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climbrocks
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Sep 4, 2016
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CodyToombs
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Sep 6, 2016
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climbrocksre: using video to achieve a viable flash meter.
This kinda gets into the complicated details of implementation, but it's basically not going to work very well for a couple of reasons. If all you care about is that it's not going to work, feel free to skip the rest of this, but I'll cover the reason why below. Note: I'm not an expert, but I've done some research on this in the past. I'm explaining to the best of my abilities while trying to keep this moderately short, so forgive me for skipping a lot of details.
So, as you already know, there are two modes for capturing anything from the camera sensor: still images and video. Since it's all electronic (even the "shutter" is electronic), the two are actually very similar. However, there are a couple of differences.
Still images are captured at relatively slow speeds while video captures images very quickly (currently topping out at 240fps on the iPhone 6S, Nexus 6P, and the last handful of Samsung flagships, and others). Still images are the best choice for measuring ambient light because it's possible on recent devices running both iOS and Android to get the raw data from the image sensor. If the app is pulling the jpeg image, it will have been heavily processed and basically unusable. (I'd almost bet money this is the reason some people have had wildly variable results with Luxi, their phones probably don't support RAW in the API.) However, even the fastest smartphone can only knock out maybe a dozen shots per second in burst mode, and timing it to a flash is nearly impossible, which is naturally the reason you would ask about video. Unfortunately, video (including that live view we use as a viewfinder) is processed and compressed well before it ever reaches any apps, making it effectively useless for accurate readings of ambient light.
The other problem with video is that there actually is a shutter speed, even if it is electronic. There's a span of time between each frame where light is ignored or split between frames, even for the cameras that can hit 240fps. I don't know quite enough about light meters to be sure, but I believe they take several thousand samples per second, which is easily accomplished when it's all done with basic electronics and there's no image to record or process. Video can't achieve those speeds, and no matter how fast or slow a frame is, variables like the flash duration and existing ambient light will throw off the results.
Again, I'm skipping a lot of little details and probably overlooking a few other relevant issues, but hopefully this gives some insight for why current smartphones aren't suitable for using something like Luxi to measure a flash.
So, as you already know, there are two modes for capturing anything from the camera sensor: still images and video. Since it's all electronic (even the "shutter" is electronic), the two are actually very similar. However, there are a couple of differences.
Still images are captured at relatively slow speeds while video captures images very quickly (currently topping out at 240fps on the iPhone 6S, Nexus 6P, and the last handful of Samsung flagships, and others). Still images are the best choice for measuring ambient light because it's possible on recent devices running both iOS and Android to get the raw data from the image sensor. If the app is pulling the jpeg image, it will have been heavily processed and basically unusable. (I'd almost bet money this is the reason some people have had wildly variable results with Luxi, their phones probably don't support RAW in the API.) However, even the fastest smartphone can only knock out maybe a dozen shots per second in burst mode, and timing it to a flash is nearly impossible, which is naturally the reason you would ask about video. Unfortunately, video (including that live view we use as a viewfinder) is processed and compressed well before it ever reaches any apps, making it effectively useless for accurate readings of ambient light.
The other problem with video is that there actually is a shutter speed, even if it is electronic. There's a span of time between each frame where light is ignored or split between frames, even for the cameras that can hit 240fps. I don't know quite enough about light meters to be sure, but I believe they take several thousand samples per second, which is easily accomplished when it's all done with basic electronics and there's no image to record or process. Video can't achieve those speeds, and no matter how fast or slow a frame is, variables like the flash duration and existing ambient light will throw off the results.
Again, I'm skipping a lot of little details and probably overlooking a few other relevant issues, but hopefully this gives some insight for why current smartphones aren't suitable for using something like Luxi to measure a flash.
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To me the true value here is combining the Luxi with the awesome Cine Meter II (iPhone) app. At $25, the app is easily the most expensive one I've purchased, but we're not talking Angry Birds here folks. Using the Luxi (or similar products or possibly even some color-neutral filter, tho I haven't tried those) and after going through a relatively complicated calibration, you can turn your iPhone into a quite accurate color meter. These will typically set you back at least $400. Great when shooting with multiple different light sources, strange lighting situations, video, etc. Sure, you can shoot in RAW and batch process in Lightroom from a gray card...or you can set the color temp in K at the time of shooting. I'm going to have my iPhone with me anyhow, why not really use it?
Of course, as others have pointed out, the Luxi/iPhone combo doesn't work as a flash meter, and if you're already shooting with, say, a 5D III, you've got a decent spot meter and just need to know how to translate stops into aperture/speed/ISO changes. (User tip: use a lanyard or some other cord to keep the Luxi attached to your bag/in the same spot--I'm not the most organized guy, change lenses and gear very quickly, and this sucker can be hard to locate when you need it most!)
If someone could figure out a way to use this as a flash meter (maybe using video?), I'd definitely drop another 25 bucks on that app!