How does ISO work technically

[Yoddle Laheehoo]

Disclaimer: if the answer is already on this forum somewhere, I apologize for not being able to find it. I did search, though.

I'm trying to understand the technical aspects of my E500 and hope someone can explain to me how ISO is handled, from a technical perspective. With my analogue camera's I used to buy film with different ISO-values. These films where physically different. I would adjust the camera's ISO setting according to the type of film. On some occasions I would "upgrade" the film by using a high ISO setting (e.g. 800) with a lower ISO film (e.g. 400). When developing the film, I would correct the development times accordingly. When pushing this too far, there would be a drawback with the granularity of the film becoming visible in the prints.

In the digital world, there's no such thing as different films, so I guess that the camera uses a mechanism similar to upgrading an analogue film and taking the ISO value into account when processing the image. The drawback here for high ISDO values would be the noise issue. However, it's not totally clear to me how the ISO is processed in the picture. Is it something that is done afterwards or is capturing the light already influenced by the ISO value (does the CCD "know" the ISO value)? As ISO cannot be altered for RAW images (like wb), it must already be "in" the RAW image somehow. I really would like to understand, since understanding the tech things of the E500 makes it easier for me to determine when to change what settings.

 

[Louis_Dobson]

As I understand it, the sensitivity of the elements of the sensors is turned up. As they are more electrically active, they are more prone to intefering with each other's signals. That's noise. The effect is somewhat like film grain, and the beneficial effect is similar to faster film, hence the pleasing and witty use of the ISO analogy.

The densor the sensor, the more prone to noise. Four Thirds sensors are smaller than most other standards, 8MP is two more than most of the competetion, and whoever it is that designs and builds the Kodak branded sensors hasn't really got on top of it yet.

Hence your E500 doesn't perform as well at high ISOs as some of the competetion, although it does nicely at lower ones.

 

[Yoddle Laheehoo]

Thanks for the explanation, Louis. It's very helpful.

So, we got the aperture value and shutter speed to control the amount of light that reaches the ccd. We got the ISO value to control the ccd's sensitivity w.r.t. the light that reaches it. But how does the eV work, then? I know a raw image changes when the eV setting changes but I also have about 2 eV available for processing the raw image. Does it have a little bit of both?

 

[silentstorm]

i have read it somewhere that the ISO remains at the smallest default value available. what changes the ISO is just simply electronics & software boost.

if you take a shot at ISO 100 & its underexposed by 2 stops, you can boost it in Studio & you will see the noise creeping in. the final image noise is as though you have taken the shot at ISO 400 with correct shutter speed & aperture size.

so by setting ISO to 400 or 800, the image is actually being captured at ISO 100 internally with a faster shutter speed at the given aperture but boosted within the camera software & hardware. the output will show ISO 800 with other EXIF data.

hope i am not misleading..

 

[Leon Wittwer]

At higher ISO values the analog readout of a ccd (or cmos) cell is amplified prior to going to the analog to digital converter. It is always best to get the correct exposure by collecting enough light using the f number and shutter speed. In those cases where not enough light can be collected, then increasing the ISO amplification is necessary. The amplification is necessary so that the image data is converted to digital with sufficient tone levels for normal image processing. This is particularly necessary for jpg shooters but is also best for raw shooters. There is increased noise at higher ISOs. This noise is usually dominated by photon noise coming from the lower numbers of photons (less light) collected. The amplification, itself, and add some noise but in a well designed system, this should be small.
--
Leon
http://homepage.mac.com/leonwittwer/landscapes.htm

 

[Louis_Dobson]

To be honest, I don't know.

I assumed that changing the EV at the raw stage must have quality implications, and that data that isn't there cannot be processed.

One of the reasons these forums are useful is they challenge one's assumptions

 

[Yoddle Laheehoo]

Ok, so I understand the aperture, shutter speed and ISO are all hardware related. Aperture physically changes the opening of the lens. Shutter speed physically changes the timeslot in which the mirror goes up and ISO amplifies the analogue signal from the ccd prior to digitizing. But what about eV? In this forum I learned that wo raw images shot with different eV differ, so in that respect it's similar to aperture, shutter speed and ISO. But I also have - I'm talking about Olympus master - a 2 eV frame to change exposure in the raw image which makes it similar to wb, saturation, etc. Can someone explain to me how eV is handled by the camera?

 

[BorisK1]

When you change ISO settings in camera, you change the gain on the amplifier that's placed between the CCD and the analog-to-digital converter. This causes the same CCD signal to register higher numeric values. So changing ISO records higher values in your raw file.

Changing Ev in Master modifies how the values from the raw file get converted into jpeg. So again, we are dealing with amplification, but it happens at a later stage and off-camera.

The two processes are similar (and both increase noise), but they are used for different purposes. The main reason to crank up ISO is to get higher shutter speed while you are shooting. Moving up Ev is done to pull the details from shadows.

Generally, it's better to shoot with the lowest possible ISO. If it's not possible (because the shutter speed is too low), you get two choices: Either crank up the ISO and expose the shot properly, or underexpose and compensate with EV in postprocessing. In most cases, the first method gives better results.

