Widefield Lens DSLR

[adamw]

I have done a bit of reading up on the interweb but I am still confused.  For wide field astro images with a canon 60D on an equatorial mount, which lens is going to be better (give a wider angle image)?  For example 100mm f2, 100mm f2.8 50mm f1.4 or 24mm f2.8?  I always thought the longer the lens and the lower the f ratio the better

[Purplehayze104]

hi there

as far as I know the shorter the lens an F length gives you a wider field of view, I know its bit of a crude explanation but if you get a toilet roll & a kitchen roll an look throu em the toilet one gives you a wider view than the kitchen roll, so in theory from the list provided I would expect the 24mm to be the widest field, but can cause some vignetting may be wrong on that, mite get someone with a bit more knowledge shooting my idea out of water

hope it helped.

clear skies

john

[PeterCPC]

For widefield you want the lowest f number you can and the widest lens - 24mm in your list or you could try the 50mm. Try it and see what you get.

Peter

[MartinB]

The focal length determines the magnification and for a given focal length the larger the chip the greater the FOV. The F number is the focal length divided by aperture. The larger the aperture the lower the F number and the greater the light gathering ability. For wide field you want a short focal length. How low you go with the F number will ultimately come down to cost and quality. Wide angle lenses can introduce a lot of distortion especially when fully open, good ones aren't cheap

[Knight of Clear Skies]

The lower the focal length the wider the field of view. Focal ratio is a measure of how much light reaches the sensor, the lower the better. However, depending on the lens you may need to stop it down a bit to reduce coma (elongated star shapes in the corners).

[Jonk]

In simple terms:

• The larger the focal length, the more zoom. 24mm is wide, 100mm is zoomed in, 200mm is zoomed in more.
• The f number on the lens (f2.8, f4, f5.6 etc.) is the focal ratio which is a result of focal length divided by aperture (size of the opening). For example a 200mm f2.8 lens has a 71mm aperture. 200mm/71 = 2.8. The lower the focal ratio, the 'faster' the lens, meaning it lets in light 'quicker' than a 'slower' lens. if you had a choice between a 100mm f2.8 or a 100mm f4, the f2.8 will be faster, meaning you can take a shot 'quicker' and have less blur.
• With wide lenses as explained above, a popular 14mm f2.8 is great for wide angle shots of say the Milky Way if you open it fully wide (i.e. f2.8) but you will get coma (comet shaped stars at the edges). Closing the aperture to 4 or 5.6 (known as 'stopping down') give you a sharper image across the field of view, but you need to expose for longer to get the same amount of light captured. Depending on the photo you're taking this can be a good thing or bad thing.
• The size of the camera sensor also matters, a full frame DSLR has a larger sensor than an APS-C ('cropped') sensor (1.5 or 1.6 times smaller depending on the brand). The larger the sensor, the larger the field of view.

Hope that helps!

[adamw]

Ok thanks for that, I think I get it now. Shorter the lens wider the field faster the f ratio more chance of coma. I have seen a good review of the Rokinon 16mm f2. I think I will try to pick one up second hand off flea bay.

[Jonk]

I'm waiting for a Samyang 24mm 1.4 to come up onnebay for widefield stuff. Faster the better. May even buy a new one!

Do you have a full frame camera?

[Knight of Clear Skies]

Shorter the lens wider the field faster the f ratio more chance of coma.

Not necessarily, it depends on the individual lens designs. Typically zooms lenses will produce greater distortions than primes but that's not always true.

[adamw]

I'm waiting for a Samyang 24mm 1.4 to come up onnebay for widefield stuff. Faster the better. May even buy a new one!

Do you have a full frame camera?

No its APS-C size chip. I am going to keep looking on ebay for something to come up at a reasonable price secon hand.

[kenny k]

If you want really widefield then 10 to 14mm will give you alot of skies.Check out the Samyang range,there resonably priced.

I went for the 10mm Samyang,here's my first attempt to give you a feel.

Kenny

[Ruud]

Hi Adam,

This Sigma 8-16 mm is not the fastest lens, but you can use it wide open. It's actually at its sharpest there. Excellent optics and very wide.

[Phillyo]

I would HIGHLY recommend the Samyang (Rokinon) 14mm F/2.8. You can use it at 2.8, especially on APS-C sensors and have lovely images. Another one is the Samyang 24mm F/1.4 as already mentioned, not quite as wide but better light gathering ability.

Depends if you're going to be doing static wide angle shots or tracked? If tracking then the speed isn't as important because you can stop it down to F/4 to get lovely sharp images but track the sky for longer than normal so as not to introduce star trails.

A good rule is the Rule of 500. Which roughly equates to dividing your focal length (14mm or 24mm) into 500. This will give you how many seconds you can expose for, untracked, before star trailing becomes noticeable.

Phil

[Mav359]

I would HIGHLY recommend the Samyang (Rokinon) 14mm F/2.8. You can use it at 2.8, especially on APS-C sensors and have lovely images. Another one is the Samyang 24mm F/1.4 as already mentioned, not quite as wide but better light gathering ability.

Depends if you're going to be doing static wide angle shots or tracked? If tracking then the speed isn't as important because you can stop it down to F/4 to get lovely sharp images but track the sky for longer than normal so as not to introduce star trails.

A good rule is the Rule of 500. Which roughly equates to dividing your focal length (14mm or 24mm) into 500. This will give you how many seconds you can expose for, untracked, before star trailing becomes noticeable.

Phil

Second on the Rokinon 14mm, i love mine

[octavenoize]

sigma 11-35mm F/1.8....

[octavenoize]

*18-35mm

(higher price tag.. but something to consider)