5.2 Lens types by focal length
Focal length determines how much angle and how much magnification according to the size of your subject, the desired framing proportions at your standing position (distance from subject) accordingly. Note that the first two factors are equipment factors (angle of view, magnification factor) whereas the other three are subject and user factors, again illustrating that the user is not everything.
5.3 Ultrawide angle
Nomenclature
This category of digital lens come into place after modern DSLRs adopt a smaller sensor size, as such many focal length now becomes more tele on the new bodies using the same old lens. With a 1.5x crop factor, a 24mm (wide) becomes 36mm (mid wide) and a 35mm (mid wide) becomes 50mm (standard). The previous wide angle lens now loses the wider end of the wide angle, and started from the mid wide portion onwards. New digital lens are thus invented to cover the wider end of the previous wide angle, as well as a portion that is really wide, called the ultrawide angle.
As there is no objective definition to the angles, it is determined by two things, by what is commerically available, and secondly by what you can subjectively set as a cut off. The current ultrawide angle lens are mainly between 10-24mm which on 1.5x crop factor becomes 15-36mm. As for personal subjective cut off, I notice more distortion and apparent differential sizes of near-far items at around 14mm and below on my 1.5x crop factor Nikon D50, which is about 21mm on 35mm equivalents.
Application
The use of ultrawide angles are used for various purposes, which is usually done NEAR to the subject.
1. need to encompass many separate activities or details on a horizon.
- for panaromic crop of human activities
- for landscape with vertical details, e.g. mountains, with which the wide angle requirement is determined by how far you are from the subject. If you are really very far away from the subject, such as mountain ranges where you are standing like a few kilometres off, very often wide angles itself are sufficient and in some cases, isolation with longer focal lengths may be applied instead.
2. need to cover a broad cluster of activities or subjects in an enclosed space
- small exhibition rooms through a window
- indoor with limited space, group photos (distortion would be apparent if it is too wide)
- ultrawide angle is very useful but beyond a certain threshold, distortion becomes very bad and is not nice for faces near the periphery, the less wide end of a ultrawide angle lens is thus better applied if the space constrain allows
- once out into the streets and alleys, there is often a little more space to move around, thus a conventional wide angle to standard focal length lens (with 1.5x crop factor bodies) are better suited for this purpose instead of ultrawide.
3. need to cover a tall or big structure very near to you
- usually on buildings where one hope to capture the roof to the bottom, usually a chapel or something, about 2 floor high
- you may be able to move to the farthest end, on a road opposite this building, often with your back against the building that is opposite your subject.
- distortion especially of converging verticals is acceptable, and can gives good effect depending on your taste. failing which the only other option is to invest in a tilt shift lens or to move to a floor that is directly opposite to the centre of the building.
4. to emphasize size differences between near far effect
- it can be applied on many things, such as the face of an animal. it can achieve very comical effects, and less suitable for solemn subjects, e.g. human portraits.
- the application on sports, provided safety is ensured, when being very near to the subject, can bring drama to the activity.
Choices
A good thing about ultrawide angle lens is that it has not changed much over the past 2-3 years. I have been looking at the same lens since I've started on DSLRs. Apart from Canon/Olympus/Pentax/Sony-KM mounts, there is currently 5 common ultrawide angle lens available for Nikon mounts.
Nikon 12-24mm
Tokina 12-24mm
Sigma 10-20mm
Tamron 11-18mm
Sigma 12-24mm (usually not in comparison, as the 10-20mm lens is more commonly compared for its advantage of being wider)
To date, 2007 Aug 20, this few lens are still the only ultrawide zoom for Nikon mounts.
For good reviews, one can read the following,
Kenrockwell: Very directional guide to lost souls.
Nikonians : More objective guide to more experienced users.
Issues
Problems with ultrawide angle is of cos due to the fact of its extreme optics
1. Due to its difficult optics which is a long story, ultrawide zooms are large and heavy, yet difficult to give very good optic quality as compared to the wide and standard focal lenths.
2. There is a higher likelihood of barrel distortion as well as converging verticals when pushed to extreme.
3. Faces are distorted at the peripheries and presented a main source of visual disturbance to some as human face are one of the most familiar recognisable subject that is least tolerant to distortion.
4. Light fall off may occur towards the peripheral due to difficult optics.
5. Ultrawide are more prone to physical vignetting, which depends on the thickness of the extension, which are usually in the form of extra screw on filters and cokin rectangular filters. As a rough guide for me, a normal thickness UV filter + a single slot cokin holder will physical vignette at 12mm and below, and clear safely off the shadows at the corners from 14mm onwards.
6. Ultrawide emphasize the uneven polarisation even more, when the sun is on one side of the sky near or within the frame.
7. Ultrawide brings in a lot of horizontal details which test one's judgement on composition.
8. Ultrawide can make things really small. If there is a lack of vertical interest, an ultrawide landscape can be boring, flat and undramatic, with a large empty sky, large empty foreground and tiny subjects at the horizon.
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