BetterPhoto Q&A
Category: Traditional Film Photography

Close-up lenses

Please advise.

I am just thinking about getting
started in macreo type photography
and do not want to spend much money
at this point because I do not
think that I would want to do
much of this type of photography.

Thanks.

Paul

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August 01, 2008

The close-up attachment (supplemental lens) is a filter like attachment that mounts directly over an existing camera lens. Close-up lenses are handy indeed.

Until recently, most cameras did not allow close-up photography. The reason is complex. As you move-in close, the subject becomes out-of-focus. You must re-focus the camera as camera-to-subject distance changes. Focus at close distances is achieved by physically moving the lens forward, away from the chip/film. What is happening is, as you move in close the working focal length actually changes, it gets longer. The forward movement to focus the lens is compensation.

Because at close distances the working focal length is extended, the f/numbers engraved on the lens are invalidated. This is true because they are derived by dividing the working diameter of the lens (aperture) into the focal length. Since the working focal length has changed, so must the f/numbers. By tradition camera makers stoped the forward movement when the f/number error exceeded 1/3 f/stop. Thus most vintage cameras limit close focusing to maybe 24 ~ 36 inches. Through-the-lens metering changed all that. Nowadays, the camera’s exposure metering system and chip logic blows away this limitation.

Mounting a supplemental close-up attachment allows close focusing. These lenses are by tradition are labeled in a unit of optical power called the diopter. This is the same unit used by eyeglass makers and eye doctors. Use of this unit eases the burden of math when calculating the power of a collective lens system. 1 diopter = 1000mm -- 2 diopter = 500mm -- 3 diopter = 333mm --4 diopter = 250mm. How calculated: Take a 4 diopter lens (4÷ 1000 = .004 now take the reciprocal 1/.004 = 250 thus a 250mm lens has the power of 4 diopters.

This unit saves time and brain power because it allows calculations based mainly on addition and subtraction. As an example a 50mm camera lens can be expressed as 20 diopter (50/1000 = .05 thus 1/.05 = 20 diopter). Combine this with a 3 diopter supplemental. The math is 20 + 3 = 23 diopter = 43.48mm. (23/1000 = .023 thus 1/.023 = 43.48).

OK what does it mean? Mount a 4 diopter supplemental on your camera. Set the focus to infinity (as far as the eye can see symbol ∞). At the infinity position the lens is positioned as close to the chip/film as it can go. Due to the power of the 4 diopter supplemental the camera now focuses on a subject 250mm (9 ¾ inches)from the front of the lens. Now you can move in even closer. As you do, you must re-focus. The close-up attachment fools the camera it thinks it is operating within normal limits. No exposure compensation is required as the f/number error never exceeds 1/3 f/stop.

The bad news: Supplemental close-up likely induce a lens error known as chromatic aberration. This is recognizable by a rainbow or purple fringe surrounding the subject. The countermeasure is to make the supplemental a two element lens known as an achromatic (without color error). These are costly but readily available.

The extra glass in front of the main camera lens induces flare. This is stray light, the source is reflections from the back side of the glass in the supplemental lens. Flare trims down contrast. A coated supplemental is the countermeasure.

Lastly a supplemental induces other optical errors. However, they work quite nicely at smaller apertures like f/8 ~ f/22 etc.

One more piece of information: The standard camera lens is designed to function best at infinity. The macro lens is designed to function best at unity (life size i.e. 1:1). When the standard lens is forced to work close-up it is compromised. When the macro is forces to work at infinity it is compromised. These performance compromises may be small indeed nevertheless they exist.

I keep a set in my gadget bag. .

Alan Marcus (truly marginal technical gobbledygook)
ammarcus@earthlink.net

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August 01, 2008

http://www.the-digital-picture.com/Reviews/Kenko-Extension-Tube-Set-Review.phpx

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August 01, 2008

The idea is to make available spacers that mount between the lens and the camera body. Thus when employed they extend the lens to focal plane thus allowing close focusing.

Their advantage is price. A set of extension tubes will convert any lens in your gadget bag thus allowing extreme close focusing.

Their disadvantage is they defeat the camera’s automation. Modern cameras are electrically and mechanically coupled to the body. All manor or things happen when the body is thus connected. If the extension tubes allowed body to lens communications, they would be magnificent.

As it is they are wonderful but they force the photographer to work manually. That’s OK it just places more demands on the photographers skill.

The optical quality of a normal lens, when used in an extreme close-up position, is slightly compromised. A normal lens is designed to focus a 3 dimensional world (objects at different distances) onto a flat film/chip. In close-up work the subject is generally a flat field that needs to be focused on a flat film/chip plane i.e. flat to flat. Special lenses called “process” lenses are best for this application. An attachment called a revering ring is available. It allows mounting the normal lens backwards on the extension tube lash-up. Since the rear of the lens is designed to work on a flat field, a backwards mounted normal lens is likely to perform better if reversed, in extreme close-up situations.

Today’s macro lenses are highly corrected for close-up work and all of the camera’s automation is retained. Additionally lens makers have solved most of the problems related to the macro being compromised when asked to work at far distances. Sully, if your budge allows, this is the best solution. In the meantime close-up lens and extension rings work. Additionally a more flexible method to extend the lens for close-up work is a bellows attachment. Bellows are indeed wonderful but they place greater demand on the photographer’s skill.

Photography is both and art and a science. Experimentation to achieve your art is a wonderful experience. I hope you experience all.

Alan Marcus (truly marginal technical gobbledygook)
ammarcus@earthlink.net

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August 01, 2008