What's Going On?

RxForB3 · Mar 14, 2014

I have a Canon 6D, but I'd like people to chime in that have other cameras, as well. I asked this question long ago when I first got the camera, but I don't think many people had experience with it, so I'll ask again...

I do lots of night shots and set the exposure manually. However, I want to do timelapses of sunset to stars and such. For this, I would prefer for the camera to automatically correct the exposure. My problem is that the camera can't seem to auto-expose properly in dark settings. For instance, if I set the camera in Av mode, ISO at auto with a max of 3200. f/8, and leave the lens cap on, it thinks the appropriate exposure should be 2.5 seconds at ISO 3200. Why?? Even at night when there IS an appropriate amount of light to expose at 25 seconds, ISO 3200, f/2.8 or such, it chooses to way underexpose.

Any thoughts or suggestions?

RxForB3 · Mar 14, 2014

A couple of other things to note: I've tried this while covering the viewfinder and if I change the aperture to something absurd like f/22, it DOES increase the shutter speed to 20 seconds. So it seems to be an issue with the camera determining correct exposure rather than any menu setting...

Dao · Mar 14, 2014

Can you describe the scene and what meter mode you were using? Is your camera has problem during the day? Is your night photos overexposed or underexposed? Do you have a sample photo?

RxForB3 · Mar 14, 2014

Scene is anywhere particularly dark. For instance, a moonless night with lots of stars out (which could be properly exposed at f/2.8, ISO 3200, and a 25 second shutter). Or during the day with the lens cap on. Sooo...dark, but not necessarily so dark that a 25 second shutter wouldn't properly expose the scene.

Metering I've tried all modes and it seems to happen regardless of metering mode.

My camera doesn't seem to have any problems in well lit situations.

It is drastically underexposing (and I don't have exposure correction adjusted at all), but only when the light falls to a certain point. For instance, if I try to do a timelapse series on Av mode with either an auto ISO set with a max of 3200, or just set at 3200, the shutter speed will increase over time as it should until it reaches a certain shutter speed (doesn't always seem to be the same), and from then on it shoots at the exact same settings and the resulting images get more and more underexposed.

Dao, I don't see what camera you have, but if you get a chance, try setting it on Av mode at an ISO of 3200, f/8, with the lens cap on, and tell me what it tries to set the shutter speed at...

NedM · Mar 14, 2014

If setting the exposure mode to automatic gives you absurd settings maybe you should override the settings and switch it to manual mode. Sometimes the cameras metering system, especially in automatic mode, can give false readings. Especially at night!

RxForB3 · Mar 14, 2014

That's what I do when I just shoot single images, but the allure of the auto mode is when trying to do a "holy-grail" timelapse (which is one going from daytime to nighttime, or vice versa). Touching the camera is undesirable as you might bump it out of position. So if I could just "set it and forget it," it would make my life much easier.

For instance...just before sunset, let's say I set it on Av mode with an auto ISO with a max of 3200. I could start the intervalometer and leave it. I should think I could be confident that the camera would start at an appropriate ISO of 100 and some appropriately quick shutter, and then progress from that until the camera reaches ISO 3200 and a shutter of 25 seconds (which would be fine for star shots depending on the lens)...

RxForB3 · Mar 14, 2014

It's as though it's incapable of metering anything that requires more than an exposure of 2.5 seconds at ISO 3200, f/8. If I set it to manual with the lens cap on, according to the camera that is properly exposed. Same thing if I point it into a dimly lit corner with the lens cap off. Do all cameras have this sort of limitation?

Interesting, though, it is dependent on the lens. The first example is a Sigma 50-500. With the 100mm on, it does 4 seconds at ISO 3200, f/8. With my 24mm, it shows 15 seconds at ISO 3200, f/8 to be proper exposure. Mind you, these are with the lens cap on.

So what this means in practicality is that if I'm out taking a timelapse with my 24mm lens at f/2.8 (as I would like to), it will appropriately meter until the shutter speed needs to be over 1.6 seconds. Everything after that will become progressively underexposed. Is this normal?

KmH · Mar 14, 2014

Understanding Camera Metering and Exposure

Derrel · Mar 14, 2014

I think maybe you're running into the bottom end of the light meter's measuring range, which is from LV 1 to LV 20. Here's a thread about the same problem you are running into.

6D: Bad exposure measurement in low light: Canon EOS-1D / 5D / 6D Talk Forum: Digital Photography Review

This problem is NOT a "modern era problem"...f/8 is a SMALL f/stop...at the extreme low-light end of a light meter's capability it has long been the custom to measure the dim prevailing light with a lens set to a WIDE aperture, such as f/1.4 or f/1.8 or f/2, in order to get a shutter time that might be as long as 16 or 8 seconds or 4 seconds, and then to manually (either in your head, on your fingers, or on paper), compute the extended equivalent exposure for small apertures like f/8.

Putting a lens cap on is in no way a solution...a capped lens will give an exposure wayyyyyy below LV 1 (Light Value), so...that's just a red herring kind of issue.

In the distant past, outdoors at night under dim nighttime lighting (absent a lot of moonlight) it was not uncommon to use a reflected light meter to measure a WHITE piece of typing paper, and then to increase the exposure from 16 to 20 stops "more". This was because many reflected light meters are simply not sensitive to measure really low light levels off of dark subjects.

runnah · Mar 14, 2014

My camera works so much better with the lens cap off.

RxForB3 · Mar 15, 2014

The lens cap example was a means to an end...

In any case, thank you KmH for the article. I knew most of that already, but did learn a few things from it I didn't already know.

Derrell, I think you answered my question, but I'm a bit confused. I looked up metering range and found the definition of LV 0 (f/1 for 1 second). If the 6D is capable of LV 1 with a 50mm at ISO 100, it can only expose to 1 second at f/1.4, correct? Which would translate to, for instance, 2 seconds at f/2. If I repeat the scenario with the lens cap on a 50mm at f/2 (since I only have the 50mm f/1.8), it comes up with a shutter of 20 seconds. Is this, as you said, just a red herring, or am I misunderstanding something? Why 20?