Maximizing DOF with A-DEP
In one of thread discussion at the photo.net, I discovered one method to get a maximum depth-of-field (DOF) using one of the Canon EOS exposure modes, i.e. the A-DEP mode. This mode is placed in the creative zone modes, according to the Canon manual, though I think it is fit more in the automatic zone, but I bet not many people use it oftenly.
What is A-DEP?
A-DEP, or Automatic Depth of Field, is an exposure mode where your camera sets automatically a proper exposure for several objects simultaneously, whenever auto focus (AF) points are fired on these objects. In A-DEP, all AF points are active (you cannot select one particular AF point). When the camera has already given which AF points are fired, then the camera selects from these AF points, which is the nearest and the farthest objects from the camera. After that, a proper exposure is set to get DOF from these two points. Thus all of those objects are guaranteed to have reasonably sharp image.
Quite nice huh? Actually, it is dumb. Why? Because sometimes not all objects that we want to focus are aligned with the AF points. You must somewhat rotate your camera, but not zoom or changes your camera position. Otherwise your objects will not be in the DOF area. Sometimes also that your lens cannot get focus on all objects you want.
At other Canon SLR cameras, such as 1V, Elan 7/7E, EOS-3, 1D and 1Ds, there is the DEP mode, which is similar with A-DEP, but it is not automatic. You control yourself which is the nearest and the farthest objects by half-pressing the shutter button twice and then the camera sets the proper DOF. This is genius, because you just can perform the hyperfocal focusing easily without even using the DOF scale. (I do not understand why Canon engineers replace this beautiful feature with the dumb automatic DEP one?)
Nevertheless, we still can use the A-DEP mode to maximize DOF without the availability of DOF scale on your lens.
The A-DEP/Av mode
Here's the technique:
The first 3 steps are clear, we use the A-DEP mode to select objects and let the camera turns the focus ring automatically. You might miss some objects in between the nearest and the farthest ones, but that's fine. If you don't satisfy, then just try to recompose again and half press again the shutter button.
The fourth step is done to avoid the camera turning the focus ring again when we press the shutter button at the last step. So we lock the focal length.
In the fifth step we maximize the DOF, because A-DEP (and DEP) tend to set wide aperture. Since the camera has set the focus to a certain length and set the aperture to gain enough DOF for objects that are focused on, stopping down the aperture will expand the DOF more. If you want to maintain the same exposure as has been set in the A-DEP step, then you must switch mode to manual (M) and calculate the proper shutter speed after you stopping down the aperture.
Experiment 1: All objects are sharp with A-DEP/Av modes at f/22.
It is a picture of Vail mountains at the farthest position, pine trees and a creek in the middle and finally some flower at the lobby hotel as the nearest object. We want objects from flowers to the mountain are reasonably sharp, or say they are in the DOF area.
We used Canon EF 28-135mm IS USM camera, set at 28mm focal length, IS on, handheld. Exposure bias was set to -1.667. Figure 1 gives the result using the above method. Four insets at the right are 100% croping from different places: flowers, the black curtain along the creek, the pine tree and the mountain. It seems that the method works because all subjects are sharp, but not really useful because the DOF of f/22 is deep enough to contain all the subjects.
It's a pity that there is no information of distance-to-object from that lens so I cannot use it to calculate the DOF area. Nevertheless using Sean's online DOF calculator with f/22 @ 28mm and 1.6x multiplication factor, the minimum focus point to include the mountain at infinity in the DOF is about 1.8m, where the nearest object is at 0.889m. This is fine for the flower since it has a distance more than 1m from me.
If I pointed the mountain as my focus point with only using the Av priority mode, then according to the DOF calculator, the nearest point of DOF is 1.73m. So if the flower has a distance less than 1.73m from me, then the flower will not be sharp.
Thus if I set my aperture to f/22 at 28mm for 1.6x DSLR camera (like Canon 350D), then I have to focus on an object that is far more than 1.8m from me in order to include all objects from 1.73m to the infinity to be in the DOF. So we don't really need the A-DEP mode for this setting. The problem arises if I want to get smaller DOF or using longer focal length.
What is A-DEP?
