1/50 shutter speed implications at DVinfo.net

Go Back   DV Info Net > The Tools of DV and HD Production > Open DV Discussion

Open DV Discussion
For topics which don't fit into any of the other categories.


Reply
 
Thread Tools Search this Thread
Old April 24th, 2002, 12:13 PM   #1
Tourist
 
Join Date: Apr 2002
Location: Italy
Posts: 4
1/50 shutter speed implications

To trasfer tape (DVCam in my case) to 35 mm you should use 1/50 shutter speed. Shutter speed means time exposure, right?
For a correct exposure (and I need the right exposure) once the exposure time is fixed the IRIS goes in consequence (there is only a value that matches, right?).
I would like to decide the IRIS to obtain a certain result, but how can I do that with a fixed exposure time? Is there anything wrong with this statement?
Consider that I live in Italy and I spent the last month asking around to every tech and nobody was able to clarify me these points (and who knows does not want to let the others know).
I am going to shoot my first short and there are more than 100 persons involved.
I do not understand few important points:
1)Why I have to use 1/50 shutter speed (the technical reason)
2)There are 25 frames per second - PAL - with all the shutter speeds? Yes! What happens exactly with shutters speed inferior to 1/50 sec? Are there less frames? Or there is duplication of fields/frames (the total number of fields per second does not change). Example: with 1/25 sec shutter speed every field is exposed for so much time that the following one is the same field copied by the camera from his memory. Regarding the memory I read that many DV cameras use the digital frame buffer of the camera in conjunction with a variable clock on the CCDs to accumulate more than a field's worth of light on the face of the chip before transferring the image to the buffer and thence to tape. This can do two things for you: more light integration, and slower frame update rates. What does slower frame update mean? (in terms of fields, copied fields (if any), frame rate) ??

Thanks to whoever will help me in advance.
Sincerely,
Massimo.
Massimo Scappato is offline   Reply With Quote
Old April 24th, 2002, 07:18 PM   #2
Warden
 
Join Date: Mar 2002
Location: Clearwater, FL
Posts: 8,267
Hi,

Television as it exists today is an interlaced system. In 1/50 a second all the even numbered lines (625 lines TOTAL in PAL) are drawn from the top to the bottom of the screen. This is called a field. Then all the odd numbered lines. The combination of the odd and even fields makes 1 frame of video (1/50 plus 1/50 equals 1/25 second).

If you speed up the shutter to stop action or change exposure the chip sees a shorter duration of action. It is still recorded and displayed for 1/50 of a second. This shorter duration of action can create a flickering effect with fast motion.

If you slow down the shutter the motion will become blurred. The image on the chip is not moved to the buffer every 1/50. Fields and frames become interpolated. Different manufactures use different algorthyms to do the interpolations. Different cameras will produce a different look when slowed down.

If you need to reduce the light to achieve the proper apperature you would use a neutral density (ND) filter. They reduce the amount of light hitting the chip thereby forcing the apperature to open up so that more light hits the chip.

Some of this will become clearer if you roll some tape in a camera and view the various results on a monitor. If something is still unclear or my explanation confussing, post back and I'll try to explain a little differently. Good luck.

Jeff Donald
Jeff Donald is offline   Reply With Quote
Old April 26th, 2002, 05:31 AM   #3
Tourist
 
Join Date: Apr 2002
Location: Italy
Posts: 4
1/250< x <1/50. About the flickering.

Sir,
first thanks for the useful reply.
To every reader. Please, partecipate: this is a very important issue.

I still do not understand if with the shortest exposure times (<1/50) there is a flickering too. There is more blur and interpolation, but I heard that there is a lack of frames also or generically something in this direction with the visual look of the scene. Seems strange to me, however.
To be clear, after the inter-field and the intra-frame interpolation the resulting frame starts and ends upon the tape in the same points as the others? Please forgive me if I am quite imprecise here. Or there are less frames? Or the flickering is caused by the lack of real fields (not the interpolated ones)? I am trying to guess.
NB: I read that there is a stroboscopic effect under 1/50 sec and over 1/250 sec of exposure. Why if I transfer to film everyone is advising me to shoot at 1/50 sec and not to shoot between 1/50 and 1/250? I do not understand why if with the DV PAL the blur/flickering level is fine between 1/50 and 1/250, this would be not after the transfer to film!


Thanks.
Massimo Scappato is offline   Reply With Quote
Old April 26th, 2002, 07:09 AM   #4
Warden
 
Join Date: Mar 2002
Location: Clearwater, FL
Posts: 8,267
The slower shutter speeds produce motion blur. The blur is from the subject moving. While the light is accumulating on the chip longer or in the interpolation method the subject moves and blur results. If there is no motion I do not notice a flicker.

Frames are a must. You must have frames for the monitor to display the images properly.

Film is projected at a specific speed or frame rate. The frame rate is usually 24 frames per second. If you shoot PAL at 25 frames per second you have a frame rate that almost matches the film frame rate. this makes for a much easier transfer than NTSC and its 30 frames per second. The PAL conversion to film should not produce a flicker.

