Auto-Focus Techniques and Focussing Points

Last Updated:- 15th December 2021

Introduction

In this short article, we will study the image look at different focus points, focal lengths, and apertures. How does the focus point influence the look of the image and what happens if we change the aperture, focus point, or the size of the sensor?

For example, if you are looking for a new lens and the salesman mentions Depth Of Field or Hyperfocal Length, are you OK with these terms? In this short article, we will also look at how to select a new lens and why it's a good idea to select a lens for the right application.

Let's look at a definition for Depth-Of-Field (DOF) so that we are all on the same page. See the definition from Wikipedia below:

For most cameras, depth of field (DOF) is the distance 

between the nearest and the farthest objects that 

are acceptably sharp in an image.

Why would photographers want background blur or a nice bokeh? Why would they isolate subjects from the background? What is the so-called cinematic look? How does videography impact the use of depth of field or what do videographers want from the various focusing techniques?

Over the past 10 years, we saw how photographers progressively gave away more control to the camera. Automated functions became increasingly more dominant each year and in the process, photographers even started to question proven photography principles. 

Mirrorless cameras now apply built-in lens corrections and mobiles phones made it possible for anyone to now isolate subjects with a simple App. Do we still need to consider old-school photography techniques or should we only rely on the digital camera?

Canon 650D with 15-85mm - f9, 1/125, ISO200 (Used Kodak Porta film simulation with DXO FilmPack 5)

Let's ignore AI for a moment, mobile phones, and things like sensor size, dynamic range, and Auto Focus (Contrast/Phase Detect). Step back 50 years and let's look at photographers from that era. I bet, they will challenge us with questions like:-

• What criteria will you use when you select and purchase your next lens?
• Hyperfocal distance, Depth of Field, acceptable sharpness, circles of confusion
• Focus twice the distance to the nearest subject or focus one third into the scene
• Lens design, number of lens elements, lens groups, and how it impacts the lens?
• Testing your lenses, how to determine your lens sweet spot and diffraction point?
• Video recording introduced new rules with new lens variables, or are they new?
• Start using full Manual Exposure or continue using Auto Mode in A or S-Mode.
• Did you ask yourself, why does the Olympus wide-angle 12mm lens have f2?
• What about the backlighting or from the side, how does it impact your image
• How about the right lens, focal length, and using infinity FL on your lens

Those, not 100% familiar with Depth of Field (DOF), HyperFocal distance, or Circles of Confusion, or the infinity method, please have a quick look at the articles below:

Link 1:- A great article about sharpness, lens sweet spot, and the App we will discuss...
Link 2:- A great article about focusing techniques, all you need to know about focus...
Link 3:- A quick and simple technique to determine Hyper Focal Distance... great article
Link 4:- An article discussing how to use Hyperfocal distance for landscape photography
Link 5:- The App (OptimumCS-Pro) we will be discussing in this article... pls study
Link 6:- One final idea and a great practical example using the CS Pro App... a great read

Link 7:- Great presentation on infinity focusing techniques at Photography on Line...

Olympus EM1 with 60mm f2.8 Macro Lens - f7.1, 1/125, ISO100 - (Fuji Velvia film simulation with DXO FilmPack 5)

Here is a short summary of the key points in the above articles:

• Top lens sharpness (sweet spot) is typically +2 stops above the widest aperture
• Lens sharpness is determined by the aperture de-focus blur or its diffraction blur
• A long focal length offers a better illusion compression than a wide-angle lens
• Wide-angle lenses are more effective when used with large depth-of-field applications
• Large sensors need a smaller aperture for the equivalent DOF as smaller sensors

Different focusing features and techniques with MF and AF:

• Use full Auto Mode, AF (SAF or CAF) when doing street or landscape photography
• Selecting the focus point at double the distance from the closest "sharp" subject
• Thirds focussing rule, for max DOF, select a point one third into the scene and focus
• Using the camera preview button, available on most OMD cameras - see Link 3
• Manual Focus - various MF assist features like Pre-MF, magnify, and peaking
• Selecting the right lens and infinity focusing technique to ensure an infinite DOF

The OptimumCS-Pro and Simple DoF Applications

A quick look at the OptimumCS-Pro App. I trust you studied the information in the above articles and are familiar with the CS Pro App. Did you install the above Apps on your mobile phone?

When we photograph at a too wide aperture, we could lose image resolution because of defocusing (the blurring of objects in the front and back of the focus point); and vice versa with a too narrow aperture, we could lose sharpness due to diffraction. So we always need to think about the optimum aperture setting and focusing distance. The CS-Pro application can help us with that.

