W. D. Grey said:Quoted message said:Quoted message said:Try 20 secs (Aperture priority) at f 8 and check the histogram.
Digital is so user friendly :-)
That would come out almost black in those conditions!
Both Paul and I missed my faux pas above ..!
I should have said try Aperture priority f 8 and check the histogram.
You can't set 20sec in Aperture priority - you set the aperture. The
camera decides the shutter speed.
Quite! Also, aperture priority decides the exposure, whereas the whole
point of checking the histogram afterwards is for a manual exposure.
Since night shots usually require longer exposures than 30 seconds, you
have to use the B setting. I haven't checked, but I suspect the B
setting only works in manual mode, for obvious reasons.
Again, if testing night exposures, start with the widest aperture and
highest ISO rating to quickly determine the correct exposure, then
change to a lower ISO and smaller aperture and compensate the shutter
speed accordingly. This probably could be done using aperture priority,
but you'd still have to check the histogram and correct if necessary,
then switch to manual mode for the longer exposure.
An important point about apertures and star trails. As we know, a
smaller aperture is compensated for by a longer shutter speed, so no
matter how small the aperture, we can make the scene as bright as we
want by increasing the exposure time (longer shutter speed).
However, when photographing stars, this is only true up to a point.
That point (if you excuse the pun) is when the apparent movement of the
stars starts to create a trail, so you get a line rather than a point of
light. Since the stars move (I know it's not really the stars moving),
a longer exposure time won't make a star any brighter once it's moved to
a different position. Therefore, the aperture you use determines the
maximum brightness of the stars, and consquequently the limiting
magnitude (the faintest stars that can be seen).
So if you want the brightest stars (and more faint stars), you'll need
to use the widest aperture. Using f8 may give you better depth of
field, but f4 will give you more and brighter stars.
In fact, I read that for this kind of night photography you should use
the widest aperture, in spite of the widest aperture usually producing a
lower quality image. In fact, if you're really serious about night
photography, it would be best to use fixed focal length lenses with very
wide apertures. A 50mm f1.4 lens would be superb for this, although not
very wide (80mm equivalent). You can get a 35mm f2 I believe, and maybe
even a 28mm f2, but they're not cheap and still not very wide on a DSLR
(28 is equivalent to 45). I don't think wider lenses go as bright as
f2.
With this in mind, my Canon G3 was rather good with its f2 lens,
equivalent to 24mm with the wide-angle adapter. Unfortunately there was
a maximum of only 15 seconds exposure time and no B setting, so no good
for star trails.
Currently I think the the most practical option for DSLR night
photography is the new Sigma 18-50 EX lens, which is f2.8 throughout the
whole range. Not as bright as the fixed lenses, but considerably
cheaper and reasonably wide, and of course it's a zoom, saving money
weight and bulk on a collection of fixed lenses.
One more thing, there's a rule for star trails, the 600 rule. If you
divide 600 by the focal length (equivalent focal length for digital),
that's the maximum number of seconds you can expose for before trails
start to appear, so for example;
600mm lens - 1 second
300 - 2 secs
200 - 3 secs
135 - 4.4 secs
80 - 7.5 secs
50 - 12 secs
35 - 17 secs
24 - 25 secs
16 - 37.5 secs
These exposure times, combined with the widest f-stop, will give you the
brightest possible stars without trails appearing.
Oh, and for best results, choose a moonless night and a location far
away from city lights.
Paul
--
http://www.wilderness-wales.co.uk
