Using Extension Tubes
An extension tube is a hollow metal tube that attaches between the camera body and the lens. By adding an extension tube you increase the distance between the film plane (or sensor image plane if using a digital camera) and the lens. This allows a closer focusing distance and increased magnification compared to without any extension. This can be really useful for close up photography and is far less expensive than a dedicated macro lens.
An extension tube fits onto your camera in the same way as your lens, which is typically by bayonet fit. On the front of the extension tube there is a female bayonet mount to receive the lens in the same way as the camera mount. The simplicity of this means that you can stack multiple extension tubes onto each other to increase the magnification even more! A benefit of extension tubes is (since they are hollow), is there is no loss of image quality. There is no lens elements in the way, degrading the original optics.
Great! But what’s the catch?
Well there had to be one, this isn’t a specialist macro lens after all. Despite these benefits, there are some factors to consider when using an extension tube. Unless you go for the camera’s fully dedicated extension tube you lose out on most automation. What I mean by automation is auto-exposure and lens coupling as extension tubes typically do not feature a lens CPU (processor) for all the electronic functions to be retained. So forget auto-focus! Unless of course your extension tube is fully compatible with the camera and lens that you wish to use it for. Without any automatic diaphragm coupling you will be forced to use the lens’s widest aperture, unless it has a manual aperture ring on it – which many modern digital SLR lens do not have. So you need to be aware and accepting of their limitations.
The other issue of the increased magnification is the darker image through the viewfinder, making focusing more difficult. This presents an additional challenge when attempting to photograph moving subjects such as insects. This is less of an issue when photographing still life, flowers on a tripod or in studio light conditions for example.
So which one to choose?
Well it all depends on what you wish to use it for. To start with a tube with a modest amount of extension should do just fine and provide a useful amount of magnification increase for most situations. The amount of extension is measured by distance, typically in millimetres and there is a direct correlation in magnification between the focal length of the lens you use to use it with and the tube’s distance in extension.
Increase in magnification = Extension (mm) / Focal length (mm)
So adding a tube with 25mm extension will increase the magnification of a 50mm lens again by half. So if that 50mm lens focused to a half-life size (0.5x) to start with, it will now focus to 0.5x + 0.5x = 1.0x. That’s full life-size! Most 50mm lens don’t offer full-life size to start with, unless it is a dedicated macro lens. So it’s a simple and cheap entry into macro photography.
Consider another example. Two Canon 25mm II extension tubes stacked together would increase a Canon EF-S 60mm f/2.8 Macro (1:1) lens by 25mm+25mm / 60mm = 0.83x, providing you now with a useful 1.83x magnification.
Extension tubes provide a simple way to increase the magnification of a lens. They can be very useful in most situations without too much compromise, especially when using a tripod or when you can control the lighting conditions.
The Close-up Lens
This is again a simple way to increase your lens’s magnification. It screws onto the front filter thread in the same way as any screw-in filter. They are relatively inexpensive depending on the make. There are also no lens coupling compatibility issues or darkening of the image when trying to focus, as there are with extension tubes.
The major problem I have with them though is the image quality. The optical quality of the lens is typically inferior compared with any original camera lens and adding yet another layer of glass in front will only degrade the sharpness and image contrast. This is especially so when you stack the lenses on top of each other when trying to increase magnification. Stacking lenses also makes your pictures more prone to ghosting and lens flare. Although in fairness, many modern close-up lens do offer excellent optical quality for the price especially if you go for a multi-coated, double lens version or achromatic model to reduce any chromatic aberrations. The best quality close up lens, those that are made to the highest standards with multi-coated glass are also quite expensive.
Unlike extension tubes, the magnification or “power” of the lens is measure in dioptres, like spectacles. The higher the +d the more “powerful” the lens and the closer it will focus. To confuse matters, some manufactures don’t publish dioptres factors on the lens but use other figures and models designations. Nikon for example has released the new N1-CL1 close up lens (offers 3.8x magnification if in case you didn’t know), while Canon uses 250D and 500D (4+ and 2+ dioptre respectively). In all cases however, the magnification of the lens or dioptre factor can be measured by 1000/f, where f = focal lens of the lens. A close-up lens will also reduce depth of field. It will also shift the focusing range of the lens closer, meaning it won’t focus to infinity anymore. Not an issue when you consider the point of it is to focus for close-ups anyway!
The next option to consider is reversing your lens.
Lens reversing ring
A lens reversing ring is an inexpensive metal ring that has a camera male-bayonet mount on one side and a filter thread on the other side. They are so cheap that you can buy one on ebay for as little as 0.99p! The downside is that they are simply a metal ring. There is no auto-focus, no diaphragm coupling - no functions whatsoever. They simply allow you to reverse the lens and securely mount it to your camera.
The biggest advantage (apart from the cost) of a reversing ring is you can achieve much higher magnifications compared with close-up lens and extension tubes. The smaller the focal length of the reversed lens, the greater the magnification. A reversed 50mm lens for example provides about 1:1 life-size magnification, whilst a reversed 28mm wide angle lens gives about 3x magnification which is great for more extreme macro work. In all cases of course the greater magnification the shallower depth of field so if you want any discernible depth of field ensure you use a mid to small aperture. On the flip side, this lack of depth of field can work in your favour when photographing extreme close ups with a completely out of focus background.
The best option if you specialise in macro photography is to use a dedicated macro lens for your chosen camera. They are designed to focus at closer distances so offer the best optical quality. There is also no compromise when it comes to auto-focus, metering or for ease of use. Some of my best macro images were taken with the highly regarded Tamron SP 90mm f/2.8 1:1 macro. The Nikon 200mm f/4 ED-IF Micro-Nikkor is also regarded as one of the best macro lenses for photographing insects and other wildlife. Not everyone can afford the hundreds and even thousands of pounds it costs for these specialist lenses, but luckily there are cheaper alternatives. If you are willing to factor in the above considerations when using extension tubes, close up lenses or reversing rings, then there is no reason why you cannot achieve stunning macro images on a small budget!
Unless your lens has its own aperture ring, there is no way to control the aperture, so this is worth bearing in mind. I currently use my cheap Nikkor Ai-S 50mm f1.8 standard lens (which has its own manual aperture ring) with a 58mm reversing ring. You can still use modern camera lenses that lack an aperture ring, it is just that you won’t be able to control the aperture. A get around to this to actuate the diaphragm lever by sticking a blob of blu-tack to keep the lever in place. A DIY fix but it works. Keeping the aperture fully open or mid-way works best, allowing sufficient light into the camera viewfinder in order to compose and focus manually.