Here are some useful facts and figures that may shed new light on your understanding of illumination. Light is one of those mysterious things we all know, but rarely stop to think about in much detail. More than ever we’re concerned with low light performance, and creative cinematographers are becoming more flexible than ever when it comes to how they use light. Emmanuel “Chivo” Lubezki recently did the seemingly unthinkable for a Hollywood feature, shooting The Revenant using only available light. It wasn’t his first time either, and with continual improvements in sensitivity, we’ll see an increase in the number of high-profile films executed by smaller crews shooting only in the natural light that is available to them. Of course for documentary and news, as well as most of us without Hollywood-sized crews and budgets, we’ve been shooting in available light for a long time, and will continue to do so. With that said, here are a few basic concepts that should be at the forefront of what we do. Understanding Light: Lumens and Lux The Lumen is a measure of the total quantity of visible light emitted by a source (luminous flux). Lux is a measure of illuminated area. The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. 1 Lux = 1 Lumen per Square Meter. Remember also that light intensity falls off inversely to the square root of the distance from the source. So, just how bright is 1 lux? Understanding Light: Illuminance Typical lux values of surfaces illuminated by: 0.0001 lux – Moonless, overcast night sky. 0.002 lux – Moonless clear night sky with airglow. 0.27–1.0 lux – Full moon on a clear night. 3.4 lux – Dark limit of civil twilight under a clear sky. 50 lux – Family living room lights. 80 lux – Office building hallway/toilet lighting. 100 lux – Very dark overcast day. 320–500 lux – Office lighting. 400 lux – Sunrise or sunset on a clear day. 1000 lux – Overcast day; typical TV studio lighting. 10000–25000 lux – Full daylight (not direct sun). 32000–100000 lux – Direct sunlight. What does this mean in terms of exposure? Of course, it depends on frame rate, shutter speed and ISO (or EI). Understanding Light: Exposure Value Exposure Value (EV) is a scale representing the combination of aperture f/stop and shutter speed. Any combination of aperture and shutter speed that results in the same EV value will result in the same exposure. EV = log2(N²/t) Where: N is f/stop number t is shutter speed Here’s an example: Let’s say we know we are planning the shoot to be interior nighttime. It’s a living room, and we will be shooting at 24fps with a standard 180-degree shutter (or 1/48th sec) and we want to maintain a nice, shallow DoF with an f/stop of f/2.8. So we know our general illuminance level is likely to be somewhere around 50 lux, that’s how much light is likely to be falling on our subject. So lets plug that into the equation to arrive at an EV value. N is 2.8 t is 1/48 or 0.0208 EV = log2(2.8²/0.0208) EV = log2(7.84/0.0208) EV = log2(376.92) EV = 8.558 So we have an EV value of 8.5 Now we can look this up on a EV/Lux conversion chart such as this one by Sekonic. The chart gives us corresponding illuminance values (lux) for EV values at 100ISO. The chart gives us an illuminance value of 900 lux at 100 ISO for a correct exposure at f/2.8 with 1/48th sec shutter speed. From this point it’s easy, we won’t have 900 lux, we’ll only have 50 lux but we won’t be exposing at 100 ISO either. So let’s adjust this figure for ISO. Every time we double the ISO, we double sensitivity, and so our illuminance requirement is halved. ISO 100 @ EV 8.5 = 900 lux ISO 200 @ EV 8.5 = 450 lux ISO 400 @ EV 8.5 = 225 lux ISO 800 @ EV 8.5 = 112.5 lux ISO 1600 @ EV 8.5 = 56.25 lux So, if we want to maintain f/2.8 aperture, we need to expose for EI 1600. If we know we don’t want to exceed 800, we can open our aperture up to f/2.0 (one stop) and we’ll be properly exposed for a typical nighttime living room interior—using only the available natural light. Understanding Light: Latitude Dynamic range (latitude) is all about contrast ratios. We’re so used to quoting how many stops of dynamic range our cameras have, but rarely is that put into some sort of real world context where it takes on some relatable meaning. Dynamic range, or latitude, is measured in stops. Every stop is a doubling of light intensity. 