Continuing our discussion of understanding digital exposure, lets move on to Step 2. Aperture.
If you ever hear someone talking about f-stops, shooting wide open, bokeh or depth of field, chances are they are referring to Aperture.
Aperture (AKA F-STOP) is the opening of your camera’s lens. This dictates THE AMOUNT of light (think How Much) allowed in to hit the camera’s sensor. Aperture also dictates Depth of Field.
Depth of field as defined by Wikipedia:
is the portion of a scene that appears acceptably sharp in the image
You will use the Aperture to dictate what areas of your image will be sharp and which areas will fall off out of focus. In the world of Digital SLR photography the visual difference is huge when you change your aperture from f/2.8 to f/8. In the Point n Shoot Digital photography world, the difference is not as extreme (in my experience).
Here is an example of different aperture sizes:
Image from Wikipedia
Aperture is a little tricky at first, because the SMALLER the f-stop number, the MORE light is allowed in. The LARGER the f-stop number the LESS light is allowed in. The SMALLER the f-stop number = SMALLER depth of field (less areas in focus). The LARGER the f-stop number = LARGER depth of field (more areas in focus).
Here are a few examples of how I use Aperture:
- Low Light situations such as when a Bride is getting prepared for her wedding, I do not like to use flash. To ensure a good exposure, I am using LARGE f-stops (f1.4 to f2.8) and a HIGHER ISO. I need to allow MORE light into the camera’s sensor.
ISO: 1250 Aperture: 2.8
- When creating headshots, I like to use (f2.8 to f3.5) because I can focus on the eyes to really make them stand out from the rest of the subject’s face. The effect is beautiful as the eyes are tack sharp and everything else falls off out of focus softly. This also naturally softens the skin so less post production is necessary to artificially smooth the skin.
ISO 400 Aperture: 2.8
- If I am creating a Scene Setting Shot, such as a reception hall that is being prepared by the venue’s staff, I like to Drag my Shutter (Explained in Step 3. Shutter Speed). I do this because I like to imply the hustle-bustle of the staff and it is not important to me that they are sharp. They are blurred from their movement and the slower shutter speed. My goal is just to document the environment. In order to do this, I have to use HIGHER f-stop numbers (ie, f/8 – f/11) which allow LESS light into the sensor. This enables me to use a slower shutter speed and lower ISO to acheive desired results. Each venue is different and a steady surface or tripod is necessary for these shots.
Aperture f8 Shutter 1 second
So the relationship between ISO, Aperture and Shutter speed is THE FORMULA for you to achieve desired exposures. There is a difference between Correct Exposure and Creatively Correct for your given situation. We will talk about this later. For now, here is an exercise for you to try.
1. Put your camera on APERTURE mode (Turn the dial to the BIG A or Av)
2. Set your camera’s ISO to the lowest setting allowed (probably 100 or 200)
3. Set your Aperture/f-stop to the lowest setting as well. **NOTE:: If you are using a Variable Aperture Lens, please keep in mind that you will need to stay at the lens’s widest focal length in order to use the Lowest f-stop setting. This is because when you zoom, the f-stop will automatically get smaller and there is nothing you can do about it. If you have a 18-70 f3.5-5.6 lens this means that to use f3.5 you need to use the 18mm focal length. If you zoom to 70mm the lowest f-stop you can use is 5.6.**
4. Place an object on a table, such as can of soup, a teddy bear, a book, etc… (Make sure the background is as neutral as possible, such as a wall. For instance do not shoot facing a sliding glass door that will heavily backlight your scene and throw off your exposure.)
5. Take a few shots of the object and view your exposure.
6. Adjust your aperture to f5.6 or f8.0 and increase your ISO to 400 or 800
7. Again take a few shots of the object and view your exposure.
8. Finally stop down your aperture to f11 or f16 and increase your ISO to 1600 or more
9. Repeat Step 7
10. Put the files on your computer and view the difference in the depth of field.
Because you are shooting in Aperture Priority Mode, the camera is making its best guess as to what shutter speed to use. Ideally you will have 1 correct exposure at each f-stop used. By comparison you should see the depth of field get greater and greater. You may notice digital noise and color degradation as the ISO increases as well. We will talk about all of this.
Questions for you:
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Please let me know in the comments.
I get a lot of questions about flash that I will definitely work on in the future. Firstly I want to help those of you that struggle with basic exposure. Flash is an entirely different beast that we add to the equation. When we add flash, we consider flash power, distance to subject, distance to background, balancing ambient light, killing ambient light, on camera, off camera, how many flashes, etc… etc…
Please keep the feedback coming. It has been extremely useful to us and the readers alike.