If you set "1/60" shutter speed, what exposure time do you expect?
Do you expect the actual exposure time to be different in stills vs. video?
To be different for electronic vs. mechanical shutter?
I knew for a while that a camera I have (Panasonic S1) uses 1/64.0 for stills with mechanical shutter, and 1/60.0 for video at a "1/60" setting. Some inconsistencies in RAW exposures between different shutter types led me to dig a bit more.
It seems that on this camera (and other Panasonic S-series cameras, and I suspect MFT cameras), when electronic shutter is used, a different set of exposure times is selected for shutter speed settings in the range "1/10" - "1/125".
Presumably other manufacturers do similar things, particularly for video, where no-one wants an exposure time of 1/64.0 s, or 1/128.0 s.
Comments:
This feature would be a lot more useful if it were documented !
It would be helpful to have "anti-flicker" shutter speeds with mechanical shutter. Flicker manifests different artefacts with a fast mechanical shutter vs. much slower electronic shutter, but there are still problems with flicker with mechanical shutter.
It would be nice to have - and straightforward to implement - a menu option for flicker-based vs. EV-scale-based shutter speeds for stills.
Some of the electronic shutter errors are surprising: I found larger errors at "1/30" and "1/60" than at "1/120", when I would expect the opposite. The electronic shutter speed of 1/59.73 s at a "1/60" setting appears to be a bug.
Panasonic offers "Synchro Scan" (Ability to specify shutter speed with roughly 0.1Hz resolution) in video (only) on its GH-, S1H, and S5ii models. I have not had an opportunity to measure the behaviour of this, but the accuracy of some of the shutter speeds implemented by Panasonic - notably "1/60" - raises questions about the implementation.
For the Panasonic cameras that behave like this, there's a significant practical implication when shooting under lighting with mains-related flicker, such as many mains-powered LED lamps, incandescent lamps, and magneticallly balasted gas-discharge lamps: it may often be preferable to use electronic shutter, rather than mechanical shutter or EFCS, to reduce flicker-related banding and exposure variation.
The difference between 1/60 and 1/64 is less than 7%. When I started in photography, even the very best film cameras often had much larger errors than 7% in both shutter speed and aperture settings.
These days, quartz clocks have made the measurement of time vastly more accurate than it used to be using mechanical timing devices. However, I wonder what is the typical accuracy of aperture settings. I suspect that the errors are often much more than 7%. In addition, except for cine lenses, no account is taken of transmission loss in the lens.
An error of 7% is insignificant for most practical purposes and I can understand why the camera manufacturers do not worry about these errors.
The mech speeds follow a much nicer log scale than the nominal values - I'm quite impressed that the delicate mechanical mechanism is getting <0.5% error over several octaves.
You think someone might get injured by a low shutter speed?
With practice - which is pretty much free - it's quite possible to record pleasing images down to "1/4" or slower.
Image stabilisation in modern lenses and bodies helps a lot.
It seems sad to reject the possibilities of such photography out-of-hand. For example, a blurred hand gesture can bring an image to life.
My post doesn't have any bearing on that.
My post does somewhat bear on choice of shutter type when shooting under lighting which exhibits mains-related flicker, on Panasonic cameras.
I doing consistency checks on shutter times, I came to the conclusion that Nikon DSLR had 1s to quartz precision and the other times in precise
powers of two, respectively 2^1/3, steps of actual time values. So 1/60s nominally is actually 1/64s etc. I assume other cameras do the same. (nominal 30s was 32s -0+0.3 hand stopped on shutter sounds)
If the quartz clock runs the frequently used "32" kHz, precisely 32768 kHz, this would allow exact 1/8192s timing, but then 1/6400s nominal and 1/6502s expected would be 0.8% off with a 5 clock step approximation, and 1/5000s nominal and 1/5161s expected would be 5.8% too long with a 6 clock step approximation, 1/2500s nominal and expected 1/2540s would be 2.3% too short with 13 integral clock steps.
