Light meters with reflective, incident and spot metering

138S · Jan 14, 2021

Alan Edward Klein said:
Isn't there an issue with incident readings since a cell phone has no "globe"?

IMO it is not an issue because the front camera has a lens+pixels substituting the globe+sensor, and this substitution is fair.

Another thing is if the smartphone reading comes from the light sensor for the scren auto-brightness, in that case reading is very directional, and reading may change a lot when tilting the phone. Some Lux meter Apps use that sensor... I would not use much that kind of readfing. But we speak about incident reading from the front camera (screen side camera for selfies...), to me this one is fair.

The globe and the "front camera" readings are not exactly the same because the globe "integrates" the light rays received in all directions hitting the dome, while the front camera has a more limited angle of view.

Probably the light illuminating the subject from the side or at similar angle, (integrated by the globe but not by the front camera) is not of interest for the metering exposure anyway, because it will have a very small impact in the light power reaching the subject and reflected to the lens.

Anyway the globe is not a perfect light integration way, or at least it is not better or worse than integrating (say) a 60º angle on the incident light. Integrating the lateral illumination may make sense or not, but the principal information is how much light comes from the directions that will bounce to where the camera is.

Please check instructions for the R-27 Kodak Grey card, you will see that it is suggested that the card has to be oriented with a particular tilt-swing... this is a refinement compared to how the globe meters.

At all I'm saying that the globe is not a good way... What I'm saying is that the interaction between light direction, direction of the subject's faces, camera position, and the exposure we want... this is a complex matter, and while the globe is a good way to simplify the reading it is not perfect and it may not be the best. Instead measuring the light comming in a (say) 60º angle (centered in the camera-subject axis) may not be better or worse that the globe integration.

In practice... we have to check if the App+Phone we have works well for us...

Alan Edward Klein · Jan 14, 2021

Bob S said:
Many incident meters used both a flat plate and a dome. The dome for metering 3 dimensionally, such as scenes. Flat plate when taking readings of 2 dimensional objects, like copy work.

So a phone app does not work the same way as a standard incident meter would where you normally use the globe which is most of the time. Most people take pictures of scenes not flat images. What adjustments to the readings should one make when using a phone app?

138S · Jan 14, 2021

Alan Edward Klein said:
So a phone app does not work the same way as a standard incident meter would where you normally use the globe which is most of the time. Most people take pictures of scenes not flat images. What adjustments to the readings should one make when using a phone app?

IMO, of the choices Bob Salomon mentioned, the cell phone App meters more like a dome than like a plate, but limiting the sensitivity angle to the front camera angle view, which is not good or bad.

Alan Edward Klein · Jan 14, 2021

138S said:
IMO, of the choices Bob Salomon mentioned, the cell phone App meters more like a dome than like a plate, but limiting the sensitivity angle to the front camera angle view, which is not good or bad.

Not sure what you mean. So if I'm using a phone app in incident mode, how do I hold it differently than a dedicated incident light meter with a globe? What adjustments do I make to the readings it gives?

138S · Jan 14, 2021

Alan Edward Klein said:
So if I'm using a phone app in incident mode, how do I hold it differently than a dedicated incident light meter with a globe?

IMO the phone has to be oriented in the same way than a regular incident meter...

You know that some people point the meter to the camera others point it to the light source, and others point to an intermediate direction.

If you point to the camera you are metering for the surfaces that are facing to the camera. If you point to the light soruce then you are metering for the surfaces that are inclinated to reflect the incident rays to the camera (if they were a mirror)...

IMO this has some analogy with the Kodak R-27 gray card instructions for the card orientation, we may use that recipe or another one, feedback will refine how we interpret the reading.

Alan Edward Klein said:
What adjustments do I make to the readings it gives?

This is about comparing side by side like in the image I had posted:

If I had a Sekonic I would compare with it, as I have a Lux meter then I compare to it, this is about finding different illumination situations (direct, diffuse...) and comparing to se what nuances we tilt-swing the meter.

I mostly use spot meter, when I consider the reading with the smartphone (close to subject) oriented to the camera and oriented to the light source, trying to realize what surfaces I consider...

but the thing is quite complex, light can be direct/difusse, and surfaces can be bright/mate...

Incident light metering looks straight but it has many nuances. IMO it's the more difficult way to meter if we want to exploit all the information it delivers, we have to interpret the reading in the direct_vs_difusse_vs_bright_vs mate context. Specially in portraiture this requires having been trained from feedback. A master may read incident Fill/Key/Fall, and from the illumination kind he visualizes the result...

