ISO Settings in Digital Photography

ISO Settings in Digital Photography
by Darren Rowse

What is ISO?

In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you’ve probably seen them on films - 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you’re taking.

In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography - the lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example an indoor sports event when you want to freeze the action in lower light) - however the cost is noisier shots. I’ll illustrate this below with two elargements of shots that I just took - the one on the left is taken at 100 ISO and the one of the right at 3200 ISO


100 ISO is generally accepted as ‘normal’ and will give you lovely crisp shots (little noise/grain).

Most people tend to keep their digital cameras in ‘Auto Mode’ where the camera selects the appropriate ISO setting depending upon the conditions you’re shooting in (it will try to keep it as low as possible) but most cameras also give you the opportunity to select your own ISO also.

When you do override your camera and choose a specific ISO you’ll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example - if you bumped your ISO up from 100 to 400 you’ll notice that you can shoot at higher shutter speeds and/or smaller apertures.

When choosing the ISO setting I generally ask myself the following four questions:

1. Light - Is the subject well lit?
2. Grain - Do I want a grainy shot or one without noise?
3. Tripod - Am I use a tripod?
4. Moving Subject - Is my subject moving or stationary?

If there is plenty of light, I want little grain, I’m using a tripod and my subject is stationary I will generally use a pretty low ISO rating.

However if it’s dark, I purposely want grain, I don’t have a tripod and/or my subject is moving I might consider increasing the ISO as it will enable me to shoot with a faster shutter speed and still expose the shot well.

Of course the trade off of this increase in ISO will be noisier shots.

Situations where you might need to push ISO to higher settings include:

* Indoor Sports Events - where your subject is moving fast yet you may have limited light available.
* Concerts - also low in light and often ‘no-flash’ zones
* Art Galleries, Churches etc- many galleries have rules against using a flash and of course being indoors are not well lit.
* Birthday Parties - blowing out the candles in a dark room can give you a nice moody shot which would be ruined by a bright flash. Increasing the ISO can help capture the scene.

ISO is an important aspect of digital photography to have an understanding of if you want to gain more control of your digital camera. Experiment with different settings and how they impact your images today.


source:
http://digital-photography-school.com/iso-settings

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Center Your Subject for Action Shots

Center Your Subject for Action Shots

By Brian Auer

We hear a lot about things such as the rule of thirds and not centering your subject for better composition. But there are times when you should actually center your subject to ensure that you get the shot. Action shots are typically a one chance situation. This can include sports, racing, performances, etc.

The problem with these action scenarios is that the main subject is usually moving quite fast and you only have one opportunity to capture a given moment. Spend too much time thinking about composition rules will ultimately result in missed shots. Here are a few reasons why you should think about centering your subject (and some tips for action shots):

* It’s easier for your AF camera to focus on the subject when centered — nothing worse than a sharp background and blurry subject. The caveat to this is if you have your camera set to spot focus somewhere other than the center.
* Most manual focus screens have additional feedback at the center of the frame — use it!
* Center your subject and you won’t miss a shot due to over-thinking the composition.
* Leave a bit of extra room around the main subject so that you can crop for better composition later.
* Use continuous AF to track the action — especially when the subject is moving toward or away from you.
* Get the dang shot!

source: http://blog.epicedits.com/2009/05/12/center-your-subject-for-action-shots/

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So You Think You Know What An F-Number Is

So You Think You Know What An F-Number Is
By Brian Auer

Most of us have heard, and probably used, the term f-number. Most of us have also probably used the term interchangably with “f-stop” or “aperture”. What are all these things, really? Yes, they have to do with the size of the holeallowing light to pass through the lens, and they affect depth of field and light intensity. But I’m talking aboutwhat these things really are.

First, let’s start with a few definitions. An aperture is simply a hole which allows light to pass through it.A diaphragm is the mechanism inside your lens that forms an aperture, and most modern lenses have an iris diaphragm made up of several interlocking blades. An f-stop is a discrete step in the f-number, and it refers to the physical stops in the diaphragm adjustment. So how are all these things different from an f-number?

