7D video and shutter speed

Dan7312

I'm trying to understand how shutter speed affects HD video recording on the 7D. Moving up the ISO make the video brighter and so does opening the aperature. Changing the shutter speed doesn't seem to have any effect. Is that because, in effect, the shutter speed is somehow fixed by the frame rate?

If I use an ambient light light meter, like a Sekonic 758DR (not the Cine version), how should I set it up to get an meter reading, i.e. what shutter speed do I use?

Thanks in advance,

cab.in.boston

Dan, I'm no video expert at all, but when shooting DSLR video, I'm pretty sure that the shutter speed means nothing. You're not actually firing the mechanical shutter in video mode (I don't think the 7D - or other DSLRs - can handle 30 frames per second or faster ). I think the sensor is just sampled at a rate determined by the video speed you are shooting, and the whole time the actual shutter just remains open. That would explain why changing the ISO and aperture affect the picture, but the shutter speed has no effect.

Hopefully some video guys can jump in and better answer your question.

Dan7312

I think I understand what I was missing now. The frame rate you choose determines the slowest shutter speed you can use. For example at 60fps you can't set the shutter to less than 1/60th of a second, but you can set to 1/60 or faster. Makes sense now, the shutter can't be opened for more time than the camera spends taking the picture, duh.

So if I shoot video in M mode I can use my light meter as I would for stills, but I'm limited in the shutter speed I can use.

The reason I want to shoot in M mode is a white board is going to be taking up most of the image and my guess is that will confuse the camera's auto metering. So far except for the board itself the image has been a bit dark.

Dan7312

Tried out some more things and as long as you don't pick a shutter speed that is not supported by the frame rate, the settings the ambient light meter says to use work as expected with M setting.

Dan

cab.in.boston

Just so I understand, it is a "virtual" shutter, isn't it? The sensor is collecting data for the amount of time you specify as shutter speed and then transferring it, it's not actually operating the shutter? Seems that would be both beyond the capability of the shutter and even if not, it would be gross abuse of the shutter and shorten its lifespan tremendously.

DavidTO

Not to mention noisy!

Dan7312

No, the physical shutter is not operating, it is open the whole time. The electronics is sampling the sensor. I don't know if the electronics "wipe" the sensor after each frame or the sensor has some kind of continous mode of operation or what.

On a real film movie camera the shutter is a disk with a "V" cut out of it that spins. When "V" flies by the film it is exposed. The spinning disk is mechanically sync'd up with the sprockets that advance the film during the time when the V isn't flying by. On some file movie cameras you can change the disk to change the shutter speed.

Film projectors work in a similar way except that is something happens and the film stops moving it starts to burn. That's what safety film is all about, the burning stops when the heat goes away, but it generates poison gas while it burning. Oh hummm, can't have everthing.:D

ziggy53

Canon has a 7D video mode article which helps to explain some of the video settings and results:

http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=3050

The 7D uses an "electronic" shutter for video and it polls the imager by pixel rows. What this means is that it is similar in treating rapidly moving subjects and fast pans to the problems induced by a focal plane shutter. While the exposure "duration" of each pixel is determined by the electronic shutter, the scan rate is determined by the video frame rate. The result is a "rolling shutter" issue and horizontal motion is distorted, sometimes visibly so.

More useful information on this page:

http://www.imaging-resource.com/PRODS/E7D/E7DVIDEO.HTM

Dan7312

Thanks for the article pointers Ziggy, that help's clear up what is going on.

So it looks like the CMOS sensor is being used like CMOS memory, i.e. being read a row at a time but skipping the refresh. So after the row has been read the cells are empty and ready to do another photon collection. The only way to read CMOS memory is a row at a time so it looks like the rolling shutter problem is intrinsic to the technology.

Do the high end cameras, like Red, have a rolling shutter problem? If not how do they get around it?

I've seen some things where Red people say that the Canon technology has some issues, but they don't mention rolling shutter, I think the call the problem something like dropped lines or frames but I'm not sure because I wasn't really sure what they were talking about. What does Red have that Cannon doesn't, besides some of the obvous things like 12 min limits on video?

ziggy53 wrote: »

Canon has a 7D video mode article which helps to explain some of the video settings and results:

ziggy53

I'm not familiar with the "Red" imagers but I'll try to find out.

I do know that the Red One, for instance, is part of a system, designed for video cinematography. As such it has special fixtures and adapters allowing all sorts of lens options and focus control, recording options, remote camera control, mounting options, audio options, etc.

Understand too that Red system cameras are in a whole different league and it's not really fair comparing it to a Canon 7D and vice versa.

I met someone from "Full Sail University" last month and they had a video that was produced using a Red One. What I saw was awfully nice, but it was displayed outdoors and I couldn't really scrutinize the video quality too much.

Dan7312

Not so much for comparison but for curiosity. The director (@GregYaitanes on twitter) of the television series "House" used Canon DSLR's exclusively to shoot the last episode of House last season. He waxes almost poetic about how good it was to use them. One reason was cost, not the camera but the sets, but in general found it easier to get the results he wanted using the Canon's rather than film. He certainly could have used Red cameras if he wanted to.

ziggy53 wrote: »

Understand too that Red system cameras are in a whole different league and it's not really fair comparing it to a Canon 7D and vice versa.

ziggy53

As I understand it part of the season finale of "House" was filmed in a stable but collapsed building. Instead of building a full set to simulate the actors inside the small working spaces, the producers were able to use the smaller Canon cameras and compact lighting to manage inside the wreckage.

I don't think anyone "enjoyed" the shoot as it posed a tremendous number of new problems, but it was successful and the episode was rather unique. I do think that everyone involved enjoyed the publicity it offered.

I don't think that there are plans for "House" or any other regular series to use dSLR cameras for regular production. Some documentaries and several larger "indie" style productions are getting good use of the new technology. These new video capable dSLRs certainly are proving themselves to be good niche solutions.

nvisiblephoto

To the original question; yes, shutter speed does have an effect when shooting video. Think of shooting video as simply shooting many photos continuously. Higher shutter speed means less motion blur but less light.