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 Re: A7R6 readout speed | |
speedmaster20d wrote:
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duncangr wrote:
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Fred Miranda wrote:
Steve Spencer wrote:
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speedmaster20d wrote:
For those interested I just measured the readout speed for the A7R6 (as well as A7R5 and A1 II that I have at hand). I had preordered the camera and it came yesterday.
I used a pulse generator driving a high speed LED to generate a precisie optical waveform and capture it with each camera (will put more details on my channel)
summary
--------------------
A1 II : 3.9 msec
A7R5 : 94.35 msec
A7R6 : 19.7 msec for RAW (Lossless compressed or compressed HQ) / 14.1 msec for lossy compressed
it is quite a bit behind A1 series
one pleasant surprise : In A7R6 the EVF resolution does NOT drop like all other Sony cameras when you engage the continuous AF by half-pressing the shutter. It remains high and the EVF brightness is great as well
BTW thanks, you didn't happen to test in APS-C mode did you.
Hopefully Arash can give us an actual measurement, but it should be basically a function of the crop factor so something close to 13.1 msec (i.e., 19.7 / 1.5) in RAW or lossless compressed or compressed HQ and 9.4 msec (i.e., 14.1 / 1.5) in lossy compressed.
This seems right to me, and getting around ~9 ms in APS-C (lossy compressed) actually makes it a very usable option for action shooting. I don't remember exactly, but I think the original A9, the one that made a lot of action photographers switch over pretty quickly, was around 5-6 ms.
unfortunately that does not help. it is the scan slope (time to read a single row) that matters not the total readout time. The distortion is a function of panning speed relative to the scan slope. I checked and the slope does not change in APS-C crop. the distortion will look identical i.e. it will look same as just cropping the FF image in post.
I agree the ciritcal readout time (beyond which is almost don't care) appears to be about 5-6 msec for a FF sensor (or 3-4 msec for APS-C).... so the A7R6 falls short by 2.5X in best case scenario
the issue I take is the strong emphasis Sony has put on "ready for pro action" for this camera... not true advertising IMO. Otherwise it's fine for what it is.
But if you have the same FOV in APS-C mode as in FF mode then the slope will be quite a bit less. As you rightly point out cropping alone will make no difference.
the slope does not change in APS-C mode, it's the same. Slope is limited by sensor electronics not FoV or anything like that.
if you want to get same FoV in APS-C as FF you have to walk quite a bit back from the bird that will reduce the rotational speed as you pan from much farther away and makes the distortion less .... but no one would switch to APS-C and the walk back from the bird to end up with a low resolution image of a far bird.... doesn't makes sense really
It depends on what's being compared. People use crop mode because they can't fill the frame. If one is comparing 400mm at crop with 400mm at full frame, then yes, the slope is the same. If one is comparing 400mm at crop with 600mm at full frame because they don't want to afford 600mm, I think the slope is better on 400mm at crop.
This is how I think about this problem
1. The sensor scan slope is the same value regardless of the APS-C crop or FF (at least for stills that I tested, I cannot test it for video) it does not change whether a 100mm lens is mounted or 800mm lens is mounted or whether you are closer or farther away (it's a fixed physical quantity)
2. the distortion is a function of : 2.a. the scan slope (fixed) 2.b. the horizontal movement of a fixed reference on the sensor plane during the read out time as result of photographer panning , let's call it dx
3. dx = V x readout time, where V is velocity of a fixed reference on the sensor plane when panning.
4. V can be calculated from 4.a panning speed (angular velocity) which is in sync with BIF 4.b the distance of the bird to the photographer 4.c the focal length of the lens ( 4.c is a weak factor) using laws of optics and geometry
the user cannot change the readout time, nor can they change the bird's flight speed (they have to pan as fast as the bird if they want to capture the bird), But user can change 4.b by getting closer or farther from the subject, or 4.c by using a different lens
I won't bore you with the math, but the result of the calculation is shown in the table attached. I defined, somewhat arbitrarily, distortion as the slant angle of perfectly vertical feature like the image below. I have subjectively found that an angle of 10 degree or higher is quite obvious and thus color coded the table.
you can map your scenario to this table to see whether it improves or degrades the distortion
e.g. you use crop mode, it makes no difference as the crop is not a variable in the math
but say you use the crop made then walk back from the subject using the same lens the distortion becomes less because the velocity of the background on sensor plane decreases ....
hope this helps
I generated this table for 600mm lens in the first attachment and the 2nd attachment shows the same table but for a 400mm lens. the difference is no that large as in both cases the subject distance is >> focal length which is always the case for bird photography
I used AI to find estimates for each species flight speed. it may not be 100% accurate
Thanks for the inputs. I agree len's angular movement and the horizontal distance traveled stay the same regardless of the focal length. However, in my thinking, the vertical length in the frame is a function of the len's focal length. And slope is a function of both horizontal movement and vertical length. Let me throw up some numbers for illustration.
