If I receive the correct question, is it about whether in the infinite approach I should use wider openings like f / 1.4 or narrower openings like f / 11 to get the sharpest results? The answer is, it depends … First let's see what depth of field is really in the infinite focus.
Depth of field depends on three things:
- Focusing distance,
- focal length,
- opening (the number f) and
- sensor or film frame size.
The focus distance is the distance between your camera (specifically the sensor or the film) and the subject you are focusing on. That is usually expressed in feet or meters. The focal length is the distance between the sensor / film and the point of convergence of the light on the lens. It is usually expressed in millimeters (50mm main lens, 28-135mm zoom lens …) and determines the field of view. The number f is the ratio of the focal length to the apparent diameter of the aperture diaphragm as seen through the front element of the lens. You usually see this expressed as the inverse of that with "f /" in front. A 50 mm lens with an apparent iris diameter of 10 mm would have an f number of 5 (50 mm / 10 mm), usually denoted as f / 5. The same 50 mm lens with its iris open at 25 mm as go from the front would have a number f 2, denoted as f / 2.
Depth of field decreases with wider openings. For a given focal length and focal length, you get a lower depth of field in f / 2 than in f / 8. The depth of field decreases with a greater focal length. For a given focal length and number f, a focal length of 50 mm would give a greater depth of field than a focal length of 100 mm. The depth of field increases with the focal length. For a given f-number and focal length, focusing on a subject 20 meters away gives you a greater depth of field than a subject 10 meters away. Details on why this can be found on this site and in many other resources. All this is due to how the lenses and projections work, combined with physical limitations of materials such as sensors and films.
The sharpest focus will be anything in the field of view at the focus distance. Then, if you focus your lens at 3 meters away, things at 3 meters will be sharper regardless of focal length or aperture. In front of that imaginary spatial plane (between the camera and the focus distance) there is an acceptably sharp focus area, and also behind (between the focus distance and the infinity). The sum of these two distances is the depth of field. It is important to know that the "sharp" area between the camera and the subject is less deep than the one behind the subject. Let's see some examples. I will use an online DOF simulator. In case that link is disconnected, you can easily find many more or download an application for desktop computers or mobile devices.
Imagine we are shooting with a 50mm lens. The subject is 3 meters away and we focus correctly. The opening is set in f / 3.2. The camera has a full-frame sensor (or 35mm film). The total depth of the field is 66.5 cm. 29.6 cm of that range are in front of the subject, 36.9 cm behind the subject.
Move the subject to 5 meters and focus correctly, and you will get a depth of field of 190 cm, 77.6 cm in front and 113 cm behind.
Keep focusing further and the depth of field will increase. The depth of field behind the subject will also increase at a faster rate than in the front. With a 50 mm lens and f / 3.2 focused at 20 meters, the depth of field becomes 65.59 m with 8.51 m in front of the subject but 57.08 m behind the subject.
At some point, the part of the depth of field behind the subject becomes essentially infinite, even before focusing on infinity. This is known as the hyperfocal distance. For a focal length of 50 mm in f / 3.2, the hyperfocal distance is 26.94 m. This means that if you focus your lens at that distance or more, you have the guarantee that "acceptable sharpness" extends to infinity behind the subject. You still get a great depth of field in front of the subject too, but not all the way to the camera.
Hyperfocal distance is very useful for certain types of photography. For example, in landscapes, if you know the hyperfocal distance for the chosen focal length and aperture, then, if the subject you focus is at least at that distance, you know for sure that you are also getting everything behind (such as mountains, distance clouds, stars …) in focus.
When you focus on infinity, the depth of vision extends from hyperfocal distance to infinity. So that is a second useful aspect. Focusing on the infinite and knowing the hyperfocal distance tells you how close something can be to you before it starts to blur.
That brings us to your question. If you are filming things that are far enough away that you need to focus at infinity or at least very close to that setting, you should not worry about a wide aperture that makes you lose focus. Even with a 200mm lens at f / 1.8 (if there is such a thing) the hyperfocal distance on a full-frame sensor would be about 775 meters. Focus for infinity and everything beyond 775 meters will be clear. Concentrate to 775 meters and you will get a depth of field from 387.3 meters to infinity.
Assuming things are as far away as you think and manage to focus at least at the hyperfocal distance, you will not get images that lack sharpness due to a "very shallow" depth of field. So what could make you get images that are not as sharp as they should be?
First there is, of course, equipment. Quality lenses and a camera with a good sensor (or a high quality film if you become analog) will tend to get better results.
Assuming it is at acceptable levels, you should be able to keep the camera stable. Manual shots will rarely be sharp at longer shutter times, especially as the focal length becomes longer. With a 50mm lens and no other stabilization technology, any shot with a shutter speed slower than 1/50 of a second is a bet. If the camera is stable (on a solid tripod, not on something heavier than a breeze), you can increase the sharpness by using a remote control to shoot the shutter and using the lens lock function if the camera has it.
Narrower openings lead to longer shutter times because you get less light on the sensor / film. That is detrimental to manual shots. This could be the reason you found better results in f / 1.4 than in f / 8. But even if your camera is mounted on a tripod, longer shutter times can reduce the sharpness through motion blur. This could be due to the vibrations of the mirror and / or the shutter (more pronounced at specific speeds), a slight movement due to the wind or simply the movement of the landscape. When shooting stars or the moon are taken, it is easy to underestimate the speed at which the night sky changes. An exposure of even a few seconds would be enough for the stars to begin to scratch and remove the moon's details. Therefore, a narrow aperture may not leave a shutter time short enough to obtain a clear shot. When you do, you may have to raise both the ISO in a digital camera that introduces a lot of noise.
But always going for the widest opening is not necessarily the best option. Again, due to the physics behind the lens design, the sharpest area of the projection will be in the center if the image with quality decreases towards the edges and corners. That's where distortion, chromatic aberration and blur begin to play a more important role. The lenses tend to have an optimal point in their aperture range, where this is minimized. Typical numbers are f / 5.6 and f / 8. There is a point of diminishing returns by reducing the opening. In values such as f / 16 and higher, you may begin to lose sharpness again due to diffraction. You can usually find information on which apertures a lens works best, make a conjecture, or experiment to find out.
Then, in the end, you should consider things in this order if you want to shoot things at a long distance.
- Which is the topic? Are they only things in the distance or are there elements in the foreground that you want to be clear? Compose to find the right focal length.
- Take the closest thing that needs to be sharp and calculate the distance (or use assistance such as autofocus and a reading of the resulting focus distance). Find the widest aperture you can use to focus on that thing and still make the DOF extend to infinity behind it, or focus behind it and still have it in the DOF before the focal plane. If it is beyond the hyperfocal distance, it can focus to infinity.
- Now discover the appropriate shutter time at that aperture and for the given lighting conditions. If it is fast enough to reduce the aperture a little more and sacrifice some shutter speed, do so if a narrower aperture brings you closer to the ideal value for sharpness.
For a digital camera, you should also keep in mind that you want to keep the ISO as low as possible to reduce image noise.