Exposure Compensation vs ISO adjustment

[cauger61]

Can the members pls. take up this topic for discussion? My point is: are these two settings of dslr -- exposure compensation and iso adjustments, alternatives to one another? When and where they should be used? Do they give same result with a different approach. It appears to me a bit confusing!!

[William W]

In general and simple terms, for most not all cameras:

Exposure Compensation is most often used when the Camera is being used an Automatic Mode: Av; Tv; or P (Aperture Priority; Shutter Priority; Program) and for these following examples the ISO is a set chosen value.

When in one of these Automatic Modes, the camera's TTL Light Meter (Through the Lens Light Meter) controls the exposure settings automatically. In Av, the Aperture is chosen by the Photographer and the camera selects the Shutter Speed; in Tv the Photographer selects the Shutter Speed and the Camera selects the Aperture automatically; in P Mode the Camera will select both the Aperture and the Shutter Speed. Some cameras have an additional function in P Mode to allow the Photographer to change one or both of the Shutter Speed and Aperture and the Camera will adjust the other, automatically.

When Exposure Compensation is used in any of the above scenarios, the camera automatically adjusts the varying exposure parameter according to the exposure compensation selected.

For example, if the Camera is in Av Mode and the Photographer has chosen F/4 and the Camera selects 1/100s as the 'correct' Shutter Speed based upon the TTL Meter:

a) if EC = -1 is selected, then the Camera will set 1/200s as the Shutter Speed (i.e. the resultant exposure will be one stop under the 'correct' exposure calculated by the TTL Meter)

b) if EC = +1 is selected the Camera will set 1/50s as the Shutter Speed (i.e. the resultant exposure will be one stop over the 'correct' exposure calculated by the TTL Meter)

Similarly in Tv Mode the Camera will adjust the Aperture up or down to suit the EC selected.

When the Camera is in P Mode and EC is selected, most cameras will have a functionality that follows a program which will prioritize adjusting the Shutter Speed fast enough to allow hand holding and inhibiting Motion Blur; although it is not uncommon that a Camera will adjust the Aperture in some lighting conditions usually when fast lenses are attached and detected.

***

ISO Adjustment is a selection made by the Photographer. (This is assuming that you do not mean setting the Camera to "AUTO ISO")

In most situations best practice dictates that the Photographer will choose an ISO based upon the intensity of the light on the scene and suitable to address two criteria:
1. Choosing an ISO low enough to reduce the effects of noise (unless noise is wanted)
2. Choosing an ISO that will allow the RANGE of Shutter Speeds and Apertures required to make the image as required.

Often, it is suggested that ISO be chosen simply on the light's intensity - for example outdoors sunny, then select ISO 100 and when overcast select 400 and when inside select 1600 or 3200. Although this simple approach may be a basic FIRST STEP it is fraught with danger to make it your rule and not take into account the type of pictures that you are making and the lens that you have to make those pictures.

For example, we might be lucky enough to have tickets on a bright sunny day watching India play Australia in Field Hockey. Let's say we have our DSLR and our Kit Telephoto Lens - say a 70 to 300 F/4.5~5.6. Using the F/16 Rule, typically in front lit full sunlight, we would be pulling shots at F/5.6 @ 1/800s @ ISO100.

However, for a fast moving game like Hockey played at an elite level, a Shutter Speed of 1/1600s would be better and a shutter speed of 1/3200s would be very safe: so we would choose ISO 400 to allow us to make F/5.6 @ 1/3200s @ ISO400.

However, thinking about this a bit more, we know that our kit lens, whilst good at F/5.6, is actually quite a bit sharper if it is used at F/8, so, if our Camera is very good at ISO 800 we might choose to use ISO 800 to allow us to make F/8 @ 1/3200s @ ISO800.

Of course when we see the clouds moving over the sun we might choose to select ISO1600 for this particular genre of outside photography. I have shot quite a lot of Field Hockey and much of it was at ISO800 on sunny days and light cloud days, with fast (F/2.8) lenses: selecting an higher ISO allowed me to have a RANGE of Shutter Speeds and Apertures available especially when transitioning from shooting with the light to in the shade of the Grandstand to shooting into the light.

