Smartphone Syncing — The Best of Two Worlds

written for the stereosite by Pascal Martiné, Germany

Introduction

This arti­cle is a sort of sequel to my tuto­r­i­al Stereo pho­tog­ra­phy to go. There, I explain how I take, com­bine, edit, and share stereo pho­tos entire­ly on a smart­phone with­out hav­ing to move files to a PC. The whole process usu­al­ly takes me less than 3 min­utes per picture.

That way I got used to the ben­e­fits of the smart­phone and hard­ly took my twin cam­era set­up out of its bag any­more. But I had to pay a high price and always regret­ted that I was lim­it­ed to sequen­tial stereos.

Sequential versus Simultaneous

If you are used to the tech­ni­cal terms and the prac­ti­cal issues, you can direct­ly jump to the next chap­ter. For every­one else, I’ll briefly sum up the basics. If we talk about stereo pho­tos tak­en with a smart­phone we usu­al­ly mean sequen­tial stereo pho­tos, i. e. those tak­en with a sin­gle cam­era by tak­ing at least two pic­tures, one after anoth­er, while side-shift­ing the phone. ‘Sequen­tials’ tak­en with a phone have sev­er­al advantages: 

1. You can con­trol the amount of depth by freely choos­ing the base­line or by tak­ing a sequence of three or more pho­tos (See David Kuntz’ arti­cle Get­ting the Right Depth in 3D Pho­tog­ra­phy).
2. You don’t need any spe­cial equipment.
3. You can act spon­ta­neous­ly, because your phone is always with you.
4. Edit­ing can be done on the same device.
5. Data trans­fer and shar­ing is rel­a­tive­ly easy.

Since we most­ly dis­play stereo pho­tos in a small size, new­er smart­phone cam­eras are most­ly suf­fi­cient even for expe­ri­enced stereo photographers.

But, there is one huge draw­back to all sequen­tial stereo pho­tos: Every move­ment that hap­pens between the first and the sec­ond shot will cause so called rival­ries. In the best case, this will result in a mod­i­fied depth per­cep­tion. But more often it caus­es annoy­ing flickering.

Even a small gust of wind can dis­turb a beau­ti­ful nature view. Touris­tic sites are almost impos­si­ble or have to be cropped to the detri­ment of the com­po­si­tion. A sin­gle cam­era most­ly lim­its you to non mov­ing scener­ies, like emp­ty streets, archi­tec­ture or still lives.

The solu­tion for this prob­lem are simul­ta­ne­ous stereo pho­tos. That means that both pho­tos for the stereo pair are tak­en exact­ly at the same time. Every move­ment is per­fect­ly cap­tured, birds float in the air, water foun­tains look like they’re frozen. It increas­es your pos­si­bil­i­ties tremendously.

In most cas­es, simul­ta­ne­ous stereo pho­tog­ra­phy requires ded­i­cat­ed stereo equip­ment. Recent­ly, Masu­ji Suto devel­oped an App for the iPhone that lets you fire two lens­es of the back-cam­era at the same time. More infor­ma­tion can be found in the iPhone Close­ups gallery.

Preliminary considerations

If you want to move to simul­ta­ne­ous stereo pho­tog­ra­phy, you always have to sac­ri­fice one or more of the advan­tages of sequen­tial stereo pho­tog­ra­phy. Either you loose con­trol about the base­line, you have to trans­fer the pho­tos to anoth­er device, or you always have to plan to take a stereo rig with you. Espe­cial­ly if you have been using your smart­phone, you will have to car­ry some extra weight.

Per­son­al­ly, I always avoid­ed spe­cif­ic stereo cam­eras because of their fixed base­line. I also want­ed to keep my equip­ment light­weight. Addi­tion­al­ly, I real­ly enjoy keep­ing the whole process on one device that allows me to instant­ly free-view my new­ly tak­en stereo pho­tos. In 2021, when my ‘old­er’ iPhone 11 Pro became cheap­er as a sec­ond hand mod­el, I start­ed think­ing about build­ing an iPhone twin rig once again. This time, I took it seriously.

