Shutter Speed vs Max FPS

[Joseph]

Hi, Everyone

Sony A7R V is limited to shooting a maximum of 7 FPS in uncompressed raw and 10 FPS in JPEG
[Mechanical Shutter].

My Question:

If your shutter speed was set at 1/100 'theoretically' if my math isn't off, that's 100 FPS - please you are more than welcome
to correct me if I am wrong?

Can someone explain what is happening here?

It makes no difference if you select 1/60 of a second or even 1/1000 of a second it's still 7 FPS max?
I broadly understand the mechanics and technical limitations of sensor read / write speeds.

But I still can't get my head aound the 'theoretical' MAX FPS and the obvious difference say between 1/60 and 1/000
all resulting in the same max FPS.

I figured the limitation is the time the processor takes to read / write off the sensor in this regard the shutter speed
would become totally irrelevant.

Nevertheless, I still find myself struggling with the concept. The Mechanical shutter is capable of actuating say 100 times a second
but only records 7 FPS due to the limitations of sensor read / write speeds?

In other words... If you held the shutter down for exactly 1 sec @ 1/100 where do the rest of the images
go if you only get 7 FPS?


Many thank's for your thoughts and responses
in advance,
Regards

 

[Richard A]

You can get 10 FPS in compressed RAW also. My understanding is it it limited by the sensor readout speed but I may be wrong.

 

[iamdlewis]

The "rest of the images" don't go anywhere because they never were. It isn't the shutter speed that determines the FPS it is the processing capabilities of the CPU. In the development of the CPU it will be coded to know the size of the images that the sensor creates and how much onboard storage capabilities it has, so it will only allow in an image when it has space for that image, as well as a timer feature to say how often it can take in that image data so that you don't just capture the 1st 7/100ths. So you take an image at 1/100th and that get passed to the CPU and the CPU then tells the sensor that my door is closed for the next 14.28/100th so don't bother sending me any data, so the sensor doesn't even generate the image during that time.

 

[AlphaWorld]

First off, the shutter speed is just of the things limiting your frame rate.

Others are:

• How quickly the shutter can reset (if using a mechanical shutter - this involves physical movement, so it’s not instant).
• If you are in AF-C, how long it takes to focus before the next shot (assuming AF priority is set to Focus - if it’s Release, doesn’t matter)
• How long it takes the camera to read the data from the sensor (on the A7RV that is roughly 1/10, so you are never going faster than 10fps!)
• How long it takes the camera to process the image (into a RAW or JPEG, or HEIF), although the images can accumulate in the buffer for a bit, once they have been read from the sensor)

Also bear in mind that you might have a shutter speed of 1/2000, but the shutter still takes a certain amount of time to travel across the sensor - something like 1/250 for the mechanical shutter or 1/10 for the e-shutter (on the A7RV) - only the A9 III grabs every pixel at once.

 

[AlphaWorld]

BTW: the mechanical shutter is not capable of anything like 100 actuations per second.

Maybe it would help if you think of it like this:

1. You press the shutter button
2. The camera close the shutter
3. The camera clears the sensor (it has been used for the viewfinder or rear screen view until now)
4. The shutter first curtain starts moving
5. The shutter second curtain starts moving - possibly chasing the first curtain, possibly after the first curtain has been open “a while”
6. The second curtain closes - about 1/250 after it started moving
7. The sensor is read - this is a very slow sensor, so this takes about 1/10
8. The CPU processes the data from the sensor into a form fit for the buffer (probably a RAW file or a JPEG)
9. The CPU writes the file from the buffer to the memory card (thus can be overlapped with other operations).

 

[Joseph]

Thank you for your response I did understand the limitation was due to the sensor and processing read write speeds but it was just the 'theoretical' FPS versus the actual max FPS achievable that I was struggling to understand.

Your helpful response was appreciated and much clearer now.


Regards,
Joseph