Strange aperture behavior

[SrMi]

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Photojoejoe21 first noticed this strange behavior on his Q2. I was able to reproduce it on Q2 and SL2.

SL2 usually keeps the aperture wide open while focusing manually, regardless of the working aperture. However, in some instances, SL2 suddenly closes the aperture to something like f/8 (shutter not pressed). This strange behavior makes manual focusing harder as the DOF becomes too large for manual focusing.

While it occurs in regular operation, I created an artificial setup that demonstrates the issue:

• manual exposure mode
• low ISO (ISO 200)
• fast shutter speed (1/4000)
• a small aperture (f/16)

I would expect SL2 to close the aperture only when fully pressing the shutter. As you decrease the aperture from f/4 toward f/16 or less, the aperture suddenly closes. The closed aperture does not correspond to the working aperture as it does not change when changing the aperture.

Does anyone have an explanation?

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[jaapv]

Your setup suggests a deliberate design choice, not a bug. It may have to do with the speed of the aperture closing not keeping up with the shutter speed (speculation)

[BernardC]

7 hours ago, SrMi said:

• manual exposure mode
• low ISO (ISO 200)
• fast shutter speed (1/4000)
• a small aperture (f/16)

That's a ISO/speed/aperture combination corresponds to a scene 16x brighter than direct mid-day summer sunlight (1/ISO @ f:16). Do you see the issue at exposure combinations that are closer to what one would expect?

[SrMi]

5 hours ago, jaapv said:

Your setup suggests a deliberate design choice, not a bug. It may have to do with the speed of the aperture closing not keeping up with the shutter speed (speculation)

I was also thinking of that reason. However, the selected ISO should not matter. At ISO 800 or higher, the phenomenon does not occur. BTW, it happens with CL as well. 
It looks like work-as-intended, but I wonder why. It can be a problem in practice.

[SrMi]

44 minutes ago, BernardC said:

That's a ISO/speed/aperture combination corresponds to a scene 16x brighter than direct mid-day summer sunlight (1/ISO @ f:16). Do you see the issue at exposure combinations that are closer to what one would expect?

So far, I see the issue only when I hit the described constraints.

[BernardC]

I wonder if it's a strategy to protect the sensor from extreme over-exposure. The most likely way to get that kind of exposure reading is by pointing your camera directly at the sun.

[SrMi]

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39 minutes ago, BernardC said:

I wonder if it's a strategy to protect the sensor from extreme over-exposure. The most likely way to get that kind of exposure reading is by pointing your camera directly at the sun.

I was thinking of that too, but why does Leica reduce the aperture only at lower ISOs. 
Maybe there is an issue with very bright scenes, where ISO is low, and the system can’t properly process the photons hitting the sensor unless reducing the light.

[BernardC]

8 minutes ago, SrMi said:

I was thinking of that too, but why does Leica reduce the aperture only at lower ISOs. 
Maybe there is an issue with very bright scenes, where ISO is low, and the system can’t properly process the photons hitting the sensor unless reducing the light.

Probably because 1/4000 F:16 at 1600 ISO isn't an unusual brightness level. It's just one stop past "sunny sixteen" (1/1600 at f:16 and ISO1600). The sensor can deal with that. As I mentioned before, the exposure you used is 16 times (4 stops) brighter than mid-day sunlight, which may be beyond the sensor's design specifications. The solution is to stop-down and bring brightness levels closer to what Earth-bound cameras are built for.

Anecdote: the "sunny 16" rule technically applies to Rochester, New York (home of Kodak). Rochester is at 44 degrees of latitude. You should use "sunny 22" near the equator, and probably "sunny 11" near the poles.

[SrMi]

21 minutes ago, BernardC said:

Probably because 1/4000 F:16 at 1600 ISO isn't an unusual brightness level. It's just one stop past "sunny sixteen" (1/1600 at f:16 and ISO1600). The sensor can deal with that. As I mentioned before, the exposure you used is 16 times (4 stops) brighter than mid-day sunlight, which may be beyond the sensor's design specifications. The solution is to stop-down and bring brightness levels closer to what Earth-bound cameras are built for.

Anecdote: the "sunny 16" rule technically applies to Rochester, New York (home of Kodak). Rochester is at 44 degrees of latitude. You should use "sunny 22" near the equator, and probably "sunny 11" near the poles.

Nice story about "sunny 16", thanks.
The ISO setting does not influence the sensor. It is applied after the camera reads sensor data (except dual-gain). Low ISO does correspond to more light, and the camera likely expects it. It is possible, as you said, that the camera closes the aperture to protect the sensor; or facilitate processing at those settings. 
Also, there is a hysteresis in the behavior. It is not at the same setting that the aperture closes and opens. It closes at the same shutter-speed/aperture combination but opens at a much lower shutter speed, f-number.

