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My Science Project!!!! (PLEASE help me!)
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Hey people! I am a student at Sebastian River Middle school looking for some help on my science project for this year!!
Okay here is the question, PLEASE help me!Does the angle of the flash affect the apperance of red eye?!
I know it does, but i need to know how to find the angle between the flash and the subject!!!
PLEASE post, anything is better than nothing~
No easy answer to this one, although basic physics comes in to play here. Red eye is caused by the flash bouncing of the retina, which is full of capillary vessels, hence the reason for the colour red. The basic physics part of it is that the angle of incidence is equal to the angle of reflectance (or the other way around if you prefer).
But of course, it’s not quite as easy as that because the retina is not a flat surface but concave in shape, although I guess for the purposes of most calculations the curvature of the retina may be ignored and it can be assumed to be a flat surface. The other factor that comes into play is the size of the pupil. If the pupil is wide open then of course the propensity to red eye is much higher than if it were narrow. This is the principle on which red eye reduction flash works. A pre-flash emitted before the actual exposure causes the pupil to get “close-down” thereby blocking the flash from entering the eye in the first place and blocking any reflection from coming back out in the second. So, lot’s of things to consider in your calculations.
Red eye usually happens when the flash is near the camera’s optical axis, and the person being photographed
is looking more or less at the camera. One way round this is to move the flash off the camera’s axis; another, as Thorsten says,
is to pre-flash, closing the iris down.You could try a set up with a camera mounted flash and the victim straight ahead, looking at a fixed point – perhaps the camera’s
name plate; then gradually move the camera off axis until you don’t get red eye. Keep the flash parallel to the optical axis.Then repeat, but tilt the flash towards the iris, gradually moving away from the camera.
Measuring the angles ought to be simple enough, a protractor, and knowing the distances – sin, tan etc
This is for one fixed camera-subject distance; and this might need to be repeated for differing distances.
But your victim isn’t going to enjoy this much, and you will have to allow enough time for the iris to dilate.
Alternatively, get a bull’s eye from the local abbatoir and try using it.
Confused?
You will be…Middle school Thorsten, not medical school-
There probably is an easy answer to this one,
and like all good science projects, you can find it by experiment-You’ll need a torch, and an assistant-
Find a darkened room, and attach the torch to the side of your head,
as close to your eye as possible, and shine it at your assistant.You should see their eyes glow brightly,
exactly as Thorsten said they would-As you move the torch away from your head the effect should abruptly disappear.
The angle you’re looking for is the angle between the light and your lens-
when that angle is very small, almost coincidental,
then that’s when the incidence and reflectance conspire to make the eyes act like mirrors.Small cameras need to use pre-flashes, as Thorsten mentioned,
because the physical distance between flash and lens is very small-Larger cameras, particularly those with external flashes,
don’t need pre-flashes at all-
at normal distances-Although human eyes reflect red,
other animals reflect different colours-
you never know, maybe there’s a few extra marks if you mention that-Welcome to the site-
j
ah, edit-
like what rc said-One of these http://www.discoverthis.com/eyeball-model.html might be helpful with some of the preliminary work. I’m thinking of your victim here!
Confused?
Another idea; beg or borrow an opthalmoscope. This is used for looking at the retina, with
the observer up close to the subject. But it can also be used to look for the ‘red reflex’ which is what
happens with flash – you simply stand back a bit, and point it at the eye.The light from an ophthalmoscope is exactly in line with the viewing axis. Move off axis,
see when the red reflex disappears, measure the angle.OR; get one of the head mirrors that ‘real’ doctors use; this has a circular mirror with a hole
in it through which you look. The light source is beside the subject, pointing to the viewer.
Get the red reflex, move off axis, measure.Good luck!
Slightly OT, but you might be interested:
http://www.newscientist.com/article/mg19726382.600-eye-opener.html
Thanks to you all who answered my last post on my science project and the camera what evers.
But I have to let you know I am in MIDDLE school, and can’t get a grip on the exact meaning of what
you said. SO… this is what I ask you,How do you find the angle between the subjects PUPIL and the FLASH??
But please put it into the simple-est words you can, unless my brain gets wacked!!!
Thanks!
So in other words, how do find the angle of the reflection off the person’s pupil….
RIGHT??
You should probably know this from trigonometry by now (I guess)
Anyhow, visit this site for the formula:
http://www.regentsprep.org/Regents/Math/rtritrig/LtrigA.htmOf course before applying the formula, measure the distance from the camera to the subjects eye.
Then the height of the centre of the flash from the centre of the lens. Then simply apply the formula
in the link enough. It’ll probably be accurate enough for the purpose of your project.1. get your victim/subject to sit/kneel at a table or bench which probably needs to be 1.5 – 2 meters wide
2. get them to look at something directly opposite with one eye
3. shine your torch or ophthalmoscope across table/bench, see red reflex.
4. while victim still looks across bench/table, move light/ophthalmoscope to one side [preferably their ear side] You might need to angle the light a bit
5. make sure that your light moves at right angles to their line of vision.
6. use trig as per Mark ^^ – that is, measure distance across bench [=adjacent], distance along bench on your side until red reflex disappears [=opposite]
7. tangent angle of reflection = opposite/adjacent
8. QEF
One more Question!
Say if I had the camera pointing exactly parallel to the ‘victums’ eye and I moved it left and right, what would be the angle. So if I moved to the left the angle would decrease, and if I had moved to the left the angle would increase, right?? BUT… how do I find my starting angle. Would it be 90 degrees for a half circle or 180 for a full circle angle??
I AM SO CONFUSED!!!!!
When you start, you and your camera/torch or whatever you are using, are looking directly into the victim’s pupil:
they should be looking directly in front of themselves;
the angle of reflection will be straight back at you, and will be the same as the angle of incidence – the angle at which
the light from the torch hits the pupil of the eye. At the start, you call this 0 degrees.When you then test, you move the light or whatever to the right or left, measure the distances.
Using the tangent function, as above, you then get an angle; this will be the angle of reflection.
You start at zero degrees, and as you move to the right or left, so you get, say 30 degrees — that is
30 degrees away from the straightahead position. It doesn’t matter whether you move to the right
or left, you should get the same answer more or less in degrees [ but you might find that the victim’s nose
blocks your torch before the red reflex disappears — since I haven’t done this, I don’t know]Either way, to right or left, the angle will be [say] 30 degrees. You don’t have to call one +ve and the
other -ve.Imagine a line sticking out from the subject’s eye; straightahead, call this zero degrees. Imagine a line
from the subject’s eye – which must stay still – to the torch when you can’t see the red reflex; the angle
between them is what you are looking for, but you measure it from where you are.It should be easy enough to go vertically upwards from the starting position, and you should get the same
answer; going downwards might be a bit trickier to set up the equipment.BTW, make sure your victims aren’t wearing contacts or glasses – you might get different answers. And
make sure that the pupil of the eye stays a constant size – give the victims time to relax – because smaller
pupils are less likely to show red reflex – as in ‘pre-flashing’ ^^If you want a gold star, try seeing if there is a difference between blue and brown eyes; as I remember
brown eyes usually have bigger pupils, so they ought to show red reflex over a greater distance.Still confused?
jb7 wrote:
Although human eyes reflect red,
other animals reflect different colours-
you never know, maybe there’s a few extra marks if you mention that-Australian Saltwater Crocodiles reflect green. Dogs orange, cats yellow.
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