What Lens – What Object?
By Greg Walton
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:
|
Degrees of sky captured |
Pentax K-x 2/3 sensor Pentax ist 2/3 sensor 22mm x 15mm sensor |
35mm Film Full frame SLR 35mm x 22mm sensor |
Rough guide |
||
|
Lens |
Length |
Width |
Length |
Width |
|
|
18 |
80 |
50 |
100 |
70 |
Many constellations |
|
28 |
45 |
30 |
60 |
40 |
Large constellation |
|
50 |
27 |
18 |
37 |
24 |
Medium constellation |
|
80 |
16 |
11 |
22 |
17 |
Small constellation |
|
135 |
10 |
7 |
14 |
10 |
Very small constellation |
|
200 |
7 |
5 |
10 |
7 |
Large Star clusters |
|
300 |
5 |
3.2 |
7 |
5 |
Pleiades M45 |
|
400 |
3.5 |
2.5 |
5 |
3 |
Eta Carina nebula |
|
500 |
3 |
2 |
4 |
3 |
|
|
800 |
2.3 |
1.2 |
3 |
2 |
Orion nebula |
|
1000 |
1.5 |
1 |
2 |
1.5 |
Globular clusters |
|
2000 |
0.7 |
0.5 |
1 |
0.7 |
Full diameter of the Moon |
|
12000 |
|
|
|
|
Planets |
|
Note these are my old fixed length lens and I have roughly measured the degrees from my picture. |
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