The consistency of exposure times can be checked with good accuracy with a uniformly and constantly illuminated light patch as source photographed with fully open aperture. The average value for the center 10'000 pixels in a Bayer subset measure relative exposure to better than 0.01% accuracy on high exposure. It is according to Poisson statistics of the electron number and digitization error, so it degrades some for shorter exposure times.
Right. But it turns out that "1/60" may not be 1/64.00 s, by design.
I guess that the reason behind the non-standard speeds with electronic shutter is the way lighting flicker manifests on cameras with shutter travel time AKA "rolling shutter" longer than the lighting flicker period (currently typical for Electronic shutter), versus cameras with shutter travel time shorter than the flicker period (typical for mechanical "focal plane" shutters). I guess that the actual shutter speeds with Electronic shutter were fudged to eliminate (complaints about) banding.
When the "rolling shutter" is greater than the lighting flicker period, lighting flicker (filtered by the sinc() temporal MTF of the exposure time) is intra-frame - it manifests as banding.
When the "rolling shutter" is much less than the lighting flicker period, lighting flicker is inter-frame - it manifests as shot-to-shot variation in exposure, which for most consumers is less objectionable, but may be quite undesirable in some photographic contexts.
When the "rolling shutter" is less than the lighting flicker period, but not so much less that illumination, filtered by the exposure time, is essentially constant during the shutter travel time, flicker is both inter- and intra- frame. This is the typical case for good quality modern focal-plane shutters at shutter speeds much faster than the lighting flicker frequency.
For a given set of pixels exposed at the same time, and for a given lighting flicker waveform and frequency, the shot-to-shot variation in exposure is determined by the exposure time. Unless exposures are synchronised (phase-locked) with the lighting flicker cycle - which is sometimes done when shooting movies.
That had been my assumption too, outside of video.
I tried to measure exposure uniformity with Electronic, Mechanical, and EFC shutters, but I found that Electronic shutter was giving different exposures to Mech & EFCS at lowish shutter speeds, but very similar exposures at high shutter speeds. That led me to make the measurements above.
Without detailed understanding of the design, assuming a 32kHz reference is a bit speculative. 32KHz crystals are not great precision references, and the shutter mechanism likely needs faster clocks (for the logic) to control the linear motors.
I haven't finished my measurements of high shutter speeds, but so far I'm seeing significant inaccuracy at "1/4000" (i.e. 1/4096.0) and above with Electronic shutter. I want to confirm my direct shutter speed measurements with tests of relative exposure.
Did you mean "consistency" rather than "accuracy" ?
Such tests are also quite demanding of the stability of the light source. I've been some using (relatively) inexpensive video LED panels (GVM-672S), which turn out to be extremely stable.
0.01% = 0.014EV.
On The Panasonic S1 I refer to in this thread, my current model for shutter-related standard deviation of exposure, with mechanical shutter, is roughly 0.00020EV + ((shutter speed)/600000) EV - e.g. giving about 0.007EV standard deviation at "1/4000".
In my earlier exposure tests, I found that the standard deviation attributable to stopping a modern 85mm f/1.4 fly-by-wire lens down to f/2.5 was about 0.0045EV - twenty times greater than the mechanical shutter noise at "1/60".
So I use fully manual lenses for such tests now.
Also, any black level errors.
In moving pictures it's very important to be able to use the nulls in the sinc() temporal frequency response of the exposure time to very heavily attenuate flicker.
Very small frame-to-frame differences in exposure, which would rarely be important to a stills photographer are very obvious to a moving picture audience. They also interfere with inter-frame compression.
The flicker problem, and its solution, have been well understood by the movie folk since the 1930s. For example, the (24.00 frames-per-second) Mitchell BNC camera, designed in the 1930s, and in use into the 1990s, had (rotary) shutter calibration marks at 172.8° (1/50.00 s) and 144° (1/60.00s).