A-DEP, or Automatic Depth of Field, is an exposure mode where your camera sets automatically a proper exposure for several objects simultaneously, whenever auto focus (AF) points are fired on these objects. In A-DEP, all AF points are active (you cannot select one particular AF point). When the camera has already given which AF points are fired, then the camera selects from these AF points, which is the nearest and the farthest objects from the camera. After that, a proper exposure is set to get DOF from these two points. Thus all of those objects are guaranteed to have reasonably sharp image.
Quite nice huh? Actually, it is dumb. Why? Because sometimes not all objects that we want to focus are aligned with the AF points. You must somewhat rotate your camera, but not zoom or changes your camera position. Otherwise your objects will not be in the DOF area. Sometimes also that your lens cannot get focus on all objects you want.
At other Canon SLR cameras, such as 1V, Elan 7/7E, EOS-3, 1D and 1Ds, there is the DEP mode, which is similar with A-DEP, but it is not automatic. You control yourself which is the nearest and the farthest objects by half-pressing the shutter button twice and then the camera sets the proper DOF. This is genius, because you just can perform the hyperfocal focusing easily without even using the DOF scale. (I do not understand why Canon engineers replace this beautiful feature with the dumb automatic DEP one?)
Nevertheless, we still can use the A-DEP mode to maximize DOF without the availability of DOF scale on your lens.
The A-DEP/Av mode
Here's the technique:
- Set exposure mode to A-DEP and turn on AF mode in your lens.
- Compose your picture to select certain objects, rotate your camera if necessary and press half the shutter button. Do not too worry if not all objects you want to focus are not aligned, as long as the the nearest and the farthest objects are at focus points.
- Release the shutter button. Do not take a picture.
- Switch into manual focus (MF).
- Set exposure mode to Av priority and stop down into, for example f/22.
- Recompose your picture while keeping the same distance of your camera to the objects and press the shutter button.
The first 3 steps are clear, we use the A-DEP mode to select objects and let the camera turns the focus ring automatically. You might miss some objects in between the nearest and the farthest ones, but that's fine. If you don't satisfy, then just try to recompose again and half press again the shutter button.
The fourth step is done to avoid the camera turning the focus ring again when we press the shutter button at the last step. So we lock the focal length.
In the fifth step we maximize the DOF, because A-DEP (and DEP) tend to set wide aperture. Since the camera has set the focus to a certain length and set the aperture to gain enough DOF for objects that are focused on, stopping down the aperture will expand the DOF more. If you want to maintain the same exposure as has been set in the A-DEP step, then you must switch mode to manual (M) and calculate the proper shutter speed after you stopping down the aperture.
Experiment 1: All objects are sharp with A-DEP/Av modes at f/22.
Figure 1: Vail view from hotel lobby (28mm, f/22, 1/50s, ISO 100)
We used Canon EF 28-135mm IS USM camera, set at 28mm focal length, IS on, handheld. Exposure bias was set to -1.667. Figure 1 gives the result using the above method. Four insets at the right are 100% croping from different places: flowers, the black curtain along the creek, the pine tree and the mountain. It seems that the method works because all subjects are sharp, but not really useful because the DOF of f/22 is deep enough to contain all the subjects.
It's a pity that there is no information of distance-to-object from that lens so I cannot use it to calculate the DOF area. Nevertheless using Sean's online DOF calculator with f/22 @ 28mm and 1.6x multiplication factor, the minimum focus point to include the mountain at infinity in the DOF is about 1.8m, where the nearest object is at 0.889m. This is fine for the flower since it has a distance more than 1m from me.
If I pointed the mountain as my focus point with only using the Av priority mode, then according to the DOF calculator, the nearest point of DOF is 1.73m. So if the flower has a distance less than 1.73m from me, then the flower will not be sharp.
Thus if I set my aperture to f/22 at 28mm for 1.6x DSLR camera (like Canon 350D), then I have to focus on an object that is far more than 1.8m from me in order to include all objects from 1.73m to the infinity to be in the DOF. So we don't really need the A-DEP mode for this setting. The problem arises if I want to get smaller DOF or using longer focal length.
posted by Pak Sui at Tuesday, August 16, 2005
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