Resolution & Contrast

The two most discussed performance issues are sharpness and contrast. Both characteristics are closely related and in combination they form usually define whether a picture appears to be "sharp".

The resolution of a lens is quite easy to define - basically it is the ability to transmit a certain degree of details of a target object - usually measured in lines (or line pairs) per millimeter. In the real world resolution is no 0/1 thing - it is sort of fuzzy - at high magnifications a line isn't just visible or it's not - it is "more or less" visible/fuzzy - dependent on the quality of the lens (the wavelength of light is the limit here in any case).
And this is also the point where we can define image "contrast" - it is the measured difference between two different colored image areas. The illustration below visualizes the difference resolution and constrast.
 

Resolution vs. Contrast (imagine a dot at high magnification)
perfect sharpness  & contrast	not so sharp  but quite contrasty	quite sharp but  not so contrasty	bad sharpness  and contrast	Total loss of  contrast

In a real life picture this may look like this:
 

high resolution and contrast	bad resolution but high contrast	high resolution but bad contrast

As you can see a high contrast is in fact often more important than high resoltion because "washed out" colors are usually much less acceptable. As mentioned above both characteristics are closely related so a lens with a high resolution is usually also very contrasty. Many lenses are often just quite contrasty and bottom end lenses tend to be pretty bad in both respects.

If you want to find out more about the technical background you may check out the article about "MTF" .

Apart from the principal quality of the lens design image quality is also a function of f-stop (technically due to light dispersion and diffraction effects). Usually the maximum picture quality is reached somewhere between f4-11, depending on the max. possible aperture (~ max. Aperture + 2 f-stops - e.g. f5.6 for a f2.8 28-70mm).
 

Stopping down further than f16 will enlarge the depth-of-field but reduce the sharpness/contrast in the focus-plane. The wide-open as well as the fully-closed performance (this means e.g. f2.8 and f/22 for a f2.8 28mm lens) is much worse compared to the optimal point - depending on the quality of the lens. The picture above assumes that the max.quality of 28-70/2.8 & 28-70/4 lenses is about the same!!! THIS IS UNLIKELY!!! A manufacturer will take much more pains over improving the quality of "pro"-speed lenses than with a cheap standard-zoom.

However this "rule" is NOT true for super-high speed fix-focals like 50mm/1.0 or 28mm/1.4. Usually a lower speed counterpart (50mm/1.8 or 28mm/2.8) will easily outperform his ultra-high-speed brother. It is extremely difficult to achieve an acceptable performance with wide-open aperture for high-speed lenses. As a compromise the potential peak performance seems to suffer a bit.

Please note that the effective light transmission may differ between two lenses with the same max. aperture. Different coating quality and front element diameters may be the reason for this. Usually these differences are small but it can be as extreme as 1 EV (1 f-stop). Due to TTL metering this is no problem except maybe for flash photography.

What kind of distortions can be present ?

Distortions are usually well corrected in fix-focal lenses but they are still a problem with zooms. The table below shows the three types of distortions applied to a rectangle. The common characteristic of a zoom lens is that is shows barrel distortions at the wide end of its zoom range and pincushion distortions at the long end. Somewhere in between there's usually a small range that is free of distortions. Barrel and pincushion distortions are not really nice but usually acceptable unless they're too extreme (as the exaggerated distortions below). Some lenses also show wave like distortions which are really ugly and disturbing - fortunately wave like distortions are relatively seldom.
 

pincushion	wave	barrel

Apart from these design related problems there're also distortions due to perspective which are most obivous with wide angle lenses. If you shoot an object other than on the same high it gets distorted. The two pictures to the left may severe as examples here.  Such kind of distortions can only be surpressed (more or less) if you center your shooting height with your main subject. A more elegant solution offer so-called "Shift" lenses which allow either to correct or stress perspective distortions. These lenses tend to be a little bit expensive for most mortals though and are usually used by serious architecture photographers.

The specifications of fisheye lenses look pretty promising - e.g. 16mm f/2.8 at a very cheap price. Unlike corrected wide-angle lenses these lenses show extreme barrel distortions - just for the effect of it. Looks interesting but nothing that should be frequently or it gets boring. The picture to the right below illustrates the effect of a full-frame fisheye lens.

wide-angle lens tilted upwards	wide-angle lens tilted downwards	Fisheye Lens

What is "vignetting" ?

Vignetting means that there's a decreasing (=darker) illumination towards the image corners. Vignetting problems can easily be recognized in plain colored area like the sky as shown the the pictures below - the pictures in the 2nd row illustrate the effective illumination as it would be present when shooting e.g. a white paper.
 

vignetting	no vignetting

Vignetting is a quite normal characteristic and is most obvious at wide-open aperture, especially with wide-angle lenses. However, even with lenses showing a high degree of vignetting the problem can be minimized by stopping-down - 1-2 f-stops down from the max. aperture setting are usually sufficient to suppress vignetting effectively. Some lenses don't even show any significant vignetting at all so, as usual, a smart lens design is a key issue here as well.

