Couple questions...

[William W]

Basically, IMO, metering off a gray-card for still photography, sort of transforms the functioning of your reflected camera meter into something like an incident light meter.

What I mean by this is that you need to adjust your exposure differently when metering the scene or subject as opposed to when metering off of a gray card.

When metering a scene or subject, the camera's meter tries to convert everything to gray. Therefore if you are shooting a snow scene, you need to add extra exposure to make the snow white, not gray. When shooting the proverbial black cat in a coal bin, you need to decrease exposure so the cat and the coal will be black - not gray.

However, if you meter off a gray card; you need to subtract exposure when shooting a bright scene (like the snow) and add exposure when shooting the black cat in the bin.

Hi Richard.

I don’t understand what you mean by the underlined portion.

If you use a Grey card to meter the light where a White Dress will be located or a Black Tux will be located – then the 'correct' exposure to use, will be what the grey card meters it to be: and will be the same for the Black Subject as it is for the White Subject.

Isn’t that the idea of using a grey card, so the camera's light meter doesn't get confused by the white or the black?

WW

[jcuknz]

I have not read the last few entries but there seems to me a situation where EC might not work as one thinks it would and this is where one is working at the limit of the range the lens is capable of such as at full zoom with a f/4<f/5.6 lens. Does the lens zoom back to wide so that it can use f/4 ... OK a silly suggestion

[Mike Buckley]

No, the lens doesn't zoom back to achieve a wider aperture. Instead, depending on the settings you are using at the time, increased exposure will be achieved by either using a slower shutter speed or by increasing the ISO value.

[Mike Buckley]

However, if you meter off a gray card; you need to subtract exposure when shooting a bright scene (like the snow) and add exposure when shooting the black cat in the bin.

Richard,

I think you meant to include one very important word and you reversed the use of two other words:

However, if you meter off a gray card; you don't need to add exposure when shooting a bright scene (like the snow) and subtract exposure when shooting the black cat in the bin.

[Inkanyezi]

No, the lens doesn't zoom back to achieve a wider aperture. Instead, depending on the settings you are using at the time, increased exposure will be achieved by either using a slower shutter speed or by increasing the ISO value.

Please don't chastise me for nit-picking here. There is one thing that I would like to stress, and in the above quote, I regard it as a significant error, that confuses more than it clarifies.

Exposure is NOT changed by fiddling with the ISO setting. The only thing the ISO setting does is altering the exposure index, EI, by raising the ouput level of the sensor. Exposure is what you set the camera to in manual, or what the automatic functions set and display in the viewfinder or screen. In fact, by increasing ISO, with any of the automatic functions, exposure will be decreased.

However, by increasing the exposure index of the sensor, without changing exposure, the jpeg image SOOC may come out brighter, and it will also lose dynamic range if the scene dynamic range is larger than the resulting dynamic range of the sensor at that ISO setting.

It is worth the mental exercise to fathom that ISO and exposure are two very different entities, and they are not related in the way that many people might gather from the fact that in auto modes, exposure will be influenced by the ISO setting. Exposure is the time the shutter is open and the amount of light the sensor receives during that time, completely unrelated to ISO setting.

[Mike Buckley]

Urban,

I'm not familiar with the term, "exposure index." Never heard of it. Perhaps that erroneously influences the precise words that I use when writing about exposure. Having said that...

Exposure is the time the shutter is open and the amount of light the sensor receives during that time, completely unrelated to ISO setting.

I completely disagree that exposure as you define it is not at all related to the ISO setting. The sensitivity of the material that the light falls onto, whether it is an electronic sensor, film, glass, or whatever, directly affects the amount of time the shutter is open and the amount of light that falls upon it to achieve the photographer's desired results.

[William W]

. . . seems to me a situation where EC might not work as one thinks it would and this is where one is working at the limit of the range the lens is capable of such as at full zoom with a f/4<f/5.6 lens. Does the lens zoom back to wide so that it can use f/4 ... OK a silly suggestion

YES. You are correct, JC.
There are instances where EC does NOT work as expected.


NO. There is no lens system which zooms the lens back, to allow a faster aperture.


Different camera brands may have different functionality.
I will outline how CANON EOS cameras will function in the exact situation to which you are referring.

IF - a Canon EOS camera is in Tv Priority Mode (Shutter Speed Priority) and a Varying Maximum Aperture zoom is used (for example EF 35 to 135 F/4~5.6) and the lens is framed at FL = 35mm and the camera is framed on a scene of an EV (Exposure Value) such that the combination of:


the Camera’s TTL Light Meter Reading;
the User-determined Shutter Speed;
the Exposure Compensation,

necessitate that F/4 be used to make the shot

AND – then the lens is zoomed to 135mm, (where F/5.6 is the MAXIMUM Available Aperture),

THEN - the camera will still allow the shot to be made, even if the Camera’s TTL Light Meter determines that the shot will be underexposed.

