Stop action of a falling rain drop

It has been raining in Toledo and that has reminded me of an ongoing quest of

mine. I was told by two different engineers that they saw a picture of a

raindrop at terminal velocity and it is in the shape of a donut. I had the idea

to focus at 10-15 feet and use the flash to stop the action of a falling

raindrop. I guess the flash duration is about 1/1000. Maybe shorter. I got

drops in focus but they were blurred. A streak of about a relative 1/4 inch,

not stopped.

 

Has anyone stopped a falling raindrop? Does anyone have any ideas? Does anyone

know the terminal velocity of a falling raindrop?

 

Cordially,

Frank

That sounds a bit too far to resolve the shape of a raindrop. I'd use a fast lens, wide open and focused close. This will allow you to blur the background and you might be able to resolve them. The flash is a good idea. Perhaps you could try some shots during the day or night time hours to see if you get some differences. Good luck.

I'd focus a lot closer you'll probably need at least 1:1 if not greater greater macro capability so use a macro lens and Nikon's R1C1 set up where the flash duration is a lot shorter. I'd also set up a black background (yes outside in the rain) to isolate the rain from the background.

 

I also suspect that wind conditions have a lot to do with the shape of the raindrop.

You need to use flash, I think wide open to get speed is a bit too narrow to focus on rain drops.

The formula (simplied) for the velocity in feet/second is v = gt where g is the acceleration of gravity (32 ft/sec2) and t is the time in seconds since the drop formed and began to fall. There would be an effect from air resistance but I think you can ignore it for your purposes. The problem as you can see is determining how long the drop has been falling. You could start by trying to find out the altitude of the clouds which are producing the rain. The time in seconds will be equal to the square root of -2y/g where y equals the altitude in feet and g as above.

 

Have fun.

When electronic flashes first came out they were promoted by showing how they could freeze a bullet as it left the muzzle of a gun and with pictures of the "crown" type ring that formed in a liquid splash.

 

Those experimenters knew exactly how far away the bullet or splash would be from the lens because they were controlled experiments. A little tougher to freeze a "natural" falling raindrop because the dept of field would be really shallow and you never know exactly where the raindrop will fall.

You can get a good estimate of the terminal velocity by using stokes law .. if you guess

what the typical size of the drop is. I don't think it should be a donut .. if you get a

donut .. post here or email me as i'd love to see this.

 

(4/3) pi r^3 * (1000) * 9.8 = 6 * pi * r 1.73 * 10^-5 * v

 

err let me simplify this a bit for you (sorry i don't have a calculator on me)

 

velocity = (2/9) r^2 / (98000 * 1.73 * 10^-5)

 

where r is what you think the radius of a drop is in meters

 

the answer you will get is in meters/second

 

if this sounds like gibberish give me the size of a droplet (or a range of sizes) and i'll give

you a velocity :)

Sorry, Richard, but you cannot ignore the effect of air resistance to a falling body. There is a point, called the "terminal velocity" at which the force of air resistance balances the pull of gravity. This varies with the shape, size and density of the falling object. If I were to guess, a raindrop tops out at about 15-30 mph. Cats and dogs, literally, probably fall at 50-80 mph, a human at 80-125 mph.

 

At minimum power, an SB-800 has a flash duration of 1/80,000 second, which should be sufficient to get a sharp image of a spashing raindrop. You just have to get close enough to the subject. You may have to "kiss a lot of frogs" to get a "prince" of a picture.

 

In order to get a lot of power into a very short (~1/1,000,000) flash duration, you need special equipment. This includes high-voltage operation (~15KV), low-impedance components (e.g., oil-filled capacitors), and an up-to-date will.

oops too many zeros:

 

velocity = (2/9) r^2 / (9800 * 1.73 * 10^-5)

oh hell i may as well do this properly, there is a bouyancy factor as well that I was ignoring..

thinking about this you get

 

velocity = (2/9) r^2 (density_water - density_air) / viscosity_air

 

as a pretty good estimate .. if you give me a range in sizes i'll give you the range in velocity...

there are still a slew of approximations here but it will okay.. as you are using this as an

estimate..

You will need to backlight your rain to make it stand out from the background for the easy way to do it. Perhaps grid/snoot your strobe so it comes across a predetermined position of focus. To get any resolution on a rain drop, you'll need a macro lens, too.

 

Attached is a diagram of how I might shoot it.<div></div>

Hmm...here's a link allowing you to graphically figure out the rain speed:

 

http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=146

 

Large raindrops are 20mph, small are 9mph. Mix that in with the extremely small area of interest and a flash strobe at ~1/4 to 1/16 power will work nicely. Don't bother trying to capture it with your shutter. Use the Edgerton technique.

I agree about the use of a flash and micro lens... ohh, and as Aaron say "Nikon of course!".. ;)

 

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D70/55mm 2.8 Micro AIS and one SB-600

with all my respect... for your efforts... to all of you...

but I think you are trying to invent the invented thing...

the shape of the liquid - when falling - is quite well known

 

when the liquid is falling there is no gravity - like in space, and as we can all feel when we are in very fast moving elevator

all liguids, when put in no gravity situation have the form of the perfect sphere

but... they are falling, and they meet in their way the air - and the air shapes them - the form is similiar to sphere, but a little longer at the end - I don't know tha esact word in english - in my native language the word i 'droplike' :)

Iossif,

 

Rain drops are not sperical, nor tear-drop shaped. They tend to be flattened discs, thinner in the center, twisting around as they fall. Tune in on some of Edgerton's archives - things are not always as they seem.

You guys are amazing. Thank's for participating so avidly! It's delightful. I need time to digest everything and check out the links to other references.

 

I'm thinking, was thinking, that a drop from a low cloud would not reach terminal velocity and may be somewhat teardrop shaped. but if the speed is between 9 and 20 MPH then they must reach terminal velocity quickly. The reference to a disk shape somewhat thinner in the middle is about like a dognut. That's what I was told they look like.

 

The picture of the water drop is excellent! I'm guessing it was taken in a room. Not a high velocity. It's round. HIgher speed, teardrop like. Terminal, disk shaped. Do you agree?

 

I may not have the equipment to implement the suggestions with the most probability of success. Anyone with the right equipment with the interest and opportunity, please take the shot and show us all.

 

Thanks

Frank

 

PS I will keep trying.

no to desapoint you but I took the picture in my kitchen sink, the speed was 1 second and the aperture 2.8, and I shot the SB-600 manually when the water drop was coming down from the faucet. It took quiet few tries...;)

Here is another one taken the same day/same kitchen sink/same method, the strips are from a plate.

 

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