***

In the strictest sense of meaning, Exposure Compensation and Aperture Selection cannot give the same results, but that question and answer are academic and probably not very useful the real world of making Photos, I think more important to understand is that the former is the last operation of an automated function based upon the TTL Meter and the latter is a fixed selection made by the Photographer.

WW

[pschlute]

Can the members pls. take up this topic for discussion? My point is: are these two settings of dslr -- exposure compensation and iso adjustments, alternatives to one another? When and where they should be used? Do they give same result with a different approach. It appears to me a bit confusing!!

Exposure is dependent on three things : Shutter speed/Aperture/ISO. Changing any one of those will make your image lighter or darker.

Exposure compensation works by taking the cameras meter reading, and adjusting the exposure by the amount you tell it to...eg +1 stop or +2 stops.

How does the camera apply those stops of compensation ? It will vary depending what mode you are in. In Aperture priority with fixed ISO the camera can only change the shutter speed to compensate. In full Program mode with Auto ISO the camera can change any of the settings. In Manual mode with auto ISO the camera can only change the ISO to compensate.

So no, ISO and EC are not alternatives, but in certain circumstances can have the same effect.

[DanK]

First, a suggestion: look at the tutorials on this site. They are very clear and well-written. You will find one that discusses this. In general, it works best to read the the relevant tutorial first and then ask questions here. The tutorial will usually be more thorough and organized than our responses, and it saves duplication.

I have to disagree with Peter on one point: the only things that affect exposure--that is, the exposure of the sensor to light--are aperture and shutter speed. ISO does not. This may seem like nit-picking and just a matter of wording, but it's an important distinction. So leave ISO aside for the moment.

The rest of Peter's posting is precisely what I would suggest. His point about modes is critically important. There is a tutorial on metering modes as well, and that may help clarify his point.

ISO is different because increasing ISO does not increase exposure as such. Rather, it simply amplifies the signal from the sensor. When you increase ISO, you are essentially compensating for an exposure that is too low. Within reason, this can work fine, but it has two important drawbacks: reduced dynamic range and greater noise, particularly in shadows.

As a result of this last point, I use EC simply as another way of changing either shutter speed or aperture. One could just as well change them directly, and the effect would be the same, but sometimes EC is more convenient. For example, if you are in a setting in which you decide to use aperture priority, which I use a lot, and you find that the lighting is consistently giving you shots that are underexposed by a stop, it may be easier to dial in +1 EC than to switch to fully manual mode and manually set the exposure to +1 EV, but the effect is identical. To do this, I have ISO set to a fixed value, generally as low a value as I can manage in that setting.

[William W]

Sanjib, another point - I cannot stress enough that I have seen so many Photographers who have no idea the basics of the different Metering Modes and the basics of how to use same - so my suggestion is to learn that, first.

"Metering Modes" selections are termed in words like: 'Evaluative' 'Centre Weighted Average' 'Matrix' 'Spot' et al

WW

[DanK]

Post deleted

[MrB]

If we assume reasonable lighting conditions, the ISO should be set at its lowest value. When using the semi-auto modes, Av or Tv, the camera's lightmeter will set suitable values of shutter speed or aperture, resp., for an exposure in which the light on average is a medium brightness. However, the camera doesn't know what your subject is so this might produce an image which is too bright or too dark. E.g. The settings might brighten a black cat so it looks grey in the image, and similarly they might darken a winter scene to give snow that looks grey in the image. These are the sort of scenarios in which you could apply exposure compensation - negative in the former case and positive in the latter.

It is important to keep an eye on the exposure values of aperture and shutter speed. E.g. In Av Mode if you select a small aperture (to give a large depth of field), the shutter speed might become so low that the image is blurred by your own movement (camera shake). Similarly in Tv Mode if you select a fast shutter speed (to freeze the movement of your subject), the aperture might open to its widest giving a very short depth of field, so that most of your subject is out of focus. These are the sort of scenarios in which you could increase the ISO value - this would increase the shutter speed in the former case and give a smaller aperture in the latter.

