OM-D EM-5 - High Iso Banding?

Started May 8, 2012 | Discussions thread
Detail Man
Forum ProPosts: 14,837
Re: OM-D EM-5 - High Iso Banding ? - Lighting Pulsations / Scan Rates
In reply to Vlad S, May 9, 2012

Vlad S wrote:

Detail Man wrote:

Vlad S wrote:

I doubt that light pulsation itself is the cause.

Why, specifically ? Note that the Shutter Times that were noted to exhibit the problem the most (1/30 and 1/60 Second), are exposure-windows that are directly integer-related to 60 Hz (CPS), or a sub-multiple. We know for a fact that indoor lighting is modulated by 60 Hz AC line currents ...

So are the RF emissions... I do not know exactly how the Live MOS sensor data are read out, but CMOS sensors read it line-by line. If that is the case with the Live MOS sensor as well, then the pulsations would have to occur many times during either exposure or readout. The reason why I thought that high frequency harmonics would not show up in the visible range is that in my personal experience the fluorescent lights glow for some time after the power is cut, so I assumed that this glow would smooth out any high frequencies. This is just an assumption though, so I do not insist that this is the answer, I just thought it was a reasonable hypothesis.

Maybe. I don't know a lot about flourescent lamps. I have (since authoring that post) been somewhat swayed by Anders W 's arguments (in general) that it seems (perhaps) unlikely that higher order (predominantly odd-numbered) harmonic multiples of 60 Hz or 120 Hz would be likely to be able to cause the large number of amplitude variations per unit image-height that we see in the case of the OPs's posted image below (as somewhat enhanced using high-pass sharpening):

But the electric discharges within the lamp are a source of radio waves, and the oscillators in the ballast also might be a source.

The possiblity of EMI at 60 Hz or its harmonics seems extremely unlikely. Vague theories regarding "radio waves" in the microwave frequencies higher than 2 Ghz (cell phones, Wi-Fi) seem possible, but (likely) a lot more far-fetched, less likely, and thus more nebulous ...

Why are the harmonics in the electronic circuits unlikely? If a circuit resonates at a certain frequency then it will necessarily produce overtones.

You mean harmonics of 60 Hz or 120 Hz, I guess. Well, it is the magnitude of the currents that flow as well as their rate of change in time which determines the strength of any electromagnetic fields. At 60 Hz or 120 Hz (or their harmonic multiples), the wavelength of such frequencies is extremely long. Thus, they do not easily radiated (or are they received) by electrical conductors.

As far as other emitting devices are concerned, I have not experienced interference from microwaves, but I experienced interference from my cell phone clearly audible on my ipod and care stereo, and by the sound of it I'd say it's higher than 60 Hz. 2Ghz and above is just the carrier signal, what you see/hear is the modulation of the carrier, rather than the carrier signal itself.

What you are observing/experiencing is the demodulation of high-frequency carrier frequency modulated RF/microwave energy, where the (AM, FM, or PM) modulation of the higher frequency carrier waves can (in any non-linear component or circuit made of components, particulalry in the case of AM, but also in the case of FM or PM where the component or circuit exhibits frequency-dependent properties) be demodulated - where the modulating frequencies are as a result translated downwards in frequency closer to lower frequency "base-band" (nearer to DC).

And, it was you (yourself) in a recent post on this thread who stated that the camera likely is well shielded against the electro-magnetic coupling of EMI . Thus, you are now contradicting yourself ...

It's only shielded if there's no conduit from the outside of the shield. If the signal is picked up by the circuitry in the lens, then it can be transmitted by wires, and if that particular conductor does not have a choke, then the high frequency will pass inside even a shielded circuitry.

Yes, once RF energy manages to "get into" a shielded environment, it can, indeed, be re-radiated. I was just somewhat puzzled in that you had seemed to have a "change of mind" on the matter.

What is your own background, knowledge, and experience in analog/RF electronic design, Vlad ?

With the electronic design - just a middle school electronics club. But I do have a couple of graduate level physics courses under my belt, and I studied quantum physics long enough to know properties of electromagnetic waves fairly well.

Well, that's certainly farther than I managed to get in college in the late 1970s. Got through differential Calculus and introductory Physical Chemistry, but withdrew the following quarter ... Around 10 years ago I earned a 2-year AAS degree in Electronic Technology (ironically, after the end of my career in electronics). Wiil be looking to you regarding solving differential equations !

I really would not like to turn this into "who's stick is bigger" contest. I don't need to be right. I participate in this thread because I am going to get this camera, and I am interested in its limitations. So please don't question my background, if you think that I am saying something wrong just present your argument - that's quite sufficient.

Surely. When I was younger, I "wanted" to be right - but time and tides have proved beyond a doubt to me that I am only (eventually) somewhat right after I have wrestled with first being somewhat wrong many more times that my mere mortal ego would prefer (to say the least !).

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