Understanding Lens Properties

Upgrades aside, purchasing additional lenses is for most of us, driven by the desire to capture images that our existing lenses fall short of. There is nothing quite like taking a new lens into the field and shooting a bunch of pictures that have a different perspective to anything previously taken. The best fun I had was getting into wide angle photography many years ago; I used a 28mm lens for some years but found it was not capturing the expanses I wished to photograph. This heralded in 24 and 21mm focal lengths which addressed the problem. The ‘wow’ factor when looking through the 21 for the first time is something I will not forget in a hurry, and it opened the door to some great subject matter. As this was back in the film days, it took me a while to understand fully the properties of the lens simply because the results were not instantaneous. I also had to take notes of camera and lens settings for consultation when my slides were returned, something that is taken for granted in these days of easily accessed EXIF data.

All lenses have optical flaws - some more than others, and they usually result from compromises made in their design, the quality and geometry of glass used and of course the price point of their intended target market.

Inspired by the recent purchase of a Pentax 100 f2.8 macro lens, we decided to put together an article that explains a lens’ properties in terms of general characteristics that can be found in most lenses, and not relating just our recent acquisition. It is fair to say that the majority of lenses achieve optimal sharpness when stopped down one or two stops, but some lenses such as those manufactured by Leica and Olympus’ Pro and Top Pro Four Thirds offerings are optimised for use wide open. Also many lenses, with the exception of macro optics, are optimised for use at infinity although some have a group of floating elements that move independently to others when focussing closely.

Lens Properties

Aperture has a direct impact on depth of field and shutter speed; a lens set to f2.8 will have significantly less depth of field (the distance in an image that is in focus), than when set to f8. A much faster shutter speed can be used at f2.8 than f8 as more light is entering the lens and striking the sensor/film.

As eluded to previously, using as lens wide open may not achieve the sharpest image, but the quality of the lens will have a great deal to do with this. It would be reasonable to state that a budget ‘kit’ lens will be softer wide open that a professional grade optic and I can base this statement on my own experiences over the decades. But budget lenses aside (some of which turn in remarkable performances when stopped down) deliver very good results for their price point.

Stopping a lens down one or two stops usually ensures that the lens is performing at its sharpest and reduces the presence of optical aberrations such as vignetting, CA (Chromatic Aberration) and coma. This is usually referred to as the ‘sweet spot’ – the place where all of the lenses properties conspire to provide optimum results. Shutter speeds are lowered accordingly, but can be compensated for by increasing the ISO a little. Once stopped down beyond this, performance begins to drop off slightly due to diffraction caused by the light striking and bouncing off the lens aperture blades.

If your subject matter is predominantly portraits, diffraction will probably not be much of a concern as you will shooting with the lens wide open or shut down a stop. If your subject choice is macro you will be shooting stopped down (sometimes as far as f22 or even f32 depending on the power of the macro lens) to increase depth of field, so factor in the effect of diffraction and see how much it impacts on your results. Remember, all lenses perform differently so there is no hard and fast rule as to where diffraction becomes noticeable. I have a Zuiko 80mm macro lens that is perfectly acceptable down to at least f11, possibly f16. There again I have used others where the effect becomes quite apparent at f11. Landscape subjects will suffer likewise as a small aperture is necessary to create increased depth of field, which produces images where the rock in foreground is as sharp as the horizon many miles distant.

What crawls up my pipe is Chromatic Aberration or colour fringing, and is the curse of many lenses, particularly those that fall into the category of wide or super wide. It usually makes itself known when shooting such lenses at open aperture and more so in the corners of images that have high contrast such as tree branches against a light coloured sky, or the edges of silhouettes. Simply put, it is the inability of a lens to focus different light waves in the same place. As they strike the sensor, they do so at an angle; the red wavelength focusses in a slightly different place to green, resulting in the purple of green fringing often seen in areas of high contrast.

Stopping a lens down one or two stops usually goes a long way to minimizing the problem, and in those situations that call for a wide open shot, many image processing packages provide a method of removing it during the post processing stage. We have lenses that exhibit this problem in varying degrees; The Zuiko 11-22mm lens can suffer badly when wide open at 11mm, as can the Pentax DA* 16-50. The Leica X1’s Elmarit 24 displays a little in the corners wide open, but is well controlled and gone when stopped down, as are the Zuiko 50mm and Pentax 100mm f2.8 macros. My Zuiko 180mm f2.8 for my OM cameras is also bad, demonstrating that telephotos can be just as susceptible to the problem as those providing a wider view.

Vignetting is the term given to areas of light fall of in the periphery of an image, when compared to the centre. Again, this is usually visible more in super wide angle lenses when shooting wide open and most easily seen where the subject is predominantly the same colour e.g. a snow scene or large area of blue sky; corners may appear darker than rest of the image and the problem is exasperated by the use of incorrect lens hoods or deep rimmed filters. Stopping down to f4 usually resolves things and interestingly, image processing software usually contains algorithms that can add the effect for artistic reasons.

Coma is an aberration that renders points of light as ‘teardrop’ shapes towards the edges of an image whilst light points in the centre resolve correctly. Naturally it becomes more apparent when taking night shots that contain street lights or when photographing star fields. My experience of coma is that it can affect lenses of all focal lengths and stopping the lens down helps greatly in illuminating it. The worst examples I have seen relate to budget zoom lenses where the light is passing through lesser quality elements.

There are several other types and sub types of lens aberrations which I will not cover here. All can play a part in the resolution of the final image and it is not the purpose of this article to delve into each one individually. After using a lens for a while its strengths and weaknesses are soon learned. Given all that has been written above, I do not shy away from using any apertures when an image calls for it. But I am mindful that the result may not be as perfect as it should be – I would rather take the image and rely on a little post processing than not take it at all.