TUTORIAL – 2
ALL ABOUT LENS AND FOCAL LENGTH SIMPLIFIED
Read PART I: FOCAL LENGTH & LENS Here
Read PART II : TYPES OF LENS Here
PART III: QUALITY OF LENS & LENS ABERRATIONS
QUALITY OF LENS: The high quality of lens depends on many things like design/construction, coating, image quality (sharpness, color rendition), maximum/constant aperture, fixed lens size, auto-focus speed, weather sealing etc,.
1. Optical Design & Construction: Good quality of lens requires more optical elements to reduce lens aberrations like lens flare, chromatic aberration, lack of sharpness. Lens like Canon 70-200mm f/4L IS with 20 elements in 15 groups, Nikon 14-24mm f/2.8 with 14 elements and 11 groups, Nikon 35mm f/1.4G with 10 elements in 7 groups are known as lens of excellent high quality for sharpness, fast focusing, contrast and fine image quality reducing lens flare, and keeping minimal softness along the edges. More importantly is the type of lens elements used within lenses like aspherical, Extra-Low Dispersion ( ED ) and Fluorite with special coating such as Nikon’s Super Integrated Coating (SIC) and Nano Crystal coating which improve sharpness, contrast, colors, reducing ghosting and flare.
2. Image Quality (Sharpness,Contrast And Colors): High quality of lenses are designed in such a way that these produce image with high quality, with sharp center to corner resolution. Such lenses are able to reduce optical aberrations like distortion and vignetting ( dark corners of image ) and yield super colors by keeping chromatic aberrations minimal.
3. Constant Maximum Aperture: Good quality of lens will provide the maximum aperture constant at all focal lengths of a zoom lens allowing more f/stops for low light photography. For example in Nikon 70-200mm f/2.8G we can use f/2.8 at both end 70mm and 200mm while cheaper lenses have variable maximum aperture. Nikon 18-105mm f/3.5-5.6G ED has variable aperture in which maximum aperture f/3.5 can be used at wide end 18mm only while at 105mm the maximum usable aperture would be f/5.6.
4. Fast Autofocus Speed: Professional or high quality lenses are equipped with very fast autofocus motors that brings subject into focus immediately. Such fast autofocus system is very useful rather necessary for wild life, sport and candid photography.
5. Fixed Lens size: Because cheaper or consumer grade lenses change in size (extend/collapse) when zooming in and / or focusing these are prone to potentially malfunction or break in the future. High quality of lenses remain fixed in size due to their internal focusing system.
6. Vibration Reduction(VR)/Image Stabilizer: Good quality of lenses contain such device which( small gyro stabilizer sensors and servo-actuated lens elements) enables to correct camera shake that occurs with longer focal length lens or shooting with slower shutter speed in low light conditions. It is claimed that these lenses enable the photographer to shoot handheld at 2 to 4 stop slower shutter speed.
7. Weather Sealing: Lenses of good quality with their tough construction are mostly sealed against dust and moisture so they perform well in extreme temperature, humidity and even in rain.
LENS ABERRATIONS: (courtesy : kenrockwell.com)
Distortion: Ideally straight line would stay straight in a photograph but in reality lines often curve a little bit In or Out when these run along the sides of an image. When the lines curve out from the center of the image it is called Barrel Distortion (wooden barrel); while inward curve refers to ‘Pincushion’ (pillow) distortion. Most zoom lenses have barrel distortion at their shorter end and pincushion distortion at their longer end but a point in the middle is usually free from distortion. So we may lessen the distortion by avoiding extreme zooming. Perspective distortion can also be seen while shooting tall buildings by using very wide focal length. Perspective can be corrected by using Tilt-Shift lens/PC or Shift lens.
Flare & Ghosts: Lens flare is the most commonly known aberration which is caused by taking shot directing the lens towards the sun or other brilliant source of light. The resultt is bright spot and streaks in the frame near light source. We can avoid lens flare by using proper lens hood (petal-shaped hood for wide angle lenses and round hood for telephoto lenses). Multi-coated lens elements too reduce undesirable reflections. Ghosting is the result of light bouncing off sensor, reflecting off of the back lens elements and returning to the sensor. Ghosting usually creates a soft, a slightly blurred off centered copy of the scene. Multi-coated lens elements reduce reflection as much as possible.
Spherical Aberration: It is another kind of optical aberration which results from the difference in refraction at the edges of a lens compared to the center. It causes softer focus rather than sharp focus. We can avoid spherical aberration by using high quality lens designed with Aspherical lens elements which cause less refraction at the edges and more in the center, resulting in proper convergence at a given focal length.
Chromatic Aberrations: This occurs when all colors do not come to focus at exactly the same place. Chromatic Aberration causes loss of sharpness and appearance of various color fringes on bright edges of an image. In other words chromatic aberration is caused 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. As a result color fringing(like red, green, yellow, purple, magenta) can appear around objects especially in high contrast situation in the photo. There are two types of chromatic aberration- Longitudinal Chromatic Aberration and Lateral Chromatic Aberration.
1. Longitudinal(Axial) Chromatic Aberration(LoCA) occurs when different wavelengths of color do not converge at the same point after passing through a lens. In LoCA red, green, blue or a combination of these colors can appear around objects. This aberration can be reduced by stopping down the lens ( using small aperture like f/16 ). Prime lenses with fast aperture are much more prone to LoCA than slower lenses.
2.Lateral/Transverse Chromatic Aberration occurs when different wavelengths of color focus at different positions along the same focal plane.
Unlike LoCA, Lateral Chromatic Aberration is not seen in the center and is only visible towards the corners of the image(blue and purple fringing) in high contrast areas when using fisheye, wide-angle and low quality lenses. It can be reduced in post processing software. Though by using achromatic/ apochromatic optical designed and extra-low dispersion lens elements(ED) chromatic aberration can be reduced to certain limit it is still an issue on most prime and zoom lenses. Many good DSLR’s have in-camera post processing technique to tackle with chromatic aberrations.
Diffraction: This sort of aberration happens when light passes through tiny hole of small aperture like f/22. It ( light ) bends and interferes with itself resulting in loss of sharpness. But when light passes through large aperture like f/2.8 it does not bend or disperses therefore no diffraction also. Large full-frame sensors exhibit diffraction problem less than smaller sensors.
About the Author:
A.FIAP, Hon FIP, Hon.ECPA
M.A.Philosophy (Gold Medalist), Ph.D., Retd.professor & Head, Dept Of Philosophy. Photography is his passion. Got more than 200 awards including Kodak, Nikon & 10 Gold medals. 20 slide shows at Singapore, MIT U.S.A., Mumbai. Ahmadabad, Varanasi,Indore etc. Published pics in ‘Popular Photography’ U.S.A., Practical Photography. Amateur Photographer England, Invited as a member of jury by Lalit Kala Academy, Lucknow and Ahmadabad and about 60 International salons held in India.