However, from the specs of Kodak CCD, it seems that ISO values on it can't go past 800. So ISO 1600 is done by the camera firmware, by underexposing and then applying compensation. This means that shooting well-exposed ISO 1600 and postprocessing 1-stop underexposed ISO 800 should give similar results.

Some people posted shots with E-1 when they had to combine the two techniques - set the ISO to 3200 and underexpose by a stop, then recover that stop in postprocessing. This gives effective ISO of 6400. The results are, as you would expect, noisy - but they are better than not getting any picture at all. After noise reduction these shots look a lot better, and converted to b&w, they often look quite good.

Hope this helps!

Boris

 

[steve16823]

Exposure Value (eV) is not really a property in and of itself, but a calculation of the shutter speed, aperture value, and film/sensor ISO. The purpose of making such a calculation is to be able to put all of the exposure settings (shutter, aperture, iso) into common units, so that you can calculate equivalent exposures by varying the three exposure parameters.

For example, two different camera settings:

Setting number 1: f4, 1/60, ISO100
Setting number 2: f5.6, 1/125, ISO400

These 2 setting have the SAME eV, because while the aperture is reduced by one stop, and the shutter speed is reduced by one stop, the ISO is increased by 2 stops. Since they have the same eV, the density of the exposure will be the same -- ie, the range of light and dark tones in the image will be equal.

 

[Mountaineer]

The properties of the photodiode can't change, so you get the same amount of charge (or voltage) for a given amount of light at any ISO.

I'm just guessing here, but they probably just change the reference voltage on the Analog to Digital converter that converts the analog signal from the CCD to a digital value.

Another possibility is that they have a 16 bit A2D converter, but only save 14 bits in the RAW file (or something to that effect). Which bits get rejected could depend on the ISO setting. This method only allows ISOs that are powers of 2, i.e. 100, 200, 400, 800, etc.

-Scott

 

[Yoddle Laheehoo]

I posted this before, but somehow it didnt show up.

Exposure Value (eV) is not really a property in and of itself, but
a calculation of the shutter speed, aperture value, and film/sensor
ISO.

Thanks for the clarification. There is also an eV setting on the camera. I would like to know what happens when we change this setting on the camera. As eV is a combination of shutter speed, aperture and iso, what changes when the eV is changed? Or isn´t it as simple as that?

 

[David 247]

ISO = International Standards Association. - It is the international standard for measuring the sensitivity of any photographic recording medium. The standard is the same for both filim and digital, from the end users perspective. The predecessors to ISO were ASA and DIN but using two different standards of measurement in the world created come confusion among casual photographers, so an international standard was created. For all practical purposes ISO and ASA are the same.

FILM ISO: As you increase film ISO, you also increase "grain" and reduce resolution. Films whoever have been evolving for over 100 years now, and some very substantial improvements made. However, the basic rules remain the same for a particular brand. Higher the ISO, the higher the grain and the lower the resolution.

My memory is a little fuzzy but the original ASA and subsequently the ISO standard is based upon the formula of light at a specific time, date, location and the shutter speed required at F/16. What I can't remember is if that is based upon "physical tests" at the "equator or at Rochester NY, since the standard was originated by Eastman Kodak.

However, assuming it is Rochester, NY, the ASA/ISO formula would read this way: Sensitivity (ASA/ISO) is the shutter speed required to properly expose an average subject with normal processing methods at high noon, on a clear day. (Clear day means no pollution, no haze, no clouds, and low humidity). So theoretically at ISO 100 and the optimum conditions described above you would shoot at 1/100 second at F16 or any shutter/aperture equivalent. (1/200@ F11, 1/400@ F8....)

(NOTE: as you change altitudes, latitude, humidity, seasons, and pollution influences, you must adjust some just based upon experience.)

TRIVIA: For the first half of the 1900's, this is how a great many photographers "guestimated" their exposures. You used to be able to get "cardboard" exposure wheels where you indicated the speed of the film, and then it would give recommendations for starting exposure under different lighting conditions. That method still works with the ISO ratings of today, but with modern metering technology really isn't necessary for most people to know.

While there are some differences, in general terms it remains the same with Digital. A sensor will have a "base" ISO which will be the maximum quality. As you increase ISO, you then increase noise and with the noise comes a loss of resolution. Much the same as with film.

--
Denver David
My photos at:
http://eaglesnest.smugmug.com
http://www.pbase.com/brdavid/
http://homepage.mac.com/brdavid/

 

[Louis_Dobson]

Hmm?

There is a setting to decide whether any EV compensation you do yourself (ie over riding the metering by telling it to over or under expose) is set in incrmenets of one thrid or one half stop (I changed mine to a half, but I'm sloppy and normally in a hurry).

Is that what you mean?

 

[David 247]

EV does not impact ISO directly. EV adjustments do not alter the gain level of a digital sensor or the base sensitivity of a film. ISO 100 remains ISO 100. However, they will impact the actual aperature or shutter speed used.

