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Home >> Learning >> e-Book on Photography Table of Contents Photography e-Book Chapter 6 Subsection - Lens Technical
This is pretty dry stuff to work through and understand and truth be told I never bothered to until I decided to do this project. It is insomnia busting material, especially when looking at equations and I assure you that I still wonder how I got into university with my math grades. If you can slog through the reference material I link to at the end of this chapter, you will actually benefit in understanding a little of what an optical engineer and designer has to take into account to create those wonderfully sharp lenses in our camera bags. The aficionados of fine lenses often use MTF charts to get a measure of a lens's characteristics but for me and others who are less technically inclined, the results on film are what count. History The first lens was not even a lens, as we know it now but actually a hole in the wall, specifically a pinhole camera also known as a camera obscura. This pinhole camera goes back to ancient times when it was discovered that light passing through a tiny hole would project an image of an object that light passed by first before entering the pinhole. The light was projected in a dimly lit or darkened room; yes, camera obscuras were huge room sized projectors. With camera obscuras, there was the recognition of what the subject was but not a whole lot of definition or resolution to the subject. This changed when the camera obscura had an optical lens placed in front of it, around 1550 AD. Now definition of the details of subject were detected but it would be hundreds of years later before a viable means of capturing the image in permanent form would be developed in the 19th century with the Daguerreotype in 1839. The physical properties of light were not developed until around 1000 AD, by an Arabic physicist, Ibn el-Haitam. With the properties known, development of optical lenses progressed so that by the time the Renaissance era was in full bloom, microscopes and telescopes were being used by the leading scientists of the time. By the time Daguerreo came up with the idea of bringing together optical lenses with a relatively portable camera box and light sensitive silver salts to permanently record the image, a whole new means of expression was born and a new art form was created. The latter 19th century saw significant improvements in optical lens production but the real golden era was in the first half of the 20th century when two venerable German optics companies gave the world the first usable cameras using the 35mm film format. Carl Zeiss (Contax) and Ernst Leitz (Leica) had some remarkable scientists working for them during this early stage of the 35mm format camera. Lenses in this era were/are capable of fine image quality but they were plagued with some problems that all optical companies had to contend with,
These and other problems were not resolved until the lens coating process was developed, which allowed optical designers to work with more complex lens designs to correct the problems in the favored simple lens designs. With the lens coating process in place, the differences between old classic lenses of the 1940s and 1950s are considered minimal to those of the current era. Of course, we have had some meaningful progress since the first half of the 20th century. Such improvements as the multi-coating process (apparently co-developed by Zeiss and Pentax, T* Coating and SMC respectively) and the computer aided design of lenses has made for enough improvements that even Leica considers its modern designs to be superior to their old ones. If you are the proud owner of elderly optics (1960s and 1970s) from companies such as Nikon, Canon, Olympus et al, and if they are in good shape, there is no need to get rid of them. Proper technique with your present equipment will often be more meaningful than an overhaul of the kit. Consider that my Nikkor 35-70mm f2.8D AF lens is getting old in design and upgrades to it have been more about features such as auto focus and Nikon's "D" chip rather than optical improvements. This lens dates from 1987 but is still considered to be among the best zoom lenses produced by Nikon and holds its own against the Nikkor 28-70mm f2.8D AF-S when comparing Photodo's ratings. The 35-70mm is still being produced in parallel to the 28-70mm and at less than half the price of the motorized lens 14 years after being introduced. Modulation Transfer Function or MTF MTF graphs can be a difficult process of understanding optical theory. This is hardcore technical data used by lens designers to formulate the compromises inherent in optical design. Improvements are made on a regular and gradual basis but we still live in an imperfect world and building the perfect lens is not possible at this time if ever. We want a high level of resolution and a high level of contrast. Resolution refers to how well we can detect all the fine details on film but contrast refers to how well we can define the boundaries of similarly toned subjects at a micro level. Humans being greedy by nature want the perfect lens with 100% resolution and 100% contrast but this is impossible and so the trick or art is how well the lens designer compromises to achieve the result photographers find pleasing. The basis of measuring resolution and contrast is via closely spaced pairs of black and white lines. One black line and one white line equal one line pair and how many of these pairs can be resolved by the lens is taken as line pairs per millimeter or lp/mm. The more lp/mm a lens can record, the better its resolution. The greater the ability of the lens to differentiate the lp/mm at the micro level, the better its contrast. Resolution and contrast may sound similar but they are actually different and the physical representation of this is the MTF graph. Another way of describing the MTF is that it represents just how much of the subject is actually resolved by the lens on the film, the loss being represented by the MTF graph. On the MTF chart, we have an X and Y grid box with both starting from the same 0 point at the lower left corner of the box. The 0 and extreme left vertical line represents the center of the film where resolution and contrast should be the highest and as we follow the measurements outwards towards the film edge, this represents how much resolution and contrast drop off. The X coordinates are horizontal and measures resolution. The closer it is to 100, the better the resolution in lp/mm. The Y coordinates are vertical and measure contrast. The closer it is to 1, the better the contrast. There are usually two graph lines that are shown in a MTF graph, one solid line and one dotted line. The solid line refers to sagittal and the dotted line refers to tangential measurements. I will be honest here and admit that I am still trying to grasp just what the differences between sagittal and tangential measurements are. What I can tell you is that you do not want to see a MTF chart in which there are sudden drops in the lines resembling a waterfall that either bulges inwards or bulges outwards. An inward bulging line indicates good resolution but poor contrast. An outward bulging line indicates good contrast but poor resolution. What you want to see is a gentle drop off for the contrast as you move further outwards on the horizontal X scale.
