AstroMedia Hands-on Science Series Instructions de construction - Page 11

Parcourez en ligne ou téléchargez le pdf Instructions de construction pour {nom_de_la_catégorie} AstroMedia Hands-on Science Series. AstroMedia Hands-on Science Series 12 pages. Camera obscura

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Tips for using the camera obscura:

Tip 1

Make sure that the subject is well lit and that

the camera obscura itself, especially the tracing pa-

per screen, is in the shade or in the dark.

Tip 2

You can see the picture more clearly by cov-

ering yourself and the screen with a dark cloth or a

piece of clothing.

Tip 3

If the inner lens tube moves too easily, you can

glue some strips of (black) paper into the outer tube

to make the fit tighter.

Tip 4

Unwanted reflections, which cannot be avoid-

ed despite the black inner walls, can be greatly re-

duced with the additional diaphragm provided. Cut

it out of the cardboard and fold the eight tabs back-

wards. Push the diaphragm, octagon first, into the

open end of the inner lens tube, opposite the lens,

until the tabs are flush with the edge of the tube.

You can secure it in this position with a few drops of

glue or, if you want to experiment with other dia-

phragm sizes, just leave it loose.

Tip 5

The lens has a large aperture and with a focal

ratio of 4.8 a high light intensity. The focal ratio or

f-number is calculated by dividing the focal length

of 275mm by the 57mm lens diameter. A trade-off

for the bright picture is the low depth of field, i.e.

only objects that are at a similar distance are imaged

sharply. By moving the lens in or out you can adjust

the camera to different distances. You can increase

the depth of field by reducing the lens aperture with

a diaphragm made from dark cardboard. The small-

er the diaphragm, the larger the depth of field and

the dimmer the image will be.

Tip 6

The lens cannot be pulled out far enough for

very close objects. However, you can get around this

with a makeshift extension made of black paper,

which you fold into a suitable octagonal tube and

insert between the inner and outer lens tubes.

Photo prints of your drawings

The camera obscura is not suitable for direct exposure

of photographic paper, but you can make very impres-

sive photographic prints from pencil drawings you made

on the tracing paper. There is a simple method without

having to use a darkroom or chemical developers. Take

a sheet of AstroMedia's Solar Photo Paper (article No.

411.FSP) and place the front side of the sketch, which

is mirror-inverted, on the photo paper when exposing.

When developing (with normal tap water!), a white neg-

ative appears on a dark blue background, now showing a

right-sided image.

The Camera Obscura as a pinhole camera

Your Camera Obscura can also be turned into a pinhole

camera. To do this, remove the inner tube with the lens

and cover the outer tube with dark paper, in which you

have pierced a hole.

Start with a small hole (approx. 1 to 2 mm), which you can

then gradually enlarge. Cover yourself and the screen as

lightproof as possible with a dark cloth and allow your

eyes to adjust to the darkness. Unlike those obtained

with the lens, the images you see have a continuous

depth of field – which is the great advantage of pinhole

cameras. However, the picture is also much darker. The

bigger the hole, the brighter, but also the blurrier the

image becomes.