Basics of Photography #01: ISO Sensitivity

Yes, I know, they all look the same!!!! But look closer/read on and they are in fact all different, and all the same!!!!.......................

 

I thought I would do something a bit different and start sharing a few basic photography tips, which will hopefully be of use to someone. I aim to keep things simple and concise in subsequent uploads but I need to ramble on a little bit here to set the scene. No doubt some of you will know all of this already, but I'm willing to bet there are also plenty who don't. Including those with high-end equipment who don't know how to use it!! :-))

 

A brief introduction

Every photograph has an exposure value (EV) which is dependent on three things: Aperture, shutter speed and ISO sensitivity; often referred to as the exposure triangle. In fully automatic mode a camera will choose the most "suitable" values for each variable for a given scene and calculate the "correct EV" accordingly.

With DSLR and bridge cameras the user can have direct control over aperture and shutter speeds but this is rarely the case with compact "point and shoot cameras". However, these usually allow the user to adjust the ISO sensitivity so ISO seems like a good place to start. And to keep it very simple I am only going to look at ISO here.

 

What is ISO sensitivity?

ISO is a measure of how sensitive the camera sensor is to light. The higher the ISO, the more sensitive it will be. OK, but what does that actually mean in practice? Put simply, when the ISO is increased the shutter speed must also be increased (faster) so the final image still looks the same. Camera manufacturers and retailers will often draw attention to how high the ISO will go, and how great a camera is in low light conditions. "Get sharp images even in low light" etc. Faster shutter means less camera shake.

 

So that's it then, we just crank up the ISO and hope for the best? Or is there more of a pattern to it? Yes, there is a pattern and it's incredibly simple. The images above reveal the pattern!....................

All eight images were shot in Aperture Priority mode (camera calculates shutter speed), with aperture fixed at f/8. Lighting was from a lamp to ensure a constant light source. The only thing which was manually changed each time was the ISO. But the resulting images all look virtually the same. As can be seen above, when the ISO is doubled from 100 to 200, the shutter speed also doubles from 1/5 seconds to 1/10 seconds. That is, the shutter is twice as fast. "But 1/10 is half of 1/5, not twice!" I hear you cry. Hmmm, yes, we are dealing with fractions. Put another way, at 1/10 seconds, the shutter is only open for half as long as at 1/5 seconds. i.e the speed doubles. Or from ISO 1600 to 3200, the shutter speed doubles from 1/80 to 1/160.

 

The key point here is: DOUBLE ISO = DOUBLE SHUTTER SPEED

 

A good analogy to this relationship between ISO and shutter speed is to think of worker bees gathering nectar. Assume all the bees work at the same rate; If 100 bees can collect 1kg of nectar between them in an hour, then 200 bees can get the job done twice as quickly. They can collect 1kg in half an hour! I've no idea if the numbers are realistic, but that's not the point!! :-))

Done