Steve Cushing Impresionist Fine Art Photography

Embracing imperfection, recording emotions, one impression at a time…

Helicoids


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Many photographers use extension tubes to turn their lenses into macro lenses. An extension tube is just a spacer that sits between the lens and the camera body to alter the MFD (minimum focus distance). The MFD of a lens is a measurement of the closest point a subject can be from the camera’s sensor, whilst still being able to focus. If a subject is closer to the sensor than a lens’ MFD, you cannot focus on that subject.

The thicker the extension tube used on a lens, the higher the lens magnification becomes as the photographer is able to move the lens closer to the subject and still achieve focus. I tend to use a Helicoid rather than tubes as I can then change the minimum focal length at will.

See Extension Tubes HERE
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Adapters

If possible I always create a lens mount that fits into a helicoid, this is usually possible with lenses from old SLR cameras as they have a large flange distance, I can thus always use them on a helicoid rather than an adapter basically consisting of an extension tube. Yes some are harder to make than others as you need to create the correct lens mount on the end of the tube. Many lenses I use here, for example projector and enlarger lenses, do not have an internal helicoids so I need to create this type of adapter anyhow.

By creating lens mounts on adapter rings that screw into the helicoids I do not need to buy lots of helicoids which can be quite expensive.

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So what is a helicoid?

If you have ever used manual focus lenses before, you have already used a focus helicoid as every manual focus lens has one built-in. It is the part that allows you to turn the focus ring until the lens is focused. It basically consists of threads that when turned move the front of the helicoid forwards or backwards. In a lens the thread moves one or more element forwards or backwards.

An extension tube and focus helicoid does not have any elements inside; they are just the lens barrel where you can change or turn to extend or retract. In essence, you use them to increase, or decrease the amount of space between the lens you attach to it, and the camera’s sensor (or film plane), until focus is achieved. A focus helicoid is basically a tube whose length can be adjusted to vary the distance between the lens and the camera’s sensor or film plane in order to achieve correct focus.

Sizes

Focus Helicoids come in different focus sizes, and mounts. The size of the helicoid usually refers to opening or “throat” of the helicoid. This “throat” is the size of the hole in the middle based upon the thread size. Most common ones are M42, with a 42mm, M39 39mm, M52 52mm, M58 58mm and M65 65mm. You need a helicoid that has a big enough throat size not to interfere with the light from the lens so causing vignetting. In some lenses the rear of the lens has to fit inside the throat of the helicoid to achieve infinity focus.

Focus Range

Since the focus helicoid changes the length as it focuses, it’s usually defined by two numbers, such as 12mm-17mm. The first number tells the length of the helicoid at its thinnest position, while the last number indicates the maximum length the focus helicoid can extend too. The difference between these two numbers is the amount of extension the helicoid is capable of, which in this case, is 5mm. Usually, the longer the focus range, the closer you can focus the lens.

Image Quality

When manufacturers design the optics inside a lens, they take into account distortion and try to correct for it as much as possible. Focusing a lens does move the optics inside a lens, so that usually means that the amount of distortion varies depending on how far away your subject is from the lens. Lens designers try as hard as they can too correct for distortion at the most important points in the focal range of a lens. The result is that some lenses are sharper than others when they are used at the minimum focus distance.

But in general the helicoid makes little difference to the image quality of the lens.

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