(Some cameras have more sophisticated user-definable controls which allow the photographer to set how the ISO behaves - read the manual!)

Philip

[William W]

If we assume reasonable lighting conditions, the ISO should be set at its lowest value.

I disagree.

Taking only the lighting conditions as the criterion for setting the ISO is, IMO and IME not a very good idea at all.

Please see details describing an Hockey Game in Post #1 as one merely one example of why.

Noted that later follows an explanation to watch the aperture and the shutter speed values, but the quoted opening sentence, as an opening sentence, still stands alone and is, IMO, poor advice and exactly the type of commentary which was addressed in Post #1.

WW

[MrB]

I disagree.

Taking only the lighting conditions as the criterion for setting the ISO is, IMO and IME not a very good idea at all.

Please see details describing an Hockey Game in Post #1 as one merely one example of why.

Noted that later follows an explanation to watch the aperture and the shutter speed values, but the quoted opening sentence, as an opening sentence, still stands alone and is, IMO, poor advice and exactly the type of cokmentary which was addressed in Post #1.

WW

Haven't I stated something similar in my second paragraph? Start from the base ISO and increase it if necessary to achieve the aperture and/or shutter speed to suit the shot.

Philip

[William W]

Haven't I stated something similar in my second paragraph? Start from the base ISO and increase it if necessary to achieve the aperture and/or shutter speed to suit the shot.

Philip

Yes you did. That was already acknowledged. The point is, that the opening sentence is a definitive statement and as such, especially for a beginner, IMO poor advice, or if you will not a very good opening for a couple of paras of advice for the OP.

For clarity, no offense is meant and it is not an argument about what you know or how you make photos: I think it is an important point that the first sentence has a very strong impact, and it was worthwhile to the OP to disagree strongly with it.

WW

[cauger61]

For example, we might be lucky enough to have tickets on a bright sunny day watching India play Australia in Field Hockey. Let's say we have our DSLR and our Kit Telephoto Lens - say a 70 to 300 F/4.5~5.6. Using the F/16 Rule, typically in front lit full sunlight, we would be pulling shots at F/5.6 @ 1/800s @ ISO100.

WW

Thnx a lot William, for an excellent exposition of the topic. Your writings are appearing to be a "class note" for a photography course!! (Ha ha). Anyway, as you have given the India-Australia hockey match example, I am putting that in a different context. Suppose that someone is taking a portrait shot with Aperture priority mode in front of an open window through which light is coming from the back, what he/she should do — increase exposure compensation or with regard to iso, what to do? one cannot go below 100 at the most 50 in some cameras.

[MrB]

It is NOT unreasonable to suggest that a beginner starts with the lowest ISO setting in reasonable lighting conditions, and alters it from there as required to reach appropriate exposure settings for each shot. How would a beginner know to start with ISO 400 or 800 until he has gained the experience that you have?

Philip

[DanK]

Thnx a lot William, for an excellent exposition of the topic. Your writings are appearing to be a "class note" for a photography course!! (Ha ha). Anyway, as you have given the India-Australia hockey match example, I am putting that in a different context. Suppose that someone is taking a portrait shot with Aperture priority mode in front of an open window through which light is coming from the back, what he/she should do — increase exposure compensation or with regard to iso, what to do? one cannot go below 100 at the most 50 in some cameras.

I'll let William answer in more detail, as he has vastly more experience with portraits than I do, but I will make one suggestion first. I think you are asking the wrong question. Forget Av mode and ISO for the moment, other than assuming that you should be shooting at base ISO in this case (100 in your case). The first question is not what settings to use. The first questions are how to expose the shot, given the bright backlighting, and what depth of field you want. The first is a complex question with numerous choices, which I suspect William will address. But only after you decide on an exposure and depth of field should you worry about the choice of settings, which is just a way to get to the exposure and depth of field you have decided you want.