Rotary-shutter film movie cameras had "rolling shutter" comparable to good 21st century mechanical focal-plane shutter cameras, perhaps a little slower. A few milliseconds, rather than tens of milliseconds.
Edit:
At 24.000fps, the shutter angle for 1/50.00s exposure is 172.8°, not 172.6° (which would be suitable for 23.976 fps).
Add paragraph about typical case with mech shutter at high shutter speeds.
You may not have given hints at how you do direct shutter speed measurements. It would be interesting and educational for interested readers like me.
On the approach via relative exposure, you seem to have gone along similar lines like me. Your mention of black level variations may be a hint that you have thought of corrections that escaped me so far. Please tell.
This is a placeholder - I'll add/rewrite in a bit.
I have 3 white LEDs in series, in series with a 33R resistor on a breadboard. The LEDs are close together, in a triangle pattern.
I drive them with a signal generator (inexpensive FeelTech FY6800). Max current is ~30mA. Current rise/fall time is ~45ns.
I use a PWM waveform with very low duty cycle - such as 0.02%.
Set the PWM frequency close to what I think the exposure time is.
Take a very defocussed picture of the LEDs.
- Light bands in image? PWM freq is too high.
- Dark bands in image ? PWM freq is too low.
Close to correct frequency, bands become faint or intermittent. External monitor on HDMI out helps.
Duty cycle needs to go up a bit at high shutter speeds.
I tried a few iterations. The first was attaching a few LEDs with attached series resistors, in parallel, to the signal generator. That's what I used for the measurements above. But the light output wasn't great, so I looked for more efficient LEDs, and making more efficient use of the available power from the signal generator. Greater efficiency makes it easier to detect frequency errors. Using only series LEDs seemed important to avoid skew between parallel strings of LEDs. Some signal generators might work better with 2 LEDs.
I did try searching for methods for measuring shutter speed. I couldn't find anything as precise. I can currently get 1/50k precision, depending on the shutter speed and rolling shutter.
Pictures of setup:
Overall:
The monitor is an Atomos Ninja V. Usually used with 2x zoom, and the brightness turned up a bit.
LEDs:
The tile was just a convenient rigid mounting.
debating anything longer than 20milliseconds indicates a non-European mains connection. I presume that the 60Hz flicker risk on videos is the main concern, not a 7% exposure deviation.
I could not resist checking again on my earlier tests about exposure precision mentioned in my previous post here. While it may barely matter for much of normal photography, it does for scientific purposes like measuring external quantum efficiency of a sensor. There you like to have things accurate, if you can.
In toda's test with result reported below, I used two LED panels illuminating white paper with 'continuous light' and took images with a D850 and a 105mm lens fully open at f/1.4 using electronic shutter 'silent lifeview SL1 mode'. Exposure at 1/1600 s nominal resulted in about 22'000 electrons per pixel and correspondingly less for shorter exposure times. My reported averages for Bayer green 1 and for green 2 are the average coming from 4 takes where the pixel values of the 10000 central pixels of the Bayer subset were averaged. From the 4 takes I can know how precise these average readings are, this is reported as rms1 resp rms2. The table shows these 4take averages normalized to the shown assumed exact exposure time and the average count one from the first take. This allows to see the deviation from expected exposure easily. The greatest seen rms deviation was at 0.03%, which is close to expectations from Poisson statistics. A part of it may be due to residual time dependence of the light sources and residual jitter of the electronic shutter. The hypothesis that a common "32kHz" quartz clock may be used, uttered in my previous post is clearly dismissed, as this would result in a~6% deviation for the 1/5000s exposure.
The mechanical shutter adds unavoidably some jitter and and slack. In the case of the pro Nikon D850, I find a little more than 1% jitter and a few percent slack towards the shortest exposure times. In the enthusiast model D7200 (with a wild history...) more of both.