Jeff
Jeff Donald is offline   Reply With Quote
Old April 26th, 2002, 08:02 AM   #5
Major Player
 
Join Date: Sep 2001
Location: Las Cruces, NM
Posts: 343
Be Careful What You Wish For...

Hi Massimo,

<To every reader. Please, partecipate: this is a very important issue. >

Please forgive me for not actively participating in this thread. I'm sure it is important and I do find it interesting, but I know just enough about this topic to follow what is being said by others. Certainly not enough to contribute! Soneone once said "It is better to stay quiet and appear stupid than to open your mouth and remove all doubt". (I probably did just that by misquoting) Oh well... I'll shut up now.
__________________
Ed Frazier
Ed Frazier is offline   Reply With Quote
Old April 26th, 2002, 11:35 AM   #6
Contributor
 
Join Date: Sep 2001
Location: Italy Milan
Posts: 59
ciao

Ciao Massimo
dove stati girando questo film???
magari ti vengo a vedere
io vivo a Pavia e lavoro a Milano.

per il tuo dubbio purtroppo non so aiutarti, ma per esempio mi pare che nei film su pellicola il tempo sia un 48 esimo, forse solo una questione di somiglianza con il tempo/luce/diaframma/sensibilita della pellicola.

Ciao

Francesco
Francesco Marano is offline   Reply With Quote
Old April 26th, 2002, 12:57 PM   #7
RED Code Chef
 
Join Date: Oct 2001
Location: Holland
Posts: 12,514
Francesco... Please keep it english on this board. A lot of people
have no clue to what your saying now.

According to altavista translation engine:

Hello Maximum where be turning this film? I to you even come to see I live to Pavia and job to Milan. for your doubt unfortunately I do not know to help you, but as an example he seems to me that in films on film the time is 48 esimo, perhaps is only one issue of likeness with the tempo/luce/diaframma/sensibilita one of the film. Hello Francisco
__________________

Rob Lohman, visuar@iname.com
DV Info Wrangler & RED Code Chef

Join the DV Challenge | Lady X

Search DVinfo.net for quick answers | Buy from the best: DVinfo.net sponsors
Rob Lohman is offline   Reply With Quote
Old April 26th, 2002, 02:15 PM   #8
Hawaiian Shirt Mogul
 
Join Date: Nov 2001
Location: northern cailfornia
Posts: 1,261
you state this is your 1st project. you are just learning your camera, shooting , audio, lighting etc. HOW can you be thinking tape to FILM ??

free yourself !!!! you can shoot 1/100 , 1/250 ... any speed you like - you should see some difference between 1/50 and 1 /250 perhaps you'll like it ? perhaps you won't. THAT is how you learn BY doing .... learn from this project and take what your learned to your next ....


you have other things to worry about then 1/50th shutter , contsant F stop etc... like how are you going to deal with 100 people ? you might want to get a cameraperson to help you out on this one ?

if you are shooting progressive frame shoot any shutter you want ... HOWEVER if you are shooting interlaced then transfers to film tend to be better at 1/50 ....... PAL is made up of 625 scan lines ( top to bottom) .. a video frame is made up of 2 fields... field 1 records the odd lines 1,3,5,7, to 625 .... field 2 records all the even lines 2,4,6,8, to 624 .... in interlace mode the ccd records field 1 to tape then a split sec of time later it records field 2 to tape and this equals 1 frame , if you try to put field 1 and field 2 together they are NOT a perfect match because they were recorded at different times. when you go to film they must combine the 2 fields together to a frame of film FROM testing they have found the 2 fields go together BETTER if 1/50 is used vs. 1/250 = it's just the way it is ....

if you shoot PROGRESSIVE frame then BOTH fields are captured at the same moment in time ( even and odd scan lines ) and when you put these fields together in a frame they are a perfect match ...

PAL interlaced video is also referred to having 50 fields of motion in a sec ( because ALL 50 fields are captured at different time - 1 second is divided into 50 bits of time/motion ...

progressive is also referred to as having 25 fields of motion as during a sec it only recorded 25 bits of motion ( it divided 1 sec into 25 bits of time/motion )

when pal video is transferred to film it is transferred at 25fps ( field 1 and field 2 are combined together to make 1 frame on film.
Don Donatello is offline   Reply With Quote
Old April 27th, 2002, 09:33 AM   #9
Tourist
 
Join Date: Apr 2002
Location: Italy
Posts: 4
INACCURACIES

Donatello, thanks for the reply.
But there are few inaccurancies with your statements!


