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Understanding f-stop

understanding f-stop in photography

what is a "STOP' means in photography

Stop means in photography is the measurement of exposure, and it is relating to shutter speed, ISO or aperture settings. In other words the "f-number" generally known as focal ratio, f/ratio or f/stop.
For example; if you increase the exposure by one stop, you would be doubling the exposure. As a result, lets take a look how it would effect your exposure.

At normal settings:
Shutter speed is: 1/100
Aperture Setting: f/8
ISO setting : 100

At increased by one stop
Shutter speed is: 1/100
Aperture Setting: f/8
ISO setting : 200

Doubling the ISO makes you sensor twice as sensitive, therefore you jumped 1 stop and your image will be brighter.

At decreased by one stop
Shutter speed is: 1/200
Aperture Setting: f/8
ISO setting : 100

Doubling the shutter speed, the shutter moves twice as fast so your image will be darker.

1. What is ISO Stop?

As for ISO stop, provides the increase and decrease of light signal intensity, hence how much light is required in addition to produce an image at optimal exposure. This might most simple to understand. ISO 100 to ISO 200 is 1 stop. ISO 200 to ISO 400 is also one stop. If you change ISO 100 to ISO 400 is 2 stops. The actual intervals may not be exact, because different cameras have different increments set by manufacturers.
As an example: a Nikon D810 ISO settings are much more intricate and are smaller in increments (ISO 100 > 125 > 160 > 200 and so on).

2. What is Exposure Stops (Shutter Speed Stop)?

Most of the time on a well lit day you are shooting at a fraction of a second. If you shoot let's say at 1/200 of second and the clouds role in you might change your shutter speed to 1/100 second to recover some of the light. You halved your shutter speed 1 stop.

3. F-stop (Aperture) In Relation To Shutter Speed

When you change your f/stop number, or commonly known as "Aperture" number, the changes reflects in the size of the iris opening. Iris is a circular set of blades opens /closes and controls the amount of light travelling into the camera through the lens to the sensor. Just as your pupil of your eye, it gets smaller and bigger depending the amount of light it senses.
The size of the iris is in partnership with the number setting for the f/stop. Any time you change the f/stop number your stopping it up or down or should say double or halve the amount of light entering the lens, therefore creating a lighter or darker image.

As a practice;
Set your camera to A (Aperture Priority) and take picture with the same f/number at a light and dark object and see how your shutter speed changes to compensate for the amount of available light.
Next do the same thing by setting your camera to S (Shutter Priority) and take a couple of shots.

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As a practice;
Set your camera to A (Aperture Priority) and take picture with the same f/number at a light and dark object and see how your shutter speed changes to compensate for the amount of available light.
Next do the same thing by setting your camera to S (Shutter Priority) and take a couple of shots.

In the images below, note that the first image took longer time to capture, because there was less light available than the second photo. The light conditions were different.

F/8, 1/20 second, @ 300mm

F/8, 1/125 seconds @ 300mm

You probably noticed the change in the shutter speed number when you change the f/number, example - from f/9 to f/20 since the iris opening is significantly smaller at f/20, the amount of light is naturally less as the speed of the shutter will have to compensate by slowing it down to allow enough light to enter to through the lens for the photograph to be balanced in light.

This brings us to another element of the F/stop, "Depth Of Field". Will be covering this in another article, but first lets talk about what happens with the ISO settings when the F/stop is manipulated

f/20 1/13 second @ 200mm

f/9 1/160 second @ 200mm

4. F/stop (Aperture) In Relation To ISO Settings

This brings us to another element of the F/stop, "Depth Of Field". Will be covering this in another article, but first lets talk about what happens with the ISO settings when the F/stop is manipulated.

The other option is select a higher ISO setting instead slowing your shutter speed to allow the sensitivity of the sensor to reflect more light into the lens. This also means that in the darker part of the image (where the shadows are) will notice significant noise or in other words the image will be grainier. This might not be the solution you are looking for so use it sparingly. Of course you can use higher ISO creatively and set a certain tone for your image. Might want to check out "Understanding ISO In Photography" blog.

In the example below the ISO setting is at 1250 for both images, but by changing the f/stop from f/2.8 to f/22 also allowed me to slow the shutter speed down to create the light streaks as the cars drove by.
So as you can see the 3 main camera functions worked in unison to create the effect I was after. By changing one of the settings will effect the other two.

f/2.8 ISO-1250 Exposure time: 1/80 sec

f/22 ISO-1250 Exposure time: 2.5 sec

In the example below the ISO setting has been adjusted to light conditions. The first image was taken in ideal light condition, as you can see the sunlight is brushing against the subject.
The image on the right was take late in the afternoon in a valley in an overcast day. With this in mind, the ISO settings were changed to satisfy the darker gloomy circumstances.

f/7.1 ISO-100 1/100 sec

f/6.3 ISO-640 1/125 sec

conclusion

Practice, Practice and Practice

What is F/Stop
What is Exposure Stops (Shutter Speed Stop)?
F/stop (Aperture) In Relation To Shutter Speed
F/stop (Aperture) In Relation To ISO Settings
Set time delay between 1-2 seconds after pressing the shutter button to avoid camera shake

To really understand the co-operation between F/stop, ISO and shutter speed, take the time to practice. Change one setting in small increments at a time and take a shot at each step. Examine your "Histogram" how effects the tones in the bright and dark area of your image.

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