them.

Lens Aberrations

Using lenses that provide wide or telephoto fields of view bring with them certain other problems that have to be taken into account, besides that of perspective, in order to get the best use out of them and produce worthwhile shots, and some of these relate to general lens design.  

Barrel and Pin-cushion distortion

Two basic lens distortions that occur are called pin-cushion and barrel distortion because that is the effect they have upon an image. Symmetrical lenses, as exemplified by the standard lens, have complementary elements at the front and back which cancel out distortion, but telephoto and wide angle lenses, which are asymmetrical, do not. Barrel distortion is found in wide angle views, and pin-cushion in telephoto. Corrective Lens elements are used to reduce these faults as much as possible and some lenses exhibit much less distortion than others. The quality of a lens, and it's type, single focal length [prime/monofocal] or zoom, usually determine how much distortion occurs. A full frame fish eye lens for example, one where straight lines appear to bend through 180degrees, produces the kind of distorted views it does because, deliberately, no corrective elements are used. It is not made to be a rectilinear lens. Prime lenses tend to produce less distortions because there are fewer elements and less need for optical compromise and it can be optimized for it's particular focal length, while a zoom involves many elements and some compromise, and the wider the focal range, the more compromise is involved. 

Corner Shading / Vignetting

Another problem that can affect lenses is corner shading, the darkening of the corners of an image. It is most noticeable when a lens is used at it's maximum aperture and where large areas of low variable tone occur - the sky is a prime example - and reduces as the aperture is closed down. Although it an be found in any type of lens it mostly occurs where the optical limits have been pushed in order to reduce the size and weight and tends to be more prevalent in wide angle lenses. It is an effect that occurs because the amount of light that passes through a lens reduces from the centre outwards. Mostly this goes unnoticed because the difference is small but if it becomes large enough to be seen, corner shading is the result. As the lens aperture is stopped down, and more light is taken from the middle of the lens, and light dispersal becomes more even, the effect disappears.

Using a lens hood together with flash when focusing at close distances can also cause this effect as the lens hood can block light reaching the image area. When this occurs the effect is known as vignetting. Lens hoods alone can cause vignetting with wide angle lenses, and is the reason many have 'dedicated' petal shaped hoods to prevent it.

The examples we have provided of the distortions and corner shading/vignetting have been exaggerated to make them clear to see. All lenses produce aberrations but in the very best lenses made they are hardly seen whilst in the cheaper types they may be quite prominent.

Corner shading has become a more significant problem with the use of digital cameras than was the case with film. This is because digital sensors are more sensitive to differing light levels. It is a particular problem with Full Frame sensor DSLR's.

Flare

Flare is another common problem that can afflict all focal lengths and results in an overall loss of contrast in an image. It is caused by secondary light rays - non image forming light - bouncing around causing reflections within the lens and sometimes the camera body, and occurs when a very bright light source, usually the sun or it's reflected rays, are present at an oblique angle to the lens image. In other words it's not part of the light that forms the image. A lens hood is often used to help reduce it, as are lens coatings designed to produce a second reflection from the lens surface which interferes and mostly cancels out the first reflection. It is called destructive interference.  Wide angle lenses suffer much more from flare than other types. This is mainly due to the combination of a large front element, which is required to collect the wide field of view, and the resultant small lens hoods that can be used. Very great care needs to be used with ultra wide angle lenses outdoors, as in most cases lens hoods are non-existent [they would cause vignetting], and excluding the sun's rays is difficult because of the view captured. 

Because of the highly reflective nature of digital sensor surfaces as compared to film, extra lens coatings are now being applied to lens elements, and particularly the rear element, to reduce light reflection on the sensor and the degrading effect this has on image quality.

Chroma and Colour Fringing

As the primary colours of light are of different wavelengths, they travel through lens elements at slightly different speeds and angles. This causes the light colours to focus at slightly different lengths, called Chromatic Aberration (CA), and results in loss of definition - un-sharpness -  and colour fringing at the focal plane. It is a problem that is particularly noticeable in long focus - telephoto - lenses. The solution is to use lens elements composed of differing materials with different indexes of refraction to offset chromatic aberration and special glass that includes rare earths in it's manufacture, called extra low dispersion, to offset colour fringing. Lenses that use this glass are usually designated as such by the inclusion of the letters ED or LD [extra dispersion/ low dispersion] in their product description. A lens that is designed and made to focus all three colours is classified as an apochromat - APO. Lenses that included low dispersion elements, and especially those also classified as APO, are expensive to produce, and to buy. 

There are actually different types of CA. That which we have just mentioned is the most well known, and is called LCA, Longitudinal Chromatic Aberration. LCA occurs when different colors focus on different planes. Where they don't register together at the same point and the focal plane. For example, the red channel will be out of focus, while the green and blue channels are in focus. Transverse Chromatic Aberration. TCA, (also known as Lateral Chromatic Aberration), occurs when the colors focus on the same plane, but with different magnifications. For example, the red channel may be magnified relative to the green and blue channels. These are both primarily concerned with the design of a lens and the optical quality of the lens elements.

