How did you work out a 0.4 second shutter speed?
I would have thought that to achieve 25fps, the camera will be at least taking 25 stills within that second? The camera just boosts the iso to accommodate this?
This doesn't then explain how slr cameras cant achieve 25fps, even top level ones under normal conditions. Maybe thats down to the mirror, but even mirrorless arent that quick yet...
Yes I think there's an error in there some where...
25 frames per second... would imply an effective shutter speed of something above 1/25th of a second, not 1/2 a second wouldn't it? (missing zero, perhaps, 1/25th s = 0.04s)
35mm movie cameras... I think the 'standard' projector frame rate is 24 frames per second... obviously for slow motion, they 'under-crank' of shoot film faster than the projector will show it to slow it down, or vica verca to speed it up like a Benny Hill chase.
1/25th is a fairly low shutter speed but exposure takes longer than the shutter speed.. 35mm movie camera, between frames, the film has to be advanced, held perfectly still, and clamped against the focus plane, the shutter cocked, then the shutter released. So practically, to do everything that needs to be done between frames, you cant really have the shutter open for more than half the time you have to get the film through the camera, so perhaps 1/50th or 1/60th second is slowest possible shutter speed.
That's still a fairly modest shutter speed though; hand holding shutter speed for 35mm SLR with standard 50mm lens on the front.
Why cant many even top 35mm film cameras shoot at movie FPS?
Wll, shutter speed isn't really the critical factor, but a 35mm stills camera, has to advance more than twice the film through the trap in the same time as a movie camera.... 35mm film camera exposes an oblong 24mm tall by 36mm wide along the length of the film, the film running through the camera horizontally in landscape mode, where a movie camera using same 35mm film, will expose an oblong 16mm tall by 24mm wide across the film (Half Frame), running through the camera vertically, with the camera in landscape.
Exposing a larger area of film in the same time, having to feed twice the film past the gate for the same frame rate, means it has to travel at least twice as fast, and the film isn't traveling at constant speed, its having to stop start between frame exposures; this means very large forces and accelerations on the relatively weak film, and the film risks tearing, getting stuck or jammed, the emulsion, rippling or tearing, or getting scratched.
The 24 frames per second was a practical compromise on movie cameras dictated by the mechanics of film transport.... which would suggest that for double frame size still camera, same limits would start to be reached at around 12 frames per second.
In the days of the 35mm SLR, the 4fps frame rate was well beyond what most pro-am cameras could deliver with an electric accessory winder. These usually only gave 2 frames per second. Full Pro-grade motor-drives, though would offer 'over' 5 frames per second, and I believe some could get up to around 16fps, with bulk-film cartridge backs fitted, essentially making them a horizontal movie camera! (Remember Movie cameras are view-finder cameras, they aren't SLR's they don't have the mirror mechanism to get in the way; and I suspect that for some of the super-high-speed fps 35mm SLR's to get the frame rate up that high, they did so with the mirror locked up)
Transport speed, was always the limiting factor to movie projector speed though; and if you look at the IMAX system, running horizontally to maximise film area and resolution, the one that was hardest to tackle.
Electronic Analogue or Digital Video? - No Film, no transport problems, right?
Well, yeah, sort of. There's no film to move around, so in theory, frames per second need be a lot less limited. But you still have to capture an image and some-how get your image 'signal' off the sensor and to whatever other medium you use to record it.
Bottle-Kneck now pops up in the 'Sampling-Frequency', if not the actual sampling method.
Movie camera; takes a series of still images in very quick succession, then displaying those stills sequentially, you get an aparently moving image, like a flick-book......
Video does NOT make moving images in the same way from a series of quickly displayed stills. (normally)
Old fashioned Cathode-Ray TV Screen. The signal, captured by the TV camera was piped to the screen, where a particle accelerator (the cathode ray) got a lot of electrons very exitable, then fired them in stream at the front of the glass tube, to strike phosphorescent paint, making it glow. The beam moved side to side across the screen firing or not firing electrons to make dots glow or not glow, or not glow so brightly, in lines, moving down the screen, one line at a time....
Only trouble with that is, that by the time the beam has got all the way down the screen, then back to the top... the dots that it did first have stopped glowing... so it used a thing called 'interlacing', where it would do 1/3 of a screen at a time, firing every 3rd row in each pass, then doing the next row down on the next pass, then the bottom row on the third pass... evening out the 'lag' between dots being lit up.
And you DO NOT get a 'still' image from the TV system... you get a dynamic image, continually showing changes in CERTIAN areas of the picture.
