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Lesson 3: Exposing Properly Using High ISO

May 24th, 2010 · 3 Comments

A great feature in newer Digital SLRs is High ISO capability.  This is a wonderful feature and really opens up the doors in the digital world.  Some still remember the film days of being stuck in a particular ASA (film equivalent to ISO) for an entire role of film.  In the digital world, we can change our ISO on every single image (if we choose to take the time).  This is a great tool, when used properly.   As with everything in life, there are downsides to all of these great features.  In this lesson, I will go over how to shoot in High ISO to obtain the fast shutter speeds we are all after, but in a way to minimize the bad (NOISE!)

First let’s discuss the basics of ISO and the noise that goes with it.  ISO basically makes the digital sensors in our cameras more sensitive to light.  When the sensor is more sensitive to light (higher ISO), this means for a given light source, the shutter is open for less time (faster shutter speed).  Fast shutter speeds are key when photographing wildlife, especially birds!  When I’m shooting birds in flight, I strive for 1/1000th of a second at the very least so in cloudy situations when this is typically very hard to achieve, I look to ISO to improve my shutter speeds.  This increase in shutter speed is great but the high ISO adds unwanted noise.  There is a fine line that one must not cross.  ISO menus on most Digital SLRs will allow the shooter to go as far as ISO 1600, 3200, and sometimes way beyond.  This is a crazy amount of ISO range and the biggest piece of advice I can give is “know your limits!”  When talking about noise, let’s look at some examples shot from a low-end Digital SLR with a 1.6x cropped sensor (PLEASE CLICK ON THE IMAGES FOR LARGER VIEWS):

Canon Rebel XTi, f4.5, Shutter Speed= 1/160th, ISO= 100

Canon Rebel XTi, f4.5, Shutter Speed= 1/2500th, ISO= 1600

In the examples above, notice the noise difference between ISO 100 & 1600.  There is major noise visible in the ISO 1600 example.  The good side, however, is the difference in shutter speed.  At ISO 100, our shutter speed was 160th of a second.  When we changed over to ISO 1600, look at how fast our shutter speed is; 2500th of a second.  That is a huge jump!  We must, however, weigh the downsides in this ISO change.  The shot will always look great on the camera’s small LCD Screen but when you get the image home, you will most likely be disappointed in the noise.  You may have a great subject in the frame but if noise takes over the image and that’s all the viewer sees, what’s the point?  Now let’s look at the image below shot at ISO 400 (the acceptable ISO limit for most lower end digital SLRs, in my opinion):

Canon Rebel XTi, f4.5, Shutter Speed= 1/640th, ISO= 400

The example above shows noise, but not enough to detract from the important elements of the image.  By knowing the limits, you can use higher ISO to your advantage but not go overboard.  By increasing the ISO to 400, we gained a lot of shutter speed (from 1/160th to 1/640th) but also minimized the amount of visible noise.

Another factor to consider when using High ISO is your depth of field (DoF).  When shooting Wildflowers a couple of years ago in shaded lighting, I needed to capture a specific shot but with a faster shutter speed due to winds.  I decided to take my ISO to the limit on my Rebel XT (ISO 1600).  I knew there would be crazy noise but decided to go for it.  At the time, I was stopped way down to f22 in order to have my entire foreground & background in focus.  When I brought the image home, I was amazed how little noise there was.  I zoomed in to 100% in Photoshop and noticed the noise was most definitely still there but because my background was in focus (adding depth and CONTRAST), I didn’t notice the noise.  The added contrast effectively hid the noise and allowed me to still capture the intended image.  The following example shows how the noise is hidden with an aperture that is stopped down:

Canon Rebel XTi, f25, Shutter Speed= 1/80th, ISO= 1600

This technique is not going to work with action shots as you still won’t have fast enough shutter speeds, given your stopped down aperture but when shooting landscapes or flora from a tripod, this can be a lifesaver in certain situations.

Now that we’ve explored the limits of a low end digital SLR with a 1.6x cropped sensor, let’s see how the noise factor changes with a full frame sensor.  The example below shows how a full frame sensor still at ISO 1600 shows way less noise:

Canon 5D Full Frame, f4.5, Shutter Speed= 1/2500th, ISO= 1600

The noise melts away mostly because of the full frame sensor.  The larger sensor allows more room for the pixels that make up the resolution of the camera body.  If you compare an 8mp digital SLR with a 1.6x crop to a full frame digital SLR with the same 8mp resolution, the full frame body’s sensor is physically larger but still holds the same 8,000,000 pixels.  With the 1.6x crop, the 8,000,000 pixels are packed tighter which doesn’t allow the body to shoot cleanly at high ISO.

Another way to minimize the visible noise is ensuring that you’re not underexposing.  When an image is underexposed, we end up with more shadow areas, which tend to show noise more than highlight areas.  A general rule when shooting at high ISO is to expose for the highlights.  This is actually beneficial on many levels.  The right side of your histogram (highlights) always holds more data which will not only yield better results, but allow more exposure adjustment in post-processing after the fact.

With digital, we are able to change the ISO for every shot but don’t go crazy here.  Just because we have the capability doesn’t mean we need to do this.  Lighting is changing constantly and whether you are moving around a lot or staying put, we need to constantly monitor what our exposure is doing.  Don’t spend all your time changing your ISO settings constantly.  Be cognizant of your lighting and exposure and adjust your ISO as your given situations change.  The beauty of digital is we’re not locked into a full roll of film with a given ASA/ISO.  Take advantage of the jump in digital but use the features wisely.  It all goes back to one of the best things Dad ever taught me, “Don’t be a slave to the camera, make the camera your slave.”  This means learning, reading your manuals, and taking the features to their limits in order to get the best possible images around.

Top 4 things to take away from this lesson:
     1)  Know your limits!  Some cameras/exposures can’t handle super high ISO
     2)  Stopping down in Aperture will hide Noise
     3)  Avoid underexposing as shadows amplify the appearance of noise
     4)  Change ISO based on what you’re shooting and the available light

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