Astro Exposure Calculator
by Michael Oates
This exposure calculator is designed for both astronomical and terrestrial use.
- First download each of the 3 discs and print out onto thick paper or even
card if your printer will allow this.
- Paste the prints onto card.
- Cut out the 3 disks, then carefully with a sharp knife cut out the 2 shaded
- Next pierce the centres of each card with a sharp point.
- Pass a paper fastener through all 3 disks, the smallest on top and the largest
on the bottom.
- Spread out the fastener on the back an secure with tape.
- The calculator is now ready for use.
How to use the Calculator
I will explain with an example...
Suppose you wish to photograph the moon at quarter phase and the film you
are using has a speed of 100 ISO.
For planetary photographs use the PLANET pointer.
- First use the pointer marked LUNAR and move the outer disk until it is aligned
- Then making sure the two outer disks don't move rotate the inner disk until
'100' is seen in the small window.
- You can now read off the exposure required from the 2 scales 'f' and 'sec',
such as 1/60th sec at f8, or 1 sec at f64.
You can even work out the correct exposure for ordinary daytime photography
by using the DAYLIGHT pointer.
Exposures should be bracketed either side of the indicated exposure for a
number of reasons.
- If the object is low in the sky, the atmosphere is thicker and absorbs more
light from the object.
- The planets vary in brightness depending on the phase of the planet its
distance from us and its distance from the Sun.
- Different makes and types of film respond differently to exposures over
1 second in length. The emulsion becomes tired, with the result that the image
is not as dense as it should be. This is called reciprocity failure. That
is why, on the calculator the exposure lengths over 2 seconds do not double,
but in fact treble with each increase in exposure. You should only use this
as a guide, as I can only approximate since many different films will be used.
- Slide film needs to be exposed much more accurately. Print films however
can stand a one stop over or under exposure.
The figures used in the exposure calculator were based on tables published in
Michael Covington's book "Astrophotography for the Amateur".
I've taken many photographs with this calculator and I find the exposures
always spot on. (but then I may be slightly biased :-))
You are free to use the calculator and pass it on to friends. But I do ask,
please do NOT re-distribute, without permission, or sell/pass this on
to other parties for profit.
Copyright © 1995 Michael Oates
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Maintained by Michael Oates. Email
19 January, 2005