Help deciding what lens for pictures of stars

[Nocluewhattodo]

Hi guys I'm hoping some of you could help give me some ideas about what lens to get my girlfriend for shooting pictures of stars. Or at the very least what to look for. She is new to it and I wanna get her something to experiment with. Please try to remember I'm not wealthy so as best as you can on price and quality! Any help is appreciated!! (P.s. photography isn't my thing so you might need to explain stuff)

 

[weepete]

What camera does she have?

 

[Nocluewhattodo]

A Nikon D3300. Sorry common sense should have said you'd need that info!

 

[KmH]

The 18-55 mm lens that probably came with the camera would work nicely set to the the 18 mm focal length.
I assume she already has a good steady tripod and ball head.
Oben BE-117 Ball Head
Manfrotto MT055XPRO3 055 Aluminium 3-Section Tripod with Horizontal Column (Black)

A tracker would allow her to make exposures longer than 15 or so seconds.
iOptron SkyTracker Pro Camera Mount with Polar Scope

 

[Nocluewhattodo]

Thanks this is helping a lot so far!

 

[weepete]

A Tokina 11-16mm f2.8 possibly. But yeah, a good sturdy tripod is a must have

 

[TCampbell]

The two popular tracking heads are the Sky Watcher "Star Adventurer" head (that's probably the favorite these days) but iOptron recently redesigned their old "SkyTracker" head and it's now the "SkyTracker Pro". It should be much better but I don't know anyone who has used that head yet.

I use a Losmandy StarLapse tracker but that head is no longer made.

Here's a sample shot... this was taken using a 135mm f/2 (a Canon lens, but Nikon makes one too):


Lower Region of Orion by Tim Campbell, on Flickr

Note that it's only possible to get a shot like this with a tracker head. You wouldn't be able to expose long enough if it were on a stationary tripod - everything would smear due to the rotation of the Earth.

These tracker heads run about $300... but there are a few accessories that are helpful and it would boost the price a bit to the upper $300 range. The accessories are not required so she could always get them later.

As for the lens... low focal ratio is very helpful because it means you collect more light in fewer seconds. However if you shoot at the lowest possible focal ratio then you'll likely see every weakness of the glass. Stopping down - even if just slightly - is helpful if you can get away with it.

Rokinon makes a number of very inexpensive lenses which are wide angle and low focal ratio. The quality is a bit hit & miss (there are complaints of owners who buy a few and find that the optics aren't properly inserted. So if they try to test a flat field, they'll notice focus is better on one side of the image than it is on the other. But if you get a good copy... they're great.

Sigma has a new 14mm f/1.8 Art lens ... but it's not available to buy yet and I haven't seen a price for it. But that's a nice low focal ratio and wide angle... a killer combination if the glass is good, and Sigma's "Art" lenses have a solid rep for good glass (so our expectations are high). The price tag, however, may also be high.

Barring that, I'd probably take a look at the Samyang or Rokinon f/2.8 lens (probably about $310-320). On a Nikon APS-C body you stretch an exposure to about 23-28 seconds. At 30 seconds you'll probably notice the stars aren't quite "round" anymore as the motion caused by Earth's rotation causes them to elongate.

The formula is: 600 ÷ lens-focal-length = number of seconds you can expose

However there are two important points.

1) That "600" value is the generous value and some imagers think it's too high and they prefer to use "500" as a more conservative value.

2) That rule assumes a full-frame camera. Since you have an APS-C sensor camera (with a crop factor of 1.5) you have to multiply the focal length of the lens by the crop factor to come up with the value you should use to divide.

Example... if you had a 10mm lens then
600 ÷ 10 = 60
But since you have an APS-C camera it would be 10 x 1.5 = 15, so
600 ÷ 15 = 40
And if you're seeing non-round stars using 600, you'd use 500, so
500 ÷ 15 = 33.3