Where Your Focus Goes, So Does Your Exposure
The subject line up there is a bit of a generalization, yet true for most cameras, be they DSLR or point and shoot. And not a lot of people realize this fact. Many of my students just assume the camera makes one determination about the lighting in the scene and that is that. If only it were that simple (and it is when you use Manual mode and don’t let the camera decide, but that’s another topic).
The truth is, most of the time your camera will alter the selection it has made for shutter speed, aperture and ISO (depending on which variable you have allowed it to control based on mode settings) because you focused in one spot or another. This happens often in the matrix or evaluative metering modes, when the camera is allowed to use all of its metering point as it sees fit. It will not happen when using spot metering, but can, occasionally, when using center-weighted. Concentrating on the full metering mode then, be aware of where you are focusing as that may not be where you want to meter.
For instance, take this unedited scene from a coffee shop concert I shot this weekend for an ukulele band named The Castaways:
The exposure looks about right. The settings are Canon 7D, ISO 6400, 35mm, f/5.0, 1/60. For reference sake, I was using evaluative mode (using all the metering points) and my camera has three vertical areas to focus on in the focus mode I was using (the following effect is existent in most any of the focusing modes, be it single point or in a group). Those areas look like this:
In the shot up top, I am focused on the red location. Now, let’s move to the green location, right in the center.
The settings are Canon 7D, ISO 6400, 35mm, f/5.0, 1/160. That’s 1 and 1/3 stops faster. Can you guess why?
It’s because the area of focus, when metered, has more of the light banner (which is also being lit by direct outdoor light) to contend with. It sees more lightness, as compared to the first focus point, which had more black and dark areas, and compensates. What’s going to happen when I focus on the banner, or the blue location?
Well now, things are looking bleak. Canon 7D, ISO 6400, 35mm, f/5.0, 1/250. 2/3rds of a stop darker than before and two full stops darker than the image we started with, which was a decent exposure.
If you are not shooting on Manual, and there is no hard-fast rule that says you need to, then be aware that your camera may change its decision on metering when you change focus. Give it a try yourself to see if your camera changes its mind with your focus.
Set up a target with black, gray and white sections that will align with your focus points. Keep light constant. Move the focus points to the black, then gray, then white and see if your metering changes. Try this in Aperture Priority mode to easily see the shutter speed change. Make sure you tap the shutter half way to start meeting anew after moving the focus points. Don’t know how to move your focus points? It’s time to bust out your manual.
This effect is neither good nor bad. It’s simply something to be aware of.
16 Responses to “Where Your Focus Goes, So Does Your Exposure” - Add Yours
June 11th, 2012 at 4:42 pm
Its interesting because this effect can sometimes be a bit of a hassle. To get consistent exposures for me I usually spot meter off a key tone and then use “exposure lock” to make sure it doesnt change when i focus on different things. Works great pretty much everytime, unless youre pressed for time, then evaluative metering is better i think.
June 11th, 2012 at 7:23 pm
This is why cameras have exposure lock, to be able to lock exposure (biased) on something else than your focus point.. But it also depends on how you set up your camera, you can have exposure lock as you lock focus, or let the two be independent.
June 11th, 2012 at 10:00 pm
I was aware of it but to see it demonstrated so dramatically is quite astounding. Sad thing is you’d wouldn’t realise what you’d missed out on! I tend to have plenty of time and shoot with manual focus for landscapes but when you’re rushing to get candid portraits this will certainly jog my memory to pay attention!
June 11th, 2012 at 11:43 pm
Go with Partial Metering and meter by the focus area, Press AE lock (exposure lock) and re compose your frame, now take the shot.
Much easier
June 14th, 2012 at 7:32 am
Such insightful information. Thank you for the examples. I will be looking more carefully and trying harder in the future.
http://www.livingdisney.com/2012/06/photographs-of-week.html
June 14th, 2012 at 9:36 am
Ya, i experienced this during my first belly dance shot.
June 14th, 2012 at 2:17 pm
Hey, I’ve seen that band at C&P Coffee in West Seattle!
June 15th, 2012 at 1:51 am
It seems to me so easy to bracket your shots automatically (not that I’d like to do that with film), that I don’t understand what the fuss is about. Shoot in RAW and you get even more control.
When I started photographing with a 5X4 film camera I would take a Polaroid first but it’s so simple now.
June 15th, 2012 at 3:35 am
Isac nailed it. His suggestion of metering off a particular spot, then locking the exposure with the “exposure lock” function and recomposing/focusing is what I would have suggested doing. I’m new to dslr’s, but am guessing that most have a feature like this. Would be worth looking through the manual.
My particular Sony allows you to set the button to either toggle the exposure lock to on or off with one press, or keep the exposure locked by holding down the button (much harder to do while re-composing).
June 15th, 2012 at 4:19 am
Interesting article for those who never realised the phenomenon, but not for those who knew it happened but were unaware of the option to disconnect the two. As Erik already said: what your camera does in auto modes depends how you set the thing up. I use a Pentax K5 so I do not know about the Canon D7, but I assume that alike Pentax, at least Canon’s DSLR’s allow through menu set-up to let the metering (whatever metering-mode you’re in) occur without any connection to where you focus (whatever focus-mode you’re in).
It would have been useful to include this information as well in the article so people do not learn only half the “truth”. Whether you use one function or the other should depend on what and how you photograph or on your personal preferences. The way I understood the article however, is that the photographer better be aware of the “problem” so he can work his way around it.
June 16th, 2012 at 8:31 pm
This just illustrates why shooting in manual mode and having an incident meter is not only VERY useful, it’s essential if you want shots that are more than “pretty close guesses until you get into Lightroom”
It wouldn’t bother me if they stripped every DSLR of every auto mode and just left 3 dials, shutter speed, aperture, and ISO. People wouldn’t be so confused as to why their shots came out wrong and they’d quickly learn about exposure.
June 16th, 2012 at 11:48 pm
It is not possible to disconnect metering from the focus point in the Canon 7D, unless you switch to CWA or spot metering. A lot was made of the fact that exposure follows the focus point as a big plus feature when the 7D was released. Canon need to fix this in a firmware upgrade by providing a “disconnect” switch in the CF menu.
June 18th, 2012 at 11:40 pm
Wow, I can´t find the logic in having the metering dependent on where you focus when the metering mode considers the overall image, and being able to switch off the connection between metering and focus point ONLY when your metering mode is spot or center weight, as it seems to me that the only occasions you might want to have the metering dependent on the focus are exactly those where you meter in particular areas within the frame…
June 19th, 2012 at 12:25 am
The Panasonic Lumix G3 allows you to lock the exposure and then control the focus by simply touching the LCD panel in the place you want the focus. I normally bracket by +/- 2/3 of a stop and, shooting RAW, these can be blended in Photoshop so you get a printable photo. Remember that the range of contrast that your camera can capture is limited but is more than you can print.
June 19th, 2012 at 5:20 am
Ninety percent of what my Canon 7D can do, it will never do – but my Nikon F only did one thing, and did it well!
June 19th, 2012 at 4:39 pm
I know what you mean and I miss my OM2 but you do need digital as it is a vast improvement over film and scanned negatives are just HUGE files.
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