Sunny f16 Bollocks? Mea Culpa...

A couple of weeks ago I wrote a post describing how I'd used the Sunny F16 rule to arrive at an exposure that my handheld meter subsequently told me should have been about three stops lower than my calculation. I've used this rule in the past with success although, being quite far north, I've always given an extra stop just to be on the safe side.

The huge underexposure left me a bemused so I decided to look into the process a little more deeply. The earliest reference I seem to have to the Sunny f16 rule is a 1947 copy of the British Journal Photographic Almanac, a compendium of all things photographic for that year. No doubt the rule will predate that by several decades or more but I don't think we need to know the precise start date.

A few readers in the sunny parts of America left comments to the effect that Sunny f16 worked for them and one or two from darkest Britain said something like Sunny f4 is nearer the mark over here. Well, turns out they're probably both right. Let me explain.

When I was taking a picture of the dogs in my back garden on a dull day, I had 800 ISO film in my Pentax MX and worked out the exposure from the starting point under the Sunny f16 rule of 1/800th at f16. There being no 1/800th speed on the MX, that's about the same as 1/1000th between f11 and f16. I opened up three stops for cloudy dull weather (one stop for hazy sun, another for cloudy bright and the third for cloudy dull) and then gave it my usual extra stop for Scotland.

Those four stops extra gave me an exposure of 1/125th at between f8 and f11 so I made it f8. The correct exposure indicated by my handheld meter should have been 1/125th at f2.8.

Now, the BJ almanac lists an exposure table for England in the summer and gives the correct setting for a shot of landscapes and beach scenes on a sunny day as 1/25th at f16 with 700 H and D speed film. That method of rating film was way before my time (lower those inquisitive eyebrows) but it seems to be roughly equivalent to 20 ISO. The Almanac also says the exposure table is good for 24 or 25 Scheiner but doesn't say whether it's the European or US Scheiner scale they're talking about. The difference here is between 10 and 50 ISO so 20 ISO would seem reasonable. Or approximately the same as the Sunny f16 rule, then.

Since I was measuring a scene in my back garden on a dull day this far north in January, there are a few modifiers the BJ publication suggests. For hazy sun, plus 1 stop, for dull weather plus 1.5 stops and for very dull it's plus 2 stops. For Scotland in January, another 3 stops have to be added and for the shady photograph of the dogs I was taking it's plus an extra 2 stops.

If you've been counting, you'll see that it adds up to an extra 7 stops over the BJ's sunny weather recommendation of 1/800th at f16 for 800 ISO film, or, say, 1/100th between f11 and f16. Adding the seven stops gives a modified exposure of 1/125th at between f2.8 and f4 - almost exactly the same as my handheld meter reading.

So Sunny f16 is indeed viable when it comes to shooting in the UK but only within very restricted parameters - at the height of summer, on a good day and when taking a nice open landscape or beach scenes. At other times of the year and depending on the time of day and latitude Scotland needs considerable extra exposure - plus due allowance for shady photographs - and it was underestimating this that led me astray.

I'd say that up here a safe interpretation of the rule should be Sunny f8 in the summer, Sunny f5.6 in spring and autumn and Sunny f4 in winter. Sticking with those values, I don't think we'd go far wrong.

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