Today with digital people are less used to Visualization...

grat · Jan 14, 2021

I use the "Light meter - Free" Android app, and using the front camera meter, I get almost exactly the same reading as a sekonic with the 3D dome (5.4 vs 5.6). However, I had to adjust the app by -1 EV-- it seems to be consistently off by 1 EV. I tested this with a DSLR and with my spot meter. The spot meter function in the app can produce similar results to my spot meter, but it's finicky.

A real light meter works better and more reliably, but the app will work in an emergency.

138S · Jan 14, 2021

grat said:
I get almost exactly the same reading as a sekonic with the 3D dome (5.4 vs 5.6). However, I had to adjust the app by -1 EV-- it seems to be consistently off by 1 EV. .

It would be interesting to know if missmatch comes from the phone or from the app... by installing another app and installing that app in another phone.

Sirius Glass · Jan 14, 2021

How would one get a phone based app calibrated? If it is off what would one do?

wiltw · Jan 14, 2021

Sirius Glass said:
How would one get a phone based app calibrated? If it is off what would one do?

What I would do...try out a DIFFERERNT APP, until you find one of approprate degree of accuracy. Then hope your next photo works just as well as the older phone.

Sirius Glass · Jan 14, 2021

wiltw said:
What I would do...try out a DIFFERERNT APP, until you find one of approprate degree of accuracy. Then hope your next photo works just as well as the older phone.

I have light meters in cameras, camera prisms, hand held incident and reflective, and a Pentax Digital Spot Meter and every one has at some point needed to be calibrated. A phone app with or without attachment will have the same problem. So what would you do if you had to send your phone out to be recalibrated? Was your phone app ever calibrated? Does the phone app change its calibration when the phone is dropped?

138S · Jan 14, 2021

Sirius Glass said:
How would one get a phone based app calibrated? If it is off what would one do?

In the app I posted you can adjust offset and gain of the reading, just going Settings -> Correct Sensor

grain elevator · Jan 14, 2021

138S said:
IMO it is not an issue because the front camera has a lens+pixels substituting the globe+sensor, and this substitution is fair.

Another thing is if the smartphone reading comes from the light sensor for the scren auto-brightness, in that case reading is very directional, and reading may change a lot when tilting the phone. Some Lux meter Apps use that sensor... I would not use much that kind of readfing. But we speak about incident reading from the front camera (screen side camera for selfies...), to me this one is fair.

The globe and the "front camera" readings are not exactly the same because the globe "integrates" the light rays received in all directions hitting the dome, while the front camera has a more limited angle of view.

Probably the light illuminating the subject from the side or at similar angle, (integrated by the globe but not by the front camera) is not of interest for the metering exposure anyway, because it will have a very small impact in the light power reaching the subject and reflected to the lens.

Anyway the globe is not a perfect light integration way, or at least it is not better or worse than integrating (say) a 60º angle on the incident light. Integrating the lateral illumination may make sense or not, but the principal information is how much light comes from the directions that will bounce to where the camera is.

Please check instructions for the R-27 Kodak Grey card, you will see that it is suggested that the card has to be oriented with a particular tilt-swing... this is a refinement compared to how the globe meters.

At all I'm saying that the globe is not a good way... What I'm saying is that the interaction between light direction, direction of the subject's faces, camera position, and the exposure we want... this is a complex matter, and while the globe is a good way to simplify the reading it is not perfect and it may not be the best. Instead measuring the light comming in a (say) 60º angle (centered in the camera-subject axis) may not be better or worse that the globe integration.

In practice... we have to check if the App+Phone we have works well for us...

I'm highly doubtful of the incident metering with a phone camera. The problem isn't that it lacks a dome, it's that it has no diffuser at all, be it flat or domed. Have you observed the difference between readings with a meter where the dome can be removed, if you keep it pointed in the same direction? They're similar if there are no light sources shining on the dome/cell, but if there are (as an incident meter is supposed to be used at times), they are worlds apart. So if an app works reasonably well that way, it must be performing some image interpretation to look for light sources and somehow evaluate them, seems error prone. As you said, one would need to test it. On my phone, none of the apps I've tried work in incident mode.