A good lens should have high contrast (Y) going far out along the X coordinates. Contrast falls off as you try to resolve ever more detail as the "noise" between the lp/mm will make it harder for the lens to distinguish between the lines. MTF charts are a good place start to get a measure of a lens but they are not conclusive in actual real world use. MTF charts are usually given for two apertures only, wide-open and one stopped down to a middle aperture and usually at infinity. To get the full measure of the technical aspects of lens you need dozens of charts at all apertures and at varying degrees of focus from close up to middle values to infinity. This is not practical obviously but what we want to see is consistency from the lens company for their choice of compromise between excellent contrast and excellent resolution. To be quite honest, it would be hard to believe that any reputable lens company would turn out any dogs these days, outside of their lower end consumer-oriented products. Purchasing a lens from a company’s top range pretty much guarantees you an excellent piece of glass no matter what the brand name. It is just a matter of flavor and which company’s you prefer for your use. Does this mean all lenses are equal in resolution and resolving power? No, but are the small differences worth the extra cost for you? Having said that many people appreciate that companies such as Leica and Zeiss do not compromise on any of their lenses and none can be considered less than professional in build and quality produced, unlike the Japanese brands that do build consumer lenses to obvious price points. Diffraction F8 and be there, is a common refrain heard by amateur photographers starting out in photography but why f8? When early optical scientists were discovering the various properties of light, it was discovered that light passing through a small hole produced sharper looking images than from light passing through a large hole. However, there is a limit to how small the hole can be before a nasty artifact known as diffraction rears its ugly head and counteracts the sharpness of a small hole. Diffraction occurs when light waves (and light does travel in waves) travel through a small hole and the light near the edge of the hole becomes bent. In photographic terms, the hole is the aperture or diaphragm and the smaller the aperture the more diffraction plays a role in masking the sharpness of small aperture settings. I have taken test shots from wide-open to fully stopped down. I found that as I went down the lens stops, the images improved and reached a maximum resolution by f5.6 or f8 and then gradually went down in quality as I moved further down the aperture scale. By the time I reached f22 and then compared it to f2.8 (on a Nikkor 80-200mm f2.8 lens), I found them almost identically poor in resolution compared to what I received with the mid-level apertures.. The loss of resolution from f11 on down for the 80-200mm lens is a result of diffraction as the light waves at the edges are bent more when they pass through the aperture. The light at the edges become softened and are added to the unaffected light waves, the smaller the aperture, the more softened-light is added. This softening effect occurs whenever light travels near a hard edge, as aperture blades are. There are other reasons to use moderate apertures instead of wide-open. When the diaphragm is fully open, the entire lens surface is used and the slight imperfections of the glass can cause problems. Stopping down a lens allows the use of smaller portion of the lens surface and produces higher quality results. Stopping down to middle apertures can also alleviate other factors such as light fall off. In theory, if you could build a lens with a diaphragm that had a setting of "0", you would obtain zero diffraction and 100% contrast. In reality, it is very hard to design and produce lenses of exquisite quality at fast, wide-open apertures even without shooting for the unattainable f/0. Lenses that can perform as good wide-open as they do stopped down are also exquisite in price, read Leica optics. Other Problems Afflicting Lenses
Have you stayed awake through this chapter? Obviously, these comments are very basic from a person trying to understand and explain them to the best of his abilities. If you want to read the actual material I used to write these comments, they are below. References - this section has been heavily influenced and in some parts, merely regurgitated, by a few articles on the topic of optical theory and history. In particular,
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Correspondence & About this website Copyright © 1998-2008 Edwin Leong |
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I
commented on some basic types of lenses and how or why they are used
for different types of applications but what of the lens itself? At
the risk of overlapping some other sections of this e-book project,
I will try to provide some rudimentary basics about the lens.
For
fast lenses it is likely that you will see more sudden drop offs at
wide open apertures than for when the lens is stopped down to middle
apertures. It takes a bit of time to try and understand what a MTF
graph is telling you but one thing to look out for is how closely
the solid and dotted lines are to each other. The closer they are
the better the out of focus elements of the image will be whereas
the farther apart they are, the harsher the out of focus elements
will be and will indicate some astigmatism in the lens design. Note
though that astigmatism for optical lenses is not the same as astigmatism
for human eyes. Optical astigmatism has to do with off-axis focusing
problems of the lens whereas human astigmatism has to do with on-axis
focusing problems. Not that this really matters all that much in the
whole scheme of things like trying to end world hunger and bring about
world peace but some people care about this stuff.
