Using An Integrating Sphere To Test Camera Metering In Low Light – Project Overview

(This article is a supporting part of our ongoing testing of low-light camera metering reliability)


What is the dimmest light level that a given camera can meter? For a nightscape photographer, and particularly for someone who films timelapse video with day/night transitions, this question is critical. Normally when I film a timelapse video that starts during the day and ends at night, I start with the camera in aperture priority mode, and the camera compensates as the sun sets.

However, as twilight fades into night, at some point the camera’s metering fails, and I have to switch the camera to manual mode and adjust the settings until night has fully fallen. Innumerable exposure errors have occurred because I didn’t switch to manual at the right moment, or irregularly changed settings in manual mode.

In recent years, some cameras have become able to meter at night, so switching to manual mode is unnecessary. Manufacturer metering specifications are completely wrong and useless. Therefore, while stuck in quarantine in 2020, Sean Goebel and I set out to build a standardized comparison of the low light metering abilities of different cameras.

To answer this question, we built an integrating sphere. An integrating sphere is a sphere that is painted white on the inside and has LEDs for internal illumination. The camera lens pokes into the sphere. Through the use of baffles, light from the LEDs has to bounce at minimum twice before entering the camera lens, and this causes the inside of the sphere to have almost perfectly even illumination.

In other words, it doesn’t matter where the camera is pointed inside the sphere–it’s all the same brightness. The sphere is carefully sealed so that light cannot leak inside from the outside.

 

 

Gray image showing what the camera sees

 

Here are the basics of the metering test: we use an Arduino (an open-source microcontroller) to control the LEDs inside the sphere and trigger the camera being tested. We calibrated the maximum light level of the sphere to be exactly 0 Exposure Value (EV0).

For reference, EV0 is about the light level of a very dim restaurant or a landscape 20 minutes after sunset, and “correct” exposure settings for this are 1/8 s, ISO 6400, f2.8. The camera is set to aperture priority mode with an f/2.8 lens and takes a photo of the inside of the sphere.

The light level is dropped by half (to EV-1), and another photo is taken. Ideally, the camera detects the dimming of the light and compensates with a longer exposure time and/or higher ISO. The light level is dropped by one stop again (to EV-2), and this repeats until the ambient light level reaches EV-10. The correct settings for EV-10 are 30 sec, ISO 25,600, f/2.8, which are appropriate for a light level achieved under a cloudy moonless night without light pollution. Most manufacturers report that their bodies can meter to light levels of EV0 or EV1, but as our tests show, most bodies can meter well darker than this.

After collecting the 11 photos in Aperture Priority mode, each with half the light level of the previous photo, we use Python’s RawPy software library to calculate the average brightness of each raw file. We plot this against the EV that the photo was taken at. If a camera was perfect, the average brightness of the raw file would be constant, since the camera would double its exposure time or ISO with each new photo to compensate for the decrease in light. In practice, plots look like this:

For the first few dimmings, the camera effectively compensates, and the average brightness of the resulting image is constant. However, beyond some point (about EV-6 here), the camera only partially compensates for the decrease in light level–when the light drops by 1 stop, the camera only adjusts by 2/3 stop. Further along, the camera can no longer detect any change in brightness at all, and it does not adjust its settings at all.

Because cameras don’t abruptly change from perfect metering to completely nonfunctional metering, we define two criteria for metering failure.

First, we report at what light level the camera adjusts its settings only 2/3 of a stop for every 1 stop decrease in light. In other words, the first time it fails to compensate by at least 1/3 of a stop.

Second, we report at what light level the camera produces images that are half the brightness of (one whole stop dimmer than) its EV0 image.

Once you know this information about your specific camera, you can translate it to a specific ambient lighting condition (using this article here) that you can safely capture in Aperture Priority with your camera.

Say, for example, if your camera can meter correctly at EV-5, but fails at EV-6, that means your camera (with an f/2.8 lens) can correctly meter and expose a moonlit scene of ~50% illumination, but at ~25% illumination your camera will fail to meter correctly, and you should probably switch to manual exposure on such dark nights.


Integrating Sphere & Camera Metering Test Project

Main Project Page – Test Results

Project Overview – What Is An Integrating Sphere, and How We Used One to Measure Cameras’ Low-Light Metering Capability
(YOU ARE HERE)

Frequently Asked Questions / FAQ

What are EVs, and What do They Mean for Different Cameras? (Non-Technical Explanation)

The Technical Explanation of EVs, and Calibration of the Integrating Sphere

So, How Did You Build an Integrating Sphere, Anyway?

Timelapse Methods Compared: Aperture Priority VS Holy Grail Method


 

Happy 7th birthday, Canon 6D. You’re still one of the best values in astrophotography!

