Comparing Olympus 4/3lenses to FX "Full Frame" offerings

Started 7 months ago | Discussions thread
Great Bustard
Forum ProPosts: 23,240
Then understand what Equivalence says.
In reply to Doctor Lecter, 7 months ago

Doctor Lecter wrote:


"While crop factor has a use simply to compare focal lengths between formats and such, the constant comparison of a smaller format lens to its full frame ‘equivalent’ aperture is largely unevenly applied and misunderstood. It’s often used to show that a smaller format is inferior or not capable of the same things as a larger format. In some cases, this usage is correct, but it is also nearly never used the other way.


Q: Are bigger formats better than smaller formats?

A: For some specific purposes, yes; for others, no. The more specific the purpose the of photography, the easier it becomes to say that System A is "better than" System B for a particular photographer; the more broad the photography, the less easy it is for one system to be superior to the other.

Q: When are larger formats better than smaller formats?

A: To answer this question, we need to invoke the "all else equal" clause, because there are so many variables that may make one system better than another for any particular photographer. In short, the advantage of a larger sensor system over a smaller sensor system is that the larger sensor system will generally have lenses that have wider aperture (entrance pupil) diameters for a AOV (diagonal angle of view) than smaller sensor systems, which allows for more shallow DOFs (as an option, not a requirement) and will put more light on the sensor for a given exposure, resulting in less noise. In addition, larger sensors typically have more pixels which, when combined with a lesser enlargement factor from the recorded photo to the displayed photo, results in more detailed photos (at least for a given DOF). Whether or not these advantages are more important than the disadvantages (size, weight, cost, etc.) is another matter all together.

Q: Isn't Equivalence a vehicle for promoting the "superiority" of larger sensor systems?

A: Not by a long shot. If there is an agenda to Equivalence, it is to change the photographic paradigm based on the relative aperture (f-ratio) and exposure with a new paradigm based on the virtual aperture (entrance pupil) and total amount of light falling on the sensor, at least for cross-format comparisons.

Q: So Equivalence is about the lens as opposed to the sensor?

A: That's a good way to put it -- it's the virtual aperture (entrance pupil) for a given AOV that is of central importance. However, sensor size still plays a role, as larger sensors typically have more pixels and typically can absorb more light for a given exposure.

Q: Isn't Equivalence all about DOF?

A: No, Equivalence is not "all about DOF", but it is very much about understanding that both DOF and noise are intimately connected to the aperture. That said, DOF, by itself, is still a critical consideration to the captured detail in the photo, since portions of the scene outside the DOF, by definition, will not be sharp, and all systems suffer diffraction softening equally at the same DOF.

Q: Doesn't Equivalence say that we should shoot different formats at the same DOF?

A: Not at all, and, in fact, quite the opposite. That is, one does not choose one format over another to get photos Equivalent to what one would get on another format. Rather, one chooses one format over another to get photos they could not get on another format, or get better photos than they could get on another format, assuming, of course, that differences in operation, size, weight, and cost are not significant enough to be the primary consideration.

Q: Overall, then, isn't FF best the choice?

A: Again, which is best is completely subjective. While for me, personally, I prefer FF, it is my opinion that the vast majority are better served with smaller formats. As all systems continue to improve, the number of situations where FF has a significant advantage over smaller formats narrows. Of course, if size, weight, and price were not considerations, then larger is almost always better. However, since size, weight, and price not only matter, but are often (usually) the primary considerations, then it is my opinion that the advantages of FF over smaller formats are not enough to offset the disadvantages for most people in most situations.

I’ve heard many times “Yeah, your 75mm f/1.8 is crap – it’s like a 150mm f/3.6.” No, it’s not, it’s a 75mm lens with an f/1.8 aperture and a field of view that is the same as a 150mm lens on full frame.

Well whoever said that your 75 / 1.8 is crap does not understand Equivalence. However, a 75 / 1.8 on mFT is equivalent to a 150 / 3.5 on FF, as they both have the same diagonal AOV and aperture diameter.

What IS true is that the 75mm f/1.8 is not capable of the same ultra shallow depth of field as, say, something like the Sony Zeiss 135mm f/1.8 on full frame. However, this is essentially the ONLY way that it is inferior. It passes the same amount of light, and it exposes as an f/1.8 lens because it IS an f/1.8 lens"

It absolutely does not pass the same amount of light onto the sensor. You may wish to educate yourself on this matter:

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:

The exposure (light per area on the sensor) at f/2.8 1/100 ISO 100 is 4x as great as f/5.6 1/100 ISO 400 for a given scene luminance, regardless of the focal length or the sensor size. However, the brightness for the two photos will be the same since the 4x lower exposure is brightened 4x as much by the higher ISO setting. If the sensor that the f/5.6 photo was recorded on has 4x the area as the sensor as the f/2.8 photo (e.g. FF vs mFT), then the same total amount of light will fall on both sensors, which will result in the same noise for equally efficient sensors (discussed in the next section).

Let us now go into more detail on these points:




Alternatively, you can choose to remain ignorant about what Equivalence does, and does not, say, and choose to remain ignorant about the difference between the amount of light per area that falls on the sensor (exposure) and the total amount of light that falls on the sensor, and what this has to do with the visual properties of the recorded photo.

It's nice to have choices, no?

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