Larger than life 02 – supplementary lenses and stacking A. PHD

There are several methods for getting larger than life images projected onto a camera sensor though it comes down to just two practical routes for most photographers: coupling something to the front of the camera lens or behind it between camera lens and body. The former embraces supplementary lenses (diopters) and, with the same principle, ‘stacked’ lenses: the latter includes the use of extension tubes and teleconverters (multipliers).

There is a third way for Canon owners – the purchase of the Canon MP-E 65 mm 1x – 5x. that focuses by means of a lengthy, in-built extension tube. I’ll say more about ‘true’ macro lenses later in this series but suffice it to say it is not an easy lens to use. In competent hands it produces excellent results…though no better in my experience to those obtained  by ‘stacking’.

ADDING A  SUPPLEMENTARY

For many this is the first route into getting close-ups without the purchase of a macro lens. Basically, supplementary lenses are screwed into the filter holder of a lens attached to a camera: in many texts this is called the prime lens though the term has come to mean lenses of fixed focal length as opposed to zooms. Supplementaries are are sometimes called dioptres (diopter – USA) a bit sloppily since the ‘dioptre’ is the spectacle maker’s measure for lens strength, not the name of a lens. When you see +1, +2 and +3 these are the strengths of the lenses in ‘dioptres – the focal length of these is 1/ (focal length in metres) making these lenses of 100cm, 50cm and 33.3cm respectively.

They are easy to use and no light is ‘lost’ – with extension tubes light is spread out as a lens is moved from the sensor so the light intensity gets less. Images obtained can be soft at wide apertures thanks to spherical aberration (created by reflection between lens surfaces) and results are greatly improved by stopping down the camera lens to use light rays near its centre.

If you spend considerably more money and buy lenses of two elements (two different glass types cemented to eliminate chromatic aberration – the colour fringing at edges with simple lenses) called achromats: these work very well, particularly with zooms. Sigma, for example used to supply a lens to get 1:1 with their trusty 70-300mm zoom. The effective aperture of a lens plus supplementary remains unchanged: it is the figure set on the lens diaphragm.

A new range of highly-corrected supplementary lens has appeared with items from Raynox and Micro-Tech-Lab that provide a very convenient and effective way of getting up to 3x lifesize. I hope to be able to review a couple of these, soon.

MAGNIFICATION WITH SUPPLEMENTARIES

There is a useful formula that works with all kinds of supplementaries including stacked lenses.

Magnification M = focal length of primary (set to infinity) ÷  focal length of supplementary or stacked lens

Lenses such as the 1D, 2D and 3D above are close-up lenses…a 3D used with a 200mm lens would give just 2/3 lifesize magnification (less for the others and for shorter focal lengths) by racking out the focus it would go closer to 1:1

STACKING LENSES

Lens stacking is the business of adding a reversed photographic lens to the front of another camera lens (of longer focal length ) and it affords one of the best ways of getting larger-than-lifesize images, both optically and from a cost point of view by using a wide-angle or standard lens you may well have already.

You may have seen or used a macro or medium telephoto lens (100-200 mm) – here termed the prime lens – with a 50mm standard lens reversed and attached to the front of the camera lens by a male-male thread adapter that couples via the two filter rings. In this role the added lens acts as very high quality supplementary…a 50mm lens has a 20 dioptre (20D) strength and a 24mm boasts over 40D strength. It is a great way of using the old ‘standard’ lens that ‘older readers’ may once have bought with a film SLR.

With the prime lens set to infinity, the magnification “M” you obtain is given by the formulae quoted above:

M =  focal length of  camera lens ÷ focal length of lens of stacked or supplementary lens

Thus, with a 105mm lens affixed to camera and lenses of 50mm, 35mm, 28mm and 24mm you’ll get magnifications of 2.1 x, 3x, 3.75 and 4.4 respectively: use a 150 mm as the prime and you get 3.2x, 4.5x, 5.6 and 6.6…and so on.

• Quality is excellent and the wider the maximum aperture of the front lens the better for ease of focusing
• Vignetting is possible (more likely with full frame sensor and with short focal lengths especially if there is a big difference between the front lens diameters but adding an extension tube allows rays to spread out and correct this
• With a tele-zoom coupled to the camera you may (or may not get good results) so much depends on the zoom design. You need to experiment.
• wide-angle zooms coupled to a prime lens tend not to work but they can be used reversed on to a bellows (see future posts)
• Again, depending on apertures AF might function but it is better switched off in all macro work to prevent ‘hunting’ which at greater magnifications would make focusing impossible
• One important aspect is that when aperture is controlled from the prime lens a camera retains TTL exposure control.
• Whenever you deal with magnified images light is ‘lost’ – a loose way of saying that there is a reduction in intensity at the sensor due to light spreading out over a larger area.
• Most modern lenses (especially wide-angles) are asymmetric in design with a front element considerably bigger in area than rear: when reversed, the light gathering power is reduced with the smaller lens element facing forward. The effective aperture is not the marked one.

CONTROLLING APERTURE – a bit of a dilemma

There are two lens diaphragms involved so which should you use to control the lens aperture –the one in the front lens or in the the prime fixed to the camera? You will find conflicting advice: John Shaw (Close-ups in Nature) states that it is the diaphragm of the lens directly coupled to the camera you should use to control depth of field. An earlier book by Lester Lefkowitz (The Manual of Close-up Photography) who was also a professor of optics, suggests it should be the front lens diaphragm…there is truth in both but efficacy is best determined by experiment rather than theoretical discussion .

So, here’s what I would suggest in practice from my own experiments over the years:

• Stop down the front lens to around f4- f 5.6 (if it has a manual aperture ring that is) at most solely to optimise performance with a lens of wide maximum aperture: this reduces spherical aberration from the edges of the lens element of large diameter. Where there is no diaphragm ring use wide open or employ a “Z” ring that allows you to close the diaphragm with a cable release. These are tucked away in accessory lists (eg Nikon BR-6 auto ring)
• Control Exposure from the prime lens coupled to the camera body. Check through the viewfinder to see when vignetting sets in. You will then have a limited range of useable apertures up to this point on the lens before you notice the edges of the image field darken
• Connect an extension tube between the prime lens and camera body to spread the rays a bit if vignetting happens even at the widest aperture of the prime lens…(possible with a big difference in lens diameters)

ADAPTERS FOR COUPLING LENSES

These are available from the excellent SRB –Grinturn who are engineers able to make anything you want – if you do not know their site have a look at their close-up section.

Look for coupling rings – You can get a 72mm-72mm (the diameter of my Sigma 150mm f2.8 macro) or 62-62mm (a Nikon standard) and then use stepping rings – (SRB sell them and so do Speedgraphic) to fit to the lens (or lenses) you want to reverse.

You can make your own – not with lathe work but using a couple of Cokin filter adapter rings that are sometimes in the ‘junk’ basket in photo stores. Degrease them by washing in mild detergent, allow to dry and mate the large flat surfaces with superglue (keeping fingers clear…) ‘key’ them fiorst by rubbing on a piece of fine (400 grit) ‘wet and dry’ placed on a flat surface. For for a temporary fix use black plastic insulation tape round the rim. Have a dry run – superglue is unforgiving and you get no time to fiddle. Quick set Araldite may be a better bet. Do this well away from any lens – Superglue smears would be a disaster.

For part B click here (to be posted 8 June 2010)