Note: This post initially existed, unpublished, solely to allow me order my own thoughts. I shared it with one or two folks who asked me related questions recently, and they found it useful. If you find it useful, fine; however, I strongly suggest you also read the missives of those with a clue.
The below most useful if you find yourself shooting without a light meter – i.e. you’re using a slightly broken or very old camera. If you only use working modern gear, the below is merely a nice way to look “under the hood” a bit.
What’s this Frame of Reference, then?
I’ll get to that. First some background.
What is Exposure?
Exposure is, loosely, a measure the total amount of light hitting the sensor (or film) whilst taking a photograph. It can be measured absolutely (in Lux seconds), but this article is more concerned about relative measurements.
Generally, at this point, most people introduce the concept of the Exposure Triangle. I won’t*. It’s all much simpler than that … as long as you understand stops.
What is a “stop”?
Simply put: adding one stop doubles the exposure: subtracting one stop halves the exposure.
Move from ISO 100 to ISO 200 – double sensitivity – that’s adding one stop.
Move from ISO 400 to ISO 100 – quartering sensitivity – that’s removing two stops.
Change shutter speed from 1/100th to 1/200th of a second – half the duration of the exposure – that’s removing one stop.
Change shutter speed from 1/500th to 1/125th of s second – quadruple the duration of the exposure – that’s adding two stops.
Adjust your aperture from f/11 to f/16 – half the area of the aperture, so halves the exposure – that’s removing one stop.
Adjust your aperture from f/2 to f/1.4 – double the area of the aperture, , so doubles the exposure – that’s adding one stop.
Lists of aperture stops, shutter stops, and ISO stops are all over the internet.
Why are they called stops?
No idea. Finding out is on my to-do list – item #31,459,265.
Why are stops useful?
Say you have a reasonable exposure at f/8, ISO 400, and a shutter speed of 1/200th, but you wanted a shallower depth of field for artistic purposes – you decide on an aperture of f/2.8. You’ve added three stops (f/8 -> f/5.6 -> f/4 -> f/2.8) of light by opening the aperture. Now you need to get rid of three stops of light – how?
You could take three stops off the shutter. One stop would be 1/400th, two: 1/800th, three: 1/1600th of a second. Alternatively, you could remove three stops of ISO. One stop would be ISO 200, two: 100, three: ISO 50. The problem with this approach is that your camera may not go down that low (or you’re shooting film and can’t easily change). You could mix and match, ending up with f/2.8, 1/800th, ISO 200 and a well exposed photograph.
Essentially, it’s all about balance – if you’re at ‘well exposed’ already, but want to change shutter speed, ISO, or aperture: every stop you add means having to remove a stop elsewhere.
Enough background, where do I start?
The base of all of this is the Sunny-16 rule. Essentially, it says: on a bright sunny day, if your aperture is f/16, your shutter speed should be the reciprocal of your ISO. If my ISO was set to 200, my shutter speed would be 1/200th of a second. Don’t forget, this is a rule of thumb, a place to start; local conditions, or artistic desire may force you to play about. You may need to add or remove stops for best local exposure. You may not want the depth of field f/16 gives you – open the aperture, and balance that with a faster shutter.
So as far as nice days go – all sorted. But there are, as you may have noticed, many other conditions in which to take photographs. You could go away and memorise tables of data, or print those tables out and refer to them prior to shooting, or you can remember a Frame of Reference, like mine:
- 4 stops down: sunset
- 4 stops more: All the lights in my sitting room (yours may be different)
- 4 stops further: candle light
At this point I encourage you to experiment and find the reference points that work for you – if you shoot by artificial street light often, work that into your Frame of Reference. The key is to find a small number of reference points where you know the light level well. The above work generally work for me – four points, four stops apart.
Frame of Reference built. What next?
Use it. Practice until it’s intuitive. Shooting on an overcast day? It’s brighter than sunset, but duller than a sunny day – somewhere in the middle. Try two stops more light than Sunny-16. f/8, 1/200th, ISO 200. Storm moving in as you shoot? Get another stop of light to your sensor. Or maybe only 1/2 a stop for a better mood – it’s your call.
Even if you don’t get a chance for a second shot, with practice your first attempt at guessing the exposure from your (personal) Frame of Reference will be close enough to probably be recoverable in post (and if it isn’t, I’ll give you a full refund on this free advice). Assuming you’re using a digital camera – chimp your last shot. Too bright? Lose light – remove stops. Too dark? Open up – add stops. Adjust by fractions of a stop to really dial-in what you want, if necessary.
Is that it?
Basically, yes. Assuming you understand the meaning of a stop for ISO, shutter speed and aperture, that you can chimp your shots, and you remember your Frame of Reference, you’ll be able to get the right exposure dialled-in, with the desired artistic effect, very quickly. There’s a bit of a wrinkle when you add flash or neutral density filters.
My battery is running low. Can I do it in fewer shots?
Yes, there’s a shortcut. It’s also useful in hard-to-judge light, but relying on it all the time means you’re stuffed when it’s not available. Select ‘P’rogram mode, compose and shoot. Chimp. Think. Reason about what you want. Remember your camera assumes you’re taking pictures of 18-ish% grey card. Adjust for effect (more or less depth of field/increase or remove motion blur/add or decrease thermal noise). Shoot. Et voila, done in two shots – if you have a Program mode … and your camera didn’t meter off the streetlamp to the right of your shot.
So what do you do in the field?
Personally, I ignore my own advice when shooting digital! I’ll work in Program or Aperture Priority the vast majority of the time, dialling in exposure compensation based off chimps and the histogram. These days, with the recent low noise wars, I’ll even let the camera pick the ISO most of the time. I’ll only go Manual when I’m hitting the bounds of hand-holdability (and I’m happy to recover in post), or want to do something especially creative not involving resultant averages of 18% grey.
When shooting film on say a Paxette (which has no meter), “ISO” is effectively wedged. Here I will be using the Frame of Reference. I’ll also be taking notes, so that I can get enough feedback (once I dev) to keep myself calibrated. Generally, thinking of the finished picture, I’ll start with my preferred depth of field and work that back to aperture. Now I’ve got 2 of the 3 variables locked down. From here with a mk-1 eyeball and the Frame of Reference I can work out the shutter speed.
Of course the Paxette doesn’t have the range of shutter speeds of cameras made almost 60 years later, so I might have to set it to it’s upper or lower limit, and start sacrificing (or adding) depth of field to keep the exposure reasonable.
While the above is laborious to write (and probably to read), it’s only takes a matter of a second or two after a bit of practice. Now I shoot. If I’m feeling a bit iffy about the exposure I worked out, I’ll bracket the shot by about a stop either side. If I’m feeling confident about the exposure, I’ll also bracket. Pride cometh before a fall, and all that.
How long did it take you to get good at this?
Good? I’m still practicing!
Right most of the time? A few months of mostly daily practice.
You mentioned flash, and neutral density filters. What do you mean by wrinkle?
That’s a whole other article …
* I prefer the brickie’s hod model anyway.