Sirius Glass · Jan 14, 2021

138S said:
In the app I posted you can adjust offset and gain of the reading, just going Settings -> Correct Sensor

View attachment 263874

But how does one know exactly what to adjust to? I too have light meters with similar screws but I do not have a calibrated source so I sent everything to Quality Light Metric. So will you mail your phone to Quality Light Metric?

138S · Jan 14, 2021

grain elevator said:
IThe problem isn't that it lacks a dome, it's that it has no diffuser at all, be it flat or domed. Have you observed the difference between readings with a meter where the dome can be removed,.

The front camera is not the same than the naked sensor without the dome, the front camera has a lens projecting on and area with pixels, thus being sensitive for the angle of view, in a controlled maner.

Also the front camera is not the same than the sensor+dome, but it is effectively similar, still one way may be better than the other

You may try it for free,and you may calibrate offset and gain... IMO it deserves a try....

Personally I usually use spot readings in the field, but sometimes I find interesting checking incident light... We always have the phone in the pocket !

Sirius Glass · Jan 14, 2021

grain elevator said:
I'm highly doubtful of the incident metering with a phone camera. The problem isn't that it lacks a dome, it's that it has no diffuser at all, be it flat or domed. Have you observed the difference between readings with a meter where the dome can be removed, if you keep it pointed in the same direction? They're similar if there are no light sources shining on the dome/cell, but if there are (as an incident meter is supposed to be used at times), they are worlds apart. So if an app works reasonably well that way, it must be performing some image interpretation to look for light sources and somehow evaluate them, seems error prone. As you said, one would need to test it. On my phone, none of the apps I've tried work in incident mode.

I agree. There is an unquestioning blind over dependence on apps of many persuasions, not just light meters.

138S · Jan 14, 2021

Sirius Glass said:
I agree. There is an unquestioning blind over dependence on apps of many persuasions, not just light meters.

By investing $15 in a lux meter from Amazon, we get it calibrated.:

Sirius Glass · Jan 14, 2021

138S said:
By investing $15 in a lux meter from Amazon, let me post it again:

View attachment 263878

Great but has the lux meter been calibrated? If not, at this point you have gained nothing. I learned that the hard way.

138S · Jan 14, 2021

Sirius Glass said:
Great but has the lux meter been calibrated? If not, at this point you have gained nothing. I learned that the hard way.

Lux meters are calibrated, even those that cheap, but anyway you may also compare with the SLR spot reading on a R-27 gray card, to have total consistence with the other metering systems you may use. Quite straight...

Lachlan Young · Jan 14, 2021

Sirius Glass said:
Great but has the lux meter been calibrated? If not, at this point you have gained nothing. I learned that the hard way.

Calibrated & ISO certified ones are about 20x the price... I know which I'd trust.

BradS · Jan 14, 2021

JBrunner said:
Funny, a spot meter is all I use...it's hard to get a mile across the canyon with my meter and then run and climb a mile back to the shady side to make the exposure...guess I'm just lazy.

Kidding aside, if I was stuck with one meter, I'd choose a spot meter. I find it far more informative about the parts of the exposure that actually matter, and with the application of some common sense, its very easy to use it as an incident meter.

I have a 2016 Subaru Crosstrek with 40k miles and a 1997 Ford Ranger with 387k miles. The Ranger is beat to hell but runs and drives well. The Subaru has fantastic Subaru all wheel drive, power windows and heated seats! I take the Subaru just about every time I drive anywhere. If I had to choose just one, it’d be the Subaru. But I certainly do not need the Subaru.

MattKing · Jan 14, 2021

Sirius Glass said:
But how does one know exactly what to adjust to? I too have light meters with similar screws but I do not have a calibrated source so I sent everything to Quality Light Metric. So will you mail your phone to Quality Light Metric?

If you have calibrated light meters, you can adjust the phone to see if it matches their response.
If it does, it is usable.
Similar to how I make use of my Kodak Process Thermometer as a reference standard, and inexpensive digital kitchen thermometers for everyday use.
It is only necessary to regularly compare the day to day use phone meter with your calibrated reference standards.

grat · Jan 14, 2021

I admit, I assumed my brand new EOS 90D would be reasonably well calibrated-- so by using spot metering mode on the camera, I was able to verify the accuracy of my analog spot meter. I then used those two to compare with the lightmeter app on my phone, and as a few others with similar sensors had noted, the app was a bit off-- fortunately, it allows you to manually enter a fudge factor. I since picked up an incident meter to use with flash photography indoors, and it too agrees with the other two sources. There's a small amount of variance between them-- but less than 1/3 of an EV, so I just don't care.