I sac­ri­ficed the advan­tage of not need­ing addi­tion­al equip­ment and invest­ed in a sec­ond smart­phone, which was still cheap­er than two dig­i­tal cam­eras of equiv­a­lent qual­i­ty. Also, I would now have to plan in advance to take the stereo rig with me. But it’s much eas­i­er to just grab one addi­tion­al phone and some alu­minum parts as opposed to a whole DSLR twin setup.

Except that, I would keep all the oth­er advan­tages of a smart­phone that I’ve men­tioned pre­vi­ous­ly, on top of the work­flow that I’m already used to.

Selecting the components

In gen­er­al, every mod­ern smart­phone should work in a twin set­up. But cam­era sen­sors and lens­es vary from smart­phone to smart­phone. That’s why I decid­ed to get two of exact­ly the same mod­el. Since i3Dstereoid offers auto align­ment and auto colour adjust­ment, you could prob­a­bly also start with two slight­ly dif­fer­ent models.

The cam­era trig­ger method seems to be even more impor­tant. You can of course sim­ply press the shut­ter but­tons simul­ta­ne­ous­ly, but that’s not real­ly what sync­ing means. i3DStereoid also offers pair­ing two phones, but for me the delay was far from acceptable.

Blue­tooth comes to mind first, but it is in fact not an option as it’s a 1:1 con­nec­tion. I was­n’t able to find any Blue­tooth shut­ter that could be con­nect­ed to two phones at once. There are a lot of options for dig­i­tal cam­era twin setups, but these don’t work with phones. Final­ly, there are some blue­tooth beam split­ters avail­able and remote shut­ters work­ing with high fre­quen­cy tones, but for me they did not seem reli­able enough to even test them.

I’ve then thought of build­ing a mechan­i­cal con­struc­tion to simul­ta­ne­ous­ly touch the screen to shut­ter or press a but­ton on the side of the phone. In the case of an iPhone, this would be one of the vol­ume but­tons. Sud­den­ly, it came to my mind to press the vol­ume con­trol of my cable head­phones. That worked. So, iPhones have a built-in wired remote shut­ter. That would be my start­ing point for the twin shutter!

The last known prob­lem was the mount­ing of the two phones. But since there are mounts for phones avail­able that have stan­dard threads, this would only be a mat­ter of research. I would do that after the shutter.

Building the shutter

It goes with­out say­ing that there is absolute­ly no wired shut­ter avail­able that con­nects to two smart­phones. So I knew that I had to build my own. I believe that it’s prob­a­bly eas­i­er to build such a twin shut­ter for smart­phones with a stan­dard audio jack. But since I have a new­er iPhone, I had to deal with Apple’s Light­ning connector. 

I tried to actu­al­ly split the light­ning cable. There­fore, I con­nect­ed two light­ing exten­sion cables and tried to use them with my head­phone. After twist­ing 24 indi­vid­ual tiny wires in the right order, I con­nect­ed phone and head­phones. It worked. I was delight­ed. Then I con­nect­ed the sec­ond phone. At first, it only trig­gered the sec­ond phone. After that, noth­ing hap­pened any­more. Unfor­tu­nate­ly the light­ning con­nec­tor is a ‘smart’ con­nec­tor that incor­po­rates dig­i­tal infor­ma­tion about the con­nect­ed acces­sories. Speak­ing of the tech­ni­cal process, I did not ful­ly under­stand how the head­phone com­mu­ni­cates with the smart­phone. But I’m sure this is why my attempt com­plete­ly failed.

After a few days of frus­tra­tion, I real­ized that I had to come back to my first idea of a mechan­i­cal solu­tion. So I removed the ear pods of two head­phones and dis­as­sem­bled the vol­ume con­trols. Inside, I found two met­al car­ri­er plates that I removed and sand­ed down. I glued them togeth­er back to back and reassem­bled both cir­cuit boards and their cov­ers. This time every­thing worked as it should. Regard­less, I knew that a mechan­i­cal solu­tion would always be a bit less precise. 

