Frequently Asked Questions (FAQ)

This was initially an article I created for The Leica and Photography Club of Brisbane website, but as that club is closing down, I decided to re post the article here for anyone that may find it of benefit.

Originally Written in April 2007

This article we will cover some of the equipment you can use to increase the magnification of your macro photography. The Pro's and Cons of each method will also be discussed so that you can then workout which method bests suits you.

The easiest way is to just buy diopter filters (alternative spelling is Dioptre). They come in sizes like +1 +2 +3 etc. You can stack them, but always put the highest rated ones first, so if you get a set of +1 +2 +3, then they should be used: Camera+LENS+3+2+1.

Diopters work on the reciprocal of the focal length, so they have a greater impact as the focal length increases, for example using +5 diopter on the following lenses:

Magnification	Lens used (with +5 Diopter)
0.25	50mm lens (at infinity focus)
0.50	100mm lens (at infinity focus)
1.50	300mm lens (at infinity focus)
0.88	50mm macro lens (at minimum focus, 1:2)
1.25	100mm macro lens (at minimum focus, 1:2)
1.72	300mm lens (at minimum focus of 2m)

The way to get higher diopter power is to use a reverse lens. This is a normal lens mounted in reverse on the filter ring. So for example you can use a F 1.4 / 50mm lens as a +20 diopter. Always use the reverse lens at infinity focus to help prevent distortion and vignetting. The shorter the focal length the greater the diopter power, but the greater the vignetting due to wideangle lens design.

Magnification	Lens used (with 50mm reverse lens)
1.00	50mm lens (at infinity focus)
2.00	100mm lens (at infinity focus)
6.00	300mm lens (at infinity focus)
2.00	50mm macro lens (at minimum focus, 1:2)
3.50	100mm macro lens (at minimum focus, 1:2)

You can see the impact that focusing has on increasing the magnification. The greater the focal range (such as a 1:2 macro lens) the greater the increase in magnification.

How to calculate your reverse lens diopter strength: diopter Strength = 1 / Focal length of your lens in meters

EXAMPLE: a 50mm lens has the power of a +20 Dioptre
20 = 1 / 0.05

How to calculate your magnification when using diopters on your lens.

magnification = diopter Power * (Focal length in meters + extension length in meters) + magnification of lens

EXAMPLE: a 100mm (1:2) macro lens at minimum focus with a 50mm reverse lens (+20 diopter)
3.5 = 20 * ((100 / 1000) + (50 / 1000)) + 0.5

You can also buy negative diopters, and i'm sure everyone is familiar with those, they are commonly called Teleconverters, so you place them behind your lens to increase focal length. So using a 2x Teleconverter will double the length of your lens and if you use a Dioptre as well that will double the power of the Dioptre and therefore double your magnification. But you have to make additional compensation with effective aperture depending on the type of teleconverter you use. Effective Aperture is covered more extensively in the previous article.

Diopter Advantages (Pros)

• They do not impact on aperture, so no light is lost, this can make focusing easier as the view finder is normal brightness

• Easy to install, so if used on a zoom lens (say 28 - 200) you can vary the magnification achieved easily.

• Will work with any lens irrespective of lens design.

• Does not increase the length of the camera or lens to any significant value.

Diopter Dis-advantages (Cons)

• Can be very expensive as you need different sizes for different lenses, or buy the largest size you need (ie 62mm) then step up rings for your smaller lenses like 55mm, 49mm etc.

• They add optical elements into the light path, so can impact on image quality.

• Being curved elements they are prone to damage easily and stacking high power diopters can be tricky because the curvature of the glass and the width of the filter may prevent the next diopter from screwing into the filter thread.

• They usually only yield good center image quality, so verges suffer with lower image quality.

• Stacking too many makes focusing quite difficult.

• Stacking too many on a short focal length lens will yield vignetting (unless you purchase larger diameter filters and use step up ring).

• Not all lenses can be used in reverse, and some will yield massive vignetting, so you have to find a lens that works well in reverse with YOUR on camera lens.

• The largest disadvantage is they drastically reduce your working distance. This makes it more difficult to not scare away the macro subject or to get enough light (if flash required) onto the subject. For example for me to get 3.5:1 magnification with my 100mm lens, working distance is now only 25 mm.

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      Part 2: Extension Tubes & Bellows

A hollow tube inserted between camera body and lens. Use different lengths to gain additional magnification. Bellows work on the same principal except they are adjustable to give you a greater range of fine adjustment.

Each time you focus your lens you are technically adding barrel extension (which is what an extension tube does), so you can also use this to calculate the magnification from barrel extension (ie going to minimum focus):

Formula for calculating magnification : magnification = extension length in mm / focal length in mm

EXAMPLE: a 100mm macro (1:2) lens has 50mm of extension in focus, if we add a 50mm extension tube as well we get lifesize (1:1)
1 = (50+50) / 100

TIP: If you use a Teleconverter behind your extension tubes, it will double your extension tube magnification.

Extension Tube Advantages (Pros)

• They do not impact on image quality at all as they contain no optics.

• The largest advantage is they provide you with a greater working distance. For example for me to get 3.5:1 magnification with my 100mm lens, working distance is 110 mm (that's 4.4 times the working distance obtained from diopters when using the same magnification).

Extension Tube Disadvantages (Cons)

• Massive loss of light, so it's important to know lens PMF to ensure correct exposure is calculated when using a manual camera and flash.

• Not all lenses will operate correctly as they have to be corrected for focusing so close and as such some lenses will yield massive vignetting as you increase the magnification.

• Add more weight and length to camera body.

• If you have an electronic camera or auto focus, the extension tubes need to relay the electronic signals to/from the lens, so this can drastically increase their cost.

*end of article*