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09-07-2021, 05:23 AM   #1
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Under-exposure with "Sunny 16" rule

I'm a little embarrassed to say that until today, I've never tried shooting using the "Sunny 16" rule, and I figured it was high time to try it out; especially since, in the coming weeks, I'll be doing some film photography using a camera with no light meter.

For my first attempt, I used my old Pentax *ist DL and Pentax-F35-70f/3.5-4.5, since that was the camera immediately to hand.

It's a very bright, sunny day here, with cloudless skies... perhaps just the tiniest hint of haze, but really nothing to speak of. In terms of UK weather, I'd say it's about as good as we ever get. The time of my test was 1pm in the afternoon, with the sun not far past its peak. On that basis, I understand that at ISO 200 (the *ist DL's base ISO) and the lens set to f/16, a shutter speed of 1/200s should give more-or-less accurate exposure...

But it isn't. I'm seeing about one stop under-exposure. Indeed, if I shoot at f/11, exposure is pretty much spot-on.

What gives? Is this an inaccuracy in the camera's supposed ISO 200 sensitivity? Or perhaps f/16 on my F35-70 isn't actually f/16?

I vaguely recall a post some time ago - I think, perhaps, by @stevebrot - that mentioned how the "Sunny 16" rule doesn't quite work the same as you move further away from the equator. That might be a false memory, but if not, I'm wondering if the one stop difference I'm seeing could be due to that?

I intend to repeat the tests with other lenses and bodies, but won't have a chance to do so today, as we've family guests arriving later this afternoon and I have too many non-photography chores to address

UPDATE: I have an incident light meter app on my smartphone that works very well and is pretty accurate. It's giving almost identical exposure results to the *ist DL and F35-70. For ISO 200 and f/16, it's reporting a shutter speed of 1/80s, and changing aperture to f/11 gives shutter speed as 1/160s (close enough to 1/200s). So maybe the very, very slight haze is having more of an effect than I give it credit for? Questions, questions...


Any insight and/or advice would be much appreciated. Thanks


Last edited by BigMackCam; 09-07-2021 at 06:37 AM.
09-07-2021, 05:31 AM - 1 Like   #2
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It's a rule of thumb, so your mileage may vary. If you cross checked with the camera's light meter, what did it tell you?

Have you checked the image on your computer - with a calibrated display? Confirm that you didn't accidentally have any EV compensation; also check the white balance because that could be adding to the confusion.
Do you have a separate light meter or second camera body you could check with?
09-07-2021, 05:42 AM   #3
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QuoteOriginally posted by ProfessorBuzz Quote
It's a rule of thumb, so your mileage may vary. If you cross checked with the camera's light meter, what did it tell you?

Have you checked the image on your computer - with a calibrated display? Confirm that you didn't accidentally have any EV compensation; also check the white balance because that could be adding to the confusion.
Do you have a separate light meter or second camera body you could check with?
I was using manual exposure mode, so no EV comp. The camera's meter reported one-and-a-third stops under-exposure in the viewfinder, and reviewing the image in Lightroom 6, it was indeed under-exposed by about that amount, give or take a little. White balance was accurate. When I adjusted to f/11 and re-shot (still at ISO 200, 1/200s), the camera reported just one third of a stop under-exposure, and the image looked fine in Lightroom.

See my update above regarding light meter - it confirms what I found with my camera and lens, so this doesn't appear to be an equipment issue. I've possibly misjudged the almost-perfect conditions for Sunny 16 to apply?

Last edited by BigMackCam; 09-07-2021 at 06:38 AM.
09-07-2021, 05:52 AM - 1 Like   #4
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I find that anything short of noon sun with not a single hint of cloud or haze gets you f16, and even then it better be summer. So i shoot at f11 usually, and any potential overexposure hasn't been a visible issue with negative film

09-07-2021, 05:57 AM - 2 Likes   #5
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Ultimate Exposure Computer is a useful site which covers the Sunny 16 rule. Probably the most important thing to bear in mind when using it is it is based on incident light rather than relected light which a camera would use. What scene were you metering with the camera?
09-07-2021, 06:05 AM   #6
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QuoteOriginally posted by aaacb Quote
I find that anything short of noon sun with not a single hint of cloud or haze gets you f16, and even then it better be summer. So i shoot at f11 usually, and any potential overexposure hasn't been a visible issue with negative film
Interesting. Thanks for that

QuoteOriginally posted by steephill Quote
Ultimate Exposure Computer is a useful site which covers the Sunny 16 rule. Probably the most important thing to bear in mind when using it is it is based on incident light rather than relected light which a camera would use. What scene were you metering with the camera?
Thanks for the link

I tried a couple of different scenes - nearby rooftops and chimneys (including plenty of blue sky), a sun-drenched corner of my garden - both with my back to the sun. Incident metering from my smartphone light meter app quite closely matches what the camera saw with its reflective metering...

Last edited by BigMackCam; 09-07-2021 at 06:32 AM.
09-07-2021, 06:13 AM - 1 Like   #7
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Iím going to suggest that the exposure you got was more accurate than you realize. Sure it was slightly under exposed, but thatís acceptable for not metering and using a baseline heuristic rule. Furthermore digital sensors vs film is an approximation. The baseline exposure may be similar but the outcome differs.

However, as this article shows you are better off over exposing negative film than under exposing it: How Much Can You Overexpose Negative Film? Have a Look! | PetaPixel so perhaps you should consider being very conservative about what fully sunny conditions are and opening up the aperture a little or lowering the shutter speed. If your shooting Slide film this changes and a slight underexposure is fine.