Today, in 2012, the Canon 6D was announced. It only had a single SD card slot and Canon Rebel-style focus point layout, (so I didn’t count it as a top choice for my day job as a wedding photographer) …but its sensor was, and still is, a huge milestone in high ISO image quality. So, happy birthday, Canon 6D! (Also known as the 6D mk1 or 6D classic, now)

Canon 6D – Astrophotography Legend

The 6D sensor was shockingly good in its day. It forfeited just two megapixels compared to its bigger brother, the Canon 5D mk3, but it was actually significantly better than its predecessor at high ISO image quality, particularly 3200-6400 where many nightscape photographers will likely spend a lot of time.

Smart photographers who needed incredible image quality more than they needed the flagship AF and dual card slots that the 5D3 offered, opted for the 6D as soon as its image quality was extensively tested and nightscape, landscape, and adventure photographers, in general, realized that not only did it have great image quality at high ISOs, it had better dynamic range at its base ISO than all previous Canons ever, including all flagships.

Though, admittedly, that base ISO dynamic range was still 2-3 stops behind Nikon and Sony, so if you also do a ton of shooting at ISO 100, then I must stop praising the 6D for a second and suggest that you consider the similarly priced (used) Nikon D750, which recently had its 5th birthday, I  might add. The D750’s high ISO image quality is not as good as the 6D’s, (though it’s close!) but its dynamic range at ISO 100 is still considered “insane” *1 by today’s standards. (Just like the D600 and D610, BTW.)

*1 “insane” is a scientific measurement that means “way better than most photographers will ever need. In fact, you’re more likely to see a bigger difference in image quality by just making sure you use perfect technique, than switching from this camera to anything better.”

Although the Canon 6D lacks a lot of pro features, it wins big in one way- that “magnify” button can be programmed to offer 1-click 100% zooming, unlike the Nikon D600 and D610. It even plays back the zoomed-in image if the LCD is off!

Indeed, when shopping used, you can easily find a good condition 6D for $700-800, making it one of the best values on the market today for anyone who needs a hard-working full-frame sensor in a very affordable package.

Why buy a Canon 6D instead of a newer camera?

By the way, if you’re curious: why wouldn’t you buy a newer camera instead, let alone a camera for a newer, more future-proof mount? There’s the 6D mk2 and the 5D mk4 for Canon’s EF DSLR mount, both which are old enough to be found for decently good deals on the used market. Plus, there’s the Canon EOS RP which is the newest mirrorless camera body in their RF lineup, yet it debuted at a mere $1300 and can be found for under $1000 used, if you’re patient…

Glen Canyon, Utah | Canon EOS RP, Irix 15mm f/2.4

The answer is, yes, all these newer cameras are good, great even. BUT, they’re all not as “clean” at ISO 3200+ as the 6D sensor, as per photonstophotos.net. Shocking, but true.

Oh, and what about the Sony A7-series cameras that are also starting to get old, the 1st-gen and 2nd-gen A7, A7S, and A7R series cameras? You can definitely find them for under $1K, that’s for sure! But, this is because as underwhelming as the 6D’s other specs are, (autofocus, card slots, etc.) …the early Sonys are worse. Also, most of their oldest sensors are far worse at high ISO image quality.

The only old Sony A7-series cameras that have equal or better high ISO performance versus the Canon 6D are the A7R2, A7S, and A7S2. (As well as the A7R3, if you count it among the now-replaced cameras since the mk4 is here, but the R3 is still a $2500 camera, and remember, we’re shopping for a ~$700 full-frame body.)

So, if you’re just breaking into astrophotography now, if you’re on an extreme budget, and especially if you’re at all familiar with Canon cameras already, then the 6D is still your best value, despite being 7 years old. Whether or not it’s actually the right choice for you depends on your total budget for both lenses and bodies, and of course the other features you are likely looking for beyond image quality. Last, but the polar opposite of least, remember: it’s not about the gear, it’s about getting out there and shooting.

Search for a used Canon 6D on B&H (Latest price check: $689.95-$879.95, depending on the condition)

The Nikon 20mm f/1.8 G is shaping up to be the best astro-landscape lens ever!

I’ve been watching as the reviews start to come in for this exciting new lens from Nikon.  So far, the results are extremely promising!  Consensus being: this lens is highly optimized for astro-landscape work, as it ought to be, with very low vignetting and almost zero coma, the two things were almost every other lens falls flat on its face.  The one thing remaining for me to determine is, how this lens handles field curvature at infinity (star) focus.  Pretty much every lens ever made wider than 50mm has at least some issues with field curvature, either right out of the box or at least after a year or two of heavy use. Continue reading