Digital camera lenses 101: Understanding them and their reviews

Choosing a camera lens requires some basic understanding of how digital camera lenses work. In the same line, camera lens reviews are a great source of information and a big help in making the right decision. However, it is only true if you understand them well. If you don't, go on reading!

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Start understanding digital camera lenses

In the introduction to camera lens types, you learnt to understand main characteristics of digital camera lenses.

However, we didn't talk much about their quality aspects. That's what we shall remedy now. Finally, you are going to invest a measurable amount of money in your main working tool. It'd better serve you well for years to come!

This instalment will help you understand how to tell good digital camera lenses from the rest when studying camera lens reviews.

Build quality

Sometimes you only have to hold an item in your hands to tell it's something valuable. You may know nothing about what it is made for, but the way how it is made and how it feels speaks volumes.

It's no different with camera lenses. A barrel made of metal or with metal elements seems more valuable than that of plastic. How the lens lies in the hand, how it responds to its controls, even what accessories are included with it – lens hood, for example – are all part of the first experience which can be the beginning of a wonderful friendship, erm, relation – or an abrupt end to it.

As significant as outer characteristics appear, what's inside a lens is still more important.

A quality camera lens has a complicated inner structure. Rather than being a single glass lens, it unites several optical elements combined in different groups to prevent or counteract defects described in detail below.

What materials are used to produce these elements, how precise they are manufactured and assembled, is also telling. A lens with an audible rumbling of its components inside the body is hardly destined to make many friends.

Optical quality

After a lens made a good first impression, it's still its internal qualities which confirm or overturn the verdict. In a wide-angle lens you'd pay attention to the following values.

Sharpness

Though not exactly of ultimate importance for landscapes, one would expect a quality lens being reasonably sharp. Even if it doesn't have to deliver finest details across the entire frame, this definitely would be welcome.

The most common method to test lens sharpness is to measure its resolving ability. This is often conducted by photographing patterns of alternating black and white lines (yes, boring it is). The more line pairs can be distinguished in the resulting image, the sharper is the lens.

These tests take measurements in different parts of the image, as lenses tend to lose contrast away from their centre. Hence, the resolution is usually evaluated in the frame centre, in its extreme corners, and midway between the two, roughly in the middle of the long frame edge.

Lenses undergo these tests at several apertures, from wide open to stopped down. Zoom lenses have to pass the same series of tests at several focal lengths across their range. Most meticulous tests also include measurements at different focusing distances, e.g., at close range and at infinity.

Results are presented in vertical-bar diagrams, sometimes accompanied by MTF charts either copied from lens specifications or actually measured under test conditions.

MTF stands for Modulation Transfer Function. If you are interested in understanding the latter, have a look at the video below.

Distortion

Besides producing converging verticals when tilted, some optics – not only wide-angle ones – tend to bend every straight line in an image. The effect may be taken care of in-camera by its software, can be corrected to some extent in post-processing, but better lenses don't show it altogether.

This is one of the few advantages digital photography has over its analogue relative. To correct the shortcomings of a lens attached to a traditional camera was unthinkable of. Digital camera lenses – and photographers working with them – have it much easier.

Vignetting

All lenses are prone to the natural light falloff at the edges of the frame. This causes image corners to appear darker. The effect is especially evident with wide-angle lenses. The difference in brightness between the centre and the corners under 0.8 EV may go unnoticed, or even be attributed to artistic intent. Some lenses, however, are much worse than that. Falloff values of nearly 3 EV are not unheard of. This means, the level of corner brightness is below 15% of that in the image centre. This would be too much even for correcting it in post.

Chromatic aberration

The effect is caused by lens' inability to focus light of different wave lengths, that is, colours, exactly at the same pixel on the sensor. This may result in colour fringes appearing all over the image along the edges of areas with different contrast. The impression may vary from very disturbing to outright ugly. Many digital camera lenses include special apochromatic elements in their builds to prevent this from happening.

Lens flare

If you tried your hand at shooting with the sun or other bright light sources in the frame, you are probably aware of hazy and (dis)coloured spots haunting your images. This is lens flare. It is often caused by light reflections inside the lens. Some pieces manage to create it even from scattered light. Other lenses may use special coatings on their glass elements to reduce it.

Bokeh

This term is "imported" from the Japanese and describes the artistic blur quality in unsharp areas of an image. The appearance of these areas can be only attributed to the camera lens at work, as they don't even exist in the reality. Wide-angle lenses are rarely good at it, but every once in a while...

I've just had to think about it as a glimpse into virtual reality in everyday life. Isn't it fascinating?

Tags: #cameralens #photocameras #photogear

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Digital camera lenses resources

Camera lens reviews at DP Review

Lens hub at the oldest site dedicated to digital photography out there. Their camera lens section isn't as exhaustive as that for cameras, but reviews are thorough, like you would expect.

Photozone

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Camera lens scores & reviews at DxOMark

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Unattributed images on this page are sourced from Wikimedia Commons.

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