Finally, I would suggest bracketing if the film age and storage isnít well known. Typically film loses sensitivity over time but the loss isnít easily predicted without shooting some test exposures.

09-07-2021, 06:50 AM - 1 Like   #8
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That was my usual experience with fresh film in a Scottish summer. Only full Californian or Middle East sun gave me a full-blown Sunny 16 experience. I also used an incident meter back then, Weston Euromaster with Invercone (?): still got it too.

Last edited by StiffLegged; 09-07-2021 at 06:55 AM.
09-07-2021, 07:13 AM - 3 Likes   #9
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The distance from the equator should have an effect as the amount of atmosphere the light has to travel through changes with the difference in angle as you go North or South from the equator. I think time of year would play a role also for the same reason.
09-07-2021, 07:19 AM   #10
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QuoteOriginally posted by UncleVanya Quote
Iím going to suggest that the exposure you got was more accurate than you realize. Sure it was slightly under exposed, but thatís acceptable for not metering and using a baseline heuristic rule. Furthermore digital sensors vs film is an approximation. The baseline exposure may be similar but the outcome differs.

However, as this article shows you are better off over exposing negative film than under exposing it: How Much Can You Overexpose Negative Film? Have a Look! | PetaPixel so perhaps you should consider being very conservative about what fully sunny conditions are and opening up the aperture a little or lowering the shutter speed. If your shooting Slide film this changes and a slight underexposure is fine.
Thanks, Brad, I appreciate the info and article link. I'll be using negative film, so "Sunny 11" might indeed be a safer bet for me

QuoteOriginally posted by UncleVanya Quote
Finally, I would suggest bracketing if the film age and storage isnít well known. Typically film loses sensitivity over time but the loss isnít easily predicted without shooting some test exposures.
With the film camera I'll be using, there aren't too many options for bracketing; I'll only have a couple of apertures and shutter speeds - each 1 2/3rds stop apart - to play with. I'll have to choose well-kept film with plenty of latitude. Even then, it's going to be something of an experiment and leap of faith

QuoteOriginally posted by StiffLegged Quote
That was my usual experience with fresh film in a Scottish summer. Only full Californian or Middle East sun gave me a full-blown Sunny 16 experience. I also used an incident meter back then, Weston Euromaster with Invercone (?): still got it too.
Good to know - thanks I'm in the North East of England, so a reasonably similar latitude and weather. Sounds like "Sunny 11" might be a prudent assumption...
09-07-2021, 07:56 AM - 1 Like   #11
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The "Rule" will also depend on your lens and the loss of light as it travels through it. F-stops don't take this into account whereas T-stops do (and is one reason they are used on cinematic lenses and the like). Although lens coatings reduce light lost at each lens surface, some is still reflected or absorbed, and doesn't reach the film or sensor. This probably doesn't account for all of your exposure discrepancy, but may contribute to it, and as mentioned, this rule is a rule of thumb and not a solid guide you can absolutely depend on.
09-07-2021, 07:58 AM - 2 Likes   #12
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Hello,

Also keep in mind negative film has a greater latitude, making it slightly more forgiving than digital on exposure variations. On digital I try to protect the highlights. On film I try to protect the shadows.

Thanks,
Ismael
09-07-2021, 09:06 AM - 6 Likes   #13
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QuoteOriginally posted by Parallax Quote
The distance from the equator should have an effect as the amount of atmosphere the light has to travel through changes with the difference in angle as you go North or South from the equator.
I recommend bringing a Sextant along with the camera for proper application of the Sunny 16 rule.
09-07-2021, 09:19 AM - 1 Like   #14
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QuoteOriginally posted by BigMackCam Quote
It's a very bright, sunny day here, with cloudless skies... perhaps just the tiniest hint of haze, but really nothing to speak of. In terms of UK weather, I'd say it's about as good as we ever get. The time of my test was 1pm in the afternoon, with the sun not far past its peak. On that basis, I understand that at ISO 200 (the *ist DL's base ISO) and the lens set to f/16, a shutter speed of 1/200s should give more-or-less accurate exposure...

The Sunny 16 rule only requires f/16 at 1/200 at ISO 200 if you are in full sunlight in a highly reflective environment such as a sandy beach or in snow. In direct sunlight in a less reflective environment such as grass, you should use f/11 at 1/200 at ISO 200. In fact, for most practical applications I tend to think of it as the Sunny 11 rule, with the full Sunny 16 reserved only for exceptionally bright settings.

The latitude thing is a bit of a myth (even though I once believed it myself). Remember that one full stop is a doubling or halving of the light, and it becomes obvious that direct sunlight at the equator is most definitely NOT twice as bright as direct sunlight at higher latitudes. But the further south you go, the more likely you are to be in a highly reflective environment such as a beach or a desert, probably with lighter coloured buildings too, so you're more likely to need to use the full Sunny 16. But it's the more reflective environment rather than the latitude that makes the difference.

Last edited by Dartmoor Dave; 09-07-2021 at 09:48 AM.
09-07-2021, 09:21 AM - 2 Likes   #15
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Also, those older Pentax bodies did tend to underexpose. My K10D underexposes much more than my K-S1. About 2/3 of a step average, sometimes a step.

My K200D is surprisingly accurate as well, but I think it's because the viewfinder is dirty (seems stained, won't clean up) and one corner looks "de-silvered" - so it's probably adjusting for that and the exposure comes out right

Anyway, you are in England. What you perceive as a very bright day might not be as bright as you would think
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