The Naked Scientists

The Naked Scientists Forum

Author Topic: What causes 'red eye' in photographs?  (Read 11147 times)

paul.fr

  • Guest
What causes 'red eye' in photographs?
« on: 29/04/2008 16:41:23 »
And how does software remove the effect?


 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #1 on: 29/04/2008 17:07:48 »
The 'red eye' itself is caused by light bouncing off the blood vessels at the back of the eye.

The problem happens when the light source is too close to the line of view of the lens.  No matter where the light is, a strong light will cause a reflection red off the back of the eyes, but if the light is sufficiently separated from the lens, then the direction of the reflection (which will be back towards the light source) will not impinge on the image seen by the lens.  So, big cameras, with the flash well away from the lens; or when you use an off-camera flash; or when you bounce flash off a wall or ceiling, will avoid red-eye.  The problem is with many modern cameras, that are small, and with the flash unit integrated into the body of the camera, there is not a lot of room to move the flash unit away from the lens, so there is a strong propensity for the lens to pick up that reflection from the flash.

The first line of defence (if you cannot move the flash away from the lens) is to use a short preflash before taking the photograph.  The preflash is supposed to cause the subjects pupils to close up, so that less of the light from the main flash will enter the pupils, and soi there will be less light reflected from the back of the eye.  The problem with this is, other than it only has a limited effect, a subject with their pupils closed down tends to be less attractive than a subject with dilated pupils.

The software solution is the next answer.  This merely looks for an area of red in the photograph that looks like 'red-eye', and assumes that this should be the pupil of the eye, and so that area is merely painted black.
 

Offline JimBob

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 6564
  • Thanked: 7 times
  • Moderator
    • View Profile
What causes 'red eye' in photographs?
« Reply #2 on: 29/04/2008 17:41:45 »
Gee, and I thought it was the whiskey. 
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #3 on: 29/04/2008 21:43:47 »
You have to either download or purchase the correct photographic software to help you remove redeye, crop or resize your photos
and many other things that I've yet to learn about to do with this.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
What causes 'red eye' in photographs?
« Reply #4 on: 29/04/2008 22:28:39 »
The software analyses the colour of the pixels and reduces the amount of red. In photoshop, you position a circular cursor over the red part of the eye and click. If you click repeatedly the iris will go grey.
 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #5 on: 29/04/2008 23:36:25 »
You have to either download or purchase the correct photographic software to help you remove redeye, crop or resize your photos
and many other things that I've yet to learn about to do with this.

These days some cameras actually allow you to do this in camera, although personally I still prefer to do this on the computer, where I can see better what is happening, and provide better control over it.

Ofcourse, the best is to avoid it altogether, either by using natural light rather than flash, or making sure the flash is far enough from the camera lens (but you won't fit that in your shirt pocket, and probably not in your handbag).
 

lyner

  • Guest
What causes 'red eye' in photographs?
« Reply #6 on: 30/04/2008 09:40:06 »
The colour red is because of all the blood vessels on the surface of the retina (we have an inverted retina, with the photoreceptors under the nerves and blood supply - wierd). Most of the light is absorbed so you need a really bright light to get the effect - i.e. photo flash, on axis. A small angle off axis will destroy the effect.
Many nocturnal animals have a reflective coating behind the retina, to give the light sensors 'a second go' at the light, which would otherwise be absorbed. Hence, cats' eyes look bright yellow instead of red.
The best 'red-eye' reducing software responds to a very narrow colour range of red, corresponding to the colour of blood, I guess. Other reds are not corrected.
« Last Edit: 30/04/2008 09:42:53 by sophiecentaur »
 

Offline BenV

  • Neilep Level Member
  • ******
  • Posts: 1503
    • View Profile
What causes 'red eye' in photographs?
« Reply #7 on: 30/04/2008 09:53:14 »
As sophiecentaur pointed out, some red eye reduction software works by finding the right hue of red.  In other software, you have to tell it where the eyes are.  The software then desaturates the red (i.e. takes it down to greyscale) and darkens it.  In most cases this works a treat, but sometimes you will notice that the iris looks grey (from the desaturation), and you can opt to do it manually.
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #8 on: 30/04/2008 11:07:25 »
The software analyses the colour of the pixels and reduces the amount of red. In photoshop, you position a circular cursor over the red part of the eye and click. If you click repeatedly the iris will go grey.

Doctor Beaver, I have Paintshop a similar software program to Photoshop or so I've been told but removing redeyes was the first thing that I've learnt about and I don't want to keep on clicking my camera as I'll lose the photos which I don't want to do.

