The rule states that on a sunny day you can use f16 for your aperture setting and the inverse of the ISO setting for your shutter speed.
for ISO 400 that would be 1/400 or 1/500 since that is as close as you can get with standard stops. It depends on your camera. This is an eyeball approximation after all.
The proper Full Aperture F Stops are f1.4 f2.0 f2.8 f4 f5.6 f8 f11 f16 f22
The proper Full Shutter Speeds are 8 sec 4 sec 2 sec 1 sec 1/2 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000
What if your digital camera does not do f16?
So for a digital camera if the best you can do is two stops down from f16 which would be f8. Then you should move your shutter speed two stops up to compensate.
My Canon A720 IS does f2.8 f3.2 f3.5 f4 f4.5 f5 f5.6 f6.3 f7.1 f8
These increment in 1/3 stops for a total of three actual full sized stops.
My Shutter Speeds are 15 13 10 8 6 5 4 3.2 2.5 2 1.6 1.3 1 .8 .6 .5 .4 .3 1/4 1/5 1/6 1/8 1/10 1/13 1/15 1/20 1/25 1/30 1/40 1/50 1/60 1/80 1/100 1/125 1/160 1/200 1/250 1/320 1/400 1/500 1/640 1/800 1/1000 1/1250 1/1500 1/1600 1/2000.
These also increment in 1/3 stops.
You should use the proper F Stops to calculate the sunny 16 rule for your max apature. In my case f8. And since I am using ISO 400 the sunny 16 rule would set my shutter speed at 1/400. My camera does do 1/400 so now I just have to count up two full stops on my shutter speed list. That would get me to 1/1500. And that should fit the sunny 16 for my camera. If it's not quite right I can fudge a little.
My test picture came out pretty good. It looks good though as you can see. It wasn't bright and sunny all over but I had good sunlight behind me.
Another picture 1/3 of a stop back on shutter speed (1/1250) looks noticeably brighter though.
So as a rule of thumb you can use the following formula for for f8 on a sunny day. Shutter speed = 1/(4 x ISO). It will get you close to what you need if not right on.
The rest of the Sunny 16 rule.
If it is slightly overcast, shadows are soft around the edges. Back off 1 f stop. ie. f8 to f5.6
if it is really overcast, shadows are barely visible. Back off 2 f stops. ie. f8 to f4
If is heavy overcast, no shadows at all. Back off 3 f stops and punt. ie. f8 to f2.8
Couldn't test the other two. Maybe Later. I hope this is useful to someone. I am a rank amateur and this little report is my way of smart making.
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