A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,017

gsergei wrote:

Or why the light meter doesn't care.

Hello, again.

Let me repeat my point:

Given the same light source conditions and ISO setting the illumination, measured in lumens, received by any size of sensor is equal across all formats (no Ifs , no buts) , when cameras are set to the same A , S, ISO values. So, in practical terms this means, that the illumination received by my tiny c7070 at 125/5.6 and ISO 100 is exactly the same as on the so called FF sensor camera set to the same values. This will result in the same picture brightness/darkness , subject to DR and noise differences.

No one that I'm aware of claims that f/2 will not result in the same exposure regardless of format or focal length for a given scene luminance and shutter speed.

Please, do not feed me things like "amount of light", "sensor area", "light power" , "bigger lens opening" etc. and try to operate , using known physical terms and concepts.

If the difference between the amount of light per area that falls on the sensor (exposure), and the total amount of light the sensor records is difficult to understand, well...

Here is the final word. Please, do me a favor:

get hold of ANY light meter (be it a 40 year old analogue one, or a modern Sekonic) and look at it carefully and then tell me , where do you see "Canon FF" or "Olympus c2020" or Fujifilm X100s ??? And then prove me wrong.

Speaking of "final words", understanding the difference between the amount of light per area on the sensor (exposure), the total amount of light that falls on the sensor, and how these relate to the visual properties of the recorded photo is rather fundamental:


This section will answer the following four questions:

  • For a given scene, what is the difference in exposure, if any, between f/2.8 1/200 ISO 400 and f/5.6 1/200 ISO 1600?
  • What role does the ISO setting play?
  • What role does the sensor size play?
  • What does any of this have to do with the visual properties of the photo?

As mentioned in the introduction of this essay, the concept of Equivalence is controversial because it replaces the paradigm of exposure, and its agent, f-ratio, with a new paradigm of total light, and its agent, aperture. The first step in explaining this paradigm shift is to define exposure, brightness, and total light.

The exposure is the density of light (total light per area -- photons / mm²) that falls on the sensor during the exposure, which is usually expressed as the product of the illuminance of the sensor and the time the shutter is open (lux · seconds, where 1 lux · second = 4.1 billion photons / mm² for green light -- 555 nm). The only factors in the exposure are the scene luminance, t-stop (where the f-ratio is often a good approximation for the t-stop), and the shutter speed (note that neither sensor size nor ISO are factors in exposure).

For example, two pics of the same scene, one at f/2.8 1/200 ISO 100 and another at f/2.8 1/200 ISO 400 (on any system, regardless of format) will both have the same exposure, since the same number of photons per unit area will fall on the sensor, but the ISO 400 photo will appear 4x (2 stops) brighter than the ISO 100 photo since the signal is amplified by a factor of four due to the higher ISO setting.

The brightness, then, is the brightness of the final image after an amplification is applied to the exposure either by adjusting the ISO and/or a push/pull in the RAW conversion, and is often what people mean when they say "exposure". For example, pics of the same scene at f/2.8 1/200 ISO 100 and f/5.6 1/200 ISO 400 will be processed to have the same brightness, even though the f/2.8 photo has 4x (two stops greater) exposure than the f/5.6 photo.

The role of the ISO setting in exposure is in how the setting indirectly results in the camera choosing a different f-ratio, shutter speed, and/or flash power, any and all of which will change the exposure. For example, changing the ISO from 100 to 400 may result in the camera choosing f/5.6 instead of f/2.8, 1/200 instead of 1/50, f/4 1/100 instead of f/2.8 1/50, etc. Aside from that, the ISO control on the camera will apply an analog gain (which results in less read noise for higher ISOs with cameras that use non-ISOless sensors) and/or a digital push/pull (usually for intermediate ISO settings).

The total light is the total amount of light that falls on the portion of the sensor used to for the photo during the exposure: Total Light = Exposure · Effective Sensor Area. The same total amount of light will fall on the sensor for equivalent photos but, for different formats, this will necessarily result in a different exposure on each format, since the same total light distributed over sensors with different areas will result in a lower density of light on the larger sensor. Using the same example above, pics of the same scene at f/2.8 1/200 on mFT (4/3) and f/5.6 1/200 on FF will result in the same total light falling on each sensor, but the exposure will be 4x (2 stops) greater for the mFT photo, and thus the FF photographer would usually use a 4x (2 stops) higher ISO setting to get the same brightness for the LCD playback and/or OOC (out-of-the-camera) jpg.

Lastly, the Total Light Collected (signal) is the amount of light that is converted to electrons by the sensor, which is the product of the Total Light that falls on the sensor during the exposure and the QE (Quantum Efficiency of the sensor -- the proportion of light falling on the sensor that is recorded). For example, if QE = 1, then all the light falling on the sensor is recorded. For reference, the Olympus EM5, Canon 5D3, and Nikon D800 all have a QE of approximately 0.5 (50%).

In terms of IQ, the total light collected is the relevant measure, because both the noise and DR (dynamic range) of a photo are a function of the total amount of light that falls on the sensor (along with the sensor efficiency, all discussed, in detail, in the next section). That is, noise is determined by the total amount of light falling on the sensor and the sensor efficiency, not the ISO setting on the camera, as is commonly believed (the ISO setting is simply a matter of processing the signal, discussed in more detail here). In other words, the less light that falls on the sensor, the more noisy and darker the photo will be. Increasing the ISO setting simply brightens the captured photo making the noise more visible.

For a given scene, perspective, and framing, the total light depends only on the aperture diameter and shutter speed (as opposed to the f-ratio and shutter speed for exposure). Fully equivalent images on different formats will have the same brightness and be created with the same total amount of light. Thus, the same total amount of light on sensors with different areas will necessarily result in different exposures on different formats, and it is for this reason that exposure is a meaningless measure in cross-format comparisons.

Mathematically, we can express these four quantities rather simply:

  • Exposure (photons / mm²) = Sensor Illuminance (photons / mm² / s) · Time (s)
  • Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)
  • Total Light (photons) = Exposure (photons / mm²) · Effective Sensor Area (mm²)
  • Total Light Collected (electrons) = Total Light (photons) · QE (electrons / photon)

So, we can now answer the questions posed at the beginning of the section...

I know, I know -- anything that takes more than a sentence or two to explain isn't worth knowing, right?

Post (hide subjects) Posted by
(unknown member)
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow