Exposure - Amateur Astro Photography
Stills by Larry Waite
Larry Waite
Stills by Larry Waite
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Exposure
Tips
Aperture (F-Stop)
In optics, an aperture is a hole or an opening through which light travels.
This can be adjusted to let in more or less light.
For most lenses, this opening can change based on the camera's settings.
These opening have been standardized into specific sizes, called aperture stops, or F-stops.
Here's the tricky bit, The Higher the number ie:F/32 the smaller the hole. Likewise the smaller the number ie: F/1.8 the larger the hole.
Example 1:
Shooting the milky way on a dark night I would want a very low f-stop to let MORE light in. Like F/3.5 or even F/1.8 (if you have a fast lens).
Yet, during a bright sunny day, I might want less light and shoot with F/22.
I find the smaller the number (more light) the sharper my images are, (usually) but it is a juggling act.
If in doubt, put it in AUTO mode and take a look at the the camera recommends and go from there.
ISO
Basically, ISO is the level of sensitivity of your camera to available light.
The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera.
The component within your camera that can change sensitivity is called “image sensor” or simply “sensor”. It is the most important (and most expensive) part of a camera and it is responsible for gathering light and transforming it into an image.
With increased sensitivity, your camera sensor can capture images in low-light environments without having to use a flash.
But higher sensitivity comes at an expense – it adds grain or “noise” to the pictures.
Every camera has something called “Base ISO”, which is typically the lowest ISO number of the sensor that can produce the highest image quality, without adding noise to the picture. Some older Nikon DSLRs and a number of other modern cameras such as the Fuji X-T2 have a base ISO of 200, whereas most modern Nikon and Canon digital cameras have a base ISO of 100. S
So, optimally, you should always try to stick to the base ISO to get the highest image quality.
However, it is not always possible to do so, especially when working in low-light conditions.
Example 2:
Photographing stars, You would want to start on about ISO 1250 and the lowest f/stop possible, say F/3.5, F/2.8 or lower if you can.
Typically, ISO numbers start from 100-200 (Base ISO) and increment in value in geometric progression (power of two). So, the ISO sequence is: 100, 200, 400, 800, 1600, 3200, 6400 and etc. The important thing to understand, is that each step between the numbers effectively doubles the sensitivity of the sensor. So, ISO 200 is twice more sensitive than ISO 100, while ISO 400 is twice more sensitive than ISO 200. This makes ISO 400 four times more sensitive to light than ISO 100, and ISO 1600 sixteen times more sensitive to light than ISO 100, so on and so forth.
What does it mean when a sensor is sixteen times more sensitive to light?
It means that it needs sixteen times less time to capture an image!- Confused yet?
ISO Speed Example:
ISO 100 – 1 second
ISO 200 – 1/2 of a second
ISO 400 – 1/4 of a second
ISO 800 – 1/8 of a second
ISO 1600 – 1/15 of a second
ISO 3200 – 1/30 of a second
ISO 100 – 1 second
ISO 200 – 1/2 of a second
ISO 400 – 1/4 of a second
ISO 800 – 1/8 of a second
ISO 1600 – 1/15 of a second
ISO 3200 – 1/30 of a second
In the above ISO Speed Example, if your camera sensor needed exactly 1 second to capture a scene at ISO 100, simply by switching to ISO 3200, you can capture the same scene at 1/30th of a second! That can mean a world of difference in photography, since it can help to avoid camera shake or motion blur.
When to Use Low ISO
As I have said above, you should always try to stick to the lowest ISO (base ISO) of your camera, which is typically ISO 100 or 200, whenever possible. When there is plenty of light, you should use the lowest ISO to retain the most amount of detail and to have the highest image quality.
There are some cases where you might want to use low ISO in dim or dark environments – for example, if you have your camera mounted on a tripod or sitting on a flat, non-moving surface. In that case, bear in mind that your camera will most likely need more time to capture the scene and anything that is moving is probably going to look like a ghost:
When to Increase ISO
You should increase the ISO when there is not enough light for the camera to be able to quickly capture an image. Anytime I shoot indoors without a flash, I set my ISO to a higher number to be able to capture the moment without introducing blur to the image. Another case where you might want to increase ISO is when you need to get ultra-fast shots, like the bird picture I posted above. Before increasing the ISO though, you should first decide if it is OK for you to introduce some noise to the image. Remember, the bigger the ISO number, the more noise you will see in your images.
On many of the newer cameras, there is a setting for “Auto ISO“, which works great in low-light environments.
The beauty of this setting, is that you can set maximum ISO to a certain limit, so when ISO is automatically increased based on the amount of ambient light, it does not cross the set limit. So, if I want to limit the amount of grain in my pictures, I typically set maximum ISO to something like 800 or 1600 on most entry-level cameras and I can push that number even higher on higher-end full-frame cameras.
SHUTTER SPEED
Shutter Speed is one of the three pillars of photography, the other two being ISO and Aperture. Shutter speed is where the other side of the magic happens – it is responsible for creating dramatic effects by either freezing action or blurring motion. In this article, I will try to explain everything I know about shutter speed in very simple language.
What is a Camera Shutter?
Simply put, a camera shutter is a curtain in front of the camera sensor that stays closed until the camera fires.
When the camera fires, the shutter opens and fully exposes the camera sensor to the light that passes through the lens aperture.
After the sensor is done collecting the light, the shutter closes immediately, stopping the light from hitting the sensor. The button that fires the camera is also called “shutter” or “shutter button”, because it triggers the shutter to open and close.
What is Shutter Speed?
Shutter speed, also known as “exposure time”, stands for the length of time a camera shutter is open to expose light into the camera sensor.
If the shutter speed is fast, it can help to freeze action completely, as seen in the above photo of the dolphin. If the shutter speed is slow, it can create an effect called “motion blur”, where moving objects appear blurred along the direction of the motion. This effect is used quite a bit in advertisements of cars and motorbikes, where a sense of speed and motion is communicated to the viewer by intentionally blurring the moving wheels.
Going back to the photographing of the night sky again (Stars) In manual mode, you would have the following as a good starting point to photograph stars.
Aperature ,as low a number as possible Ie: F/2.8, ISO 1250 and Shutter 20- 30 seconds. or F/4.0 ISO 3200 @25 seconds. (all depedent on the capability of your lens and camera)
Enusre you camera is on a tripod, set the timer or use a remote to take the shot!
(I would be interested to see your first and last photo!)