Rig prototype

The search for suit­able phone mounts and brack­et was as easy as expect­ed and soon I end­ed up with the first ver­sion of my iPhone rig. I knew right from the begin­ning that I want­ed to keep the mount­ing as small as pos­si­ble, so this would be no more than a pro­to­type. But it was ready for its first field test.

First results — new issues

Before I tell you why this was a big dis­ap­point­ment again, let me start with some­thing pos­i­tive. The han­dling was super easy and light­weight. I would quick­ly adjust the base­line and then take a stereo pair with just one click. From time to time I would touch cor­re­spond­ing areas of the pre­views to ensure that both phones would have the same bright­ness and focus con­di­tions. Trans­fer­ring the pho­tos between the devices via Apple’s Air­Drop worked quick­ly and the stereo pairs were already almost per­fect­ly aligned. Still, i3DStereoid found a lit­tle devi­a­tion, but there was almost no crop­ping of the image.

But when it came to move­ment, it seemed that being per­fect­ly synced was rather a coin­ci­dence. I com­fort­ed myself that it still would be eas­i­er than tak­ing sequen­tial stere­os. I would just have to take 10 shots of the same sub­ject to have one per­fect­ly synced pair. But I already knew that this would not serve as a final solu­tion. For some rea­son the two phones seemed to indi­vid­u­al­ly ‘decide’ when to release the shutter.

I also real­ized that the way the phone mounts were attached to the brack­et was not opti­mal. When chang­ing the base­line you would always have to pay atten­tion that both cam­eras were not point­ing in slight­ly dif­fer­ent direc­tions. But for now, this was a minor issue.

App research

Next, I tried to find out the rea­son for the bad sync­ing of the cam­eras. Aside from try­ing to get the right feel for how to press my twin shut­ter, I did some research into dif­fer­ent cam­era apps. I nev­er had dif­fer­ences of bright­ness or con­trast between both images, so I sup­posed that it was all about the shut­ter speed. I found quite a lot of apps that let you con­trol the shut­ter speed in detail but were less easy to use. Espe­cial­ly when you have to do it twice in the same way. What’s more, most of them sim­ply did not sup­port trig­ger­ing the shut­ter with vol­ume con­trols. There was just one app, that pro­vid­ed both detailed con­trol and vol­ume key func­tion­al­i­ty. It worked, but it real­ly was a hassle.

I do not remem­ber how I found the solu­tion. Prob­a­bly it was just by ran­dom­ly try­ing every set­ting. If you tap on the pre­view win­dow, the cam­era app will adjust bright­ness and con­trast accord­ing to the select­ed area. If you tap and hold for a sec­ond, a yel­low label will appear say­ing “AE/AF lock”. AE stands for Auto Expo­sure and AF means Auto Focus. But with this, the shut­ter speed is locked, too. As a result, the sync­ing reli­a­bil­i­ty increased dra­mat­i­cal­ly. Still, from time to time, one phone seems to need a lit­tle time for what­ev­er rea­son. In the end, it’s an addi­tion­al step on both phones for almost every pho­to. But on aver­age I had a delay of less than 3 mil­lisec­onds. That could def­i­nite­ly com­pete with my tra­di­tion­al twin rig on which I used two Canon cam­eras in auto mode and a wire­less shut­ter system.

From now on, the fun began.

Final rig

As I got more and more used to my rig, my focus on the process of tak­ing a stereo pho­to changed. Pre­vi­ous­ly, I had always tak­en a sequence of 3 to 5 pho­tos. That way, I could lat­er choose the best pair based on min­i­mal rival­ries and best base­line. Now, I would still take mul­ti­ple pairs for best sync, but the base­line was always the same. That means I had to start pay­ing more atten­tion to the base­line in advance.

