Teaching Peggy Photography 101
Numbers, Numbers what are all these Numbers!
There are three different ways to control Exposure on any camera and that is what we are talking about. I will try and make sense of all of this in this lesson. There is the ISO number and that controls the sensitivity of the sensor. Shutter speed that controls how long the sensor sees light. And the f stop that controls how much light comes through the lens.
ISO stands for International Standards Organization and is a measure of light sensitivity of the sensor. If you worked with film you would by film with different speeds know as ASA or American Standards Organization. Both systems are equal so ISO 100 and ASA 100 are the same.
The ISO number is just a number that you are either going to double or half. For example I generally work at ISO 100 for landscapes. So if I want double the sensitivity I would change to ISO 200. If I would need twice ISO 200 then I would use ISO 400 and so on. The next doubled would be 800 and again 1600. So at 1600 you would be 4 times the sensitivity,
So why not set it at ISO 1600 and be done with this number? Well there is a price you pay for using a high ISO and that is quality of the image. In the film days this would be more grainy but in the digital world there is what is called Noise in the pixels. So for day time shooting ISO 100 is your best setting.
Shutter speed is the length of time the shutter is open. The shorter the shutter speed the more stop action you can capture and the slower the speed the more chance of blurring you will have, Again the hard double applies here as it does in ISO. For outdoor sunny days I use 250 or 1/250th of a second so if I need double the light I would use 125 or 1/125 of a second. If I needed more light then I would use 1/60th of a second and even more light the 1/30th of a second. The slower the speed you need a steady hand or a tripod!
The f Stop
The f number is the hardest to understand so let me confuse you to begin with. The f number is a ratio if the front lens element to the focal length of the lens. Are you intimidated yet! Well who cares about this ratio, lens designers! For use end users f8 is f8 no mater what lens we have and it will pass the same amount of light. Now the numbers are a bit confusing and it is just something for a good memory exercise. Here is the rule. f3.5 lets in twice the light as f5,6 and f 5.6 lets in twice the light as f8. The next steps are f11, f16 and f22. So again this is the rule of half and double but the number value doesn’t reflect this.
The way the lens woks is there is an ires in the lens that opens and closes just like you eye purple gets wider at night and smaller in the sun. You can see this looking at the front of the lens if you use the preview button to the right of the lens on the camera body,
So why be concerned with this number? Well other than controlling the amount of light the lens will pass it also controls the Depth of Focus the lens will see. The wider the lens opening, f3.5 the less depth of focus. So what! Well the closer you are to the subject the less the depth of focus is. So making photos of flowers you wouldn’t want to use f3.5 you would want to go to f11 or f16. Now this depth of focus works a little different that you may expect. If you focus on the center of a flower the depth will go 1/3 toward the camera and 2/3 away from he camera. So if you have a depth of focus of focus making close up of a flower of 3 inches then from the point you choose to focus on everything that is 1 inch from the focal point toward the camera and 2 inched away from the focal point will be in focus.
Well tis is enough to digest for one session. Next we can start applying these principals and why,