Using ISO & EV Effectively

Were going to cover two different, yet related subjects in this thread.

The first is the Exposure Value System, or EVS. It is an old system that is still used to help photographers. An Exposure Value is a number assigned to an exposure, which is a combination of shutter speed and aperture.

Sounds confusing? It really isn't. Think of it this way, no matter what camera you are using, if you are using an EV level of 5, its going to let in the same amount of light and thus give you the same level of exposure.

Whats the value in this? Well its easier to walk into a scene and take a rough estimate of the EV, rather then trying to guestimate at what shutter speed AND aperture combo you should use. For example, if you know that most indoor scenes look best around EV 7 then you have a quick reference for setting you shutter speed or aperture.

Or say you are taking a portrait and want a certain aperture value to get a blurred background, and you estimate you need an EV of about 6; a quick look at an EV Table will tell you what shutter speed to use.

The important thing to know is that a particular EV number always allows in the same amount of light. EV 12 is 1/80sec at f/5 AND 1/125sec at f/4, etc. And that a LOWER EV lets in MORE light.

Some of the common EV values used for various conditions are:
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EV18 and above: Bright reflection off a sunlit object, including reflections off water
EV17 White objects in full sunlight
EV16 Light objects in full sun
EV15 Mid-day sun
EV14 Early day, afternoon sun
EV13 Partly cloudy bright days
EV12 Bright overcast
EV11
EV10 Dark, overcast day
EV 9
EV 8
EV 7 Typical indoors; light outdoors 15 minutes after sunset
EV 6
EV 5
EV 4
EV 3 Brightly lit night street scene
EV 2 Typical night street scene
EV 1 Dark scene at night
EV 0 is defined as the exposure that occurs at 1 second shutter at f/1 with ISO/ASA100 speed film
EV-1
EV-2
EV-3
EV-4
EV-5 Scene lit by the full moon
EV-15 Scene lit only by starlight (near complete darkness)

Now that you know what an EV is, and how they can be used, we should be able to make use of the EV Compensation (Exp. Feature) of your camera. If you increase your EV Comp to +1 then the camera will actually change its settings to LOWER the EV by 1, and thus let in MORE light (the equivalent of 1 EV). The camera does this by adjusting the shutter speed. For example, your camera plans to capture a photo at 1/30sec @ f/2.8 which is EV 8, however you have set your EV Comp to +2/3, so the camera will actually want an EV of 8 - (2/3) or about 7.3, if you look at your EV Table you can see the A70 would decrease the shutter speed to 1/20sec in order to accomplish this exposure.




How does the ISO come into play? With these cameras there are a couple of situations that you can use the ISO setting to your benefit, but before I get into that let me describe what ISO is. The ISO value of FILM indicates the films sensitivity to light. ISO values range on average from 25 to 800. The lower the number, the MORE light is needed to expose the FILM. For example, ISO 50 film requires more light then ISO 100 film for the same exposure. Higher level ISO film (400, 800) usually produces 'grainy' photos and has less color response, at the trade off of needing less light to expose, (and thus resulting in faster shutter speeds for capturing fast action scenes).

When it comes to digital the CCD itself has a 'native' ISO usually around 100. There isn't a way to physically change the CCD's sensitivity to light, so how do digital cameras allow us to change ISO values?

Essentially changing the ISO value on a digital camera will either amplify or (de amplify) the analog signal coming from the CCD. The problem with this is not only will you end up amplifying the data you want, but also the data you don't want, Noise. Noise is basically the effects of electronic interference and random photons on your CCD that result in multicolored 'dots' on your photo.

Most of us have set our A-Series camera on ISO 400 and when reviewing the photo later are disappointed with the excessive amount of noise. This noise used to render images semi-useless to most novices.

Fortunately there is now software that does an amazing job of removing noise. The best software I have seen is called NeatImage and is available from http://www.neatimage.com/ There is a free version, along with a paid version that offers more control. NeatImage almost has been a revolution in digital images. Previously it was a difficulty and time consuming task to try and remove noise from an image. Many people (myself included) eventually became frustrated trying to remove noise from digital photos and restricted the camera's use to only the lower ISO values.

Although the process of noise removal isn't 'perfect' it is highly useable. You will loose a 'small' amount of detail and sharpness when running an image through NeatImage but it is well worth the tradeoff.


Now, tying ISO and EV together.

If you increase your ISO value from 100 to 200, then the equivalent EV has dropped by 1. Every doubling of the ISO value results in a shift of 1 on EV. For example, you compose a photo at 1/200sec and f/4.5 which is an EV of 12. If you use the same shutter speed and aperture value but at ISO 200 the 'equivalent' EV is now 11. Going to ISO 400 changes the 'equivalent' EV to 10. I say equivalent because we are not changing the amount of light entering the camera, but are just amplifying the signal from the CCD. Take a look at the EV Table for your camera to see how the ISO value correlates with the EV at a given shutter speed and aperture.


HOW can you use all this to your advantage?

Let me ask you this, how many times have you tried to capture a photo only to have some blur because the shutter speed was so long that your hand movement blurred the photo? The optimum speed to eliminate hand shake is 1/50sec or faster, however in low light conditions 1/50sec would give you too dark of a photo.

So you have two options, you either decrease the shutter speed but with the risk of blurred photos, or increase the ISO value resulting in additional noise. Which is better?

Lets look at it this way, which image is easier to 'repair'? A hand-shake blurred photo, or a noisy photo? With NeatImage, it is very easy to repair a noisy photo. It can be done with just a couple mouse clicks.

The end result wouldn't be as perfect as if you had captured the scene at ISO 50 with plenty of available light, but it will look very good. I have printed images taken at ISO 400 and then cleaned with NeatImage at sizes up to 8x10 inches and they look excellent.

Hopefully the next time you are at a sporting event, or low-light conditions you will be able to put all this information into use!