27 May
2007
27 May
'07
1:29 a.m.
If you are including any photos that show the screen,
use a long exposure
Assuming your computer/PCB/whatever is not going to run away, I would
recomend 'stopping the lens down' (setting a small aperture) to get the
largest depth of field that uyou can and having a long exposure to comensate.
Most (good) digitcal cameras have several sensitivity settings, described
as ISO/ASA settings by analogy with film speeds (sensitivities). A lower
sensitivity setting generally produces an image with less noise in it,
but of course you need a longer exposure. Assuming everything (camera and
subjext) is static, use a low sensitivity setting.
to get multiple retraces. (see the article that Dr.
Marty and I did for
Coco magazine)
my apologies if the following is too much, or not enough, technical info:
The aperture is stated as in f units, which is the ratio of the
More exactly f/ numbers (the slash is part of the name). For example, an
aperture of f/8 means what it says, the (effective) diameter of the
aperture is 1/8th of the focal length of the lens.
[effective] focal length of the lens to the
[effective] diameter.
It is logarithmic - to halve the exposure, multiply the f number by
sqrt(2).
I wouldn't call it logarithmic. There is no log or exponential function
anywhere in the definition.
The amount of light coming through the lens depends (linarly ) on the
effective area of the aperture, which is clearly related to the square of
the diameter. So to halve the amount of light, you multipy the diameter
(or f/ number) by sqrt(2),
Most lenses have the apetrure scale calibated in approximately sqrt(2)
multipes : 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45
Moving my 1 step (or 'stop' as photographers call it) halves or doubles
the eposure. SImilarly the shutter speeds are set in an approximate
doubling seires, so that changing the aperutre by 1 stop and chnaging the
shutter speed by one stop in the opposite direcion produces the same
exposure ta the film/sensor.But of course the depth of field, and the
ability to 'freeze motion' will be different in the 2 cases.
[effective] is because there are optical tricks to
make lenses have focal
lengths and diameters other than the actual measured distance (often
needed to make telephotos smaller, or to be able to have a wide angle
To be precise ,a telephoto lens is not simply a long focal length lens.
It's one where hte 'back focus' is shorted than the focal length -- the
lens has been made shorter by these optical tricks.
whose focal length won't physically fit the
camera).
In particualr, in single lens reflex cameras you have a swinging mirror
behind the lens. Obviously no part of the lens must get in the way of
that, so wide angle lenses (short focal length) need to have a back focus
longer than the effective focal length. The is a 'retrofocus' or 'reverse
telephoto' lens/
-tony