1) In PAL field 1 = even lines, field 2 = odd lines. The contrary is for NTSC only.
2) What about the intra-field and the inter-frame compression? Does it put both the fields together on the tape (each frame has its place (exactly 1/25 of the tape length for that specific second) or not?.
3) Infact you notice the strobo effect only with a movement, a strobo in the form of lack of frames (movement sections missing). Why? Are there really less frames? THAT's THE QUESTION!!!
The flickering with times inferior to 1/250 sec is different: this is due to the presence of too frozen frames, with very little blur.
4)Remember that you HAVE TO use 1/50 sec in the shooting, you have NO choice else the result transfer to film would be not with too less blur (a matter of taste) but a inaccettable result in regards to the motion! There is not any logical explanation of that... I mean, I am still in search for a technical answer.
Shooting 1/200 with DV is fine, blur is fine, 25 frames => the result of the transfer to film is NOT FINE, 24 frames (quite the same frame rate). WHY????????????
5) The slow shutter speeds (those below 50 fps) found on many DV cameras use the digital frame buffer of the camera in conjunction with a variable clock on the CCDs to accumulate more than a field's worth of light on the face of the chip before transferring the image to the buffer and thence to tape. This can do two things for you: more light integration, and slower frame update rates.
DOES SLOWER UPDATE RATES mean LESS FRAMES? Suppose that I am exposing a field for over 1/50 sec (the time dedicated to him). I can continue exposing it for the time length of his neighbour frame 2 (for another 1/50 sec): so for a maximum of 1/25 sec! But no longer! I know that there is an extreme interpolation with the second frame in this case, but with CANON XL1s I can expose for 1/8 sec !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I have to go on with the exposition, to the next "frame area!" (the tape continues to go).
6) About the progressive mode, quote from http://www.digitalfilmgroup.net/faq.htm#4: "the Canon XL-1 and PD-150 cameras often have functions called "movie mode" or "progressive mode". What this effectively does is turn your footage from 60 interlaced fields (in NTSC) into 30 progressive frames. We call this type of progressive mode a 'pseudo' progressive mode however because unlike a true progressive imaging system which captures all the image at once, these less sophisticated progressive modes capture only one field at a time. Since a single field only represents half the number of lines available, the camera fills in missing pixel information by copying lines - literally 'filling in the holes'. The remaining image is then copied and the next capture is performed". Things are not as good as you are depicting them.
Massimo Scappato is offline   Reply With Quote
Old April 29th, 2002, 01:34 PM   #10
Hawaiian Shirt Mogul
 
Join Date: Nov 2001
Location: northern cailfornia
Posts: 1,261
i'm NTSC - i'll try to put up PAL #"s but if you see a 30 or 60 just make them 25 or 50

""the Canon XL-1 and PD-150 cameras often have functions called "movie mode" or "progressive mode". What this effectively does is turn your footage from 60 interlaced fields (in NTSC) into 30 progressive frames. We call this type of progressive mode a 'pseudo' progressive mode "

as they note above these are NOT real progressive they are "pseudo" - frame mode is NOT true progressive and that is why canon calls it frame mode. the sony 150 shoots progressive at 12.5 fps. therefore neither camera can deliver the goods for true progressive.

"3) Infact you notice the strobo effect only with a movement, a strobo in the form of lack of frames (movement sections missing). Why? Are there really less frames? THAT's THE QUESTION!!! "

i only notice the stobe of 25p when i compare it to 50i ... if there is NO movement within your shot then the only difference you see is in the movement of the NOISE from the camera. a 30p will move the noise 30 times a sec and 60i will move the noise 60 times a sec. which IMO the 60i give that "video feel" .

i prefer 30P over 60i. i say what stobe ? i shot film for years and when you are use to viewing 24fps motion on the big screen 30fps looks smoother and 60 i looks smooth as glass ... but on my projects i've no intention of going to film - i prefer digital projection .. and when i shoot "frame mode" or progressive there are many times that i want it to strobe - i like the 1/1000 shutter speed ( used for effect) ...

most of these hand size camera's when shooting uder 1/50 do NOT record both fields they just double the 1st - resulting in you only get 1/2 the picture ...

for the technicals of how , why they do all this www.adamwilt.com provides some answers and many other technical info on DV ... if you are going to FILM you should be directing questions to a tape to film house that specializes in PAL ... they can provide you with what works the BEST for them

when shooting FILM on sound stages alot of DP's will decide on a F-stop to shoot at and then light for it as you have total control over lighting. when shooting on location ( house) we can tent the windows and have total control over lighting and again pick a f stop ...

i have never picked the F-stop when shooting DV as i have never had total control over the lighting. dv budgets just don't have the $$ to tent a location.
Don Donatello is offline   Reply
Reply

DV Info Net refers all where-to-buy and where-to-rent questions exclusively to these trusted full line dealers and rental houses...

Professional Video
(800) 833-4801
Portland, OR

B&H Photo Video
(866) 521-7381
New York, NY

Z.G.C.
(973) 335-4460
Mountain Lakes, NJ

Abel Cine Tech
(888) 700-4416
N.Y. NY & L.A. CA

Precision Camera
(800) 677-1023
Austin, TX

DV Info Net also encourages you to support local businesses and buy from an authorized dealer in your neighborhood.
  You are here: DV Info Net > The Tools of DV and HD Production > Open DV Discussion

Thread Tools Search this Thread
Search this Thread:

Advanced Search

 



Google
 

All times are GMT -6. The time now is 11:36 PM.


DV Info Net -- Real Names, Real People, Real Info!
1998-2017 The Digital Video Information Network