An f-number is a measure of lens speed and it is defined by the focal length of the lens divided by the diameter of the aperture — f/# = f/D where f = focal length and D = aperture diameter. So if the focal length of the lens is equal to the diameter of the aperture, you’d have an f/1. If the focal length of the lens is 8 times longer than the diameter of the aperture, you’d have an f/8.

F-Number = f/D = (Focal Length)/(Aperture Diameter)

Here’s an example with one of my fixed focal length lenses (zooms are a little more complicated so I’ll hit that in a moment). I have a 50mm f/1.4 lens. The f/1.4 designation means that it has a maximum f-number of 1.4 — so that’s as big as it gets. The minimum f-number on this lens is f/16, so it can vary from f/1.4 to f/16. So using the formula for f-number: at a 50mm focal length and an f-number of 1.4, the equation states 1.4(f/#) = 50mm(f)/(D). Do the algebra, and we get 50mm/1.4 = 35.7mm. That’s physically how big the aperture is at it’s maximum. If I had a f/1.2 lens, the max aperture would be 41.7mm. Now to the other end of the scale, the aperture on this lens has a minimum diameter of 50mm/16 = 3.1mm. Similarly, my 105mm f/2.8 lens has a maximum f-number of f/2.8, an aperture diameter of 37.5mm — which is about the same size as my 50mm lens at
f/1.4, which is two stops up from f/2.8.

On the 50mm lens, including the minimum and maximum f-numbers, there are a total of 8 full
f-stops: f/1.4, 2, 2.8, 4, 5.6, 8, 11, and 16, with half-stops between everything but 1.4 and 2
(things like f/1.7, f/6.7, and f/9.5 are half stops — f/1.8, f/3.5, and f/6.3 are third stops found
in most modern lenses). Each full stop lets half as much light in as the last full stop. This is
because the area of the aperture is reduced by half with each stop. The values in the table represent the f-numbers, aperture diameters, and aperture areas for this lens. Note the reduction of area as the f-numbers increase. Each full stop down lets half as much light into the camera, and you can see that the area of the aperture for each stop is also cut in half. This is where a lot of people end up confusing themselves over f-numbers. Higher f-numbers mean smaller apertures — just remember that. To help explain the numbers in the table a little better, the image below shows the 50mm lens at each full stop from f/1.4 to f/16 from left to right.

Zoom lenses are a bit more complicated, and they generally fall into two groups: constant f-number and variable f-number. Variable f-number zooms are most common because they are simpler and cheaper. You can spot these lenses by their markings — f/3.5-6.3 means that the lens has a maximum f-number of f/3.5 at the shorter focal length and f/6.3 at the longer focal length. This doesn’t mean that the aperture changes as you zoom; it actually means that it doesn’t change. Remember that f-number is the quotient of focal length and aperture diameter, so as you zoom to a higher focal length (and keep the aperture constant) you allow less light into the camera and the f-number changes. On the other hand, the really spendy zoom lenses can maintain a constant f-number at all focal lengths. To achieve this, they must increase the effective aperture diameter as the focal length increases to keep the same f-number ratio.

F-numbers in zoom lenses aren’t quite as simple as I’ve made them out to be. That’s basically how they work, but truthfully, I don’t know exactly how they work. If you run the numbers on a zoom lens, it turns out that you don’t get a constant aperture diameter as the focal length increases and the f-number changes. Physically, I’m pretty sure the aperture diameter stays the same. But when you zoom a lens, you shift the location of lens elements, the diaphragm, focal planes, inflection points, etc., and some of these things are factors for the effective aperture diameter. I’m not an optics expert, so I’ll leave it at that.

Basically, the major take-away from this should be that the f-number is a measure of lens speed — regardless of the camera, regardless of the lens, and regardless of the conditions. If you and a friend are out shooting, you should be able to get the same exposure of a particular subject if you shoot using the same f-number, shutter speed, and ISO value (assuming ISO’s between cameras is somewhat comparable). It takes the focal length and aperture diameter into account in order to give a value of how much light will be allowed into the camera.

source: http://blog.epicedits.com/2007/06/16/so-you-think-you-know-what-an-f-number-is/

http://blog.epicedits.com/author/Brian/

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