The camera will give a warning that the shot will be underexposed in the viewfinder: the aperture value will flash – but the shot will still be allowed.

***

Here is a very quick example comprising two JPEGs SOOC which were shot in my office a few minutes ago.

The scene is a wall where the EV is reasonably even and equal whether the shot is framed at a Focal Length of 35mm or 135mm.

I used the EF 35 to 135 F/4~5.6 Lens and a Canon EOS 5D Camera.

METHOD:

1. The camera was set to Tv Priority; the lens zoomed to 35mm; the EC set at +1Stop; Metering Mode “Partial” (which is 8% of the viewfinder at the centre and is akin to a widish Spot Metering) - AND THEN - the Shutter Speed (1/10th Second), was selected such that the camera’s light meter chose F/4 as the Aperture for a ‘correct’ exposure.

Note that F/4 is the maximum aperture capable for that lens when the lens is at 35mm

Note the tone of the paper on the Olympic Certificate and tone of the Cork Wall Covering at the centre of the frame where the TTL Light Meter was making its reading.
Here is the JPEG, SOOC:


Frame 600092_Sample 01
F/4.0 @ 1/10s @ ISO1600 EC +1; Tv Mode; Partial Metering


2. Then the lens was zoomed to 135mm. At that Focal Length, the lens is only capable of F/5.6 as the Maximum Aperture.

The viewfinder was flashing F/5.6 as a warning that the image would not be correctly exposed - that is to say the EXPOSURE COMPENSATION, which was exactly +1 Stop would NOT HAPPEN and therefore the shot would be take at F/5.6. Here is the JPEG SOOC, note the DARKER tone of the paper in the Certificate and the DARKER tone of the Cork Wall Covering:


Frame 600093_Sample 02
F/5.6 @ 1/10s @ ISO1600 EC +1; Tv Mode; Partial Metering

***

In the case of AUTO ISO -

Ignoring the separation of ‘ISO’ from ‘EXPOSURE’ which is described above and also noting I shall not debate that point:

If I were to have used a CANON camera which is capable of a AUTO ISO and that camera were set to AUTO ISO, (for example an EOS 5D MkII), then in the situation which I described above the function of ‘Exposure Compensation’ will be achieved ONLY IF the camera is able select an ISO high enough to allow the shot to be made at the ‘correct’ exposure the camera’s light meter suggests.

For example I used my 5DMkII and the same lens, the same shutter speed, the same scene, etc.

The 5DMkII selected ISO1600 for the shot at 35mm, but it selected ISO 3200 when the lens was zoomed to 135mm - thus the AUTO ISO allowed the Exposure Compensation to function, as expected.

However, ISO 3200 is the limit of AUTO ISO on the 5DMkII – and if the scene were darker or if a faster Shutter Speed were initially selected when the lens was at 35mm, then the 5DMkII would act the same as the 5D and the Aperture would blink when the lens was zoomed to 135mm - BUT - the camera would still allow the shot to be made, but the EC, in this case would not function effectively and the 135mm shot would appear darker.

WW

[Inkanyezi]

Urban,

I'm not familiar with the term, "exposure index." Never heard of it. Perhaps that erroneously influences the precise words that I use when writing about exposure. Having said that...

Exposure is the time the shutter is open and the amount of light the sensor receives during that time, completely unrelated to ISO setting.

I completely disagree that exposure as you define it is not at all related to the ISO setting. The sensitivity of the material that the light falls onto, whether it is an electronic sensor, film, glass, or whatever, directly affects the amount of time the shutter is open and the amount of light that falls upon it to achieve the photographer's desired results.

Exposure index is the setting of the exposure meter. The term was used when film was "push-processed" to a higher exposure index, analogous with our changing of ISO setting in the electronic camera. If we for example had a Tri-X film with a measured sensitivity of ISO 320/26, but decide to use it as if it were ISO 1600/33, by increasing developer temperature, we use the film at an exposure index of 1600 ASA, thereby losing about two stops of dynamic range.

When we increase the output of the sensor, its sensitivity remains the same. Just as with the silver halide film, we cannot increase its sensitivity. Increasing the ISO setting does not increase the sensitivity of the sensor, but it increases its output, much in the same way that turning up the volume of a radio will not make it more sensitive to the radiowaves that enter, but it increases the output from the amplifier.

It is a basic understanding of the photographic process that is confused by the erroneous notion that raising ISO would change the sensitivity of the chip.