How does this app work? The app looks at sensor size, focal length, distance from the closest point to the furthest point that needs to be in focus. It then calculates the aperture and the focus point at double the distance from the closest focus point.

The Simple DoF App is free and easy to use. It calculates the hyper-focal distance for each lens, focal length, distance, and aperture. For those new to the App, it's important to practice first, before using it on critical projects. Having had the app on my phone for several months, I do use it from time to time but I found it's more valuable for learning purposes than my day-to-day photography.

Let's try the App and see what we learn about the Olympus 17mm f1,8 and the 12mm f2,0 fixed focal length lenses. Which apertures and what focus distance will work best to achieve an acceptable depth of field? I noticed that the lens data you will use with the App is those displayed on the lens and not the full-frame equivalent.

There are basically 4 ways to select a focus point:

1. Using full Auto (A-Mode) and focussing on the far distance using f5,6 to f8.0
2. Use mobile phone apps like simple DOF and CS Pro app, to calculate camera settings
3. Select the closest focus point in the scene and set the focus point at double that distance 
4. Select an appropriate aperture and then use the rule of 3rds to determine the focus point

Pen F w Lumix 14-140mm - ISO200, f5,6 1/200

1. Using full auto A-Mode, AF, and f5,6 to f8

To use this method it is important to have a good understanding of what lens to select for every scene or photo application. The ideal is you tried and tested all your lenses, you know the best aperture (sweet spot) for each lens and you experimented with selecting different apertures for street, landscape, product, and portrait applications. When you apply these rules you will find they become second nature. Let's have a quick look at the following lenses:

Olympus 17mm f1,8

For example, we know when doing street photography in crowded areas, the camera will select an AF point reasonably close to the camera. Let's see if that will support a preferred DOF from 1,5m to 30m. Using the Simple DOF and the OptimumCS-Pro applications, we see when selecting an aperture from f6,3 to f7,1 and a focal distance starting from 2,7m to 4m, the DOF will be from 2m to infinity.

Testing the 17mm f1,8 you will find its sharpest aperture is between f4 and f7,1. With a little practice, you will see that you can safely dial in an aperture of f7,1 for both street and landscape photography. For portraits, you will typically work with apertures ranging from f1,8 to f4.

Olympus 17mm f1,8 best DOF settings

Olympus 12mm f2,0

Peter Forsgård proposed using a wide-angle lens with apertures f5,6 to f8 for one month. Using the above applications, we see the advantage of WA lenses is the closer in-focus distance to the camera, and the wide-angle lens DOF characteristics tend to benefit a wider range of photography applications.

Studying the above examples we see the 17mm f1,8 hyper-focal distance was 1,4m. The CS-Pro App proposed a focal length of 4m. This is important when you need the foreground sharp. The general habit of focussing on a far distant point is therefore not ideal.

Repeating the above exercise with lenses like the Olympus 12mm, 17mm, and 25mm, you will find they are all great for street or landscape photography. The 45mm, 60mm, and 75mm lenses lean more towards portrait or product photography. Applying the same process you will see the 9-18mm f4 or the 12-50mm kit lenses are great for a wide range of applications.

To set up the camera so you can safely focus at any point, then lift your camera, reframe the image and take the shot, see this article. Look for - "Customizing the camera AEL and AFL"

2. Hyperfocal Distance Method

(Using only the Simple DOF App)

Looking at a hyperfocal distance I wanted to see the results when using the settings suggested by the App. I also selected to use a fixed MF distance. See the calculated results from the app below. See if you get the same results I did.

Olympus MFT lens 12mm f2,0

- Using f7,1 everything from 1.2m to infinity will be in focus
- Set the camera FL to 2m (MF or SAF)
- Hyperfocal distance is 1.4m

Olympus MFT lens 17mm f1,8

- Using f8 everything from 1.7m to infinity will be in focus
- Set the camera FL to 3m (MF or SAF)
- Hyperfocal distance is 2.4m

Sony A7 III lens 34mm f2,8

- Using f8 everything from 2.5m to infinity will be in focus
- Set the camera FL to 5m (MF or AFS)
- Hyperfocal distance is 4.9m

Pen F w Lumix 14-140mm - ISO200 f5,6 1/400

3. Using the OptimumCS-Pro App

With this example, I again selected MF to test what the results will look like when using the settings suggested by the CS-Pro app. See the results below. Again see if you get the same results I did.