BenV what happens then if the person or people, all dark coloured eyes would that inhibit the redeye problem??
 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #9 on: 30/04/2008 11:26:59 »
BenV what happens then if the person or people, all dark coloured eyes would that inhibit the redeye problem??

OK, I'm not BenV.

The reflection happens on the back of the eye, and occurs as light passes through the pupil of the eye.  The colour of the eye relates to the iris (the area around the pupil), and has no relevance to light passing through the pupil.
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #10 on: 30/04/2008 11:51:08 »
As sophiecentaur pointed out, some red eye reduction software works by finding the right hue of red.  In other software, you have to tell it where the eyes are.  The software then desaturates the red (i.e. takes it down to greyscale) and darkens it.  In most cases this works a treat, but sometimes you will notice that the iris looks grey (from the desaturation), and you can opt to do it manually.
That one !!!
BenV what happens then if the person or people, all dark coloured eyes would that inhibit the redeye problem??

OK, I'm not BenV.

The reflection happens on the back of the eye, and occurs as light passes through the pupil of the eye.  The colour of the eye relates to the iris (the area around the pupil), and has no relevance to light passing through the pupil.
I know
That is the poster, who I was referring too. As I've mentioned before I am not expert with the redeye camera complications.
 

Offline BenV

  • Neilep Level Member
  • ******
  • Posts: 1503
    • View Profile
What causes 'red eye' in photographs?
« Reply #11 on: 30/04/2008 11:55:32 »
Another_Someone is right, iris colour won't affect how likely red eye is to occur, but a darker iris should make the 'grey eyes' that sometimes occurs with  red eye reduction less of a problem.

My apologies to Dr Beaver, I jumped in and replied without reading the whole post - you clearly beat me to it!
 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #12 on: 30/04/2008 13:01:31 »
If you care about doing it properly, you can paint the eye's in manually (probably best to use the clone tool), then you can make sure the colour matches exactly, and is the shape and size that looks natural.

I've never found the red-eye tool to be really satisfactory (but then, if at all possible, I avoid using flash, or use fill-in flash, so don't often get faced with red-eye - and my newer camera is somewhat larger than my previous, which should do more to avoid the problem even when using flash - but on-camera flash flattens the shadows anyway, so best avoided, except as fill-in, unless you really must use it).
« Last Edit: 30/04/2008 13:04:30 by another_someone »
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #13 on: 30/04/2008 13:31:35 »
Mine does not do that or more likely I've not bothered how to find out what to do as with some techy things I get confused so stop looking into it.
 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #14 on: 30/04/2008 15:11:37 »
Mine does not do that or more likely I've not bothered how to find out what to do as with some techy things I get confused so stop looking into it.

Not sure what you mean it does not have?

Do you mean that the camera does not have fill-in flash - most of the cameras I have looked at do, but I can imagine there would be some 'point and shoot' cameras that may not have that feature.

If you mean that Paintshop Pro does not have a clone tool - I know that version 8 certainly did have.  It was a long time since I looked at it (version 8 is an old version - the last version they produced before Jasc sold Paintshop to CA), but I would certainly have noticed if that was missing.  I think I may even have the manual for Paintshop version 7 around, so if you like I might be able to look it up (although I cannot say if the menu structure for the current version of Paintshop is anywhere close to that used for version 7 - but I cannot imagine that they have removed features).
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #15 on: 30/04/2008 16:11:23 »
I have the JASC version of Paintshop Pro 7 and it suits my very
non-techy knowledge/needs so far. Cropping will be something to definitely to deal with, I seem to cut half of a picture away time will tell as they say.

What is a "clone tool" anyway? I can download it from somewhere but I also do have the Microsoft's picture manager already installed and I use that too.
 

Offline BenV

  • Neilep Level Member
  • ******
  • Posts: 1503
    • View Profile
What causes 'red eye' in photographs?
« Reply #16 on: 30/04/2008 16:40:05 »
The Clone tool is a delightful little tool in photoshop and some similar programs.  It lets you select an area to copy, then you can paint somewhere else in the image and it copies from the reference point.  This makes it really easy to copy colours, textures etc, or hide things you don't want in the image.

For example, if you have a stain on a carpet in a photo, you can 'clone' over the stain, using the clean carpet as a reference - et voila! clean carpet!
 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #17 on: 30/04/2008 17:09:59 »
It is as BenV describes it.  You select where you are cloning from, and select where you are cloning to, then then as you move the mouse, you see two cursors move in parallel with each other, and the area under one cursor is copied to where the other cursor is.  Analogous to BenV's carpet, I have nicely removed a street sign from in front of a building by copying another part of the building over it (in the process, the building happened to acquire a window it never actually had, but unless you knew the building intimately in real life, you would never know there was an extra window).  I have removed people from photographs by cloning the background (grass in one photo, sea in another) over where the people were.