And in almost all cases, there are several ways to get to that exposure, and the choice among them is often just one of convenience. For example, if you want the face to be properly exposed, and if you have decided on the depth of field you want, you could spot-meter off the face, using either Av mode or manual (both of which allow you to control aperture and hence depth of field). If you find that this exposure is, say, about one stop underexposed, you could use Av and +1 EC or use manual and set the shutter speed down to expose at +1 EV. The two choices will have exactly the same effect on what the camera actually does.

I'm again going to point you to the tutorials. They are not long. At the least, read the sections on exposure, metering modes, camera modes and aperture and depth of field. Then come back to the forum with questions.

[William W]

It is NOT unreasonable to suggest that a beginner starts with the lowest ISO setting in reasonable lighting conditions, and alters it from there as required to reach appropriate exposure settings for each shot. How would a beginner know to start with ISO 400 or 800 until he has gained the experience that you have?

Philip

Firstly, we obviously disagree on the meaning of the opening sentence of Post #7.

That is probably the point on which the conversation began to come unstuck.

As already mentioned: "If we assume reasonable lighting conditions, the ISO should be set at its lowest value." is IMO - NOT - "a suggestion". It is a definitive statement. That having been reiterated...

***

On the second part, yes, I agree the suggestion for a beginner to go out and learn in good sunlight, starting with a low ISO (not necessarily the lowest, but the BASE ISO) and then look at the Shutter Speed and Aperture to reckon those suitable or not suitable for the shots that they want to make is a very good learning experience.

WW

[rpcrowe]

I adjust my ISO using exposure compensation, if needed. I have recently been doing a lot of shooting using manual exposure (I select both shutter speed and f/stop) with auto ISO (the camera selects the ISO within the parameters that I have selected).

This combination suits me best when I am shooting fast moving action in variable lighting conditions, such as when I was shooting Duke playing with his ball under a spreading oak tree. Sometimes he would be in the shade of the tree and other times in the sun, with a third lighting situation of dappled sun.

I knew that I wanted a fast shutter speed to stop the agile little dog as he ran and jumped for the ball. I also knew that I wanted to be shooting at f/5.6, one stop down from the 70-200mm lens' maximum aperture of f/4. I selected this aperture to give me a tad more DOF than wide open, yet still maintaining a modicum of separation between Duke and the background.

If I had chosen aperture priority, the camera would have selected the aperture and might have closed down beyond f/5.6 or opened up to f/4. Shutter speed priority would have put me in the chancy situation of slowing down the shutter speed to a point in which Duke would have been blurred.



The A6400 Sony camera with which I was shooting has very decent ISO capability and I was not worried about using ISO 6400 or even a bit higher. Another positive is that I can see the final result of the exposure viewed through the EVF "before" I shoot, that way, I could adjust the exposure (if needed - it wasn't) simply and quickly by using a + or - exposure compensation (which would adjust the ISO with no impact on shutter speed or aperture),

Shooting in that configuration, IMO, I get the best of two worlds... Auto exposure and a manual selection of shutter speed and aperture, combined with the ability of quickly and easily adjusting the exposure by choosing a + or - exposure compensation. It has taken me a bit of time to gain confidence in that mode of shooting but, I have become very fond of Auto ISO.

Of course shooting in this mode, you need to be able to decide what shutter speed and aperture that you deem best for the shot. IMO, one way for a beginner to learn the ramifications of different shutter speeds combined with various f/stops is to select the "A" aperture priority exposure mode and then to run up and down the gamut of apertures. The camera will then select appropriate shutter speeds for that aperture and the beginner can view the impact of shutter speed and aperture on the image. I also suggest that the photographer determine what he or she considers maximum usable ISO is for the camera being used.

Note" some cameras have the capacity to shoot at below the native ISO, such as shooting at ISO 50 with a camera that has a native ISO of 100 or 200. That gives the photographer even more leeway in shooting,

[William W]

. . . Suppose that someone is taking a portrait shot with Aperture priority mode in front of an open window through which light is coming from the back, what he/she should do — increase exposure compensation or with regard to iso, what to do? one cannot go below 100 at the most 50 in some cameras.

Sinjab,

I agree with Dan, this is the wrong question:

Dan wrote:

“The first questions are how to expose the shot, given the bright backlighting, and what depth of field you want.”

