Why do 4k video has to crop in the sensor? (Compared to 1080p)

[Baptiste B]

Hi everybody,

I have a question that haunts me for a while.

Why some cameras has to crop in the sensor when you use 4k mode?
I mean when you use 1080p there is no need to crop.

I am not considering here the crop factor introduced by the sensor size, APS-C or MFT camera.

Can somebody explain the difference between the 2 modes? I was searching but I could never find any answer out there.

Thank you!

 

[uncle dunc]

I couldn't give you the technical explanation, but in the case of Panasonic, the crop of the sensor allows them to capture a 4k image with a minimum of processing by the computer chip. If they don't crop the sensor, the processor has to do math calculations to create a 4k image out of the wrong number of pixels, and all that processing creates heat, which can overheat the camera. It only takes around 9 megapixels to create a 4k image, but photo cameras have 16 or 20 megapixels. If you get a camera designed specifically for video, like the GH5s, it only has a 10 megapixel sensor (9 megapixels when you count the pixels used in the video image dimensions) which means there is no crop in 4k and no overheating because the sensor is using the exact number of pixels needed for the 4k image. Plus, the pixels are larger, which enables better low light performance.

 

[DMKAlex]

I couldn't give you the technical explanation, but in the case of Panasonic, the crop of the sensor allows them to capture a 4k image with a minimum of processing by the computer chip. If they don't crop the sensor, the processor has to do math calculations to create a 4k image out of the wrong number of pixels, and all that processing creates heat, which can overheat the camera. It only takes around 9 megapixels to create a 4k image, but photo cameras have 16 or 20 megapixels. If you get a camera designed specifically for video, like the GH5s, it only has a 10 megapixel sensor (9 megapixels when you count the pixels used in the video image dimensions) which means there is no crop in 4k and no overheating because the sensor is using the exact number of pixels needed for the 4k image. Plus, the pixels are larger, which enables better low light performance.

I don't believe the Sony RX100M4 crops in 4K. Is that why they have a over heating issue in 4K?

 

[uncle dunc]

I don't believe the Sony RX100M4 crops in 4K. Is that why they have a over heating issue in 4K?

I don't know how they get their 4k, but I think the overheating is related to the small size of the camera body. Even Panasonic, with their great reputation for not overheating, has time limits on video recording with their smaller bodied cameras.

I've heard others mention Sony's scheme for image processing as an issue contributing to 4k overheating.

 

[DMKAlex]

I don't believe the Sony RX100M4 crops in 4K. Is that why they have a over heating issue in 4K?

I don't know how they get their 4k, but I think the overheating is related to the small size of the camera body. Even Panasonic, with their great reputation for not overheating, has time limits on video recording with their smaller bodied cameras.

I've heard others mention Sony's scheme for image processing as an issue contributing to 4k overheating.

The OH issue was a concern to me earlier on. But as I understand my shooting style, it had never been a game stopper. I shoot mostly 20 sec to 40 sec clips. Only when I had to shoot 3-4 minutes clips continuously for a while that the temperature signal would come on. But it quickly cured itself after I turned the camera off for a couple of minutes.

 

[Baptiste B]

Thank you for your suggestions, but the point is not overheating issues,

I can understand that if they want 4k (3840 x 2160) it is more convenient to have a sensor with this amount of pixels, but what about 1080p? I never heard a camera had to crop for this resolution.

Are the manufacturers downsampling the signal to match the 1080p resolution?

I really would like to have more technical explanations for that.

 

[Victor Engel]

Thank you for your suggestions, but the point is not overheating issues,

I can understand that if they want 4k (3840 x 2160) it is more convenient to have a sensor with this amount of pixels, but what about 1080p? I never heard a camera had to crop for this resolution.

Are the manufacturers downsampling the signal to match the 1080p resolution?

I really would like to have more technical explanations for that.

From experiments done to test aliasing, it has been demonstrated that some skip rasters in order to provide this resolution. They're not reading the whole sensor. Some use the whole sensor. Sorry I don't know which are which.

 

[Off The Mark]

Thank you for your suggestions, but the point is not overheating issues,

I can understand that if they want 4k (3840 x 2160) it is more convenient to have a sensor with this amount of pixels, but what about 1080p? I never heard a camera had to crop for this resolution.