Let's say there is a very tall vertical pole that from the 400mm full frame you see a segment of 6 meters tall. Let's say during the full frame scan time, the bird travels 3 meters horizontally, so you would have panned 3 meters. So the slope observed on the pole is 3/6. In the crop frame using the same lens, you will only see a segment of 4 meters tall, and during the crop scan time, the bird travels 2 meters, you would have panned 2 meters horizontally. So slope is 2/4, which is still the same. That part I think we all agree.
Now if we put on a 600mm lens on full frame. You will only see a segment of 4 meters tall of the pole in the picture. However to follow the bird, you would still have panned 3 meters horizontally during the full frame scan time. So the slope now becomes 3/4 which is worse compared to 1/2.
Let me know if you see something I missed in my reasoning. I think it's still consistent with your walking back scenario. However, I don't think people contemplating using crop frame is planning to walk back, rather they want to stay in the same spot while trying to get the framing/composition which they otherwise need a longer lens to fill the full frame. So it seems reasonable to compare shooting a shorter lens on crop with shooting a longer lens on full frame, at identical conditions.
I think we have some terminology gap here
the slope I refer to is the scan scope i.e the rate at which the sensor scans the row (please see the PDF paper to understand). we call all that sensor_readout_slope
it seems you are referring to something else as slope here. With that in mind I think you have several flaws in your logic.
1. Incorrect geometry framework : using "meters or length" for panning, panning is measured in radiant (angle) not meter. if the bird is traveling at speed V at distance d the panning speed or w = V / d [radiant / sec ] it does not depend on your lens or crop vs no crop. for example if a plane is traveling at 500 mph but it is 5 miles aways you barely need to pan when tracking it.
2. incorrect methodology (ignoring Laws of optics) : what you call slope cannot be calculated on the subject plane, but has to be calculated on the image sensor plane which is what we care about for rolling shutter and is exactly what I did. it can easily be proven that it is not dependent on the crop factor per below
let say the hight of the sensor is h (e.g. 24mm for FF and < 24mm in crop mode)
The horizontal shift on the sensor plane, dx is computed using lens maker's formula : dx = tan(w x t) x d x F / (d-F) (1) I will simplify this as it become messy to type in text but the table I provided uses the full formula
for BIF d>>F and w x t is a small number so dx ~ w x t F = V x F x t /d (2)
F = focal length (m)
d = distance to bird (m)
w = panning speed [radiant /sec ]
t = readout time (sec) = h x sensor_readout_slope
V = BIF flight speed (m/sec)
the distortion angle theta can be computed simply tan(theta) = dx / h = V x F x t / d x 1/h (3)
but we know t = h X sensor_readout_slope we substitute in (3)
than(theta) = V x F x sensor_readout_slope / d (4)
h cancels out and has no effect --> APS Crop has no effect at all
so while you description is bit intuitive it is mathematically incorrect which leads to only partially correct predictions
Good discussion. Yes, I'm using the "slope" loosely. I mean in this case, tan(theta), so bigger slope, bigger theta.
Can you clarify what d means in your definition? Does it mean from your location to the bird/pole? If so, that's on an orthogonal plane to the plane where your theta is drawn. Theta is on the image/sensor plane, and it has nothing to do with the distance from camera to the bird. If d means something else, let me know. Thanks.
d is the distance of the sensor plane to the subject (BIF)
the angular velocity is proportional to 1/d and theta is connected to d though the imaging condition of the lens -you cannot ignore the optics, the light path from subject to the sensor is not straight, it is bent (refracted) by the lens
In this case, your formula for theta is incorrect. Your formula just gives back the panning angle from the camera, that is how much your lens rotates during the scanning. It's not the angle on the image plane regarding how much a vertical line tilts.
you did not pay attentions to the math I am afraid despite my effort, the image sensor is attached to the camera and it rotates with the same rate as you rotate the camera. dx is computed on the image plane
I seems to me you are not very comfortable with the math, I don't have a better way to explain it. One suggestion is to pose the problem to AI have it walk you through and derive the same thing, maybe it helps.
Please don't get there,  I understand this math thing quite decently, and I already mentioned the issue I saw in your formula. Let's just leave at that.
I seriously suggest you pose this question to AI and see what it gives.
good luck!
Edit: out of curiosity I did it myself and attach Chat GPT's answer below. looks familiar to (4) perhaps it has already made forums like this obsolete
Yes, FVt is a function of focal length only, and DH is the height of the sensor. So that matches what my illustration and also has nothing to do with the distance from the camera to the object.
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