today I'm gonna put the 500 rule to the
test
if you're not sure what the 500 rule is
it's basically a calculation to work out
how long you can keep your shutter speed
open for before the stars start
streaking so instead of the stars
looking like round circles they start
looking like ovals if you're not sure
how to take photographs of the stars but
want to learn more click on the eye in
the corner this will take you to one of
my videos taking you step-by-step
through the process what I've done is
taken a series of shots from 15-second
exposures right through to 1-minute
exposures I have a cropped censored
camera and a full-frame camera and I'm
gonna see how much these rules need to
be enforced so you get good looking
stars well when I say I've got a crop
sensor I'm going to shoot on the crop
mode on the a7 r2 so this will represent
a cropped censored camera for a
full-frame sensor take your focal length
and divide 500 by that focal length for
a crop sensor take your focal length and
divide 300 by your focal length with a
full-frame at 14 millimeters 500 divided
by 14 equals 35 point seven so that's an
exposure of 35 point seven seconds with
the 24 millimeter I take 500 and divide
it by 24 this gives me 20 point 8
seconds with the crop sensor at 10
millimeters I take 300 and divide it by
10 which gives me an exposure of 30
seconds with the 14 millimeter on a crop
sensor that's 300 divided by 14 so
that's an exposure of 20 1.4 seconds so
if you look at the maths alone you can
see why a full-frame sensor is much
better for astrophotography at least
with wide scape astrophotography where
you want to get as much of the sky in as
possible and with a relatively low cost
of the a 73 and the really cheap a72
there's never been a better time to get
into Astro photography so here are the
images I've dropped the saturation down
to minus 100 so the color doesn't
distract I'll make it fullscreen
this is at 20 seconds it's at 25 this is
at 30 35 40 you can see the stars are
really starting to streak up in this top
corner by 50 seconds you can see those
all of the stars in the shots are ovals
even when I reduce it to this size in
the frame you can still see how streaked
those stars are up here I found with the
10 to 18 millimeter lens I was using
there was a lot of distortion which made
the stars appear oval-shaped towards the
edges of the frame with almost any
shutter speed I used at 10 millimeters
were the 300 rule I should be able to
expose the shot for 30 seconds however I
would not go above 20 seconds and I
wouldn't use the sony 10 to 18
millimeter f4 lens because of the
distortion you can see in this shot oval
stars in the top right hand corner with
a cropped sensor at 14 millimeters the
calculation comes out at 21 point 4
seconds so I've started at 20 seconds
and the stars look ok as I go through
the shots that's a 25 that's at 30 you
can see these stars over here are
starting to streak a plane came through
the shot and that's that streak at the
bottom but if you look at the stars
they're all starting to streak now so if
we zoom in to the middle you can see
they're more oval shapes than circles at
just over a minute you can see they
really start to streak so you can see
this calculation is really important if
I select the 22nd and the one minute and
six seconds shot and make it full frame
when I switch between the two shots you
can really see the difference if you
ignore the light streaks from the planes
and just look at the stars in this
center area you can see here their
streaks whereas when we go to the 22nd
one their pinpoint stars now I've
changed their a7r to the full frame
setting so I've come out of the crop
setting I'm using a 14 millimeter lens
so let's see what we can get away with
with the 500 rule
halation if i take 500 and divide it by
14 that will give me thirty five point
seven seconds so I've started at fifteen
and I've gone through to one minute and
ten seconds so at fifteen seconds you
can see the stars of really good
pinpoints they're all still circular
it's at twenty seconds 25 seconds 30
seconds when I'm using this lens 30
seconds is normally as far as L go with
a 14 millimeter on a full frame camera I
can go up to 35 seconds using the 500
calculation but I tend to keep it to 30
seconds at a maximum so we go to 35 40
45 this is 50 55 and you can really see
the stars are starting to streak now
also one thing to note with this lens
there's a lot of distortion on the edges
so the streaks will actually look longer
towards the sides this is one thing to
think about when choosing a lens you
have something like the 14 millimeter
samyang that has a lot of distortion on
the edges whereas if you spend more
money and get the lower 15 millimeter
f/2 that has a lot less distortion so
this is at 55 seconds and this is at one
minute now we have oval stars and the
Milky Way is starting to blur so if I go
back to 15 seconds you can see the Milky
Way is much clearer and the stars and
more circular so next we have the
sequence of the full frame camera with a
25 millimeter batis so you can see it's
cropped tighter on the Milky Way at 15
seconds you can see the stars are
circular I really like this exposure
length for astrophotography and if
there's enough light about I'll try and
keep it between 13 and 15 seconds this
then gives me a bit of leeway and it
means that my stars are going to be more
circular then at 20 seconds
still looking okay 25 you could probably
get away with that on Instagram because
the image is a lot smaller so this is at
30 seconds on the 25 millimeter you can
see the stars are starting to look more
I wouldn't be happy with this and I
would drop my shutter speed down and at
35 40 45 50 at 50 seconds they're really
starting to streak if you want to get
the stars streaking in your shots what
you want to do is do really long
exposures and then stack all these
together and then you'll get really long
star streaks this can look really nice
but if you want pinpoint stars you have
to follow this rule that's one minute
the stars are really streaking and you
really wouldn't want to take a shot with
this longer exposure unless you are
using a tracking system so again if I
select the 15 second shot and the one
minute shot and switch between the two
as you can see the 15 second shot has
nice pinpoint stars whereas the one
minute shot looks streaky and blurry it
doesn't look good at all so as you can
see from these series of shots the five
hundred rule is really important and if
you're on a crop sensor it's the 300
rule if you zoom in enough though you
will see the streaks so if you pixel
peep the Stars will still look oval
basically the bigger that you're gonna
print the image or the bigger the image
is going to be the more you'll see these
streaks if you're posting to something
like Instagram or a social media site
that has relatively low resolutions you
can get away with slightly longer
exposure times but as a rule I'd stick
to the 500 rule and that's about it if
you're going out and shooting the stars
make sure you follow the 500 rule if
you're on a full-frame sensor and the
300 rule when you're on a crop sensor
also if you're using something like the
14 millimeter samyang that has a lot of
distortion on the edges I'd keep the
exposure time a little bit shorter with
a 14 millimeter hug keep it to a maximum
of 20 seconds with the 24 millimeter
samyang the distortion is less but still
I'd keep that below 20 seconds as well
if you're on a crop sensor and you're
using say a 10 millimeter just see what
the distortion is like in that lens
it's actually good to go out and do this
test yourself so you can work out what
you can get away with with your camera
but as I said I tend to shoot at around
about 15 second exposures with my Astro
shots I'll bump the ISO up a little bit
but these modern-day cameras can handle
the higher ISO as always if you like
what you see give me a thumbs up if you
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