Olympus MFT lens 12mm f2,0

- Using f7,1 everything from 1m to infinity will be in focus
- Set Focal Length to 2m (MF or SAF)

Olympus MFT lens 17mm f1,8

- Using f7,1 everything from 2m to infinity will be in focus
- Set Focal Length to 4m (MF or SAF)

Sony A7 III lens 34mm f2,8

- Using f13 everything from 2.5m to infinity will be in focus
- Set Focal Length to 5m (MF or SAF)

What's really interesting about using this method was finding the right focus point. I ended up using another app on my phone that only measures distance. When the app recommended a focus point of 2 meters, I used the other app to measure that distance so I could mark a focus point. I would then manually focus on that point.

Another interesting thing I noticed was the camera reacts differently in Manual Mode and Auto Exposure. The camera seems to "automatically" optimize the DOF when in Auto Mode. When in Manual Mode this was not the case. This happened with both the A7 III and the EM1 II.

4. Using the thirds and doubling subject distance, focussing methods

Considering what we learned, I see this final method as a practical example as it summarizes what experienced photographers have been doing for years.

Setting up a home studio

Let's look at how to set up a home studio for a new YouTube channel. We will look at the lens and the position of the camera in relation to the target. What focusing method will you use?

We often see camera reviewers walking around in a video to test and demonstrate the camera's CAF abilities. Do we really need this kind of CAF capability when setting up a simple home studio? Let's assume your home study consist of a desk, a chair, and something like a green backdrop. Considering what we discussed, we can now calculate the preferred aperture and focusing distances. This enables us to use older cameras like the Panasonic GH4, the EM10 II with MF, and save money.

You can select any focal length ranging from 12mm to 25mm. I used the 17mm f1,8 and the Olympus 25mm f1,8 for this example.

• Olympus 17mm f1,8 is camera A
• Olympus 25mm f1,8 is camera B and C (One could also a 45mm f1,8 for C)

I like to have a DOF from the center of the table to +2m behind the chair. See the CS Pro results below.

With a single camera set-up, you will select either camera A fitted with the 17mm lens or camera B with the 25mm lens. The key is the lens FOV (Field of View). If you prefer an upper body with a face view, the 25mm will work well, and if you prefer to have part of the desk included you will select the 17mm. This example is not an exact design, you still need to calculate the details. Camera C is meant for product views, that is if you like to have detailed product views in your presentation video.

Selecting an aperture and focus point for landscape images

One can use any application to calculate the best focal distance for a landscape. As said, it is key you practice with your camera. The practical experience provides a good feel for what works best for you. My personal lens preference for landscape photography is 9mm to 25mm. I test each of my lenses and then record the results in my aperture "sweet spot" notes book. My preferred landscape apertures range from f5,6 to f7,1, and in some cases, I will go up to f8.

The key is not to focus on the far distance but to select the "best" focus point. I prefer to use the rule of thirds to determine my focus point. I will then use SAF in Mode 2. This is similar to the back focus technique and enables me to select a focus point, move the camera, reframe, and take the image. See this article for more on this focussing technique.

The thirds method works on the principle that the distance from your camera to the focal point should be a thirds distance into the scene. That means everything twice that distance behind the focal point will be in focus. The closest in-focus point will depend on the lens aperture and the lens FL. This is the reason we prefer to focus on the first thirds when doing landscape photography. Again, you need to practice this method and experiment until you are comfortable with the results and your camera.

One more focusing method - Focus Stacking

In more serious cases photographers prefer to use focus stacking. Some Olympus cameras do this all automatically inside the camera. Others will take the stacked images and as explained in this video, create the final image in Photoshop. When watching the video, think of where you will select the different focal points for each stacked image. In other words, break each stacked scene up into thirds and select the focal point on the first third. (What camera did the presenter use, a crop sensor, or a FF camera?)

My final thoughts

Preparing for this article I spend a lot of time and I took many images while testing the two Apps. I was trying different focussing techniques like MF, SAF, and CAF. I was looking at what works best, a single AF point, 9 group AF points, and more. Each time I came back to the single AF point placed in the center of the camera screen. I also preferred MF or SAF depending on the situation.

I often used my EM1 II together with my A7 III. Interestingly, the A7 III images are more detailed than the EM1 II and one would expect that (24MP versus 20MP). That said, the closer I get to my camera/lens combination "perfect" settings, the smaller these differences are. Practicing with your camera and finding those sweet spots and best settings will result in significant image quality improvements.

The worst thing you can do is buy a lens, have no sweet spot or preferred aperture strategy, focus in the far distance, and expect great results...