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As I have understood by this question, an exposure value of a certain scenario cought from a digital sensor (CCD or CMOS) results from:

  1. the scenario luminance
  2. the sensor sensitivity
  3. the ISO, shutter speed and F number settings

Where ISO is not the sensor sensitivity (which is proportional to the sensor pixel area and to its quantum efficiency), it is just the tunable amplification factor.

When using an on board lightmeter, I guess it, once measured the (reflected) amount of light, it adds up the sensor sensitivity contribution to tell us if the image we are going to take will be properly exposed.

Well, this conversion factor is unknown to a dedicated external light meter. So, I guess its suggestions may be not optimal for all sensors, and may even depend on the utilized lens.

I'd say that a calibration procedure would definitevely solve this question. But why don't usually people even think at this? Is it a negligible effect? Would this effect cause less accuracy than the low accuracy provoked by the reflective measurement of a TTL on board light meter?

Will an old light meter, calibrated for old sensors with lower quantum efficiency, give us proper suggestions when utilizing an efficient camera?

3 Answers 3

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My answer is based on my humble logic so some technical details may differ from the reality :)

After the sensor the amplifier is tuned on such way so if you have particular amount of light which reach the sensor, base ISO, you will have (more or less) normally exposed image (after the ADC). And because external lightmeter get as input this same light it can calculate (with some accuracy) the values of aperture, ISO and shutter speed (usually you have two of them preset, so it calculate the third one).

And these settings of course vary slightly from camera to camera which require to calibrate external light meter for particular camera/sensor. Usually external light meter provide more precise results because they can have much bigger light metering sensor inside and do not have (so much) constrains as power.

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  • Internal meters are seeing 'light reflected off the subject', external are seeing 'light from the sky'. idk the precise details, but they're looking at different things.
    – Tetsujin
    Sep 6 at 18:46
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    @Tetsujin, only if you put external on the subject. If you use it near to the camera and point it to the subject they are the same (almost :) ). Sep 6 at 18:53
  • It's not really 'pointing' at anything, though. It's an ambient light sensor. What if you're standing in the sun & the subject is in the shade? TTL would measure the subject's brightness, external would see the sunlight.
    – Tetsujin
    Sep 7 at 11:52
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    Have to admit I've never seen one of that type. I work in the film industry so I do see a lot of very expensive kit every day [even if I never get to touch it myself, I don't work on camera crew] but if they're not using the camera's built-in meter, they use a regular incidence meter, held at the subject. Anyway, I now get that it's possible, thanks.
    – Tetsujin
    Sep 7 at 12:11
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    @Tetsujin Both incident and reflective light meters (or a single meter that could measure both) have been around in still photography forever. Reflective meters aren't very common at all in cinema settings. Today they're less common than incident meters in still photography, but were much more common before cameras started including built-in reflective meters.
    – Michael C
    Sep 11 at 7:34
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Where ISO is not the sensor sensitivity (which is proportional to the sensor pixel area and to its quantum efficiency), it is just the tunable amplification factor.

This is not correct... the base ISO(s) for the sensor are based (at least closely) on the sensor's reactivity/sensitivity to light. See the japanese standard CIPA DC-004 2004. And part of that standard is that the camera's metered exposure, including ISO, closely match that which would be achieved with another metering system (external/hand held) and/or w/ external lighting (guide number).

Any ISO above base is the additional amplification required to achieve the same "standard"... I.e. ISO 800 is different amplifications above different base ISO's (e.g. it's 8x for a sensor with base ISO 100, and 4x for a sensor with base ISO 200). But in all cases it results in a similar exposure. So, the "conversion factor" is not unknown to the maker of an external meter (within a small tolerance).

I did reply to your other question... perhaps you had not seen it?

Is it a negligible effect? Would this effect cause less accuracy than the low accuracy provoked by the reflective measurement of a TTL on board light meter?

There are several "standards"; Sensor Saturation, Standard Output Sensitivity, and Recommended Exposure Index... of the three, REI is the least standardized; and it is generally underrated (overstated to protect against the digital sensor's harsh clipping characteristic)... but it is still generally w/in about .5 stops of the other methods.

But in the end none of this matters too much... there is no such thing as a correct exposure in artistic terms. E.g. with slide film it was quite common to intentionally underexpose the film by assigning it a higher ISO in order to generate more saturated colors and take advantage of the film's exposure characteristics. Similarly, there is nothing saying that the metered exposure will generate the image you want to create... as often as not it won't. Both cameras and handheld meters (generally) have a way to calibrate them; to each other, or to generate the results you prefer (offset the metered exposure readings).

This is the Nikon's Z9's REI ISO ratings compared against a more standardized SOS measurement... it has one of the largest offsets I have seen, but still less than 1 stop. enter image description here

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  • Digital cameras tend to "under-amplify" analog ISO settings and then "push" digital development in order to preserve highlights. So the camera that is "underexposing" by half a stop at a particular setting is also compensating for this in digital processing. Even when third party raw convertors are used, they all pretty much apply the correction based on the specific profile the app has for the particular camera model that took the photo at the particular ISO setting used when the raw data was digitized.
    – Michael C
    Sep 11 at 7:47
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Well, this conversion factor is unknown to a dedicated external light meter.

No, the sensor reacts to the light meter exposure parameters in EV, not the other way around. EV 0 is f/0 at 1 second, and ISO is basically a multiplication (in film, of sensitivity). Normal bright direct sunlight (not hazy, etc) is near EV 15.

In any external light meter, you have to enter into it your ISO and fstop or shutter speed, so it will know the other value (based on the brightness it sees).

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  • Strictly speaking, exposure values (EV) are light agnostic combinations of exposure duration (shutter speeds) and aperture values. If I use ISO 100, 1/60, f/4 it is EV10. It matters not whether I use that setting to take a photo of a black cat in a coal mine or of white snow at local noon on a cloudless day, it's EV10. One will be grossly overexposed, the other grossly underexposed, but both are shot at EV10.
    – Michael C
    Sep 11 at 7:42

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