[Photojoejoe21]

11 hours ago, BernardC said:

That's a ISO/speed/aperture combination corresponds to a scene 16x brighter than direct mid-day summer sunlight (1/ISO @ f:16). Do you see the issue at exposure combinations that are closer to what one would expect?

Hey! I encountered this frequently when shooting my Q2, but only when there were bright light sources taking up most of the image or, basically anywhere outside in direct sunlight. I got this camera just to shoot street photography with, so naturally I'm out when there is bright sun and hard shadows to play with. My settings are usually somewhere around the sunny 16 rule. So something like ISO 100, f/16, 1/125th of a second.

The opposite also occurred, where I would be trying to shoot at f/1.7 and would set my shutter speed and iso to the meter for a properly exposed image, and the camera would stop the lens down to around f/8 for the preview for focus peaking purposes, overriding the physical setting I had the aperture ring at.

I am not able to test my work cameras (Sony A1 and A9) for this issue because they stop down the aperture in the viewfinder for live view preview, unlike the Leica which I thought was only supposed to stop the lens down when you half-press the shutter. 

I've also gone through the whole manual for the Q2 and found nothing that fixed the issue for me! Tried different combinations of playing with exposure preview in "P A S" and "P A S M," electronic vs mechanical shutter, and AUTO ISO.

Pretty stumped at this point!

Edited December 15, 2021 by Photojoejoe21

[adan]

I think there is an obvious explanation for this. EVF or "live-viewing."

A sensor will blow out the image if there is too much light falling on it. The pixel wells fill up and overflow.

When viewing, there is no "shutter speed" - the sensor is exposed to the image from the lens continuously. The shutter is more or less on "Bulb" when viewing. Under some light, that may mean the light-flow at f/1.7-2.0. is effectively a fire-hose.

If the meter/electronics determine this is going to blow-out the viewing image significantly, even at "base" ISO (as dark as the sensor can go), it will stop down the aperture enough to reduce the constant light flow - because the aperture is the only light-control available when viewing.

I experienced this with the one EVF camera I used significantly - the Digilux-2. In bright light, it would stop down during viewing, regardless of the selected aperture. Which, indeed, occasionally meant trying to focus at f/8 (with an 9-23mm lens ) even if I was going to shoot at f/2-2.4 (wide-open).

It is not really protecting anything - except the ability to see an image on the EVF or LCD that is not mostly blown-out.

[NickT]

When I used these settings, in today's gloomy weather in the UK, the screen was completely black. Thus the camera has not enough light to produce an image. When half pressing the shutter or pressing the joystick an image appears as the camera amplifies the signal so you can at least see what you are looking at. When pressing the shutter I recorded a completely black image. So isn't the camera simply amplifying the signal rather than adjusting the aperture to help the photographer view the image?

[wizard]

vor 13 Stunden schrieb adan:

If the meter/electronics determine this is going to blow-out the viewing image significantly, even at "base" ISO (as dark as the sensor can go), it will stop down the aperture enough to reduce the constant light flow - because the aperture is the only light-control available when viewing.

I sympathize with your explanation, and have encountered a similar behaviour on an Olympus digital camera. However, your explanation does not explain why - as the OP has pointed out - this closing the aperture phenomenon does not occur when ISO is set to 800 or higher.

[BernardC]

1 hour ago, wizard said:

However, your explanation does not explain why - as the OP has pointed out - this closing the aperture phenomenon does not occur when ISO is set to 800 or higher.

Think of it in terms of light intensity.

1/4000 at f:16 is summer daylight at ISO 800, but it's brighter than anything on Earth at base ISO. You can, of course, achieve this brightness level by shooting directly into a powerful light source, but it's understandable why it could be beyond the expected range for live-view.

Supposing that there is a range of illumination levels that are compatible with live-view, it makes sense to extend that range toward low-light, and stop-down a bit if the illumination level is far beyond direct summer sunlight.

[SrMi]

1 hour ago, BernardC said:

Think of it in terms of light intensity.

1/4000 at f:16 is summer daylight at ISO 800, but it's brighter than anything on Earth at base ISO. You can, of course, achieve this brightness level by shooting directly into a powerful light source, but it's understandable why it could be beyond the expected range for live-view.

Supposing that there is a range of illumination levels that are compatible with live-view, it makes sense to extend that range toward low-light, and stop-down a bit if the illumination level is far beyond direct summer sunlight.

In this case, it would be light intensity assumed by the camera. The actual scene brightness (and live view scene brightness) does not change the point at which the aperture closes.

BTW, with Q2, ISO 100, 1/2000, the camera closes aperture at f/5.6. At 1/12,550, the aperture is always closed (f/1.7).

 

Edited December 15, 2021 by SrMi

[BernardC]

29 minutes ago, SrMi said:

In this case, it would be light intensity assumed by the camera. The actual scene brightness (and live view scene brightness) does not change the point at which the aperture closes.

