Thursday, April 1. 2010
Why red eyes appear in my photos and how to prevent them
Question: Why do those red eyes appear?
Answer: When strong white light enters human eye, some of it reflects, and most of the visible reflected light is red. So, when you shoot with flash, chances are people on your photos will have red eyes. Voila - Demon eyes made easy 
Question: How can I prevent this ?
Answer: Here is one simple trick that sometimes can do the magic.
The light enters our eyes through our pupils. Pupils actually act as eyes apertures - they open and close, allowing more or less light to enter. Tell your subject to look at something bright for a short period of time (be careful. Don't make them look at the sun or similar dangerous objects for our eyes). Their pupils will automatically close in order to lower the amount of light entering the eye. Now, tell them to look at you and BOOM! flash them out! Since pupils were smaller, less light from your flash entered the eye, so less chance to see a red eye (d)effect.
Some photo cameras have no-red-eye mode which works in similar way: it flashes few times, which closes subject's pupils, and then the photo is made.
This trick, while useful, is not always working.
Question: How to really prevent red eyes in a photo?
Answer: Imagine that you are standing in front of a mirror, and you have flashlight in your hand. (Or, if you want, do it for real with lights off! Ideal flashlights for this experiment are the ones which have focused light. Do not try this with lasers! While they are ideal for experiment, they are also ideal for ruining your eyes!)
Put the flashlight in front of your eyebrows, pointing exactly at reflection of your eyes at the mirror. The mirror reflects the light and the light will strongly shine in your eyes.
Now, take flashlight to your right (as far as your hand goes). Point its light at the same place in the mirror.
You will notice that the light that shines directly at your eyes is none.
This is something that we are familiar with.
Now, if your eyes were camera sensor, flashlight was your portable flash unit, and the point in the mirror you were lighting was your photo subject - it should be clear that:
In order to prevent red eyes you should have your light source moved further from your lens.
How much further depends on how big light source is, how strong it is, how far you are from the subject etc.
Have fun flashing your subjects 
Sunday, August 2. 2009
CPL - circular polariser filters
- Short Information
- Detailed information
- Examples
Short information
This is second article on photography filters. You can read the first one on UV photo filters.
For those of you that don't want to get too technical, here are major benefits of using polarizing filters (scroll down for examples):
- colors are more saturated
- you can get darker colors in the sky - even dark blue sky
- remove reflections from water surfaces and glass
- use it to reduce light so that you can shoot with slower shutter speeds
Detailed information on how Polarizing filters work
A physicist would say: most sources of electromagnetic radiation contain a large number of atoms or molecules that emit light. The orientation of the electric fields produced by these emitters may not be correlated, in which case the light is said to be unpolarized. Which, translated to plain English means that: almost all light is, by default, is in state that is unpolarised. Here is a illustration of unpolarised light:
When this light hits some surface, as glass or water surface - it gets reflected. Leve of reflection depends on entrance angle of the light and surface reflection qualities. Interesting part is that when the reflection occurs, very often the light is being polarized, because only one of atom generated waves is being reflected. To a naked eye - the light is the same. We can not feel any difference.
When we put a polarizing filter in front of our lens, here is what happens. Polarizes acts as filter to the polarization. It has one plane of polarization - and only light that has that plane will pass. All others are blocked. So, if we use polarizer with regular light it will pass one plane and the final image will look the same - just as we did not use any (since we cannot see the difference). However, when we use polarizing filter with polarized light, interesting thing happens. The light has one orientation of its polarization and the filter has other. When those two match - the light is passed. When they do not match - filter will block the light. Quantity of blocking depends on filter make and quality.
The filter is most effective when used at 90 angles degree from the light.
Air atoms reflect and polarize some of the light that hit them. Lets say that we are shooting some landscape. If we put on pol. filter and point the lens at 90 degree from the sun, we can benefit from darker sky. So, get the sun to your left or right shoulder and make that landscape shot with and without the filter. What happens is that the polarized light from air atoms is blocked. So, the sky itself has less light that hits the sensor and it appears darker. This is useful as usually the sky is very bright, and not so pleasant for viewers eye.
Same applies to reflected light from leaves, doors, glass, water surfaces etc
Examples
See how the filter darkens the sky, defines clouds and also makes trees little greener
Polarization filter is oned on left side. check how it removed the reflection in glass, so you can see through it
The filter has darkened the sky, and also removed reflection from sea surface so that the sea bottom is visible
Monday, June 25. 2007
Photo Exposure explained - histogram, ISO, shoot speed, aperture
Introduction
There are three things that influence on your photo exposure: duration of shot, how open is aperture, and ISO.
In this article I will explain all of them.
I suggest to have your camera with you, while you read the article, and check and test things as you read them. Choose manual mode (often marked as M), so that you will have full control over camera settings.
Exposure and histogram
Photo exposure is the term that we use to describe how bright is the photo. Simply said: photo has good exposure when it is not neither too dark or too bright. However, there are ways to scientifically describe when a photo is in good exposure. Open your favorite photo editing software. Most of them will have option to show you the photo histogram. It graphically shows the dark and bright
parts of your photo. So, by looking at this graph, you can say if the photo is overexposured or underexposured and make the corrections. Most of newer digital cameras show this histogram when you preview the photo on its display. Some even show it during the framing, so that you can make the changes in settings even before the shot. The more the graph is gathered in left part - the photo is darker, and vice versa.
