Marvellous Macro page2

Mike McNamee returns to his roots in microscopy

Lens Extension

There are four ways of creating lens extension

1. [A] Via the focusing mechanism of the lens, sometimes with additional internal lens movements to betteroptimise aberrations. In general modern, aspheric lens technology has enabled more ambitious magnifications to be provided. Almost all of today’s macro lenses can achieve 1:1, compared to only 1:2, twenty years ago.

2. [B] Using Extension tubes. These are sized specifically for an individual lens or, sometimes [C], are mixed size sets so that the changes can be rung.

3. [E] Using a bellows extension. Only Nikon currently have bellows in their catalogue. Bellows allow for the largest magnifications as well as the use (with adaptors) of specialist micro lenses and the use of enlarging lenses. They are cumbersome to use and disconnect the electronic signalling between lens and body.

4. [D] A teleconverter. This increases the effective focal length of the prime lens and a 2x converter doubles both the working distance and maximum magnification of the prime lens alone, at the cost of a 2-stop loss in aperture (which is usually not an issue as you are striving to get to f16 anyway!).

All of these methods create more magnification than that which is achievable with the prime lens alone. Changing the rotating focus barrel of the lens (or auto focusing) changes the magnification and for scientific work, in which a known value is important, it is usual to focus by moving the whole camera back and forth until the subject is sharp. For critical, higher magnification shots, a focusing rack is invaluable, as it allows for precise movement of the camera. The alternative is to keep the camera fixed and move the subject, sometimes by means of a scissor jack or some other mechanical device. Quality scissor jacks can cost as much as a digital SLR!

Depth of Field

Digital has improved many aspects of macro photography, especially the gauging of correct exposure which always involved either calculations or the use of tables or nomograms (oh happy days!). This used to be compounded by the problem that the lens was electronically isolated from the body so that the exposure meter did not function. Nowadays you simply look at the histogram after exposure and you know, very precisely, how to adjust you exposure as it is a WYSIWYG system. Depth of field though has become a little more confusing, not less. You used to be able to rely on little booklets, that came with the lens, to look up the depth of field. However, the different chip sizes and formats mean that these tables no longer apply unless you have a full-frame SLR. We can at least provide you with tables for the classical 35mm frame size and the Nikon D size of chip (23x15mm). This gets you into the right ballpark at least. We have assumed a circle of confusion of 0.016mm, arrived at by backcalculating from 5x4in, through 6x6cm and 35mm formats to the D-size chip. Probably not a perfect calculation but close for most applications outside of an optical laboratory.

You can see from the values in the table that the depth of field values are very small. For example at 1:1 and f2.8 it is a mere 0.3mm, the thickness of a fingernail. This brings along a host of other problems when trying to focus upon a subject.

Focus

As macro distances are approached, the usual rule of focusing 1/3 distance into the subject (or group at a wedding say), breaks down, so that by the time you get to 1:1, the optimum distance is half way into the subject; that is the depth of field is equally disposed to front and rear of the focus plane. From a composition point of view, however, the usual rules still apply, namely that big splodges of out-offocus foreground are very distracting. Selection of the focal plane therefore is critical and often not best served by auto focus. A better approach is to decide on the magnification required and then to manually set the ‘focus’ to that point using the lens barrel markings, then to approach the subject

Page 1 - Page 2- Page 3 -Page 4 - Page 5 - Page 6 - Page 7 - Page 8

Photo Quote: You've got to push yourself harder. You've got to start looking for pictures nobody else could take. You've got to take the tools you have and probe deeper. - William Albert Allard