That's correct, and it's a possible explanation:

• The camera uses the image sensor as an exposure meter.
• The sensor can only measure up-to a maximum amount of light. The A/D converter overflows beyond that point.
• When you set your exposure beyond that point (which is much brighter than the brightest daylight scene on Earth), the camera tells the lens to stop-down.
• This brings the light level back into measurable range.

In other words, the camera can't assume that you aren't taking pictures on the surface of Mercury. It can't assume that its light readings are correct either, because the sensor's A/D converters would be unreliable if you were on the sunny side of Mercury. The sensible thing to do is to reduce the amount of light.

[wizard]

vor 17 Stunden schrieb BernardC:

1/4000 at f:16 is summer daylight at ISO 800, but it's brighter than anything on Earth at base ISO.

I fail to understand this argument. The light intensity entering the lens/camera is, as has been pointed out correctly, the same (at any given point in time), regardless of whether ISO is set at base ISO or at ISO 800. Within the camera, however, ISO 800 means that the light gathered by the sensor will be amplified to make the sensor even more light sensitive. Why would the camera, at the same ambient light intensity, close the aperture down only at base ISO and not at ISO 800, when at the latter ISO setting the received light intensity is amplified internally, thus resulting in even more light (as seen by the camera)?

[BernardC]

1 hour ago, wizard said:

Why would the camera, at the same ambient light intensity, close the aperture down only at base ISO and not at ISO 800, when at the latter ISO setting the received light intensity is amplified internally, thus resulting in even more light (as seen by the camera)?

The camera doesn't read your mind. It doesn't know that you have deliberately set an unlikely exposure. All it knows is what you tell it: the light level out there is well over what the sensor can read reliably.

 

Let's say that the A/D converters in the sensor can read values between "0" and "100,000" Lux. In reality it's either a 14-bit ot 16-bit number, depending who you ask, but let's stick with 100,000.

You are telling the camera that the actual light level corresponds to 123,456 Lux, which is past 100,000. The A/D converter would be out of its measurement range if that was the case, so it would truncate its output and return 23,456 (it could also return another value, but this is a simplification).

As I've speculated before, I think that the camera stops-down at this point, so can be confident that the sensor isn't getting more light than the A/D converter can handle.

 

As for your question: "why doesn't this happen at ISO 800?" The answer is simple, 1/4000 at f:16 is only 40,000 Lux (or so) at ISO 800, and that's well within the A/D converter's capabilities. There's no need to stop-down, because there's no reason to suspect that the sensor might be overflowing.

By the way, light isn't "amplified internally" by the sensor at higher ISO, but that's been covered in many different threads here (and all over the web).

 

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[wizard]

vor 4 Stunden schrieb BernardC:

The camera doesn't read your mind. It doesn't know that you have deliberately set an unlikely exposure. All it knows is what you tell it: the light level out there is well over what the sensor can read reliably.

Don't get me wrong, you may well be correct. I am just interested to learn why the camera adopts such a behaviour. What I still don't understand is why you say that a certain time and aperture combination, e.g. 1/4000 at f:16, corresponds to a certain Lux level. If I set any time/aperture combination, the camera in advance does not know what the light level (or Lux level) is, nor does the camera know the result I am aiming at. Depending on the prevailing Lux level and assuming I have set a desired time and aperture combination, I may get a shot that is totally underexposed, totally overexposed or more or less correctly exposed. And I am pretty sure the camera will not interfere, regardless of the capabilities of the A/D converter, at least if I am not using any auto exposure mode. If we are talking auto exposure mode, that changes everything, as the camera will then likely try to stay within certain limits to make sure that an acceptable result may be produced.

 

vor 4 Stunden schrieb BernardC:

By the way, light isn't "amplified internally" by the sensor at higher ISO, ...

Quite correct, it is the sensor's output signal that is amplified (hence the associated higher noise), and I am aware of that. The point I wanted to make is, if there is tons of light, and you have set the camera to boost the sensor signal (by setting ISO at 800), resulting in a considerably higher signal value than if you had used base ISO, then why does the camera stop down the aperture when the actual signal value is not boosted (at base ISO, that is), but decides not to do so when the signal value is going through the roof (at ISO 800 or higher), so to speak.

Edited December 16, 2021 by wizard
addition of text

[SrMi]

7 hours ago, wizard said:

I fail to understand this argument. The light intensity entering the lens/camera is, as has been pointed out correctly, the same (at any given point in time), regardless of whether ISO is set at base ISO or at ISO 800. Within the camera, however, ISO 800 means that the light gathered by the sensor will be amplified to make the sensor even more light sensitive. Why would the camera, at the same ambient light intensity, close the aperture down only at base ISO and not at ISO 800, when at the latter ISO setting the received light intensity is amplified internally, thus resulting in even more light (as seen by the camera)?

Assume that the camera does not know what light intensity hits the sensor. So instead, it computes the light intensity from the current setting (ISO, aperture, shutter speed), assuming the image's standard exposure (brightness).