Are the manufacturers downsampling the signal to match the 1080p resolution?

I really would like to have more technical explanations for that.

Agree with Viktor above. Some are skipping lines and some are pixel binning their 1080p sensor readouts so that the processor is actually processing less data.

Apparently it is easier to process 6K input to 4K output than it is to process 6K input to 1080p output.

 

[erichK]

I don't believe the Sony RX100M4 crops in 4K. Is that why they have a over heating issue in 4K?

I don't know how they get their 4k, but I think the overheating is related to the small size of the camera body. Even Panasonic, with their great reputation for not overheating, has time limits on video recording with their smaller bodied cameras.

I've heard others mention Sony's scheme for image processing as an issue contributing to 4k overheating.

The OH issue was a concern to me earlier on. But as I understand my shooting style, it had never been a game stopper. I shoot mostly 20 sec to 40 sec clips. Only when I had to shoot 3-4 minutes clips continuously for a while that the temperature signal would come on. But it quickly cured itself after I turned the camera off for a couple of minutes.

Sensor heat can be a real issue. It is for me because a major part of my shooting is classical solo and chamber music concerts. Often, cameras "fall asleep" during loner shoots, likely because protection circuitry kicks in.

Single pieces - like Messiaen's Quartet for the End of Time - can be nearly 50 minutes long and more typical ones like Schubert's Trout Quintet well over 30 minutes. The recording environment is stately, formal and quiet, much like that in a dramatic production or some presentations.

Experience is teaching me that dslr form-factor cameras are very problematic for such projects. They produce much higher IQ than the small-sensored higher-consumer-range camcorders long used for such recording, but at a considerable to huge increase in time and trouble involved in both recording and post-processing/production.

The interim solution is to mount pairs of cameras (one needs at least two angles anyway) in tandem and quietly start each second well after the first, , and switch at the (obligatory anyway) 30 minute mark, and synchronize to a separate master digital sound track (which can be a real pain, as even professional videographers warn). Black Magic's upcoming mFT's camera with a built in fan and cooling slots top and bottom may well be a better, longer term one. It should, at least, provide a reliable, high quality "A" track.

 

[uncle dunc]

...Experience is teaching me that dslr form-factor cameras are very problematic for such projects. They produce much higher IQ than the small-sensored higher-consumer-range camcorders long used for such recording, but at a considerable to huge increase in time and trouble involved in both recording and post-processing/production.

The interim solution is to mount pairs of cameras (one needs at least two angles anyway) in tandem and quietly start each second well after the first, , and switch at the (obligatory anyway) 30 minute mark...

...Or, the interim solution is to mount a pair of Panasonic GX85s, which have no time limit for 4k recording, other than battery power or card size, and which don't overheat shooting 4k unless you get into the 100 degree range, in which case they'll run for approximately 20 minutes.

 

[erichK]

...Experience is teaching me that dslr form-factor cameras are very problematic for such projects. They produce much higher IQ than the small-sensored higher-consumer-range camcorders long used for such recording, but at a considerable to huge increase in time and trouble involved in both recording and post-processing/production.

The interim solution is to mount pairs of cameras (one needs at least two angles anyway) in tandem and quietly start each second well after the first, , and switch at the (obligatory anyway) 30 minute mark...

...Or, the interim solution is to mount a pair of Panasonic GX85s, which have no time limit for 4k recording, other than battery power or card size, and which don't overheat shooting 4k unless you get into the 100 degree range, in which case they'll run for approximately 20 minutes.

I run a G85 alongside an Olympus OM1ii, trying for good 4K (mainly to allow PP zoom and crop). No 30 minute limit and adequate battery (and I have a dummy and external power feed, if needed), yet frequent problems.. Initially shut-downs trying to operate via wiFi shooting 4K, apparently because of band-with problems (apparently solvable with a portable router - but I don't need yet another gadget!), more recently, sleep or apparent sleep (dark LCD - just as bad because diagnosing interrupts/disturbs shots.

