Using Tiny Digital Camera as Light Meter - Question

[Marko]

The response of a digital camera is nowhere the same as film. You would be better off just using a good spot meter.

The histogram is only indicative of what was captured by the sensor, not what was present in the scene. It would be valuable if sensors had the same response as film, but it doesn't.

Following this logic, the same thing could be said about a spotmeter (or any other lightmeter).

A digital camera sensor and a traditional spotmeter both use photo-electric sensors with (very close to) linear response to light. Film, on the other hand, uses photo-chemical sensor with a response very different from linear. As a matter of fact, the response of any given film is very different from any other film, that's what makes them unique.

The histogram is a graphic representation of electrical signal as a function of the intensity of light that hit it. The same as needle sway on the old style light meters and the same as the led readout on the newer light meters. The ability of the photographer to understand and correctly interpret that data according to the film used is what makes the instruments useful.

If I am missing something about spectral response of a traditional lightmeter, its dynamic range or perhaps even its reciprocity awareness, I'd like to hear a little more about it than witty one-liner soundbytes.

[roteague]

Following this logic, the same thing could be said about a spotmeter (or any other lightmeter).

Perhaps, but my understanding is that the histogram is based upon the recorded pixels, not on the scene as a whole. That is why the point about it not being similar to film. A spotmeter doesn't have to be, since it is directly metering the subject and measuring the strength of the light being reflected.

[Marko]

Perhaps, but my understanding is that the histogram is based upon the recorded pixels, not on the scene as a whole. That is why the point about it not being similar to film. A spotmeter doesn't have to be, since it is directly metering the subject and measuring the strength of the light being reflected.

Well, that's where our understandings diverge, along with our opinions

My understanding is that both the digital camera sensor and the light sensor in the majority of contemporary spotmeters (or other lightmeters) are in essence electronic chips which translate the intensity of light that falls on them into the electrical signal of commensurate amplitude.

The histogram is simply a distribution curve as a function of the light intensity, the pixel-recording happens after the light is already measured. In fact, the pixel-recording also happens as a function of the light intensity levels captured, that is the essence of digital camera.

I also don't see how exactly is the response of a spotmeter similar to film? And if it is, then which film? Color or b&w? Negative or Transparency? How about reciprocity? And why exactly do you have to calibrate your entire system for each emulsion used?

But you don't have to take my word for it, there is a very simple experiment you can do to prove either your hypothesis or mine:

1. Simply take a spotmeter, take a few dozen close measurements of your subject, from shadows to highlights and plot a distribution curve.

2. Next, take a digital camera and frame your subject to contain the same area as measured with the spotmeter and take a shot, in RAW if possible, observing the histogram.

Chances are that both curves should be very similar in shape.

[Marko]

Perhaps, but my understanding is that the histogram is based upon the recorded pixels, not on the scene as a whole.

On further reflection, there are no pixels involved really, until the image is imported into an image-editing program. Until then, the entire image consists of either electrical (in camera) or magnetic signals (on CF card or disk).

The histogram cannot, therefore, be based on recorded pixels any more than the spotmeter's readout can be based on the film grain.

[roteague]

On further reflection, there are no pixels involved really, until the image is imported into an image-editing program. Until then, the entire image consists of either electrical (in camera) or magnetic signals (on CF card or disk).

Actually not, a digital camera is in essence a signal processing device. The signal, which is analog, goes through a series of filters and processors, to remove noise, determine color, etc, before going through an A/D converter - in camera. From that point it is digital - before it it written to either storage memory, or to the CF card.

[Marko]

Actually not, a digital camera is in essence a signal processing device. The signal, which is analog, goes through a series of filters and processors, to remove noise, determine color, etc, before going through an A/D converter - in camera. From that point it is digital - before it it written to either storage memory, or to the CF card.

True. But it is still an electrical signal all the way until it gets written to the card. There it becomes a magnetic charge.

Analog in this case simply means that the signal has a continious amplitude as a function of its intensity, a quality it acquires before being processed in any way. Demosaicing does not change that, it only enables the three color channels to be formed, but that's in addition to the integral one. Digital means that the amplitude has been divided into discrete levels. But the distribution curve remains pretty much the same.

Again, the bottom line is that both digital camera and a spotmeter use the same basic principle for measuring light intensity, both have relatively linear characteristic throughout their range and both are oblivious to individual emulsion characteristic such as emulsion composition, spectral sensitivity or reciprocity failure, even sensitivity to light in general.

No matter how good spotmeter you use, you still have to calibrate it, factor-in the correct filter factor and calculate the reciprocity failue compensation. All of this for each film you are using.

What you are getting in a digital camera is basically a fixed combination of light measuring device and light capturing device already factory callibrated. I see no reason to not be able to callibrate it to any of the films if one so desires.

[Ole Tjugen]

The histogram is only indicative of what was captured by the sensor, not what was present in the scene. It would be valuable if sensors had the same response as film, but it doesn't.

Nor does a spotmeter, or even a different film. One film doesn't have the same response an another one. That's why there are so many of them and some prefer one, some another.

[buze]

I use my 350D almost all the time for it's histogram when I'm outside. The histogram represents the JPEG output histogram and the not the raw (even if you shoot raw); so it's quite conservative if you shoot film, even slides.

I looked into getting a smaller "histogram device" and I had found a P&S that had a "lightmeter" mode, but it was a lot of interpolation. The small P&S cameras have a tiny lens, and a max fstop of f4 or so. A DSLR can do f22 and still have an accurate measure. I'd rather extrapolate a couple of stops that 8 of them...
But still, I'd love to have a nice accurate enough histogram device. The 350D is cool to "preview" yellow/green/orange/red filter effects in black & white too...

But of course as with every tools, you need to be the master of it. In the same way as you do with a spotmeter, you have to factor "peaks" in and offset whatever the histogram might say.

[Graeme Hird]

The way I think of a digital camera's sensor when used in this manner is that there are 6 million (or more) digital spot meters arranged in a systematic manner over a small area. The histogram reveals the number of spot meter readings which show a particular EV.

I see no good reason not to experiment and calibrate the system to your own favourite film.

But returning to Andrew's question: no, I don't have any particular experience with these small cameras when used as light meters. As soon as a good one comes on the market, I'll be buying my first digital light meter with recording capability. Until then, I'll keep reading these threads to find out the latest news.

Cheers,
Graeme

[Frank Petronio]

I'm a terrible slob and I don't bracket, but after trying my Nikon d80 dSLR and my trusted 20+ year old Pentax Digital Spot Meter side by side for a few shoots... I sold the Pentax Digital Spot Meter. Haven't had a bad outcome yet, maybe 30-40 different set-ups and situations.

I use the smaller prime lenses that are almost matched to my LF focal lengths. The smaller primes make the d80 that much easier to carry.

I just make the ISO and aperture and fiddle until I get a pleasing histogram and a decent preview. Of course I trust the histogram more than the screen image, but both provide valuable feedback.

You know what could really blow (APUG) people's minds? Using the dSLR as a color meter. Remember just a decade ago when you had to cough up $800 for a Minolta Color Meter III to be considered an architectural photographer? ;-)