But in digital sensor days there is another form of CA that has arisen. And this is called purple fringing, because it's not to do so much with lens optical quality and the convergence of the light forming rays on the focal plane, but rather sensor reaction to them, called 'blooming', which occurs with high contrast edges. This type of CA has always been a problem with digicams, and is now starting to become one for DSLR's for one simple reason, it's related to sensor design and pixel size, and gets progressively worse as the number of pixels crammed onto a sensor increase. And as sensor counts have risen, so has purple fringing CA. 

Digicams are prone to producing colour fringing in high contrast scenes. This is often down to their small lenses and sensors and the low resolution and build quality of the lenses used. DSLR's with their higher quality lenses have been much better in this respect, although not entirely colour fringe free. However as pixels become smaller as sensor counts increase colour fringing is becoming more of a problem due to the demands being placed on the lens optics. This is a particular problem with the combination of older 35mm film lenses design lenses used on full frame sensor DSLR's. 

Diffraction

The edge of opaque surfaces such as aperture blades scatter light waves slightly. In contrast to other lens faults, reducing the lens aperture, called stopping down, doesn't reduce or correct it but actually makes it worse. In most cases lenses perform at their best when stopped down 2-3 stops from their maximum aperture. This is called their optimum aperture. When diffraction occurs in significant levels, the result is considerably reduced image quality, and more than enough to offset any increase in depth of field. Diffusion of the image detail occurs, in other words sharpness is reduced and tonal detail lost. This is particularly apparent with highlights.

Diffraction is present at all aperture settings but at large/fast one's is a small and insignificant proportion of the total light passing through the lens to the focal plane, and the film/sensor, the diffusion having no visible effect on the final image quality. As the aperture is reduced and less light passes, this diffusion becomes an ever increasing proportion of the total light and eventually does impact on image quality to the extent that reducing the aperture further to increase depth of field results in lower overall image quality, the extra diffusion offsetting any benefit of extra depth of field. Often a worse image results at say F32 than at F16. Or at F16 than at F11.

A situation that is arising with digital cameras, and is particularly noticeable with DSLR's, is that as the pixel counts have risen so the effects of diffraction have worsened, becoming visible at an earlier stage, due to the size of the pixels in relation to the aperture. In the past, and using 35mm as the yardstick, diffraction did not normally become significant until apertures of F16 and more usually F32 and F45 were used. Now it is appearing earlier at wider apertures. 

With low sensor counts - 5/6/8mp - diffraction occurred at roughly the same rate it had in the past, around the F16/F32 mark. The situation now is that with current 10/12mp capture rates diffraction is appearing much earlier. For the 4/3rds system lenses this is at around F8, and for the APS-C sensors around F11. Only with the 12mp full frame types is F16 reached before this arrives. With the higher resolution full frame sensors, 16-21mp capture, it's back to F11.  

None of this is good news for it means the perceived advantages of increased resolution through higher pixel counts are being offset by higher levels of diffraction.

Matching image magnification and Shutter Speed.

Generally, the shutter speed at which a camera can be handheld without exhibiting blurring due to camera shake varies from person to person. There are those who are capable of holding a camera quite still at fairly low speeds, whilst many find great difficulty at even high speeds. It is also dependant on the level of magnification the lens provides.

There is a long accepted 'rule of thumb' in 35mm photography that, on average, to help avoid camera shake when taking a shot hand held, the shutter speed that should be used with a lens should be at least the reciprocal of it's focal length or the nearest figure above it. In other words for a 28mm lens it's 1/30th, for 50mm 1/60th, 100mm 1/125th, for a 400mm 1/500th etc. It's a very easy and simple guide to follow, for it is only a guide, but a very useful one. However although it uses focal length as the basis, it's not really about focal length but field of view and the scale of image magnification that results, and there is a relationship between field of view/image magnification and shutter speed.

Here are some comparison shots taken from the same point at 200mm, 100mm, 50mm and 25mm focal lengths.

and here is a composite image with the different sizes framed on it

Each time focal length doubles, the field of view roughly halves, the image magnification doubles, and the area the image covers is a quarter of that previously. In the image above the 200mm focal length image covers 1/64th of the area of the 25mm shot. The magnification is 8x times. If you look at aperture and shutter speed Ev combinations - please refer to  Metering  - you will notice that they also run in a scale that doubles or halves the previous figure. So if you follow this rule and set a shutter speed of 1/250th/sec for the 200mm focal length you will notice that it is roughly 8 times the 1/30th/sec speed you would set for the 25mm focal length.

Using this rule soon becomes second nature to most photographers. It's not so much a case of rigidly checking the shutter speed with every shot but rather now and then, and especially when the light levels look low. Most cameras these days have exposure modes that are linked to lens focal length and will try to set a relevant shutter speed. But they can only do this if the light is good enough. It's useful to be able to gauge whether the light levels are good enough for the focal length you are trying to use. If you know that the light level is not high enough then it gives you the option to support the camera somehow, or chose a shorter focal length. Regrettably this is often not possible with many digicams simply because they do not display either the shutter speed or the focal length in use.