Making a Radar Zone Focus Device for Better Street Photography

I have always been impressed by the art of street photography. Where the photographer is right in the action of others’ daily lives, strangers in passing and capturing that decisive moment. Ever since I got my first rangefinder (not necessary for street photography) I have wanted to try my hand at capturing these fleeting moments. I have likened it to active people watching, instead of sitting and watching you are trying to place yourself at the perfect vantage point to capture the unique interactions of others on the street.

Giving it a go a few times I have found that I have trouble gauging the distance of things, which is an essential tool for street photography as you mainly use the camera in zone focus more. Setting the lens to give focus at a particular distance and then moving yourself to get the picture. Most photographers just practice and train their eyes to gauge accurately but what if I could design a tool to give me ranging information?

I decided to try and develop a device that can be attached to the top of a manual focus camera (on the hot shoe) to measure distance. This device would use an electronic rangefinder to acquire distance information and then used to gauge focus in conjunction with a pre-defined zone-focusing setting. To allow the photographer to capture quick action and moments that regularly happen in street photography.

Electronic range-finding is not a new or innovative technology…

Ranging technologies

The first is ultrasonic ranging which uses pulses of ultrasonic sound from a transducer to measure distance. This is done by measuring the time between transmission and reception of an echo off of an object. An example of this in photography is the polaroid autofocusing system shown below where the transducer looks like a small speaker.

This cheap ultrasonic distance sensor is common in many hobby projects as a way to do presence detection and obstacle avoidance. The HC-SR04 ultrasonic transducer is simple to use. it sends out a burst of ultrasonic pulses when the trigger pin is enabled, then will set the received pin high when the first pulse is detected back. you can then calculate the distance by measuring the time difference between setting the trigger and the received pin going high.

I used the Digilent Analog discovery and the waveforms software to analyze the pulse and return signals.

Unfortunately, the device isn’t sensitive or powerful enough to detect people more than a few inches from the output. I also tried measuring the pulse and raw echo to try and improve performance but that only extended it to about 1 foot.

Multi-object detection with ultrasonic sensor

Doing more research I found that the ultrasonic ranging device in the polaroid is much more powerful but at $60 for the raw transducer (no supporting electronics) it was a bit too pricy.

Instead of sending out an ultrasonic waveform what if we used radio frequency? The simple view of a radar works similarly to the ultrasonic transducer but with radio waves. but things are a bit more complicated. since radio waves move at the speed of light we need much more sensitive electronics to capture the time difference. but another more common way is to send out a continuous wave of increasing frequency and compare the difference to the return signal so the time difference is now coupled with the frequency which is a bit easier to measure at high frequencies.

Time-Frequency Difference RADAR

Previously these devices could cost hundreds of thousands of dollars but thanks to the ever-progressing technological advancements we have small Radars that cost anywhere from 5$ to 70$ that could be used on top of a camera for this purpose.

This first one is ~$20 operates on 24GHz and has a detection range of a maximum of 3.5 Meters or ~12ft which makes it usable for close-up street photography.

This one is ~$70 but has a much larger detection range of 20M or ~66 feet but could get confused if multiple objects are moving in the scene.

I went for the lower-range model. This is because I was fairly certain I could easily get the ranging data off the device, most of these modules have very little documentation. The ranging data is critical because most of these are only used for smart presence detection for lighting or opening doors. This means most just have a detected or not detected output and only a few have distance readout.

Zone RADAR

Now on to make this thing a reality! I decided to use an esp32 device with a nice color screen to interface to the uart of the radar and provide me with the distance information.

I then adapted a 3D-printed case for the dev board to include the radar module and battery system. The model is available on Printables at the link below if you want to make one.

Printables: radZone Radar Zone Focus Module

I also added a switch on the side to power it on and off. The switch is just connected in series to one of the battery leads so the device needs to be on to charge.

Software

With some help from an open-source Arduino uart parser for the Radar module protocol, I was able to get the distance readout to show up on the LCD!

Currently, it just displays the ranging output which you can use to decide whether to take a shot or not based on lens focus position. In the future, I may add more functionality but this is sufficient for me at the moment. You can get the code from the GitHub repo below if you decide to build one.

GitHub: radZone

Next steps

Now I just need to go test it! Haven’t been able to go out and do street photography for a wile so ill be sure to add an update on how that goes soon!

Future module updates are also in the works to use a LIDAR module instead of the Radar. Lidar modules work in a very similar way to ultrasonic rangefinders using an emitter and a detector to measure the time between the transmission of light pulses and their reception. I ordered a TF-Luna LIDAR device but took a few weeks to arrive. It is supposed to detect up to 8 meters, but I am skeptical if it can be used in bright lighting conditions. I am working on a benchtop setup to test this module soon.

Also, Look out for the new MiNT Rollie 35AF that is coming out soon, It has some kind of Lidar setup so Im curious to see its performance and what Lidar module they choose.

https://rollei35af.com/