So, PPings about about a dozen multi-camers formal recordings and a few informal pub shots, I've been surprised to find the best footage about evenly divided between the putatively superior G85 and the EM1ii, and the old OM5ii and little PenF often saving the boat, with slightly less crisp, but quite useable HD footage as the G85 or EM1 miss focus or otherwise mess-up

When I bought the G85, Don's Photo also had a used smaller-sensored, semi-pro video camera with xlr inputs, long zoom, big battery for sale at about the same price. Likely a better choice for this kind of shooting.


erichK
saskatoon, canada
Photography is a small voice, at best, but sometimes one photograph, or a group of them, can lure our sense of awareness.
- W. Eugene Smith, Dec 30, 1918 to Oct 15, 1978.

http://erichk.zenfolio.com/

http://www.fototime.com/inv/7F3D846BCD301F3

Photobook: http://www.blurb.ca/b/7525756-the-book-of-gina

 

[Markr041]

I don't believe the Sony RX100M4 crops in 4K. Is that why they have a over heating issue in 4K?

I don't know how they get their 4k, but I think the overheating is related to the small size of the camera body. Even Panasonic, with their great reputation for not overheating, has time limits on video recording with their smaller bodied cameras.

I've heard others mention Sony's scheme for image processing as an issue contributing to 4k overheating.

The OH issue was a concern to me earlier on. But as I understand my shooting style, it had never been a game stopper. I shoot mostly 20 sec to 40 sec clips. Only when I had to shoot 3-4 minutes clips continuously for a while that the temperature signal would come on. But it quickly cured itself after I turned the camera off for a couple of minutes.

Black Magic's upcoming mFT's camera with a built in fan and cooling slots top and bottom may well be a better, longer term one. It should, at least, provide a reliable, high quality "A" track.

I don't understand the attraction of the Blackmagic Camera here (for shooting long-term videos of concerts). The alternatives are not just big-sensor mirrorless cameras, DSLRs, small-sensor camcorders, and the BM mFT 4K.

The Sony FS7, for example, has a fan, does not overheat, has no time limit and produces higher quality 4K video with greater dynamic range and better low-light performance with its bigger sensor than the forthcoming BM camera (even assuming it has no bugs). Or there is the cheaper Sony FS5, with the same sensor as the FS7 but also no limits due to overheating. Are you restricted by budget? by camera size?

 

[Gary3000]

Thank you for your suggestions, but the point is not overheating issues,

I can understand that if they want 4k (3840 x 2160) it is more convenient to have a sensor with this amount of pixels, but what about 1080p? I never heard a camera had to crop for this resolution.

Are the manufacturers downsampling the signal to match the 1080p resolution?

I really would like to have more technical explanations for that.

Agree with Viktor above. Some are skipping lines and some are pixel binning their 1080p sensor readouts so that the processor is actually processing less data.

Apparently it is easier to process 6K input to 4K output than it is to process 6K input to 1080p output.

cropping the sensor also allows some flexibility in stabilizing footage in-camera. (IBIS) because it gives more wiggle room.

in the end. 4k is 4x the data of 1080, so it takes 4x the processing power!

 

[Off The Mark]

...The Sony FS7, for example, has a fan, does not overheat, has no time limit and produces higher quality 4K video with greater dynamic range and better low-light performance with its bigger sensor than the forthcoming BM camera (even assuming it has no bugs).

Yes, but you are comparing a $7,000 video camera to a $1,300 that includes a free copy of Resolve Studio (a $300 value). So yeah, the FS7 had BETTER be better camera.

 

[erichK]

...The Sony FS7, for example, has a fan, does not overheat, has no time limit and produces higher quality 4K video with greater dynamic range and better low-light performance with its bigger sensor than the forthcoming BM camera (even assuming it has no bugs).

Yes, but you are comparing a $7,000 video camera to a $1,300 that includes a free copy of Resolve Studio (a $300 value). So yeah, the FS7 had BETTER be better camera.

Also, if you look at my list of gear you will note that I have several stellar four thirds and many Micro Four Thirds lenses which should work well with the Black Magic camera. And it also at the outside limits of what I, as a photographer first and videographer second, would be willing to spend on a video camera/

erichK
saskatoon, canada
Photography is a small voice, at best, but sometimes one photograph, or a group of them, can lure our sense of awareness.
- W. Eugene Smith, Dec 30, 1918 to Oct 15, 1978.

http://erichk.zenfolio.com/

http://www.fototime.com/inv/7F3D846BCD301F3

Photobook: http://www.blurb.ca/b/7525756-the-book-of-gina