And there are more questions … a portrait of whom? A young active child standing up? A calm woman seated? The answers to these questions will probably have an impact on Shutter Speed choice.

And maybe more questions: how are we controlling the lighting? Are we using Flash as Fill? Should we use a reflector to bump in some of that Backlight onto the Subject’s face?

And I agree with Philip’s sentiments and advice about learning:

Philip wrote (my edit now)

It is NOT unreasonable to suggest that a beginner starts with . . . [SOMETHING] and alters it from there as required to reach appropriate exposure settings for each shot. How would a beginner know to start with [A PRESCRIPTIVE PLAN] until he has gained the experience that you have?

To me that means, taking a step at a time and starting with the basics, making photos and reviewing those photos and making changes that you think will make a better photo with only one or maybe two items before proceeding.

***

One very basic is the understanding of how the Metering Modes in your camera work.

***

Generally addressing the scenario that you outline –

Let’s say an adult standing or sitting in front of a window and let’s assume Available Light with NO reflectors.

In any shot my first thought is usually always, “Where should the camera be positioned?”

When working in Available Light this question is really important. It’s likely more important when the Subject is Backlit. Note also that a small change in Camera Position can make a perfectly “backlit” Subject, partly side and/or top lit.

In this shot, ‘backlit’ was very difficult both for a reasonable exposure on the Subject and on the wall that he was photographing. Moving the camera position so the was top/side lit allowed for a much more even exposure on the wall and on him, albeit that his head is OoF and, typically foreground OoF is generally a no-no for any Portrait – but that’s what I wanted.

Sample 1


***

I tend to address Depth of Field next. I pretty much know DoF by rote: it’s not difficult to learn for Portraiture, there are many discussions you can research here at CiC. I use the fact that the DoF remains reasonably constant for any Aperture and Camera Format, provided that the FRAMING of the Shot is the same.

In this shot I wanted very shallow DoF so I used the lens wide open, again I moved so the Subject was not completely backlit, and also so that the background wall was lit by the sun through the window, then I waited for her to move her head to a suitable position and with a suitable expression.

Sample 2


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Regarding Shutter Speeds, for Available Light Portraiture, I have my ‘rules’.

I like to be pulling about 1/500s for Adults and around 1/800s for Children. For Subjects who can ‘pose’ then 1/250s can be safe. That is not to say that I don’t tempt fate and pull Portraits at 1/15s, if absolutely necessary.

My point is with a modern camera and its good ability at mid to high ISO, I will typically sacrifice ISO for a faster Shutter Speed to be safe and nail a pin sharp portrait with absolutely no hint of Subject Movement.

The metering - the question is "what to expose for?" That dictated by what you want.

In this shot, I didn’t care about the window light blowing out, so long as the Bride’s Back and the details of the dress was intact, that was the shot I wanted:

Sample 3


***

Specifically to your scenario:

If I had to shoot a completely backlit Subject (such as Sample 3 above), then I would not care about the window light blowing out, but I would usually seek to minimize the amount of canvas that widow light occupied in the final image – big chunks of white in the background are quite distracting, so would choose a Camera Position to minimize the background blow out.

I would choose an ISO as per Post #1, to allow me a RANGE of Apertures and Shutter Speeds.

Typically on a bright day, for a backlit Subject indoors from window light, which is usually diffused somewhat, for me to get to 1/500s as my Shutter speed and have a RANGE of Apertures at my disposal, I would be at about ISO400 for the lenses that I use.

So, in summary, personally I would really never ever be hanging around ISO100, in this scenario.

If I had to use Aperture Priority Mode (which I wouldn’t, I would use Manual Mode, because I find M Mode much easier and quicker than using Av Mode and playing with EC), I would use SPOT METERING on SKIN TONE, usually the FACE. I would then adjust the Exposure Compensation to suit the SKIN TYPE. For a typical Caucasian Skin I would be at about EC = +1.5, for typical Asian-Sub Continent Skin I would be at about EC = +1. If I were in doubt, I would bracket ±⅔ on SHUTTER SPEED.