I would also add that precise cloning, painting, and even selection, is much easier now that I have a graphics tablet, rather than trying to do everything with the trackball (my desk was always too cluttered for a mouse).

And yes, Sally, you are right that the crop tool is also very critical in making sure that the picture does not have lots of clutter that you don't want in it.
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #18 on: 30/04/2008 18:54:59 »
I am happy with the Jasc's Paintshop Pro 7 that I have and I am still discovering as I've only had it on my computer since late February this year. I only got my Sony cybershot digi camera
August 07.
Thanks Rosalind
 

another_someone

  • Guest
What causes 'red eye' in photographs?
« Reply #19 on: 30/04/2008 19:44:01 »
I am happy with the Jasc's Paintshop Pro 7 that I have and I am still discovering as I've only had it on my computer since late February this year. I only got my Sony cybershot digi camera
August 07.
Thanks Rosalind

I am pretty certain that Paintshop Pro 7 will do all of the above - but if you only have had it for 2 or 3 months, I suspect you need to do quite a bit more exploration of the facilities it has, and how to use them.

Bring up the online help in Paintshop Pro, and search for 'cloning'.

Paintshop Pro 7 may be a bit dated these days, but it is still a pretty powerful package.  Some of the things that it would be missing would be things like colour channel separation, colour profile management, processing RAW files from some of the professional and semi-professional cameras - none of which you are going to be considering doing in the context you need it.

One thing, if I recollect correctly, that Paintshop Pro 8 introduced, that was missing on Paintshop Pro 7, that might be useful to the non-specialist photographer, was the ability to print multiple photos on a single sheet of paper (e.g. layout two or three smaller photos on a single A4 printed page, and so save on paper).
« Last Edit: 30/04/2008 20:03:17 by another_someone »
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #20 on: 30/04/2008 21:11:32 »
I have just looked at the Help bit of JASC's Paintshop Pro 7
and it does have the cloning tools. I still have to work them out but I will have fun discovering that, originally I got redeyes and removed them from some family photos. Easier than I'd thought possible.
 

Offline DoctorBeaver

  • Naked Science Forum GOD!
  • *******
  • Posts: 12656
  • Thanked: 3 times
  • A stitch in time would have confused Einstein.
    • View Profile
What causes 'red eye' in photographs?
« Reply #21 on: 01/05/2008 07:57:16 »
...and I don't want to keep on clicking my camera as I'll lose the photos which I don't want to do.

I'm not sure what you mean by that.
 

Offline rosalind dna

  • Neilep Level Member
  • ******
  • Posts: 2019
    • View Profile
What causes 'red eye' in photographs?
« Reply #22 on: 01/05/2008 10:32:39 »
The software analyses the colour of the pixels and reduces the amount of red. In photoshop, you position a circular cursor over the red part of the eye and click. If you click repeatedly the iris will go grey.
Quote
author=DoctorBeaver link=topic=14159.msg170913#msg170913 date=1209625036]
...and I don't want to keep on clicking my camera as I'll lose the photos which I don't want to do.

I'm not sure what you mean by that.

The software analyses the colour of the pixels and reduces the amount of red. In photoshop, you position a circular cursor over the red part of the eye and click. If you click repeatedly the iris will go grey.
Dr Beaver that is what you had said yesterday and maybe I'd misunderstood you. I thought that you had meant that if the camera is clicked repeatedly then the redeye problems would go. oops?
 

lyner

  • Guest
What causes 'red eye' in photographs?
« Reply #23 on: 01/05/2008 11:15:58 »
Clicking whilst using the software - not while using the camera, I think dr beaver means.
 

Offline techmind

  • Hero Member
  • *****
  • Posts: 934
  • Un-obfuscated
    • View Profile
    • techmind.org
What causes 'red eye' in photographs?
« Reply #24 on: 02/05/2008 20:47:20 »
The software solution is the next answer.  This merely looks for an area of red in the photograph that looks like 'red-eye', and assumes that this should be the pupil of the eye, and so that area is merely painted black.


Hmmm - I saw a photo recently where it looked like the subject had a missing tooth. We figured that the anti-red-eye software had mistaken a reflection off the tooth or a bit of spittle for red-eye ... and blacked it out!
 

The Naked Scientists Forum

What causes 'red eye' in photographs?
« Reply #24 on: 02/05/2008 20:47:20 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums