“Close-up lens”的意思、由来-开放百科全书

In photography, a close-up lens (sometimes referred to as close-up filter or a macro filter) is a simple secondary lens used to enable macro photography without requiring a specialised primary lens. They work like reading glasses, allowing a primary lens to focus more closely.^[1] Bringing the focus closer allows the photographer new creative possabilities with their lenses.^[2]

It is actually more appropriate to use the close-up lens terminology as it is a lens and not a filter, although close-up lenses typically mount on the filter thread of the primary lens,^[3] and are often manufactured and sold by suppliers of photographic filters. Some manufacturers refer to their close-up lenses as diopters, after the unit of measurement of their optical power.

Close-up lens do not affect exposure, whereas devices such as extension tubes which also can be used to do macro photography with a non-macro lens do affect exposure.^[4]

Optical power

Close-up lenses are often specified by their optical power, the reciprocal of the focal length. When focal length is measured in meters, optical power given in units of diopters. For close-up lens the diopter value will be a positive value such as +2. The bigger the number, the greater the effective magnification.

Several close-up lenses may be used in combination; the optical power of the combination is the sum of the optical powers of the component lenses.^[5] For example, a set of lenses of +1, +2, and +4 diopters can be combined to provide a range from +1 to +7 in steps of 1.

Working distances and magnifications

To use a close-up lens it is important to know the maximal and minimal distances at which you can focus because only if you are within that range it will be possible to take a focused image. There is not much of a range between the minimum and maximum values and the difference in magnification is quite moderate also.

Working at maximum distance

When you add a close-up lens to a camera which is focused to infinity, the focus will move to a distance which is equal to the focal length of the close-up lens. This is the maximal working distance at which you will be able to take a picture with the close-up lens. It suffices to divide 1 by D, the diopter value of the close-up lens, to get this maximal working distance in meters:

Sometimes that distance is also given on the filter in mm. A +3 filter will have a maximal working distance of 0.333{{nbsp}}m or 333 mm.

The magnification reached in those conditions is the focal distance of the objective lens (f) divided by the focal distance of the close-up lens; i.e., the focal distance of the objective lens (in meters) multiplied by the diopter value (D) of the close-up lens:

In the example above, if the lens has a 300 mm focal distance, magnification is equal to {{nowrap|0.3×3 {{=}} 0.9}}.

Given the small size of most sensors (about 25{{nbsp}}mm for APS C sensors) a 20{{nbsp}}mm insect will almost fill the frame at this magnification. Using a zoom lens makes it easy to frame the subject as desired.

Working at minimal distance

When you add a close-up lens to a camera which is focusing at the shortest distance at which the objective lens can focus, the focus will move to a distance which is given by following formula:

X being the shortest distance at which the objective lens can focus (in meters), and D being the diopter value of the close-up lens. This is the minimal working distance at which you will be able to take a picture with the close-up lens.

For example, a lens that can focus at 1.5{{nbsp}}m combined with a +3 diopter close-up lens will give a closest working distance of {{nowrap|1.5/(3×1.5 + 1) {{=}} 0.273 m}}.

In the example above, the gain of magnification at X_min will be {{nowrap|(3×1.5 + 1) {{=}} 5.5}}.

Macro photography with a close-up lens

Close-up lenses can make a telephoto lens function as a macro lens with a large working distance. This is useful, for example, to prevent scaring small animals or isolating the subject from messy surroundings. To use the filters for animals the size of the animal will determine the working distance (small snakes 1{{nbsp}}m to 50 cm, lizards 50–25 cm, small butterflies, beetles 25–10 cm), so it is essential to know what will be the favorite subject before screwing on a close-up lens. The close-up lenses are most effective with long focal length objectives and using a zoom lens is very practical to have some flexibility in the magnification. A good technique for sharp focussing is to take a picture at a long focal length first to have optimal sharpness at the essential details and then zooming out to have the desired size in the frame.

Optical issues

Some single-element close-up lenses produce images with severe aberrations but there are also high-quality close-up lenses composed as achromatic doublets which are capable of producing excellent images, with fairly low loss of sharpness.

See also

References

1. ^{{cite book |last=Meehan |first=Joseph |year=2006 |title=The Magic of Digital Close-Up Photography |publisher=Lark Books |location=New York |isbn=978-1-57990-652-8 |page=59 |quote=For real close-up work, some cameras need help—their own version of reading glasses.}}
2. ^{{cite book |last=Timacheff |first=Serge |year=2008 |title=Canon EOS Digital Photography: Photo Workshop |publisher=Wiley |isbn=978-0-470-11434-6 |page=8 |quote=...a variety of accessories increase your creative options. Additional components include Canon Softmat filters and close-up lenses,...}}
3. ^{{cite book |last=Busch |first=David D. |year=2009 |title=Digital SLR Cameras & Photography for Dummies |edition=3rd |publisher=Wiley |page=84 |quote=A lot of available add-ons can help you focus close, including filter-like close-up attachments that screw onto the front of the lens,...}}
4. ^{{cite book |last=Busch |first=David D. |year=2009 |title=Digital SLR Cameras & Photography for Dummies |edition=3rd |publisher=Wiley |page=139}}
5. ^{{cite book |last1=Grimm |first1=Tom |last2=Grimm |first2=Michele |year=1997 |title=The Basic Book of Photography |edition=4th |publisher=Plume |isbn= 0-452-27825-2 |page=137 |quote=They can be used alone, or two can be combined.}}