0 Stop 1:1 1 Stop 1:2 2 Stop 1:4 3 Stop 1:8 4 Stop 1:16 5 Stop 1:32 6 Stop 1:64 7 Stop 1:128 8 Stop 1:256 9 Stop 1:512 10 Stop 1:1024 11 Stop 1:2048 12 Stop 1:4096 13 Stop 1:8192 14 Stop 1:16,384 15 Stop 1:32,768 16 Stop 1:65,536 17 Stop 1:131,072 18 Stop 1:262,144 19 Stop 1:524,288 20 Stop 1:1,048,576 21 Stop 1:2,097,152 22 Stop 1:4,194,304 23 Stop 1:8,388,608 24 Stop 1:16,777,216 25 Stop 1:33,554,432 26 Stop 1:67,108,864 27 Stop 1:134,217,728 28 Stop 1:268,435,456 29 Stop 1:536,870,912 30 Stop 1:1,073,741,824 In theory, the human eye can adjust for vision in illumination levels of between 0.0000001 lux (10^-6) and 1,000,000 lux (10^6) but the useable range within that is much less. Practically, we can consider a maximum dynamic contrast ratio of 50 million to 1 (30 stops), from moonless overcast midnight black to the brightest sunlit white, but that contrast never occurs in nature—and we certainly aren’t able to perceive that entire range at once. The maximum ratio in a naturally occurring scene is closer to 1 million to 1 (around 20 stops), but even this is extreme and rarely encountered. Our vision adjusts to light levels constantly and our static contrast ratio, which is the range we can see at once within a static scene, is closer to 1,000 to 1 (10 stops). Real World Contrast Ratios You can calculate some likely real-world contrast ratios by finding the difference in illuminance levels from the first list and matching that up with the contrast ratios in stops. For example, our night time living room illuminance of 50 lux will be our general maximum level in that scene, the difference between this and the darkest shadow (under a couch for instance) can be no more than 50 lux—though it’s likely to be a fair bit less due to ambient light bouncing around—so our total range in that situation may be around 5 stops maximum. If we are outside, under sunlight, the range could be as high as 15-16 stops between the brightest ping of specular highlight from the sun and true black. If there is shadow detail that is dark although again, due to ambient light levels, that are well above true black in daylight, then the actual range in the scene may be less. Hopefully, these numbers have given you a bit of a reference to judge real world light levels and contrast ratios and some tools to even run some calculations yourself before you ever set up the camera—even without a light meter!Read more
Since their hugely successful Kickstarter campaign last year Lumu has been offering the smallest accurate incident lightmeter for stills photography. This week Lumu launched a new app that is targeted at videographers. The small, lightweight sensor goes in your pocket or can be carried with the practical necklace adapter. We looked at the new app with the Lumu device. The Lumu comes in a small metal box with minimalist design in line with other Apple stuff. The App’s design itself is similarly attractive looking a lot like the latest Apple iOS. Inside the box is a small leaflet, the Lumu and the necklace adapter that can be carried around the neck and hold the Lumu on demand. There’s also a small leather case where the Lumu can be stored and protected if desired. The necklace actually looks cool and only a little nerdy. The Lumu is held firmly though if accidentally pulled out on a shoot there’s a chance it can get lost. When the app is started it asks for the Lumu. All you do is plug it in and the app starts. If it is pulled out while using the app the Lumu symbol appears again. Everything is rather straight forward. The app is simple to use, probably simpler than an actual light meter, like my Kenko I’ve consulted for comparison. When compared to the readings on my Kenko light meter the Lumu iPhone app shows a value about half a stop lower than the light meter. [UPDATED:] I have re-calibrated the Kenko light meter, and I can confirm that the Lumu meter is very accurate. In any case the Lumu light meter app even has a setting to calibrate the meter in case it is off. Unfortunately when I tested the app the calibration process only worked for the lux meter (which is also a feature), but would reset the values on the video meter screen, so I could not calibrate it. [Update: The Lumu developers told me the bug is fixed and will be resolved in version 1.1 of the software which has already been pushed to the app store] The Lumu seems very responsive with no delay whatsoever, making it very easy to measure light and the experience is much like with a common light metering device. It is very easy to change framerates, shutter speed or ISO values and you can dial in ND’s quickly too. Very useful is the double tap option to allow for continuous measurement when you don’t want to hold the “measure” button all the time. Additionally all values including exposure time display, ISO step sizes, aperture step sizes, and ND filter notation can be adjusted in the settings menu. The incident meter uses a silicon photo diode. It can be used facing the front or back of your phone and the phone can be used facing up or down. There’s actually not much more to it. Just pop the Lumu on your smartphone and you have a fully functional lightmeter. In conclusion the Lumu seems like a very cool idea for those who want to measure light accurately. Two thumbs up for the Lumu. The Lumu costs $149 and is available in black or silver. www.lu.muRead more
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