It is just an Exposure value adjustment of plus or minus allowing you to "tweak" exposure for a specific subject and lighting conditions. When adjusting EV you are essentially under or over exposing a lighting range (subject) to compensate for known variables. For instance, if you know that certain conditions will result in either blown highlights or underexposed shadows and there is a need to retain detail in one of those areas, then you can adjust EV to compensate for one or the other.

Examples:

You are shooting in snow conditions. Because metering systems tend to "average out" light, lots of snow or other "bright" conditions may result in underexposure of your primary subjects. By adding a +EV value, you can increase the exposure for your main subjects, although at the expense of losing detail in the highlights. -- If the highlights however are critical, you can underexpose with EV to maintain highlight detail.

In simple terms, EV allows you to adjust overall exposure, plus or minus, to tailor it to your subject and shooting conditions. It is a global exposure adjustment effecting every shot taken while it is in effect, rather then a shot by shot exposure adjustment. 1 EV is equivalent to 1 stop or a doubling or halving of the light reaching the recording medium.

--
Denver David
My photos at:
http://eaglesnest.smugmug.com
http://www.pbase.com/brdavid/
http://homepage.mac.com/brdavid/

 

[digital ed]

why guess and/or give incorrect answers to sincere questions. This is not helpful.

I see responses to this question that are quite wrong. Who is to point this out to the innocent questioner?

This is an example of what the "old timers" on this formu are complaining about. If you don't know for sure, just keep quiet. Don't publish your guesses to prove what you do not know.

Ed

 

[BorisK1]

The ISO on the sensor is limited to 800. So setting it to 1600 really is a software boost. But setting it in the range 100-800 changes the actual data that is recorded.

To verify this, try recovering blown highlights from an ISO 800 shot - it's not any easier than from ISO 100.

i have read it somewhere that the ISO remains at the smallest
default value available. what changes the ISO is just simply
electronics & software boost.

if you take a shot at ISO 100 & its underexposed by 2 stops, you
can boost it in Studio & you will see the noise creeping in. the
final image noise is as though you have taken the shot at ISO 400
with correct shutter speed & aperture size.

After "pushing" it, the image noise from an underexposed ISO 100 shot will be a lot worse than the noise from a properly exposed ISO 400.

so by setting ISO to 400 or 800, the image is actually being
captured at ISO 100 internally with a faster shutter speed at the
given aperture but boosted within the camera software & hardware.
the output will show ISO 800 with other EXIF data.

That is incorrect. The ISO values 200-800 actually change the data that is captured, by changing the amplifier's gain.

hope i am not misleading..

Sorry, but you are. You might be confusing ISO and WB.

Boris

 

[steve16823]

Good follow up question. I realized that in my original post I didn't really address your question about the eV adjustment in Studio (but someone else did a good job of it).

The eV adjustment (or exposure compensation) on the camera simply offsets the meter to provide more or less exposure value (providing a lighter or darker exposure).

You are correct in saying that since eV is calculated from shutter speed, aperture, and ISO that one (or more) of these things must change. Which one changes depends on which mode you have your camera set to. If you are set in "A" mode, meaning "aperture priority" and you change the exposure compensation setting, the shutter speed will automatically adjust to set the total eV to the new setting. In "S" mode, the aperture will change. In "P" mode, either one or both of them may change.

In "M" mode, you'll notice that exposure compensation is disabled (at least I think this is the case). This is because in manual mode it makes no sense, since you have a display in the viewfinder telling you exactly how far the eV of your combination of shutter speed, aperture, and ISO vary from the optimum eV as determined by the light meter.

I hope this helps. It's really a simple concept -- almost so simple it makes it tough to explain without making it seem complicated!

 

[Yoddle Laheehoo]

Hmm?

There is a setting to decide whether any EV compensation you do
yourself (ie over riding the metering by telling it to over or
under expose) is set in incrmenets of one thrid or one half stop (I
changed mine to a half, but I'm sloppy and normally in a hurry).

Is that what you mean?

Yes

 

[BorisK1]

What I meant to say, "Ev adjustments (in Master/Studio or in-camera) do not modify the ISO value".

Exposure value (as many have pointed out) is by no means independent from ISO. Rather, it's determined by the combination of ISO, aperture, and exposure time.

Adjusting Ev in-camera can change the aperture, exposure time, or both, depending on the exposure mode.

Adjusting Ev in post-processing makes the picture lighter or darker. In effect, it simulates changing the ISO, while leaving the aperture and exposure time the same.

 

[Yoddle Laheehoo]

Thanks, Steve. This makes sense, especially that the setting that is adjusted when changing eV depends on the program the camera is set to. I didnt think of that (should have, though). I happen to know that with the E500, when set to M, the eV button is used to switch the dial to adapt shutter speed (default is aperture). Now this makes sense, too. Of course, one other question comes to mind. When changing eV in Olympus Master on a raw image, what value is changed? Obviously it cannot be aperture, shutter or iso.....