This result­ed in con­stant­ly hav­ing to change the base­line. I soon got tired of unscrew­ing, mov­ing, adjust­ing the direc­tion of view and tight­en­ing the screws again. It took me quite a while to think of a prop­er improve­ment for my pur­pose. Once again, it was all about research­ing the right pieces and com­bin­ing them in a spe­cif­ic way:

For all of you who want to build your own smart­phone twin rig, here is the list includ­ing links to the prod­ucts I pur­chased (these are not affil­i­ate links!):

• 2x met­al bicy­cle phone mounts
• 1x 40cm alu­minum T‑rail
• 2x 10cm alu­minum T‑slot miter bars
• trans­par­ent adhe­sive film
• 1x binoc­u­lars tri­pod mount

I unscrewed the phone clamps from the bicy­cle mount and screwed them on to the miter bars. After I had checked for a per­fect 90° angle, I screwed them real­ly tight so that they would always keep their posi­tion. To avoid the miter bars from slid­ing through the T‑rail with­out any con­trol, I applied sev­er­al lay­ers of adhe­sive film on all sides that have direct con­tact to the rail. Now, the bars fit tight­ly into the rail. They can be eas­i­ly moved but don’t move on their own.

All of this was rather hand­i­craft except one step. To con­nect the binoc­u­lar tri­pod mount with the rail, I had to cut a thread into the cen­ter hole of the T‑rail. For this I used a spe­cial thread cut­ter. Now I am able to con­nect my rig to any stan­dard tri­pod. Anoth­er improve­ment was that the bicy­cle phone mounts are wider and there­fore don’t allow any acci­den­tal rota­tion of the phone. T‑rails are avail­able in any length or can eas­i­ly be extend­ed for an even larg­er baseline.

Results

The new con­struc­tion allows me to change the base­line in sec­onds with­out hav­ing to wor­ry about the align­ment of the cam­eras. I also got more and more used to the workflow:

1. Choose base­line
2. Lock focus on both phones
3. Fire a few times to catch the per­fect moment with per­fect sync

Espe­cial­ly when I moved on to real­ly ani­mat­ed pho­tos, con­tin­u­ous motion like water or quick and spon­ta­neous shots, I real­ized how reli­able and effi­cient the iPhone rig had become. Here are some of the pho­tos tak­en with the final system:

For more exam­ples, see my gal­leries Stone and Water — A 3D Jour­ney through Bruges and Short Stop in Barcelona.

Caveats

I have not yet fig­ured out the best way to trig­ger the twin shut­ter. There are series where 30% of the pho­tos are slight­ly (~3 mil­lisec­onds) out of sync and oth­ers where 95% are per­fect­ly synced. This has noth­ing to do with back­ground process­es, because I use both phones in air­plane mode. That’s why I think that the fin­ger posi­tion mat­ters a lot and I’m try­ing to improve that.

Depend­ing of your oper­at­ing sys­tem, the cam­era app might have extra fea­tures like QR Code scan­ning. This could cause addi­tion­al delay between the two cam­eras, so bet­ter turn off any extras.

Also, I have not tried videos yet. That should actu­al­ly be eas­i­er because you can sync them lat­er, but I doubt this could be done entire­ly on your phone. If any­one has done stereo videos with two smart­phones, please know that any tips are very welcome.

By the way, keep in mind that you are not allowed to take the T‑rail in your car­ry-on bag­gage on flights because it might be clas­si­fied as a dan­ger­ous good.

Conclusion

Even though this chal­leng­ing project took me a year from first idea to rou­tine, I’m real­ly hap­py about the results. I do not have to keep ask­ing myself if a sit­u­a­tion is calm enough to take a stereo or not. I do not have to plan for pho­to oppor­tu­ni­ties, charge cam­eras, and take an addi­tion­al pho­to bag with me, I just grab the sec­ond phone and the rig. It’s light­weight, it’s sim­ple and it’s convenient.

When I start­ed exper­i­ment­ing, I had a lot of ques­tions how and doubts if it would work. If you ask me today, there is only one sin­gle ques­tion left: Do you have a sec­ond smart­phone or are you will­ing to pay for it? That’s all that matters.

Nota bene: I only tried all of this on iPhones. The­o­ret­i­cal­ly, this should be pos­si­ble with oth­er phones in a very sim­i­lar way. If any­one would like to start exper­i­ment­ing with Android phones, let’s have a chat!