What is meant with "Color Balance" ?

In principal a lens should transmit the colors of a scene without any shift - this is what we would call a "neutral" color balance. However, some lenses do indeed modify the color characteristics. If the image appears a bit "warmer" than in reality the lens stressed yellow-red portion of the color spectrum. In contrast to that a "cool" color balance emphasizes the blue side.
A warm or cool color cast is usually not too desirable because mixing lenses would also mean mixing different color characteristics which isn't exactly harmonic.
Color casts can have different reasons:

• the film - which is obviously lens independent. Most films are not neutral but using the same film keep the color characteristic constant so it is usually no problem.
• the glass - cheap glass tends transmit light a bit on the warm side. Some special APO elements may give you a little cool touch.
• by purpose - some manufacturers prefer a warm color balance for their portrait lenses - "warm colored" skin tend to look a bit better (= more healthy) which is often preferred e.g. by fashion photographers - the same effect could also be achieved by "warming" filters like skylight 1A or 1B.

warm (towards yellow)	neutral	cool (towards blue)

What is "Flare" ?

Flare can be a problem as soon as a strong light source can send light rays through the lens. This is most obvious if the light source is visible in the scene but the lens can also transmit light which originates outside the image field. You cannot do anything to decrease problems by direct sunlight but parasitary light sources outside the image field can be (more or less) blocked using a lens hood.

Flare comes in two different incarnations - contrast deterioration and ghostings. The contrast deterioration is visible by a more a less severe haze so colors appear to be washed out. Ghostings can be either multiple images of strong light sources or aperture shaped light blobs due to inner reflections. The picture to the right below shows both image defects.
 

almost no flare	heavy flare problem

What is a "Bokeh" ?

Lately there were lots of discussions in the photo newsgroups about the "Bokeh". The word has a couple of meanings but in the photographic world it describes the quality of the background blurr or in other words - the way how out-of-focus objects are rendered.
So we are talking about pictures with a very limited depth-of-field like typical portraits and especially macro shots.

Before telling you a bit about the technical background we should have a look at the following examples. I've included a portion of the sharp main object but you should have a look on the blurred portion of the pictures. The out-of-focus highlights shows slight differences as you can see.
 

The first picture to the left was shot with a mirror lens. The background looks very "nervous" because highlights look like "donuts". Such a lens has two mirrors, a main one which is visible from the outside and a central mirror which reflects the light towards the film. The small central mirror blocks the incoming light in the center producing the "hole in the donut". The resulting background blurr is not very harmonic so the "bokeh" is considered to be bad here.
The second picture, produced with a normal lens, apprears to be more smooth but it still shows a problem - the out-of-focus highlights are cornered. Basically you can see the shape of the aperture here and if you're looking hard you are even able to count the number of aperture blades. This introduces a distracting "technical" touch to the image which is not very desirable so the "bokeh" is still not good.
The picture to the right is a step better again. You cannot surpress that out-of-focus highlights reflect the shape of the aperture but by using more aperture blades resulting in virtually round "blob" which looks more natural - the bokeh is pretty nice here.

In technical terms there're two main factors for a good "bokeh":

• a large number of aperture blades (perferable rounded blades like with many Minolta lenses)
• a minimal difference between the sagitally (center to edge) and tangenitally (around the center) resolution of a lens. The more these two types of resolution differ the less symmetrical are the out-of-focus highlights.

Personally I'm not too concerned about the bokeh of a lens because IMO there're much more important factors about lens quality but your milage may vary here especially if you are more interested in portrait photography where the out-of-focus character is an important matter of style.

"Angle of View" vs "Focal Length"

For a non-distorting lens, the angle of view at non-macro distnaces is given by:
 

• Angle of View = 2 * arctan (film dimension / (2 * Focal Length))

35 mm format dimensions are 24mm x 36mm, the diameter is roughly 43.27mm. In order to obtain the angle of view numbers as specified in the manufacturer documents you have to insert the diameter setting here - e.g. let's calculate the data for a 17mm ultra-wide lens: 2 * arctan ( 43.27mm * / (2 * 17mm)) =  103.68 degrees.

What is meant by f-stop (e.g. f/2.8) ?

The "f-number" defines the relation between the focal length of a lens (e.g. 50mm) and the diameter of the opening of a lens (roughly the size of the entrance pupil = front element) ...

• f-number = (effective focal length) / (clear aperture)

or in other words: We know the focal length, -say- 50mm, and a given f-stop, -say- f/2.8. This means that the diameter of the opening is about 18mm.