A PHOTOGRAPHY QUESTION...(on an astronomy forum!!! )

Hi all... kevin from galway here... hope you all have my christmas presents bought!!! :lol:

Your opinions please... regarding camera lenses..

For milky way and star trail shots would i be correct in saying that a wide angle lens is best when piggybacked?...........if so, does the type of lens matter and wat should i look for?

ALSO.. for large objects such as andromeda, california neb etc, would a Zoom lens work or a telephoto... and wats the difference???

I KNOW, I KNOW,, YOU ASTRONOMERS NOT PHOTOGRAPHERS...But your collective wisdom never ceases to amaze me... :shock: :lol: :lol:

Thanks all ... hope you all keepin well!!

by way .. have a look at my comet holmes pic ... let me know wat you think..
farm3.static.flickr.com/2078/1988596701_2a00b36bca.jpg?v=0

CHEERS ALL!!!

Hi Kevin,

I am a photographer of a general nature as well as an astronomer so I'll try and answer your questions

First of all, see this on my website:-

www.pevans.me.uk/html/don_t_be_afraid_of__m_.html

Click through the Flash slides - you should get the gist.

To piggyback or not to piggyback depends on the effect you want to achieve - if you want the stars to move - ie trail, then a static tripod is the tool for the job - if you want long exposures of the stars staying still then piggybacking on an equatorial mount - not an alt-az - is the way to go.

A decent brand of wide angle lens stopped down a bit (1 or 2 stops below wide open) should give good images.

A zoom is just a lens with a variable focal length - they are less good for astro purposes as they contain more glass than prime lenses (a prime just does one focal length) and thus reflect light internally quite a bit more.

I've had quite a bit of success with both M31 and NGC1499 among other things with lenses in the range 135 - 300mm - these objects are too big for normal telescopic focal lengths apart from some of the short APO refractors.

Hope that helps!

THANKS PAUL,

I get the piggyback or not bit no prob.

A decent brand of wide angle lens stopped down a bit (1 or 2 stops below wide open) should give good images

wat does "stopped down" mean and how do i achieve it??

I've had quite a bit of success with both M31 and NGC1499 among other things with lenses in the range 135 - 300mm

is this the focal length of the lens, and is this the value you change in a zoom lens..?

Surely a zoom lens would work better for large (not vast) objects such as andromeda? could you elaborate?

Thanks heaps.
kevin

wat does "stopped down" mean and how do i achieve it??

The aperture is adjustable in camera lenses, unlike telescopes, and is the ratio of the effective diameter of the lens opening to the focal length. A typical prime lens will be f2.8 wide open, so if it's a 135mm focal length the effective diameter of the "hole" will be 135/2.8 = 48mm. Set to f4 it will be 135/4 = 33.75mm. Since light transmission works by area, not diameter, a lens set to f4 will transmit half the light of one set to f2.8. However, any imperfections in the optics will be reduced by stopping down, hence sharper stars etc.

is this the focal length of the lens, and is this the value you change in a zoom lens..?

Yes, that's it.

Surely a zoom lens would work better for large (not vast) objects such as andromeda? could you elaborate?

A zoom just means the focal length can be changed. There isn't much difference between a 70-300mm f4-5.6 lens set to its long end and a 300mm f5.6 prime lens, except in most cases the prime lens will give a better image.

My telescope has a focal length of 762mm and an aperture of 152mm, hence is an f5, however its focal length is too long to get the whole of M31 (Andromeda galaxy) in the frame, hence the use of something more wide-angle - like 300mm in this case - to get the whole picture. Of course in terrestial terms this would be a "long" lens - it's all relative!

Thanks paul,

jus one last thing to clear it all up in my head..

here is photo of my standard lens... farm3.static.flickr.com/2209/2120015767_b19077f27e.jpg?v=0

Am i correct in saying that the numbers printed on silver band are the focal lengths 18mm to 55mm, and 18mm is the wide angle end and 55mm is the zoomed end, and hence a wide angle lens might have a focal length of say 5 to 10mm and telephoto or zoom would be 50, 60, 100 maybe 200mm..
and further, the focal length is pretty much the only difference between a wide angle and telephoto (zoom) lens.................(PHEW, all in one breath too) :lol:

THANKS FOR YOUR PATIENCE... you great help!!

Yep, 18mm is the wide end, 55mm is the long end of your lens - it is termed a standard zoom.

Very few lenses would be as wide angle as 5 or even 10mm - there are some specialised lenses but 10mm is about the starting point for the very widest.

Consider it in terms of magnification - this is considered to be 1 - ie equal to normal vision when the diagonal dimension of your image sensor is equal tot he focal length of the lens. Your Canon camera has an image diagonal of approx 26mm (Minoltas, Sonys, Pentaxes and Nikons are all 28mm, Olympus and Panasonic are smaller - about 21mm, 35mm film is 43mm) So your lens goes from 18/26 = 0.7 - 55/26 = 2.1 magnification. A lens of 5mm would have a magnification of under 0.2 and would need to be able to see backwards to fill the whole frame!

Does this all make sense?