BetterPhoto Q&A
Category: Traditional Film Photography

+10 macro filter

Please respond.

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April 06, 2006

I bought a set of +1, +2, +4 filters. Using my 75-300 lens with the +1, I can get about 4 feet from a subject (as opposed to the normal 5.9 feet). With the +4, I get about 9 inches. The downside is that I can only focus from about 8.75 inches to 9.25 inches. To have more than 1/16 inch in focus at 9 inches away, I have to really stop-down the lens. At f/20, I get about 3-inches depth of field.

In short, a +10 on the same lens means you would be focusing at about 1 inch. If you were to snap a bug like this at 75mm lens length, you would only get a single fly-hair to focus. It is a cool effect, but the thing would have to be dead to get that close.

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April 06, 2006

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April 06, 2006

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April 07, 2006

I am a retired 50 year veteran of the photo-engineering field. I don’t have anyone to mentor so you are elected. Buckle your seatbelt however, if you don’t want the details, skip this writing.

When the camera is focused at infinity (as far as the eye can see), the lens to film/chip distance is at its shortest. Measure this distance and you have derived the approximate focal length of the camera lens.

When you aim and focus on near-by objects, you cause the lens to film/chip distance to be increase. The closer you focus more extended the lens becomes. Increasing the in-the-camera projection distance generates exactly the same situation as would occur if you move a slide projector further away from a screen. The projected image becomes larger however since the same light energy is now forced to play over a larger screen surface area, the projected image becomes dimmer. With the camera, this condition induces an error in the locations of the f-number detents engraved on the lens barrel. The closer you focus, the greater the error. Unless compensated for, under exposure will surely be the result. As you approach magnification one (1) life size or 1:1, the error is two f-stops. Most cameras mechanically restrict close-focus keeping the error to no more than 1/3 f-stop. Modern film cameras and most digital cameras allow extreme close focusing. This is possible because in-camera metering will completely and automatically make all necessary exposure compensations. By the way, the error is known as “bellows factor”.

Many cameras permit the use of extension rings and/or tubs or expansion bellows. These are mechanical devices that allow close focus by demounting the lens from the camera body and re-mounting it at an extended distance from the film/chip. Extreme close-up photography becomes possible. Generally these methods defeat much of the camera’s automation. The in-camera metering system may still function but automatic iris operation is lost. Stated another way, the burden of lens adjustment and perhaps exposure compensation, shifts entirely to photographer.

The macro lens (a marvelous design) gets around this problem by mechanically altering the diameter of the working lens as lens to film/chip distance changes. Thus the macro automatically opens up its aperture making the required compensation as you close focus.

Rings, tubes, bellows and macro lenses are expensive. A neat work-around is the supplemental lens. These are simple and comparatively inexpensive lenses not far removed from common spectacle lenses. Supplemental lens mount directly to the camera lens like a filter. They work by causing the light rays from the subject to begin convergence before entering the prime optical system of the camera. One of the advantages of the supplemental lens is little or no exposure compensation is required. The disadvantage is reduced optical performance (sharpness-resolution-color fringing-etc.). Optical performance is improved when the supplemental lens is constructed using two lens elements. This design is know as an achromatic meaning without color error. The lower power supplemental is always the best performers.
When supplemental lenses are combined or removed, calculating the final focal length requires some arduous math. Early lens maker were forced to perform multiplication, long division and the calculator wasn’t even a dream, slide rules were the norm. Any shortcut would be a blessing. Since lenses are usually measured using the metric system, a short cut was devised. This became a neat way to express the power of a lens (focal length) using a unit known as the diopter. The diopter unit remains and is exclusively used by every spectacle maker and eye doctor.

The advantage of the diopter:
When adding or removing a lens element, if all the powers are express in diopter, addition and subtraction can be substituted for multiplication and division. As an example: Suppose your camera is equipped with a 50mm lens and you desire to add a supplemental 100mm close-up lens. To calculate this combination you add diopter power.

(See table below)
50mm = 20d.
100mm = 10d.
Add and the total power is 30d
30d = 33.3mm (30 diopter is just another way of saying the focal length is 33.3mm)
Close focusing is now possible as the 50mm camera lens retains its mechanical restrictions however it now functions as a shorter 33.3mm lens, this allows close focusing.

The diopter is reciprocal of the focal length expressed in meters:

1 diopter = 1000mm
2 diopter = 500mm
4 diopter = 250mm
5 diopter = 200mm
7 diopter = 142.9mm
8 diopter = 125mm
10 diopter = 100mm
20 diopter = 50mm
30 diopter = 33.3mm
40 diopter = 25mm

Conclusion:
Supplemental positive lenses mounted over the prime camera lens allow shortened focal length thus reducing the near focusing limit.

Alan Marcus
ammarcus@earthlink.net

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April 07, 2006