Common refresh frequency for analogue TV, one used in the UK system was 60Hz. Screen would be entirely refreshed every 1/60th of a second. That's not far off the lowest shutter speed for a movie camera is it?
BUT, unlike the movie camera that captures ONE 1/60th of a second 'instant' of a scene.... on a TV screen... imagine a chess board.... before player one moves, you have the major pieces all on thier home square... but the camera only samples the top left hand corner, so see's white castle there... then moves on and samples the white knight, then the bishop, then the queen, king, etc etc .....till its done the first row. Then it jumps a row or two and samples empty squares, until it gets down to the row of black players pawns... Provided nothing moves, what you see is the 'set' chess-board.
But if you had really fast players... by the time the sampler comes back to do white row three, player one has moved thier knight, and player two thier pawn, and by the time its completed the full scan of the board.... its trying to display half a dozen extra pieces!
That's the analogue TV system, and the failing is that its using single pixel sampling, across the whole image area, and relying on doing it quicly enough nothing changes or changes much between sample-sweeps.
Digital Video..... OK... we now have a sensor 'array'. Rather than sampling one dot in an image at a time.... with one sensor, we have a bludgy big bank of them, that miracle of miniaturization can now stick on a pin head sliver of selenium.
800 sensors across, 600 sensors up, almost half a million of them, all looking at the scene at the same time.... We can get a still from this, surely?
Yup. And my first Digital Camera did, and they were suprisingly good too.
BUT... using that word a lot, aren't I!
There's always a but.
Polling each sensor, taking a brightness value from it, just to get a black and white image, you get a file size of about 300K from it. Tripple that to get three values, one for each colour, it leaps to over 1Mb.....
Ah... and we are back to time shifting again.... do we have one sensor measuring each colour, or ask each sensor to do three quick successive samples for each colour?
So are back to a situation where we may not be capturing true stills, but sampled motion, thats changing dynamically accross the image area while its being recorded....
AND we have another transport issue... because 1Mb for even a pretty low resolution image is a lot of data to handle.
If I take a sequence of shots with my D3200, in RAW-Fine I can easily lock-out, as the buffer runs out of space trying to process all that data and get it saved to SD card. Admitedly, that's firing at perhaps 4fps and trying to shift 10Mb a picture to the card, but even so..... we are only talking 40Mb/s transport rate, and pretty powerful processor is choking on it.
We take low res, 1mb images, and try running at a continiouse 24 fps, we are going to be pushing 24Mb/s.... and if we up that to the higher resolutions of common videa, what 1024? We're looking at 3-4mb per still, and 70+Mb/s to transport.
SO in Video, you don't.
In the days of film movies; they realised that they could get away with a lot more grain on a movie picture, than you could on a still, because the grain changes with each image, and over laying one after another, de-resolved that micro-detail.
You go look at the histories of the Hey-Day of the Holywood studio, and some of the anecdotes of some of the famouse Holywood photographers; when the studios wanted a 'screen-shot' to publisise a movie, maybe an advertising poster, or whatever, they did NOT take a still image from the actual movie film. It was too low quality. They employed stills photographers on set, to take promotional stills, and for many poster shots, after the movie was made and edited, they called actors back and did a stills shoot to replicate a screen shot, specifically in order to get a poster quality negative.
That 'acceptable quality' difference has been known for a very long time, and exploited in making moving pictures almost the entire time.
So, back to digital video; to reduce the 'load' on transport and sensor, shooting video does NOT take a succession of stills, and certainly not the high quality stills you would get taking stills images.
The Video encoding, uses a number of tricks; Rather than save twenty four sequential images, in full detail, it saves just 3.. and then fills in the missing frames with a list of 'changes' of individuial pixels between them. And it will mark those changes by dynamic sampling, like the old TV system.
So your 24 frames per second video... isn't REALLY 24 frames per second... its three frames per second.... and some between frame sampling to make adjusted frames to go between... and at almost no point is ANY on-screen image, a true instantaneouse record of the viewed scene.
So its all a bit of an over simplification;
The Frames Per Second rate quoted for a stills camera shooting in sequence mode, is actually a literal measure of how many stills images it can take each second.
The Frame rate of a Digital-Video file, though is NOT. Its an 'equivilent' frames per second, a 'refresh-rate' aproximately that of the frame rate of a film movie.
Meanwhile, the mechanics of how the image is captured, by way of sampling and storage, are achieved very differently for stills and video, the overall compression optimised to an 'acceptable' IQ for each media, with the individual frame IQ for movie, considerably lower.
