# Avoiding blown highlights by math?

I was thinking about how I could get the maximum amount of light from the darker areas in a scene without blowing out the highlights and I thought of using spot metering to accomplish that, here's what I thought of, please correct me if I'm wrong somewhere (I probably am, as I'm still a novice)

Assuming you have one source of light that may be blown out (or all the lights are the same brightness), you use spot metering to get the shutter speed where the highlight in question would be 18% gray. Assuming that 100% gray (might be dumb) is the brightest value your sensor can record (and that scale is linear, also might not be true) you can go up

$$x = \log_2{\left({100\over18}\right)}$$

stops of exposure, so your resulting shutter speed would be $$\text{Original shutter speed}\over x$$

$$\x\$$ here is around 2.47, so it can be rounded down to 2 and a third stops.

In my mind, at least with your new shutter speed (aperture the ISO remaining equal) you would collect the maximum amount of light from all the other parts of the scene, without blowing out your highlights.

Is there something wrong with my logic? I can't test this right now, as it's still around a week before I get my camera back from the repair shop.

• This almost seems like you are trying to re-invent Ansel Adams' Zone System (which people have adapted for digital cameras) Commented Jun 30 at 21:53

The main issue I see is with the variability of metering itself... There are two common ISO standards in use, and there are four possible that could be used (three by CIPA standards). And they give different results. Even calling them "standards" is a bit of a stretch.

Especially the the REI standard (Recommended Exposure Index); which is used for most DSLR's, and is the only standard applicable to recording raw files. REI commonly rates ISO (and exposure) at least 1/2 stop low compared to a more directly measurable standard like SOS (Standard Output Sensitivity).

This is the Canon R5 as measured by DXO. Note that while DXO disagrees with the manufacturer's ISO ratings, it's because they are using a different standard to measure by (I'm not sure if it is Sat (saturation) or SOS).

Part of the reason the recommended exposure (REI) is set low is to protect against the harsh highlight clipping characteristic of digital sensors. And that's why there is usually "recoverable" highlights in a raw file.

So I would say your best bet is to spot meter the highlights and then determine how many stops over results in actual overexposure/clipping by taking test images and editing in post. Note that highlight warnings on the camera's image review also tend to "lie" because they will typically report as clipped anytime any one of the color channels reads 255 (or close). And they are showing the processed jpeg (not raw file); so you really do want to verify by editing the files in post.

Your idea is correct in general.

In addition to the answer of Steven: you are going to have hard time if you are trying to reach that limit with no safety margins.

1. It's easy to choose brightest object incorrectly.
2. If there are two object with similar brightness but different colour camera may have different opinion about their brightness than you (metamerism failure).
3. Blue and red tones blow away later because green is most sensitive channel universally. That spare range usually gets thrown away by cameras when colour conversion happens if not recording to raw file.
4. Colour profiles having HSL maps built into them adds more uncertainty as well.

Aperture is only a physical measurement of a lens (focal length/diameter of circle aperture blades create), one that is pretty much always unrelated to the value you want to start with (t/stop, or transmission). Using f/stop for any calculation regarding light will immediately break your theorem.

There's very VERY few modern (post-auto focus) lenses where the f/stop and calibrated transmission are within 0.1 EV, and there are some hilarious examples (canon f2.8 zoom lenses come to mind) where they are often tested closer to t/4 than t/2.8. Or lenses that may be close to their f/stop at one focal length, but as they zoom out, go over +1.0EV. Or have terrible vignetting at wide focal lengths, which also hurts their transmission.

So yeah. Ignore f/stop. It's a marketing number. Think of it like the highway mpg car companies put on the window sticker. It's aspirational.

Now finding the transmission score of your lens is tough sometimes. But here's all the info you need.

Transmission (light exiting the last lens element), quantum efficiency (percentage of photons hitting the pixel that are counted), well depth (maximum photons a pixel can absorb before saturation is 100%.

It gets trickier because no sensor (that we can get, in a camera. These go to nasa, probably) has 16, or 24, or 36, million pixels with the exact same performance. Some will hit full well too early, some too late. If your lens has a smudge, it will lower some pixels photon number, increasing iso will bring more variance, as now amplified circuits certainly don't work exactly the same to the precision you're asking.

I'm not sure what camera system you have, but many (nikon in particular) did the hard work for you, and offer "highlight metering". Although turning on the highlight blinkies when you review images, and using exposure adjustment, or any variety of aperture, ss, and iso, to get zero blinkies works as well, and involves a whole bunch less math that someone else already did when they developed in-body TTL light meters

New contributor
Robin Gordon is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
• There's also like 15 other pretty major variables if you're actually curious. But I get the feeling (and I genuinely mean this as a compliment) you just miss your camera, and are thinking about photography, and how your camera works, or how to improve a part of your photography that's been frustrating you. I personally love that energy and passion, so yeah. Happy to keep chatting about this stuff, it was the physics of digital photography that fascinated me the most. Commented Aug 4 at 15:32