Auto ISO to obtain correct exposure?

[Jeff S]

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12,408 posts…

 

Jeff

[mirekti]

If the shutter is set manually and aperture is set manually and ISO is set to auto, what does the exposure compensation do?

 

 

It helps you expose for backlight subjects by dialing -EV for example.

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But the real reason I think the wish for Auto ISO in manual mode is misguided is that you needlessly throw away dynamic range that way. I often use manual mode for saving highlights by deliberately underexposing.QUOTE]

 

 

Aren't you basically saying here that auto ISO neglects your intent on the exposure? Whether to not you're under exposing to save highlights or over or under exposing to place a particular value on a particular zone, auto ISO kills that because you effectively have a moving target on exposure.

 

Am I following correctly?

[Chito]

If the shutter is set manually and aperture is set manually and ISO is set to auto, what does the exposure compensation do?

 

If exposure comp is set to -1 EV it should lower the calculated ISO (set automatically) by 1 stop.. (underexposing) same thing for +1 EV... (raises the ISO).

 

In a sense, if you guys think about it.. the exposure comp would become a sort of ISO dial.

[CaptZoom]

If exposure comp is set to -1 EV it should lower the calculated ISO (set automatically) by 1 stop.. (underexposing) same thing for +1 EV... (raises the ISO).

But is this what the M actually does?

 

In a sense, if you guys think about it.. the exposure comp would become a sort of ISO dial.

[Chito]

But is this what the M actually does?

 

At this moment no... This is why some people have been asking for auto ISO on manual.. if you set ISO to auto while everything else is on manual, it just goes back to the previously set ISO.

 

If you move the exposure comp while on manual, the metering will just compensate (it will show that you are over, under or correctly exposing relative to your set exposure compensation).

[jdsheepdog]

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.........But the real reason I think the wish for Auto ISO in manual mode is misguided is that you needlessly throw away dynamic range that way. I often use manual mode for saving highlights by deliberately underexposing. ......

 

This does start to sound remarkably like the way I used a camera when all I had was an M3 and Kodachrome.

 

I feel at times in the golly gee whiz amazement at "post film" cameras, that it gets forgotten that there are still only three ultimate controls, how big the hole is that the light gets through, how long it is open, and where the lens is focused.

 

Variable ISO may be akin to changing film in mid roll, but it does not change the fundamental three ways the camera is controlled.

[Exodies]

<snip>I often use manual mode for saving highlights by deliberately underexposing.

This is exposure compensation. Auto ISO would respect this requirement.

Auto ISO would render this void: when the camera cranks up the amplification of the sensor signals the highlights yield voltages overloading the ADC. While I manage to prevent clipping in the sensor by underexposing, Auto ISO ensures the highlights are clipped anyway – by the ADC rather than the sensor, but the result is the same.

Auto ISO doesn't have to be programmed to ensure the highlights are clipped. That would be foolish.

The only way to effectively prevent clipping is to fix ISO at a low to medium level and to adjust the gamma curve in the raw converter for brightening the image without throwing out the highlights again.

No, there is another way. Program auto ISO to prevent clipping.

[mjh]

If the shutter is set manually and aperture is set manually and ISO is set to auto, what does the exposure compensation do?

There are different views as to what exposure compensation should do under these circumstances. Some would argue that dialing in an exposure compensation should just shift the goal post for an optimum exposure, so if you select –1 EV in manual mode the actual exposure should not change at all – you are in manual mode so the camera has no control over exposure –, but the camera should indicate that an optimum exposure would require twice the shutter speed or stopping down one f-stop and it is up to you to change the exposure. Others would say that exposure compensation should change the ISO setting, so if Auto ISO would choose ISO 1600 and you dial in – 1 EV, the ISO should change to 800. As far as I know only Pentax have implemented both versions; in M mode, Pentax cameras don’t support Auto ISO and exposure compensation just shifts the goal post, not actual exposure. In TAv mode Auto ISO is an option and exposure compensation changes the ISO value.

Edited May 31, 2014 by mjh

[CaptZoom]

[mention=2705788]mjh[/mention], thank you.

Edited June 1, 2014 by CaptZoom
formatting error.

[250swb]

I feel at times in the golly gee whiz amazement at "post film" cameras, that it gets forgotten that there are still only three ultimate controls, how big the hole is that the light gets through, how long it is open, and where the lens is focused.

 

Variable ISO may be akin to changing film in mid roll, but it does not change the fundamental three ways the camera is controlled.

 

Nostalgia doesn't cloud my memory. Kodachrome aside the 'three controls' are expanded to thousands of controls when various combinations of film rating and development are employed. The instructions that came on the box were always a guide, not a rule. Yet now we have 'rules' being insisted upon with digital photography and are told what is bad for us, when the criteria should be what is good for creativity.

 

Steve

[jdsheepdog]

Nostalgia doesn't cloud my memory. Kodachrome aside the 'three controls' are expanded to thousands of controls when various combinations of film rating and development are employed. The instructions that came on the box were always a guide, not a rule. Yet now we have 'rules' being insisted upon with digital photography and are told what is bad for us, when the criteria should be what is good for creativity.

 

Steve

 

I can't argue with that, and the electronic post processing / development process is one that is for me a steep learning curve on which I am only just starting to climb. my point is that cameras are still the basic mechanisms they always were, despite the overlays of electronics.

[mjh]

It should be noted that Auto ISO in manual mode is something quite different from Auto ISO in aperture priority mode in that it serves a different purpose.

 

With aperture priority the camera takes care of an optimum exposure: you choose an aperture and the camera determines a shutter speed that ensures enough light reaches the sensor. How much light needs to be gathered during the exposure is determined by the ISO setting.

 

Now when the aperture chosen is small and/or there isn’t much available light to begin with, a shutter speed compensating for the lack of light would get so slow that you cannot shoot handheld, and that is the raison d’être for Auto ISO: When the shutter speed would need to go below a selectable minimum speed the camera would rather increase the ISO setting; only when the ISO value hits a selectable maximum value would the camera use a slower shutter speed. So the reason to use Auto ISO is to prevent the camera from choosing slow shutter speeds.

 

In manual mode this isn’t an issue at all since the shutter speed is selected by the photographer. At a given ISO setting, of all the selectable combinations of aperture and shutter speed only a small subset guarantees an optimum exposure, all the others resulting in either over- or underexposure. Now the idea is that with Auto ISO in manual mode one could turn manual mode into another automatic mode: whenever the chosen aperture and shutter speed values would result in underexposure, Auto ISO increases the ISO value and amplifies the weak sensor signals so the image doesn’t look underexposed. Unfortunately this doesn’t work for overexposure which still needs to be avoided.

 

So whereas in aperture priority mode the camera controls the exposure by varying the exposure time and Auto ISO merely serves to prevent the choice of slow shutter speeds, in manual mode there is no actual automatic exposure at all – it is the photographer controlling how much light hits the sensor – and Auto ISO serves to hide the effects of underexposure if it occurs. (Suppose you go to a pub and order a pint of lager, and the landlord only draws as much beer as he thinks he can spare, filling up with enough water so he can sell you a pint. Anyone who is fine with that should go ahead and request Auto ISO in manual mode.)

Edited June 1, 2014 by mjh

[250swb]

So you are saying Leica got it wrong with the M9 in having Auto ISO available in manual mode?

 

 

Steve

[mjh]

So you are saying Leica got it wrong with the M9 in having Auto ISO available in manual mode?

I think it’s a bad idea, yes.

[jaapv]

I think users should be allowed to choose either option.

[Berth]

The issue I have with aperture priority and with auto-ISO is that both leave me at the mercy of Leica's 'big' spot meter. In some scenes small changes in composition can produce big changes in meter readings, and images that can be over/under exposed. I much prefer more control and so have abandoned the use of either.

[Guest roey]

Now when the aperture chosen is small and/or there isn’t much available light to begin with, a shutter speed compensating for the lack of light would get so slow that you cannot shoot handheld, and that is the raison d’être for Auto ISO: When the shutter speed would need to go below a selectable minimum speed the camera would rather increase the ISO setting; only when the ISO value hits a selectable maximum value would the camera use a slower shutter speed. So the reason to use Auto ISO is to prevent the camera from choosing slow shutter speeds.

 

From your description it seems what you want here is not auto ISO but rather a minimum shutter speed. If you use auto ISO to ensure that the shutter speed doesn't drop below a certain value you will still get "watered-down beer".

 

(Suppose you go to a pub and order a pint of lager, and the landlord only draws as much beer as he thinks he can spare, filling up with enough water so he can sell you a pint. Anyone who is fine with that should go ahead and request Auto ISO in manual mode.)

 

There are situations where I want to get watered-down beer (to get any beer at all). It is my preference that the camera does it for me so that I get a decent preview on the LCD screen.

 

If you think auto ISO is a bad idea: don't use it. What's so hard about that?

[barjohn]

The T has an interesting, and I think one of the best, auto iso setups. You can elect the slowest shutter speed you are willing to accept and then it adjusts ISO based on aperture or you can select auto for the slowest shutter speed in which case it selects a shutter speed > or = to ½ x FL, then adjusts ISO to match aperture. This way when you change focal lengths you don't have to go back and change your minimum acceptable speed.

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Hallo,

Look here: Leica M

[rafikiphoto]

Yes - you have missed 12.387 posts on the subject

 

I seem to have stirred up a hornets nest. However it is very interesting to see the various positions on this. Thank you all.