Full moon = lower ISO?

[sanmatt]

HI all

Thought I'd try again with star trails, but Im wondering as its a full moon tonight, should I set the ISO quite low? How should I have the other settings? Using a Nikon D90 on tripod. 18-105mm lens

Thanks.

Sandra

[Ags]

You want a larger aperture and lower F ratio I think - so shoot at longer end of the zoom, stopped down. The larger aperture (from the longer focal length) makes the stars brighter while the slower F ratio makes the sky darker. For example, stars are four times as bright at 100mm/F4 as they are at 50mm/F4. They are about twice as bright at 100mm/F6 as they are at 50mm/F4 and the sky is twice as dark.

In theory anyway :-)

[sanmatt]

Shall I set it on 24mm/f4, 30 subs, ISO400?

Do I focus it on infinity?

I am finding this new hobby a bit over my head!

[Ags]

Focus on infinity, but if I was you I would go for 70mm-ish F6. You will pull out more faint stars that way.

Do a couple of test shots and compare my suggestion to your intuition - the framing may work at 24mm, so 70mm would be pointless... :-)

You can also process away a lot of the sky background in image processing software, but gradients are harder to get rid of.

[sanmatt]

right oh will do!

[sanmatt]

HIya, I've moved it to 70mm and it auto selected f5.3. Does that sound ok to you?

[Ags]

Just to be a bit nerdy... at 24mm F4 you have 6mm actual aperture. At 72mm F6 you have 12mm aperture - 4 times brighter stars! But for sky background, you are imaging an extended object and here the F ratio determines brightness: F4 gives a brighter sky background than F6.

[Ags]

Yes, the longer end of the zoom typically forces a slower speed. You might find sharper stars at the edges if you stop it down a little bit more, but it depends on the lens and the camera.

[sanmatt]

I had a quick look at the two lots of images and on 24mm/f4 I seem to get far more stars than on 70mm. All I could see on the 70mm shot was a few fuzzy blobs?

[Ags]

Ah - out of focus?

You may need to manually focus using the live view screen (the infinity setting might not be totally reliable - regardless of the number of stars, they should be points at any given focal length when focus is right). At 24mm you have more depth of field so focus is less critical.

[sanmatt]

I had to on inifinity both times. (Sigh, I just dont know what the hell I am doing with all this)

But I'm thinking 24mm might be the best to have it on so I get more trails?

(why oh why did Father Christmas bring me such a difficult hobby?)

[Ags]

Good luck! Post your results :-)

[sanmatt]

Thanks will do (and imaging RAW also)

Shoud I take some darks too?

[sanmatt]

It's stacking now.

edit - well it says its 'averging the foreground' but it appears to have stopped doing anything...the bar isnt moving.

[sanmatt]

Managed to get this in the end

[Rob L]

I think that's turned out very well. I thought about trying this last night, but in the end I was too lazy (and warm) to go outside. Well done

[sanmatt]

Thansks Rob.

I think this time it was better because I had kept the ISO fairly low. Last time (my 1st time) the sky was bright orange. I did need to tweak a bit in PS, but only to make the sky a tad darker. I dont really know what Im doing!

[Ags]

Well done!