A Sekonic C-700R-U SpectroMaster Colour Meter costs around $1,500. A calibrated light meter app for iPhone costs $3. The hardware is more accurate. The phone is in your pocket. So which one should you actually use?
Short answer
| You are… | Use |
|---|---|
| A photographer, videographer or content creator | A phone app (e.g. KEV) |
| A gaffer, DP or 1st AC on a feature | Both - phone for fast checks, Sekonic for hero shots |
| A studio shooting catalog / e-commerce | A grey-card workflow + a meter |
| Shooting film | Sekonic for exposure, phone for colour temp |
| Anyone who’s never owned a meter | Start with a phone app |
What each one is good at
Phone-based light meter app
A modern phone camera + AI can read colour temperature surprisingly well. Apps like KEV - White Balance Meter AI use the iPhone’s rear camera, analyse the live preview, and output Kelvin + magenta/green tint in real time.
Strengths
- Always in your pocket.
- One-time purchase (KEV is $2.99) - no subscriptions.
- Reads tint, not just Kelvin.
- Updates live as you move - good for chasing magic hour.
- Calibrated against a Sekonic (in KEV’s case, the C-700R-U).
- Works on iPad and Apple Watch too.
Weaknesses
- Accuracy depends on your iPhone model.
- It uses the camera sensor, not a calibrated spectrometer.
- No incident-light dome - it’s a reflected reading by nature.
- Battery on your phone, not on a dedicated device.
Hardware meter (Sekonic, Kenko, etc.)
A pro spectrometer is a measurement instrument. It has a calibrated optical sensor under a diffusion dome, filters for spectrum sampling, and ships with a calibration certificate.
Strengths
- Lab-grade accuracy (typically ±50K).
- Reads CRI, TLCI, R9 and full SPD spectrum.
- Doesn’t drift across phone models or OS updates.
- Trusted on union sets.
Weaknesses
- $1,000 - $2,500.
- Another bag in the kit.
- Battery to charge, firmware to update.
- Overkill for 95% of working photographers.
When the app is enough
For most situations, an iPhone-based meter is genuinely enough:
- Wedding, event and lifestyle photography
- YouTube and short-form video
- Indie filmmaking and music videos
- Real-estate, interiors, product
- Content creation, podcast video, interviews
- Setting white balance for tonight’s vlog
In all of those, you need a number you can dial into the camera. KEV gives you one accurate to within a few hundred Kelvin, which is well inside the difference your eye can perceive in the final image.
When the Sekonic still wins
- Studio strobes and HMI flicker testing. Hardware meters can sample shutter-speed-relevant exposure timing; a camera-based app can’t.
- Spectral data (CRI, R9, TLCI). If you’re vetting lights for purchase or set, the SPD chart on a SpectroMaster is irreplaceable.
- Union features and broadcast. Some production environments require certified meters.
- Incident light exposure on film stock. Reciprocity and dome metering matter more.
How a phone meter actually works
Worth understanding, because it explains exactly where the accuracy comes from and where it goes.
A Sekonic SpectroMaster splits incoming light across a diffraction grating onto a photodiode array, sampling the spectrum in nanometre-wide slices. From that spectral power distribution it computes correlated colour temperature, tint, CRI and everything else directly. It is a measurement instrument with a calibration certificate.
A phone has no spectrometer. It has a camera sensor with a Bayer filter - three broad, overlapping colour channels rather than a fine spectral sweep. An app reads the raw channel ratios before the camera pipeline applies its own white balance, then maps those ratios to a colour temperature. That mapping is where calibration lives: KEV was built by metering known sources with a Sekonic C-700R-U and fitting the phone’s response to the hardware’s readings.
Three consequences follow:
- Accuracy is best where the calibration data was densest - normal continuous lighting, roughly 2500 - 8000K. Exotic sources at the edges drift further.
- It cannot distinguish spectra that produce the same channel ratios. Two lights that look identical to three broad sensors but differ sharply in spectrum will meter the same. A spectrometer sees the difference immediately.
- It depends on the phone. Sensor and colour filter response differ between iPhone generations, and the calibration has to account for each.
None of this makes a phone meter a toy. It makes it an instrument with a known operating envelope - which is true of the Sekonic as well, just a wider one.
The honest accuracy numbers
| Typical error | Where it degrades | |
|---|---|---|
| Sekonic C-700R-U | ~±50K | Very low light; extreme sources |
| Kenko KCM-3100 | ~±100K | Narrow-band LEDs |
| Calibrated phone app | ~100 - 300K | Below 10 lux; heavy flicker; deep saturated casts |
| Uncalibrated free app | 500 - 1500K+ | Almost everywhere |
For context, most people cannot see a 200K difference in a finished image without a side-by-side reference. Below about 100K it is invisible even then. That is the whole argument for phone meters: the error is real, and it usually sits under the perceptual threshold.
The gap that matters is not hardware versus phone. It is calibrated versus not. A free light meter app that was never metered against a reference can be a thousand Kelvin out and will still display three confident digits.
Testing any meter before you trust it
Ten minutes, no equipment beyond what you have:
- Meter a known source. A 5500K daylight LED panel or a camera flash. A calibrated meter should land within a couple hundred Kelvin of the marked value.
- Meter the same source three times, moving between reads. Spread should be small. A meter that wanders 500K on an unchanging light is not measuring, it is guessing.
- Check the ends. A tungsten bulb (~2900K) and open north sky (~7500K). Errors often grow at the extremes even when the middle looks fine.
- Confirm against a grey card. Meter, dial the value into the camera, shoot the card, check the histogram. Equal RGB means the meter and camera agree - which is what you actually care about.
- Repeat after a phone or OS update. Rare, but camera pipeline changes have shifted app readings before.
Step 4 is the important one. The number is not the goal; a neutral frame is. A meter that reads 200K “wrong” but produces neutral files on your camera is more useful than one that is laboratory-correct and does not.
What it costs per use
| Sekonic C-700R-U | KEV on a phone | |
|---|---|---|
| Purchase | ~$1,500 | $2.99 |
| Extra to carry | 250g, own case | Nothing |
| Battery | Charge before the shoot | Shares the phone’s |
| Cost over 200 shoot days | ~$7.50/day | ~$0.015/day |
| Days you have it on you | The ones you remembered | All of them |
That last row is the real argument. A meter left at home has infinite error.
How to think about the trade-off
If your work hangs on white balance being exactly right - say, a $30K product shoot or a feature film day - the Sekonic earns its keep. For everyone else, the iPhone in your pocket is the meter you’ll actually use, and the meter you’ll actually use beats the one sitting at home.
The hybrid workflow
The realistic answer is both.
- Carry KEV on your phone all day.
- Use it for every shot.
- Keep a hardware Sekonic in the bag for the moments where ±50K matters: hero shots, multi-cam sync, gel selection, product, VFX plates.
You’ll meter 100x more often, your colour will land 95% more consistent, and you’ll only crack the Sekonic open when you really need it.