Photos Using Extension Tubes

---------- Photographing Ancient Coins ----------

Photographing Coins using Extension Tubes

There are eleven pages about photographing coins:

Black Background — White Background — Photographing the Edges — Using Extension Tubes (this page) — Choosing the Best ISO Setting — Choosing the Best Aperture — Choosing the Best Exposure — Editing the Background — Keeping Detail in the Highlights — Processing Your Image — Some Final Tweaks

A set of three extension tubes in place on a camera These pages do not cover choosing a lens to use, because it will depend on what camera system you are using, and I am only familiar with the Canon Eos system. But a macro lens of around 100mm focal length is a good standard choice. There are excellent lenses available from the main camera manufacturers, and some very good third party choices too. As this page is written, I am using a rather old Sigma 105mm EX Macro.

Typically, these lenses will give you enough depth of field for most coins, and a magnification of 1:1, which means the image on the sensor is the same size as the actual coin. But there may be times when you want greater magnification, to see part of the surface of a coin in much more detail. Or you might be photographing a tiny coin. In that case, the cheapest option is to use extension tubes.

These are simple hollow tubes that fit between the lens and the camera. They are cheap because they contain no optics. Do not confuse these with the specialist "EF extenders" made by Canon for their long focal length lenses, which are teleconverters, expensive precision optical instruments. You should be cautious of any cheap extension systems containing lenses, as you will probably lose too much image quality to make them worth while. On the other hand, cheap extension tubes which contain no lenses will be fine as long as they are mechanically sound.

Ideally, look for a model that carries the electronic signals between the lens and the camera, so that you retain some automatic control over the lens and the camera can still tell you what the lens is doing.

They tend to come in sets of three or more, of different sizes, and you can use them in any combination. The picture on the right shows all three of my tube set fixed to my camera. That is a third party set from Jessop's, and is designed for the Canon Eos system.

Here is the coin I will use to show the effect of using extension tubes. It is a contemporary fake of a Roman Republican denarius of M. Volteius M.f. with a reverse showing the goddess Cybele in a carriage drawn by two lions. The coin has been deeply scored with an X, probably to indicate that it was not good currency; the cut goes right through the silver coating and into the base metal core of the coin. There is every reason to believe that this coin will be different from the usual run, so it would be interesting to examine its surface in as much detail as I can reasonably get.

For normal use, I adjust the frame of the shot so that all of the coin is contained in it, with nothing missing from the shot. But my lens can do a bit better than that, if I use its highest magnification. To focus close in, I adjust the lens to its highest possible magnification and rack the camera up and down until the coin is roughly in focus — taking care not to impact the coin with the lens, which could damage both. Then I use the lens adjustment until the coin is perfectly in focus. Using extension tubes, I can get even greater magnification. The range of frames available to me is shown on the right, using the reverse of the coin as an example.

Of course, this is only the start. Those images are Photoshop thumbnails of the basic JPEG images and have not been adjusted in any way. If I look at the original photos on screen at their natural level of magnification, I can see a great deal of detail. Here is a section of the same reverse at its natural magnification from all four shots, with the image adjusted in Photoshop and sharpened a little:

Some coin photographers disagree with the sharpening (using Unsharp Mask), but with the images produced by my camera the result is not unrealistic, so I am happy to do it. I suggest you try it out, but be careful not to apply too much.

I think that for most purposes the image produced by the 105mm lens without any extensions is adequate. The 31mm extension tube also gives a fine result. Using all three tubes, a total of 65mm of extension, is fraught with problems.

First, it is very hard to focus and the depth of field is almost nonexistent. My own camera even stops giving me confirmation of focus when I use all three of these tubes. It is very tricky, and even if you get it right, the slightest relief or three-dimensionality on the coin will mean that some of it is out of focus. And there is more. The more tubes you use, the less light will reach the sensor and the longer the exposure which will be needed. The extended lens is now much longer than it was and somewhat heavier, and will wobble at the slightest vibration. With all this going on, at my best, even focusing on a flat field, I can get no better result than the rather blurred image you see here.

Second, I can see a lot of false colours on that fourth photo. I am not at all happy with that result. I do not think this photo contains any more actual information about the coin than the third in the sequence.

So I think the best practical result comes from the 31mm tube, the third photo in that sequence. It contains noticeably more information than photos 1 and 2. You can see as much of the surface scratching and pitting as you are normally likely to need. But please don't let this stop you from trying longer extensions for yourself. Cameras differ, as do tripods, lenses, environments and the skill of the operator.

If you need an even closer view than this, there are ways. One is to take photographs through a microscope. Another is to use a really specialised lens such as the Canon MP-E65mm f2.8 1-5x Macro, which can give twice the magnification of the 65mm extension tube combination at a high quality; but this is not a cheap lens and it will not take your normal coin photos. I will not be covering those techniques here.

And the coin itself? I have an interesting view of its lunar landscape of a surface, scratched and scraped, with a crater through the silver to the base metal below. There is also a spot of green verdigris, a dead giveaway of the copper alloy hidden beneath the silver.

There are eleven pages about photographing coins:

The content of this page was last updated on 27 January 2011