***

The CiC Tutorials are excellent. Have you read them?

WW

Post Script:

A little while ago, I replied to anther of your questions about Portraiture, in part I wrote - my BOLD and UNDERLINED now for emphasis:

with Available Light, (and NOT using a Reflector or Diffuser) there is no manipulation of light available to us – so essentially we are usually always dependent upon CAMERA VIEWPOINT and SUBJECT POSITION to gain the best Lighting Scenario.

To do this effectively it takes ANTICIPATION and also a lot of PRACTICE.

I think that if any fan were to ask Muttiah Muralitharan how he perfected his Doosra, they would absolutely accept that, after some technical instruction, his advice would be “now go practice with a cricket ball – any cricket ball will do – but practice, practice and then practice”

Yet I am amazed at the number of Photographers who believe they’ll make better photos by buying a new camera, an expensive lens and all they need to do is use it one hour, every second weekend.

Are you practicing? Hopefully you are. Maybe not hours every day, but I expect that you have a few results of your practice. Sharing a photo or two for critique OF THE BASICS would be a worthwhile exercise, I think.

If my memory serves me correctly, this has been asked of you before . . .

[xpatUSA]

Can the members pls. take up this topic for discussion? My point is: are these two settings of dslr -- exposure compensation and iso adjustments, alternatives to one another? When and where they should be used? Do they give same result with a different approach. It appears to me a bit confusing!!

Sanjib namaste,

The difference between two - and therefore the intended usage - is found in the meaning of the words:

1) Compensation - as in Compensation for what? It is well know that camera metering is historically stupid. If you shoot a scene, the metering will make your image an average brightness of gray (which works most of the time). That means that a picture of laundry hanging on a line will show gray sheets, not white. Unless the sheets are actually black, in which case they will still show as gray sheets. To compensate for the metering's stupidity, we apply a correction called (in most, if not all cameras) Exposure Compensation - the effect being to apply more or less exposure to the sensor than the metering recommends.

2) ISO on the other hand is a pathetic attempt by camera manufacturers to fool old film shooters into thinking that the sensitivity of digital sensors can be altered just by twiddling a knob - as opposed to changing rolls of film to suit the anticipated lighting or to suit the shooting of speedy or dark objects. Personally, I never touch my ISO knob - it is super-glued to the camera's native ISO value of 100.

So, my answer to your main question is that exposure compensation and ISO adjustment are NOT alternatives to one another, even though they can have the same effect on the image.

Of course, what would I know? - I shoot a fully ISO-less camera and am always-in-manual ...

[cauger61]

Thnx a lot William for the great answer; though you have misspelled my name "Sinjab" instead of "Sanjib" (ha, ha)!!

[cauger61]

Sanjib namaste,

Of course, what would I know? - I shoot a fully ISO-less camera and am always-in-manual ...

Thnx a lot Ted. Your reply is illuminated with 2 more stops by using "namaste"!! What a globalisation, boss? (ha, ha, ha) that by staying at the far off corner of Texas you are to address me by Hindi. Great job!!

Now, let's come to the subject. But then how do you take care of a situation where you cannot use flash and/ or the lighting is changing and also you are taking hand-held shots?

[xpatUSA]

Thnx a lot Ted. Your reply is illuminated with 2 more stops by using "namaste"!! What a globalisation, boss? (ha, ha, ha) that by staying at the far off corner of Texas you are to address me by Hindi. Great job!!

Thank you! Long ago, I could speak a little Gujerati because girl-friend's family was from there. Mostly forgotten now. My new "second language" is Tex-Mex ...

I shoot a fully ISO-less camera and am always-in-manual

Now, let's come to the subject. But then how do you take care of a situation where you cannot use flash and/ or the lighting is changing and also you are taking hand-held shots?

Good question, Sanjib. Photography is only a hobby for me, and I don't shoot sports, portraits, or at night. My camera has no flash. I usually shoot in good light; if light changes, I change the camera settings by hand. Hand-held for me is only for "snaps" (I have shaky hands), otherwise always on a tripod. In other words, if conditions are wrong, I don't take the shot.