Depth of field is a measurement of the maximum object depth that can be maintained entirely in focus. Learn how depth of field and resolution are related, and how factors such as f/# affect an imaging system's performance. Join Gregory Hollows, Director of Machine Vision Solutions, in this in-depth discussion on depth of field, where he provides imaging tips that are applicable to any optical design. You can also learn more in our Gauging Depth of Field in Your Imaging System application note.
Hi, I am Greg Hollows and welcome to the Imaging Lab. We are going to look at detail about Depth of Field during this section. Depth of Field is the ability to maintain resolution above and below the Best Focus of the system that the optics is being utilized in. We looked at some examples of this in the basic portion of the section from an earlier tape, but we are going to really get into more detail about why things occur and what we can expect out of the different lens systems under different resolution requirements. If you remember back to our original session, we were looking at the example of these two golf balls being placed at different distances away and the ability to get good focus on them at different distances where Best Focus is the ball next to my head and the area of the Depth of Field concerns the one in my right hand at a distance. If you remember back to that, the cut away example that we had showed that we were actually getting different Depths of Field depending at the f/# of the system. Very popular understanding about Depth of Field is I adjust my Iris setting, closing my Aperture down making it smaller in the lens. Thus, increasing the f/#, I can get better Depth of Field, basically better resolution at a distance further away from Best Focus. And that's a reasonable rule of thumb to use and the Laws of Physics do prove out that that is what occurs. But there are some things to keep in mind when we actually start to understand Depth of Field. As you can see in the slide, as we adjust the Iris setting up and down in this particular example, something else occurs here as we are actually adjusting the Iris, changing the f/#. We actually get more diffraction in the system. What does that mean? That means the smallest possible spot or piece of information I can resolve actually gets larger at Best Focus. My ability to actually maintain good resolution in the system at Best Focus goes down as I adjust my Iris setting. In many imaging systems this is not totally understood or even realized because the resolution in the system is low enough that these effectually aren't seen as being a problem. With the advent of higher resolution sensors, we are seeing some difficulties in this area. When we start adjusting the Iris, and thus increasing the f/#, these spot sizes get large enough that they exceed the ability to actually image onto the small pixels associated with these high-resolution systems. This will actually lead us to not be able to get resolution at any level if adjusted too far. This can be seen in low resolution systems as we push lenses to f/20 or f/30, let's say. But in high resolution systems, this comes into being a problem when we are running around f/8 or higher. It's important to understand that getting good Depth of Field is a combination of the actual design of the system and the f/# setting. As we can look in the next slide here, we can get an understanding of why this occurs and how we actually get better depth. When you look at the dash lines in the slide, you will notice that on the top portion of it, they are coming from the outer edges of the lens system. They are creating a very oblique angle going into our Best Focuses which is at the blue line. You will notice that as they come down and hit these sections, if you were trying to get focus at a certain spot size or a certain detail that is very very tiny, they are going to miss that because they are going to be much larger as they go through the focus portion in the system and not able to get good resolution there. So, we look at the bottom picture in the slide, we are going to notice that the Aperture is closed a bit as denoted by the dotted lines again showing a much smaller portion of the Aperture that is actually being used. These angles come into Best Focus at lower ray angles, and we are getting longer distances in the cones that we're actually able to see the resolution at. This is giving us better Depth of Field. What you think about though with these iris settings getting low into diffraction limited issues that come into play and the bigger spots that get caused by diffraction, eventually we are not going to get resolution at all, like I said earlier. Let's look at some real-world examples. We are going to do this on something called a Depth of Field target. You can see one of these in the slide that is up now. And you will actually notice that it’s a wedged piece of material, with lines running down it at a forty-five degree angle from the lens system. This allows us to get an area of Best Focus and see what the resolution is doing both above and below it. What we are going to do is we are going to look at a couple different lenses that have been designed here and serve different applications. We're going to see how they actually perform at the different focus settings. In the first one, we are going to see that at the Best Focus position, as we go through the iris settings, the resolution does slowly degrade over time. And that is to be expected as we talked about with diffraction. What we will also notice though as we start going through the Iris settings at a different Depth of Field position, something that is closer to the lens system, that the actual resolution does appear to get better to a point and then it goes back down again because the overall diffraction in the system is being pushed and we are not able to get resolution anymore. Let's look at a different example of a different lens that has some different capabilities to it and was designed differently for applications actually for close up work, typical of machine visioning and industrial imaging. We'll notice here as we go through the different Iris settings on this particular lens, we are maintaining good focus across the range. Resolution is starting to drop down at this Best Focus position a little bit. But when we examine the actual Depth of Field points, what we see is actually better imaging capability as we continue to step up through those focus positions. The Depth of Field is going up and we haven't pushed all the way to diffraction but we are seeing is much clearer lines. Now these two examples were both looking at the same Field of View. Some things that come into play here is the focal length of this particular lens, the actual f/# setting and the design capabilities of the lens. One was pushed way outside of its design to begin with and wasn't able to hold up even as we went through the Iris settings. The other one held up much better due to its design. Let's look at one last example. We are going to talk about high resolution imaging for a second. We are going to do this a little bit differently. We're going to look at different resolutions. We are going to look at a resolution very close to the pixel limited resolution of the system, and we are going to look at something that's about three times coarser resolution. What we are going to see here is when we look at Best Focus in these systems, we are going to compare them at two different f/#s. It's going to be the same lens but at two different resolutions. As we look at the finest resolution in the system itself, at Best Focus, we are already up against the limits of what can be done at the particular f/# setting and the resolution is fairly low, if you look at the blown-up section. As we go to a different Depth of Field point, you can see there is no information at all to be gained there. As we go to the coarser resolution setting, we can see that there is good information that can be seen at Best Focus and some amount of information at that same type Depth of Field point as the finer resolution area. As we start to shut down that Aperture setting, we are going to notice that all the resolution goes away because we have passed the diffraction limit and we're no longer able to get information at Best Focus or at any Depth of Field position. At the coarse position, the information is held fairly well at Best Focus. It has dropped a little bit in terms of contrast and resolution but not much is discernible. And we are getting better Depth of Field as we look into the higher-level focus position. What's the important thing about this? It's really good to understand that as you move to higher and higher resolution images, your ability to maintain good Depth of Field or even to get any sort of Depth of Field can be severely constrained by the Laws of Physics limits that the lens has wrapped around it. There's not much that's able to be done to improve this because there is no more room to actually improve the design of the manufacturing tolerances to get you any better. It's important to do this and factor this into your designs upfront, so that when you're going out and putting in high resolution images or very very expensive camera systems, that you have accommodated for this and that you might not be able to actually achieve what you are looking for because it could defy the Laws of Physics. That's Depth of Field and that brings us to the end of our first full module. You can feel free to click on any of the links that you see on the screen to take you back to some of the other topics that we've discussed or on to Module 2 or beyond.
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