What Lens - What Object?
How wide is my lens?
By Greg Walton MPAS/ASV
You may want to capture a whole constellation and cannot
decide which lens to use. Most planetarium software can give
you the field of view in degrees for a part of the sky you
wish to photograph. I like SkyMap but Starry night or
Stellarium will do.
My Pentax K-x and Pentax ist has a 2/3 sensor that’s
about 22mm x 15mm in size. Were as the top end Digital SLR
cameras have full frame sensors that are the same size as
the old 35mm film cameras with a sensor that is 35mm x 22mm.
Both have there advantages the full frame DSLR take a bigger
picture or wider field but are much heavier and will need a
stronger mount and tend to through telescope off balance.
Were as the smaller Pentax K-x is much lighter and causes
less strain on mount and telescopes, it has a 2/3 sensor
that crops the picture, which gives a 1/3 magnifying effect.
That is a 100mm lens would work like a 150mm lens. This is
not all bad when using wide angle lens the corners of the
image often suffer coma. That is the stars are distorted out
of shape or elongated and can also look like seagulls. The
cropping effect would cut some of this away give a more
pleasing picture. You can reduce the coma by stopping down
the lens to a larger F ratio and increasing the exposure
time. When using film I suspected that when you get your
photos prints back from the camera shop, they have had some
degree of cropping. So I think the Full frame digital prints
are slightly bigger.
When viewing my night sky shots the lens length is often
stated on them and can be used as a guide.
In the table below the length and width are in degrees:
