Auto ISO, when to use it?

[keithsnell]

We've had a couple of discussions lately that have touched on Auto ISO, and one of the questions has related to when to use, or not use, Auto ISO.  Before I attempt to answer that question, I should start by explaining that the Auto ISO function is implemented differently in Canon and Nikon cameras, so my explanation for Nikon cameras might not be applicable to Canon, and vice versa.  Still, it might be useful for folks to read the explanation for both brands of camera, since the "limitations" discussed for one camera might help explain the reasons for some of the settings/restrictions in the other brand's implementation.

Let's start with Canon.  The first point is that the AUTO ISO function implementation varies with the camera model.  With the Rebel XS, AUTO ISO is selected as one of the options when pressing the ISO button on the top panel of the camera and rotating the main dial.  In other words, you can select a specific ISO setting between 100 and 1600, or you can select "AUTO."  If you select AUTO, the camera will set the ISO automatically between 100 and 800 based on the shooting mode selected and ambient light conditions.  For flash and manual exposure mode, the ISO will be set at 400.  You can see what the camera has selected for the "auto" ISO by half-pressing the shutter button and viewing the ISO that is displayed in the viewfinder or top panel LCD.

By comparison, the Rebel XTi/400D defaults to AUTO ISO in the "basic" shooting modes, and doesn't give the photographer the option of AUTO ISO when in any of the "creative zone" modes (P, Tv, Av, M).  I find this to be a curious omission, but can understand the desire to simplify the user interface.

The 50D implementation of AUTO ISO is similar to that on the Rebel XS, i.e., AUTO ISO is one of the options that can be selected when pressing the ISO button on the top panel and rotating the main dial.  In the case of the 50D, the camera will select an ISO between 100 and 1600 based on the shooting mode selected and ambient light conditions.  Again, for flash and manual exposure mode, the ISO will be set at 400, with the caveat that "if fill flash results in overexposure, ISO 100 will be set."  I presume the camera tests for "overexposure" during the pre-flash, and resets the ISO if needed.  5DMKII settings are similar, with the camera having the ability to bump the ISO up to 3200 (or higher, if the "expanded" mode is selected).

Now we need to talk more specifically about the conditions that will cause the camera to select a higher ISO when in AUTO mode.  The easiest way to understand this is to recognize that based on the exposure mode you have selected, the camera will first adjust the shutter speed and/or aperture to obtain a "normal" exposure, up to the limits defined for that particular mode.  For example, if you are in one of the "full auto" modes, the camera will adjust both the aperture (until it reaches the largest available aperture on the lens) and the shutter speed (until it reaches the designated lower limit of 1/60th of a second), and only then will it start to bump up the ISO if needed to achieve a "normal" exposure.

On the other hand, if you are using the Program (P) or Av (aperture priority) exposure mode, the camera will drop the shutter speed all the way down to its lower limit of 30 seconds before automatically bumping the ISO up (with AUTO ISO set).  Obviously if you are hand-holding your camera, this would NOT be a situation where you would want to rely on the AUTO ISO to give you "useable" shutter speeds.  You can overcome this limitation by changing to Tv (shutter priority) or manual mode, or by setting the appropriate custom function for your camera's flash settings to limit the slowest shutter speed to 1/60.

I also mentioned at the beginning of this article that Nikon and Canon implementations for AUTO ISO are slightly different.  For Nikon cameras, AUTO ISO is either turned "on" or "off" in a custom function setting in the camera.  In conjunction with the on/off setting, you also have the ability to set a minimum shutter speed and a maximum ISO.  For the D3, I can set my minimum shutter speed anywhere between 1 second and 1/4000.  This provides a bit more control over when AUTO ISO will kick in.  For example, if I'm photographing relatively fast action, I can set my minimum shutter speed at 1/500th of a second to reduce motion blur.  If I'm using aperture priority, the camera will automatically adjust the shutter speed needed for a proper exposure until it reaches the lower limit I've defined as 1/500, at which time it will start bumping up the ISO.  I can also set a "maximum" ISO for the AUTO ISO function.  So, using the same example, the camera will continue bumping up the ISO until it reaches the maximum I have defined for the AUTO ISO setting (say 3200) at which time it will switch back to lowering the shutter speed in order to obtain a "normal" exposure.  Make sense?  If not, try it for yourself and then please ask questions so that I can help clarify how the system works.  

The Nikon system will always default to the ISO setting you have set using the ISO button/menu, until it can no longer achieve a proper exposure by varying the aperture and/or shutter speed within the defined limits.  At that time it will switch into AUTO ISO mode (assuming you have that option turned on in your custom functions) and will adjust the ISO as needed in order to achieve a proper exposure.  

It is also important to understand that the exposure algorithms change when you are using flash in conjunction with AUTO ISO on the Nikon system.  When you use flash, the AUTO ISO system assumes the flash will be your "primary" light source.  Therefore, the system won't begin to bump up the ISO until it determines (based on the pre-flash) that the flash doesn't have enough power to provide adequate exposure on the subject.  So, your flash will go to "full power" before the AUTO ISO kicks in, and only then will the camera start bumping up the ISO.  This can provide less than optimum results if you are attempting to balance ambient and flash exposures to avoid the "dark tunnel" look of images taken with flash as the primary light source.  The way to alleviate this is to first set your ambient exposures with the flash off, to include bumping up the ISO (using the user controlled ISO button/menu settings) if needed to get decent ambient exposures. I recommend setting your ambient exposures slightly lower than normal (approximately -0.5 EV) in order to leave a little bit of "margin" for the additional light that will be provided by the flash.  Then turn on the flash, and use it as a "supplement" to the ambient light.  Your system will default to the user set ISO, as long as it provides an adequate exposure.  If you have AUTO ISO set in this situation, the system will automatically adjust the ISO (up or down) only if it needs to in order to further adjust the amount of light provided by the flash (i.e., if the flash bumps up against its minimum or maximum power settings).  Again, this will make more sense once you experiment with your own camera and flash.

I'd like to talk a bit about using AUTO ISO in conjunction with manual mode.  There is a situation where you would want AUTO ISO on, and a situation where you would want to make sure AUTO ISO is turned off.  If, for example you are photographing an event (a wedding perhaps) in a relatively dark venue, you might want to make a conscious decision about what your largest acceptable aperture would be, and your lowest acceptable shutter speed, and then let AUTO ISO bump up the ISO to enable you obtain decent exposures.  

Let me give a more specific example. If I'm shooting in a relatively dark church (where flash is not allowed during the ceremony) I will probably choose to use my 50mm f1.4 lens for at least part of the ceremony.  I know from experience that images shot "wide open" at the aperture setting of f1.4 will be too "soft" to meet my criteria for quality images.  I also know that if I "stop down" the lens to f2.0, the images will be extremely sharp across most of the frame, so I'll set my camera to manual and select an aperture of f2.0.  I also know from experience that the lowest possible shutter speed where I can expect a good "keeper" rate for that lens is 1/30.  That shutter speed requires very good technique ("squeezing" or rolling the shutter instead of "pressing" it, correct camera holding and bracing techniques, controlling my breathing, etc.) and also requires me to "time" my shots exquisitely to catch the bride and groom at the moments when their movements are at a minimum.  Based on this, I will set my shutter speed (in manual mode) at 1/30.  However, this still might not provide enough light for a well exposed shot, so I will set AUTO ISO on my camera.  If the camera's exposure meter determines there isn't enough light to provide a decent exposure at f2.0 and 1/30 second at the base ISO, it will automatically bump up the ISO until it can achieve a decent exposure.  Of course I don't just "set it and leave it" but am instead always consciously balancing my aperture, shutter speed and ISO settings in an attempt to achieve optimum results.  If I'm shooting with the D2X, I'll let my ISOs go up to 400 without any concern.  If I'm shooting with the D3, I can let my ISO get all the way up to ISO 3200 and still get very good results, so obviously I would adjust my shutter speed higher (and perhaps my aperture smaller) to give myself more margin for "keepers," and would let the ISO go higher to bring up the exposures as needed.  (I will always visit the church beforehand to take a series of test shots that help me determine the "optimum" settings.)

If I were using a Canon camera, I would adjust the technique described above in order to accommodate the Canon implementation of AUTO ISO.  In a situation where I wasn't able to use flash, I would chose Tv (shutter priority mode) and set my lowest acceptable shutter speed, and let the camera control the aperture and ISO (set in AUTO) to obtain the correct exposure.  If I was using flash, I might choose to use Av (aperture priority mode) and implement the custom function which limits the lowest possible shutter speed to 1/60 when using flash.  In this way I could set my limits for both aperture and shutter speed, and let the camera adjust the ISO (at least within the narrow range of ISO 100 - 400) and flash power to achieve the best exposure.  Again, I would test exposures prior to the event to determine whether ISOs in the range of 100-400 were appropriate for the venue, or whether I needed to use Tv (shutter priority) mode in order to allow the camera to adjust ISOs all the way up to the limit.

As I mentioned, there is also a situation where you would NOT want to use AUTO ISO in conjunction with manual exposure.  It is important to recognize that if you are trying to use manual exposure to adjust the light level of your exposure or "tonality" of your subject, then AUTO ISO can work against you.  When I'm using manual exposure and a modified Zone System technique to control the tonality of my subjects, I will use the exposure scale in the viewfinder to set a specific level of exposure, i.e, +1 stop for Zone VI, +2 stops for Zone VII, etc.  However, remember that AUTO ISO always tries to obtain a "normal" exposure for your subject.  If I'm trying to adjust my shutter speed or aperture to lower the exposure on the scale to the -1 EV mark, the AUTO ISO function will be automatically raising the ISO at the same time to keep the exposure at the "0" mark.  The camera will keep "fighting against me" until it reaches the limits I have set for the highest ISO, and only then will I be able to adjust the shutter speed or aperture to the point where I can achieve -1EV on the scale.  Does that make sense?  If not, try it for yourself so that you can see what I'm talking about.  

If you are using manual exposure to achieve a specific tonality for your subject other than a "normal" exposure, then you need to ensure AUTO ISO is turned off.  This is easy to forget, so you should check your settings and do a "sanity check" to ensure your shutter speed and aperture values are in a range that makes sense for the given light levels (or simply check to ensure AUTO ISO is off when you are using Zone System techniques.  (Because Rebecca and I share cameras, we always remind each other to return the camera to our "normal" settings before putting it back in the bag.  That helps prevent us from grabbing the camera and firing off a quick series of shots, only to find out after we review our images that "the other photographer" had messed up the camera settings. )

Once you've had a chance to experiment with the AUTO ISO settings on your camera, please ask questions and give me the opportunity to clarify anything that doesn't make sense in this post.

Keith

[girod]

Thank you very much Keith for another eye-opener insight in knowing auto-ISO. When I use auto-ISO (D700) in manual exposure mode using the modified Zone System with spotmetering, I use the Exposure Compensation to obtain the desired zone or tonality rather than the in-camera meter scale. I select the aperture, shutter speed based on my photographic intent for that particular shoot then the minimum-maximum ISO and the minimum shutter speed - then I dial the Exposure Compensation based on the tonality of the subject. I have not used this enough because the occasions were not there but I've implemented it just now 5X and - at fixed aperture and shutter-speed, using Exposure Compensation, auto-ISO will render/expose the image according to the dialed Exposure Compensation and not to the default middle-tone ("normal" exposure").

What am I missing here Keith?

jaime

[keithsnell]

Thank you very much Keith for another eye-opener insight in knowing auto-ISO. When I use auto-ISO (D700) in manual exposure mode using the modified Zone System with spotmetering, I use the Exposure Compensation to obtain the desired zone or tonality rather than the in-camera meter scale. I select the aperture, shutter speed based on my photographic intent for that particular shoot then the minimum-maximum ISO and the minimum shutter speed - then I dial the Exposure Compensation based on the tonality of the subject. I have not used this enough because the occasions were not there but I've implemented it just now 5X and - at fixed aperture and shutter-speed, using Exposure Compensation, auto-ISO will render/expose the image according to the dialed Exposure Compensation and not to the default middle-tone ("normal" exposure").

What am I missing here Keith?

jaime

Hi Jaime,

Thank you for asking.  Yes, you could use that method to obtain exposures for a targeted level of brightness/tonality without having to turn auto-ISO off.  I should add a caveat here that this method will only work with Nikon cameras.  Canon disables the ability to set exposure compensation when using manual exposure.  (There are pros and cons to disabling exposure compensation when in manual exposure, which I can go into if anyone is specifically interested.)

Getting back to the method you are using (manual exposure, auto-ISO, exposure compensation set to a specific target tonality):  As you said, the camera will expose the image according to the dialed exposure compensation.  Using this method, you have essentially told the camera to adjust it's "normal" ("0") exposure setting. It will now base its determination of a "proper" exposure on what you have set for exposure compensation.  By turning on auto-ISO, you have given the camera the ability to adjust the sensitivity to obtain the target exposure level you have set, even though you have restricted the camera from changing the shutter speed and aperture settings by being in manual mode.  If you have very specific objectives for aperture and shutter speed, that would be one way to obtain a properly exposed image with those settings.

This method is also a great way to be able to target a specific tonality while still utilizing the camera's "auto exposure" functions and capability to rapidly adjust to changing light conditions.  In other words, you've decided what your targeted tonality (Zone) is, and you decide where to place the spot meter on your subject, but you are letting the camera take over from there and set the appropriate ISO (and therefore the exposure) for the metered brightness.  Again, this would be a great method for rapidly adjusting to changing light conditions for a specific target tonality.

Many people would argue that although this is a valid exposure strategy for targeting a specific tonality, it isn't really an implementation of the Zone System.  Why?  Because using these settings makes it very difficult to compare tonalities in the scene and see where the other tones lie in relation to your "targeted" tonality.   It has been a long time since I've taken the time to fully explain how one would use a Zone System methodology to determine exposure, so I should probably take a moment to explain this in more detail.  

A Zone System exposure method assumes that you are consciously placing ALL of the tonalities in your exposed/rendered image, not just a single targeted tonality.  In other words, when you are really using a Zone System (or modified Zone System) methodology, you should know the relative brightness levels of the full range of tonalities present in your scene, and make a conscious decision on how to best expose and render those tonalities in the final image.  I typically do this by setting my camera for manual exposure with auto-ISO and exposure compensation turned off.  If it is a "moderate" contrast scene and the mid-tones are the most important part of the scene, I will begin by spot metering on a mid-tone (Zone V) and setting my aperture and shutter speed appropriately so that the exposure scale is reading "0".  I will then pick the lightest area in the scene where I want to retain detail, spot meter that area and determine where it falls on the exposure scale in relation to my metered Zone V.  I want to ensure that this area will fall within or below Zone VII (the lightest area that will retain detail in the rendered image).  It this area meters higher than +2 EV on the exposure scale (in relation to the shutter speed and aperture settings for my Zone V exposure reading) then I will need to make a conscious decision about whether or not I want to adjust my exposure to favor Zone V or Zone VII.  My decision will depend on what area I judge to be the most important.  I can underexpose the Zone V areas in order to "preserve" the detail in Zone VII, and bring the mid-tones back up to their proper placement during post-processing (most likely with the "brightness" setting); however, there are limits as to how much I can effectively underexpose and still have a high-quality rendering for Zone V.  I'm comfortable bringing up Zone V by about 1 stop in post processing, but anything more than that will result in inaccurate colors and loss of detail.  So again, I will need to make a conscious decision on the relative importance of the Zone V and Zone VII areas in the scene.  But I'm not done yet, because I still need to meter on and evaluate the darkest area in the scene where I want to retain visible detail, and I should also meter on the darkest area of "deep" shadow, and make a conscious determination of how to fit these areas into my overall vision of how I want to render the scene.  By putting my camera in manual exposure with auto-ISO turned off, and exposure compensation set at "0" I can very quickly meter the different areas in the scene, see where they fall on the exposure scale, and make a quick assessment of the "optimum" exposure for the overall scene.  If it is a "moderate" contrast scene, chances are very good that I can expose for Zone V and let all the other tones naturally fall into their relative positions on the exposure scale.

If however, it is a high contrast scene, I will have to make a decision on what areas are most important, and how I might want to adjust my exposure to render the most pleasing final image.  This could also include the decision to bracket my exposures and use "exposure blending" to render the final image.   (Exposure Blending is a manual method of using layers and masks in Photoshop to "blend" areas from different exposures into one overall final image.)

Many of my earlier discussions on using a modified Zone System "oversimplified" the process by presupposing that we had already decided what the "important" Zone was, (i.e., Zone VII was the most important Zone in the image for the Zone VII assignment, etc.) and assuming that we had already made the conscious decision to let the other tonalities "fall where they may."  I should have more explicitly stated these "assumptions" and made it clear that when using a Zone System methodology of exposure, the objective is to consciously place ALL of the tones in the image.  If we were to leave auto-ISO turned on it would make it very difficult to judge where all the tones in the scene fall in relation to each other, because when we shifted the placement of our spot meter, the camera would adjust the ISO to achieve the "targeted" exposure value (even if that "target" had been adjusted by dialing in exposure compensation).  In other words, no matter where you pointed the spot meter, if you had auto-ISO turned on, the exposure reading would always be the same, since the camera was automatically adjusting the ISO to compensate for the different readings and achieve the desired exposure.  Make sense?

Again, the method you have chosen is an effective method for targeting a specific tonality while still utilizing the camera's "auto exposure" functions and capability to rapidly adjust to changing light conditions.  However, those camera settings (auto-ISO and exposure compensation "on") would make it very difficult to judge the relation of other tonalities in the scene, which is a prerequisite for implementing a true Zone System exposure method.

Hopefully all of this makes sense.  If not, please ask questions and give me the opportunity to clarify my explanation.

Keith

[girod]

Thanks Keith, got it and it all make sense. So far I have only used this combo (manual+auto-ISO+EC) in actions where the subject goes thru different shades of light and that I can place the focus point (spotmetering) to a specific tonality of the subject (eg facial skin tone). Otherwise, I've utilized the modified Zone System exposure methodology as you've described.

Can you please explain the pros and cons in disabling EC in manual exposure?

jaime

[keithsnell]

Thanks Keith, got it and it all make sense. So far I have only used this combo (manual+auto-ISO+EC) in actions where the subject goes thru different shades of light and that I can place the focus point (spotmetering) to a specific tonality of the subject (eg facial skin tone). Otherwise, I've utilized the modified Zone System exposure methodology as you've described.

Can you please explain the pros and cons in disabling EC in manual exposure?

jaime

The "pro" to disabling exposure compensation while in manual exposure is that it prevents a common mistake that many photographers make, that is exposing based on the exposure scale in their viewfinder without realizing that scale has been modified by dialing in exposure compensation.  Say for example that I have -0.7 EV exposure compensation set (while shooting in one of the autoexposure modes) then I switch over to manual exposure for a Zone VII exposure.  Normally I would simply place my metering spot over the Zone VII area and set +2 EV on the exposure scale.  If I do this with -0.7EV exposure compensation set, then I'm really exposing that zone at +1.3 EV, even when it is showing as +2 EV on the scale.  This is because the displayed exposure and exposure compensation I've set are "additive."  +2 EV added to -0.7 EV = an overall exposure of +1.3, which might not be disastrous, but it certainly isn't what I intended.  It is very easy to make this mistake when shooting in manual exposure because some of the "normal" indications that you have exposure compensation set (like the flashing "0" on the scale and the indication on the exposure scale showing the amount of compensation) aren't displayed because they have been replaced with the scale you are using to set your manual exposure.  You can't make this mistake using a Canon camera, because exposure compensation is disabled when in manual mode.  Canon assumes you will "factor in" any desired exposure compensation when setting your exposure using the manual exposure scale.

Make sense?

Keith

[girod]

Happened to me twice.......so, even though I hardly use EC (and auto-ISO), I make a conscious effort to check it everytime I turn the camera on.........thanks Keith.

jaime