Best qualify this with the words “Noob alert”. I come to drones as a photographer with an interest in flying things, rather than a flyer with an interest in pictures, and I’ve got all of two weeks Mavic mini ownership behind me. And what a marvel it is. A camera which could shoot 9MP photos and 2.7K video weighing under 250g and costing £400 would have been impressive not many years ago, and here is one which flies round around for 20 minutes, is steady enough to shoot long exposures from hundreds of feet up, and can find its own way home. But how good is it ? The first stills and video I got backed up the review verdicts of “pretty good” – ample resolution, low noise, reasonable ‘feel’. And very good if you allow for weight and cost. But I wanted to do a test...
The camera says it is equivalent to a 24mm lens on a 35mm film camera. This makes for some nice simple sums. 35mm is the distance from edge to edge the film, inside the sprocket holes there is 24mm of usable space and a 35mm film frame is typically 24mm tall x 36mm wide. We can draw a triangle with corners half-way along the top and bottom edges of the film frame and in the centre of the lens. The base of this triangle is 24mm and the height is the focal length of the lens – when a simple lens is focused on infinity. So, for the Mini the height and base are the same. If we extend the sides of the triangle through the lens and out to the subject, we’ll have two similar triangles. Meaning if the camera points straight down at the ground, the distance from top to bottom of what’s in the frame is the same as the distance of the lens above the ground (the height and base of the triangles are the same).
When shooting stills, the mini has 2250 pixels from top to bottom, so from 2.25 m above the ground it sees 2.25m of subject and each meter of subject is 1000 pixels tall. Flying at 22.5m it sees 22.5 m of subject and each subject-meter is 100 pixels tall and if you think in feet, flying at 22.5 feet gives 100 pixels for each of the 22.5 feet of subject that fits from top to bottom of the frame, At 45 feet/meters we have 50 pixels per subject foot/meter and at 90m we have 25pixels per subject-meter. For a nice round number in feet and inches at 187.5 feet, the mini has one pixel on the sensor for each inch on the ground. Kind of…
With video the numbers shrink. We get 1080 or 1530 pixels from top to bottom. In high res mode at 15.3m up, 1cm on the ground is 1 pixel on the sensor and if you approximate the 400-foot drone height limit in the UK as 122.4m 8cm on the ground is 1 pixel-height on the sensor. Or if you prefer at 382.5 feet (4590 inches) up, 3 inches on the ground is 1 pixel-height. Kind of .. because of the way the Bayer colour array works, and because the lens doesn’t have infinite resolving power, we won’t be able to resolve something as small as one pixel’s height. Cameras with small sensors can be noisy and may lose detail in their JPGs by doing noise reduction. Hence the test not the most rigorous test ever invented but one anyone can repeat, print a giant eye chart (I used 510, 255, 170, 128, 96, and 72 point) and some black and white stripes (mine were 72, 48, 36, 24, 18, 12 and 9mm wide.) Then photograph them from different heights. Stills going up, video coming down. I’ve tried stacking photos in the past, so I shot a set to stack as well. And here are the results.
The conclusion … if paparazzi shots from 100M away are in your plans, the mini won’t be. Stacking does find some more detail but not enough to let you do the full Blade Runner. At the sort of distances dictated by regulations and good sense it’s not a surveillance device. You can make a reasonable guess about make and model of car, but not read its number. You can tell there is a person with blonde or black hair, but can't say for sure who it is and so on. Personally I’m quite happy with that, the results are quite good enough for me, without accidentally spying on people.
Comments welcome
The camera says it is equivalent to a 24mm lens on a 35mm film camera. This makes for some nice simple sums. 35mm is the distance from edge to edge the film, inside the sprocket holes there is 24mm of usable space and a 35mm film frame is typically 24mm tall x 36mm wide. We can draw a triangle with corners half-way along the top and bottom edges of the film frame and in the centre of the lens. The base of this triangle is 24mm and the height is the focal length of the lens – when a simple lens is focused on infinity. So, for the Mini the height and base are the same. If we extend the sides of the triangle through the lens and out to the subject, we’ll have two similar triangles. Meaning if the camera points straight down at the ground, the distance from top to bottom of what’s in the frame is the same as the distance of the lens above the ground (the height and base of the triangles are the same).
When shooting stills, the mini has 2250 pixels from top to bottom, so from 2.25 m above the ground it sees 2.25m of subject and each meter of subject is 1000 pixels tall. Flying at 22.5m it sees 22.5 m of subject and each subject-meter is 100 pixels tall and if you think in feet, flying at 22.5 feet gives 100 pixels for each of the 22.5 feet of subject that fits from top to bottom of the frame, At 45 feet/meters we have 50 pixels per subject foot/meter and at 90m we have 25pixels per subject-meter. For a nice round number in feet and inches at 187.5 feet, the mini has one pixel on the sensor for each inch on the ground. Kind of…
With video the numbers shrink. We get 1080 or 1530 pixels from top to bottom. In high res mode at 15.3m up, 1cm on the ground is 1 pixel on the sensor and if you approximate the 400-foot drone height limit in the UK as 122.4m 8cm on the ground is 1 pixel-height on the sensor. Or if you prefer at 382.5 feet (4590 inches) up, 3 inches on the ground is 1 pixel-height. Kind of .. because of the way the Bayer colour array works, and because the lens doesn’t have infinite resolving power, we won’t be able to resolve something as small as one pixel’s height. Cameras with small sensors can be noisy and may lose detail in their JPGs by doing noise reduction. Hence the test not the most rigorous test ever invented but one anyone can repeat, print a giant eye chart (I used 510, 255, 170, 128, 96, and 72 point) and some black and white stripes (mine were 72, 48, 36, 24, 18, 12 and 9mm wide.) Then photograph them from different heights. Stills going up, video coming down. I’ve tried stacking photos in the past, so I shot a set to stack as well. And here are the results.
The conclusion … if paparazzi shots from 100M away are in your plans, the mini won’t be. Stacking does find some more detail but not enough to let you do the full Blade Runner. At the sort of distances dictated by regulations and good sense it’s not a surveillance device. You can make a reasonable guess about make and model of car, but not read its number. You can tell there is a person with blonde or black hair, but can't say for sure who it is and so on. Personally I’m quite happy with that, the results are quite good enough for me, without accidentally spying on people.
Comments welcome