Here is a histogram of underexposured photo:
Overexposured:
Correct exposure:
Of course, specific conditions or wanted feel and look can define different wanted histograms. For example, the overexposed histogram above is from this photo:
The photo is just the way I want it to be. So, the fact that the histogram shows that the photo is not correctly exposured does not mean that it is not good. However, histogram is very useful and I suggest you to use it. Learn how to read it - it can tell you things about photo exposure, contrast and other things.
Shooting speed
The time for which the photo sensor (matrix) is exposured to the light is called shooting speed. Obviously, the more time the light falls on the sensor, the brighter the photo will be. If we set the three settings to some values, and then change the shooting speed to be twice longer. Technically speaking the photo made with second settings will have one stop of exposure more light than the first one. Exposure stops are important and if you want to master the manual mode of your camera, you have to understand it. So, if you have speed setting at 1/100th of a second and you want to adjust the speed so that you will receive photo which will be two steps underexposured, we would have to use speed of 1/25th of a second. For one stop, the value is 1/50th, and from that value one stop more is twice shorter than that.
Aperture
Aperture is a measure for how much is the lens opened. The more you open it - the more light comes in for the same time. One of most important feature of each lens is the one which shows how much the lens can be opened. The values are measured in f-stops and written as: f/value. Since the value is a denominator, the smaller the value - the lens is more open (and more light is passing through the lens). Lenses are always mentioned together with their maximum f-stop value. For example: Canon 70-200mm f/2.8 IS L is the full name of one of the best zoom lenses by Canon. It means that the biggest aperture you can set this lens is f/2.8. Some lenses even have aperture values lower than 1.0 - the 'lightest' lens is made for film cameras and its aperture is 0.7.
I already mentioned f-stops. Here are the values of apertures that are full f-stops:
0.5, 0.7, 1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, 128 ...
Modern lenses often have one-third stops values for finer settings.
Now, you might wonder why those strange numbers. The F number itself is calculated by formula:
f# = f/D
where f# is the f/number, f is the focal length, and D is the diameter of the entrance pupil. The one f-step difference means that the light that passes through lens at f/2.8 is twice more than when we set the lens at f/4. Now, remember that we already had that 'twice light' in the definition of exposure.
This knowledge gives us the power to guess what speed and aperture to use in situations where we can not measure the light.
For example, one of the most popular exposure rules is the Sunny 16 rule: "An approximately correct exposure will be obtained on a sunny day by using an aperture of f/16 and a shutter speed close to the reciprocal of the ISO speed of the film; for example, using ISO 100, an aperture of f/16 and a shutter speed of 1/100 second"
If your camera (in Auto mode) says that in order to get good exposure it will use f/5.6 and 1/40, but the photo is blurred because of fast moving objects, you can make the needed calculations so that you will have correct exposure and faster shooting speed. Lets say that you want to double your shooting speed. Set camera to manual mode and the speed to 1/80 (twice faster). This means that if we don't change anything else the photo will be 1 step under correct exposure. In order to achieve correct exposure, we can change the aperture. take a look at above list of full stop f-numbers. The number with one stop bigger aperture than 5.6 is f/4. So, we will set the aperture to f/4. The photo taken with 1/40 and f/5.6 will be equally exposured as the one taken with 1/80 and f/4.
ISO
In short, ISO is sensitivity to light which measures the matrix's speed.
Sometimes you want to use very high shooting speeds, like 1/4000, but your lens does not not have aperture so big that the photo will be correctly exposed.
The part of your digital camera that catches the light is called matrix.
It has one property, called ISO and it can be changed. It also influences exposure and it also has full stops values. Here is a list of ISO values with one exposure stop difference between them:
25, 50, 100, 200, 400, 800, 1600, 3200, 6400, ..
Lets say that you set your ISO to 100, and your aperture is at f/2.8, and your shooting speed is 1/250 of a second and with these settings you manage to get photo which is correctly exposured, but you need even higher shooting speed. Professional sport photographers often use speed no slower than 1/2000. Lets imagine that we want to achieve speed of 1/1000th of a second and our lens is already at its widest aperture - we cannot open it wider. So, from 1/250 to 1/1000 are two stops (we calculate this in this way: from 1/250 to 1/500 is one stop, and from 1/500 to 1/1000 another stop). Now, in order to use same aperture of f/2.8, speed of 1/1000 and achieve good exposure we will have to use ISO 400.
Notes
All this might seem to mathematical to those of you who just want to take snapshots of friends. Actually, most modern cameras have modes which will do needed calculations for you. However, there are situations where the camera is wrong. Actually it does not make mistake, it just doesn't know what exactly you want to picture. In those situations, it is handy to know about speed. f-stops and ISO.
And for the end - don't forget to test and experiment.
In one of next articles I will focus on ISO, aperture and speed and how they influence the final result more closely.
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