The light sensor on the phone is largely useless for light metering-- at least on my "ancient" Google Pixel.

138S · Jan 15, 2021

BradS said:
I have a 2016 Subaru Crosstrek with 40k miles and a 1997 Ford Ranger with 387k miles. The Ranger is beat to hell but runs and drives well. The Subaru has fantastic Subaru all wheel drive, power windows and heated seats! I take the Subaru just about every time I drive anywhere. If I had to choose just one, it’d be the Subaru. But I certainly do not need the Subaru.

A better comparison would be: The Spot meter is like a Jeep that can go both to highways and to challenging mountian tracks with rocks in the middle. Other meters may be more convenient for the highway but they cannot go through challenging tracks.

Here you have 3 examples of challenging situations where an incident meter won't be useful:

https://www.photrio.com/forum/threa...cident-and-spot-metering.180410/#post-2360422

What would you do with an inccident meter in those cases ? Nothing...

grat said:
The light sensor on the phone is largely useless for light metering-- at least on my "ancient" Google Pixel.

Yes... many Lux Meter Apps use that sensor that it is temperamental because it's very directional. But metering Apps for photography tend to use the front camera, making them worth.

Still all metering systems have their own behaviour and photographer ends taking the feedback.

We all know this: No meter is perfect, and anyone may require some interpretation+override, in some cases at least.

grain elevator · Jan 15, 2021

138S said:
The front camera is not the same than the naked sensor without the dome, the front camera has a lens projecting on and area with pixels, thus being sensitive for the angle of view, in a controlled maner.

Also the front camera is not the same than the sensor+dome, but it is effectively similar, still one way may be better than the other

So you believe a digital camera as a light meter should read the same whether you point it towards the light source or away from it?
Imagine the extreme case of the meter being in a dim doorway. An incident meter with proper diffuser will read the ambient light, while an undiffused meter, whatever you call it, will meter the exposure that makes the outside area middle grey when pointed in that direction. Try an app with "incident metering" in this kind of situation, or a less extreme one, such as a room with a single window. With the brightness sensor, at least you have a chance; good luck if it uses the camera!

138S · Jan 15, 2021

grain elevator said:
So you believe a digital camera as a light meter should read the same whether you point it towards the light source or away from it?

I argumented my opinion about that earlier in that thread, please consider this:

https://www.photrio.com/forum/threa...-and-spot-metering.180410/page-7#post-2369017

"Probably the light illuminating the subject from the side or at similar angle, (integrated by the globe but not by the front camera) is not of interest for the metering exposure anyway"

The front camera of the phone (obviously) only reads the light taken from the (wide) viewing angle of the camera. This is not better or worse than also integrating the light comming from subject's side (as a dome does), but usually we don't want much that light in the computation.

Look, both with a dome meter and with an smartphone incident meter we place the meter on subject (or in the light path if no fall) and then we point both to the camera and to the light source, we tilt/swing the device and we evaluate those different readings, then we judge if we take one of the two or if we take some midpoint. We know how to use an incident meter, isn't it ? Difference is that the phone front camera reading will be more selective to the direction, but after some feedback we may get the same scene understanding with both.

What I opine is that using a "dome on a tinny spot sensor" or a "lens on an area sensor" is not better or worse choices, but they analize illumination a bit differently. Knowing the nuances... we may use both ways with total success, but the the App is for free.

Still you may check it in practice, take a Sekonic and a good App with corrections well made to match the sekonic. You will find that with even illumination you get the same, but as illumination is more complex you may get some discrepance, but with complex illumination none of the two gives you the exposure, in both cases you tilt-swing the meter to understand the scene, (for example when reading key/fill/fall of the portrait you change the meter orientation...)

...so, in short, it is true that the phone and the dome will deliver an slightly different reading in quite complex situations, but one reading is not better than the other, still both readings have to be interpreted slightly different, and an interpretation will always be required.

If we don't understand the nuances in the incident metering (pointing to the light source, to the camera or to the (fill) shadow) then it's irrelevant if we use a dome or a front camera, in that case we don't control much what we are doing, still result can be fine anyway.

If we are used to a refined incident metering (mostly for portraits...) then it's also irrelevant what metering tool we use, as we need to understand how each tool works.

Light meters with reflective, incident and spot metering

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