But even if we could change the sensitivity, it is still not related to exposure. The impression on the chip or the final image may well be influenced by its sensitivity, but exposure is only the light we let the sensor receive, quite regardless of its sensitivity. Exposure is the light that strikes the sensor, and sensitivity as well as ISO setting are different and unrelated entities. Mixing them up causes confusion.

[Mike Buckley]

Urban,

There are many common usages of the term, "exposure," while discussing photography. You seem to be focusing on the scientifically accurate description as it pertains to input.

I am using it in the more general sense as it pertains to output. As an example, it is standard terminology of international multi-billion dollar camera manufacturers to refer to the adjustment of the ISO value of a digital camera's sensor as one aspect of "exposure compensation." Similarly, I can view two images that were captured using the same shutter speed and aperture though with different ISO settings. It is standard practice to refer to one of those two images to be exposed less than the other image. Indeed, the tutorials here at CiC refer to "the exposure settings of aperture, shutter speed and ISO speed."

Though that may not be scientifically accurate, those with a technical and scientific bent should know that and should not be confused or mislead by it. When it comes to the practicality of making generally pleasing to great photographs, those that don't have that particular bent do not suffer any more than they would suffer not understanding the circle of confusion as it pertains to depth of field; if we can make the exposure that we like and if we can produce the depth of field that we like, that's really all that matters.

Just my opinion.

[Inkanyezi]

I regard photography as a visual art that includes technical processes. For an artist, it is necessary to understand the properties of the different tools, and how they can be used. In the process of learning the art, it is good to use a terminology that does not confuse the mind. When we change a setting in the camera, a basic understanding of what happens, at all levels, is useful for the artist.

By using erroneos concepts and fuzzy terms, we do the novice a disservice, as the concept becomes confused. We are not helped by overly simplifying for example the exposure index by equalling it to sensitivity. The concise term exposure index has a clear meaning, and sensitivity has another. We may regard them interrelated, as the exposure index apparently is the "speed" we are using for the chip or film when exposing it.

But it helps to keep those entities apart, as we can use some of the different concepts at various stages in the process, while others cannot be changed by any post production actions. We can change exposure index afterwards, by manipulation of the curves digitally in our image processing software, but we cannot change exposure. Exposure is what takes place when we open the shutter, until it is closed. Even if multi-million software companies put a control in their RAW conversion software marked "Exposure", it cannot influence the exposure. The exposure was set when the shutter was released. NOTHING will change it afterwards.

We can manipulate the image, by manipulating the data that builds it, but exposure WAS the moment when the shutter was released, and without travelling in time, we cannot change it. Ever.

So it is not basically a "scientific" idea behind my reasoning, but I regard it as an aid to understand the underlying processes, which may or may not be necessary for someone's artistic expression. You can discard it if you like, but nevertheless, it is dead wrong to have a control called "exposure" in the program we use for manipulating the image. Brightness would be a better choice, because it is what we can fiddle with.

Using concise terms makes it easier to talk about how tools are used, whether they are paintbrushes, chisels or photographic paraphernalia. Branding whatever "exposure" is a disservice to the photographic community. Using "exposure" for something it is not, is downright silly and does not lead forward. The same problem confused people in the film age too, and my opinion is that we should get rid of such sloppy use of terminology, particularly as there are correct terms that we could use instead.

When we use a higher exposure index, we may expose less, to achieve a similar result, compared to a lower exposure index. At EI 400 (ISO 400) we expose only 25% of what we expose at EI 100 (ISO 100), to get a similar result. The ISO 400 exposure thus is only a quarter of what we needed for the ISO 100 setting. We do expose it less. If we exposed it the same, the ISO 400 shot might be hopelessly over-exposed. It is easier to understand the basic properties of our equipment and our possibilities to use it in different ways, to achieve the result we want, if we stick to a coherent terminology where concepts formed by words are used in the same way, always.

... those with a technical and scientific bent should know that and should not be confused or mislead by it. When it comes to the practicality of making generally pleasing to great photographs, those that don't have that particular bent do not suffer any more than they would suffer not understanding ...

There's the problem. You don't confuse those that know, you confuse those that don't know about these things, but you make it harder to learn the basic stuff in photography by using terminology in a sloppy fashion. I am not misled, but anyone else that reads about it, and particularly those that are new to photography. It is a disservice to the novice, disinformation about the basic properties of our medium.

[Mike Buckley]

Urban,

If I am "using terminology in a sloppy fashion" and if my posts are "a disservice to the novice" and providing "disinformation" about this particular topic as you explain, I am in the camp as all of the major camera manufacturers, the tutorials here at CiC and virtually countless other forums and articles. Considering that their realm of influence is vastly greater than mine and surely deserving more respect than anything I could write, I recommend that you take it up with them, not me.

[jcuknz]

Sorry