Sensitivity (ISO) in digital imaging seems to be the subject of quite a lot of confusion - it's becoming common to hear talk of manufacturers 'cheating with ISO.' So we thought it made sense look at why sensitivity appears hard to pin down, why we use the definition we do and how it's actually not as complicated as it can sometimes seem.
ISO in Photography
Before we get too carried away with the intricacies of ISO standards, it makes sense to step back and consider how we use sensitivity in photography. Sensitivity is the connection between the physical exposure (how much light you let in) and the brightness of the final image. However, all this brightening occurs after the image has been captured on the sensor, so scene illumination, shutter speed and aperture end up playing the biggest roles in determining image quality.
Sensors and sensitivity
For ISO to relate exposure to final image brightness, we have to think about the inherent sensitivity of the digital sensor and this is where it risks becoming rather removed from photographic concerns.
Much of the complication arises from the fact that there is no 'correct' way of exposing a sensor. Sensors have a capacity for converting light into electrical charge - limited at the upper end by the point at which the sensor becomes saturated (and cannot convert any more photons of light into electrical charge) and extending down until the signal is drowned-out by electrical noise. The upper, saturation limit of the sensor's response defines the brightest light intensity that can be turned into meaningful data in the final image.
However, although exposing to this limit (a technique called 'exposing to the right') can be an effective way to expose a Raw file, few (if any) camera provide the necessary tools to expose in this manner. Cameras light meters (even 'highlight' metering modes) are based on JPEG output and the majority of them, per the ISO standard, are designed to 'correctly' expose a middle grey.
And there is an added complication before we can get to that point. To make an image that looks good on a monitor, or as a print, curves are applied that compensate for the response of the monitor (to make the tonal response look more like the original scene) and to make the image look desirably contrasty.
The camera's tone curve converts the sensor's output to the final image brightness, which means it also defines what Raw value represents middle grey and hence how the sensor needs to be exposed. (In fact there is a subtle interplay between the sensor's inherent sensitivity, its dynamic range, the tone curve and the camera's metering.)
A standard with shades of grey
So this is what ISO is defining when you use it: its combining considerations of the sensor's sensitivity with the effects of the tone curve and metering so that you can get the correct final image brightness with your chosen exposure.
However, the standard set down by the International Organization for Standardization (ISO12232:2006, as it happens), contains five separate definitions, each of which can produce a different answer for the same camera. Thankfully, only three of these definitions are widely used and only two, closely-related definitions are used by camera makers.
ISO, courtesy of CIPA
The two definitions of ISO that are actually used by camera manufacturers (and are reported by their cameras) are based on the brightness of cameras' JPEG output. Both definitions come from standards developed by the Japanese camera trade body CIPA, which were adopted by ISO in 2006. The first definition is probably the simplest and most intuitive, and it's called Standard Output Specification. Essentially, it defines ISO as the camera behaviour that renders middle grey at the correct brightness (as we've just described and pretty much the same way as it did for film).
The other definition (Recommended Exposure Index) is fairly similar but is designed to accommodate multi-zone/pattern metering systems. These metering systems aren't based on trying to represent middle grey and instead aim to achieve whatever the manufacturer considers to be 'correct' exposure. As such they can't be measured because the definition is pretty much circular: whatever the camera chooses is right, by definition.
So what about the others?
The only other definition of ISO you're ever likely to encounter is one that can be used for Raw data. The problem is that it's based on a combination of the sensor's saturation point and a generic tone curve – which isn't necessarily the tone curve your camera's JPEGs or metering are based on. So, discrepancies between this figure and your camera's reported ISOs aren't the result of under or over-reporting of ISO, they're a measure of how different your camera's tone curve is from this generic tone curve.
Why do I need to worry?
If you use the camera's JPEGs, or a Raw converter that acknowledges the manufacturer's rendering intent (and that includes many popular Raw converters), then chances are you're going to get the ISO that your camera tells you. So rather than measuring a slightly obscure aspect of sensor performance, our tests are based on the Standard Output Specification that the camera manufacturers use, that your camera is based on and that, chances are, you use.
|What happens when I change the ISO?|
|Traditionally ISO has been changed by amplifying the sensor's output before it is converted to digital data (as demonstrated if you move your mouse over the above diagram). However, it is also possible to mathematically manipulate the data once it has been digitised - many 'extended ISO' settings and some intermediate ISO values between full stops (e.g. 250 and 320) do just that.|
Jul 17, 2012
Jun 27, 2012
May 23, 2012
May 10, 2012
Sony is the world's leading mirrorless camera brand but remains third for ILCs overall, it's said in a presentation to investors. A focus on high value cameras and lenses should boost operating income, it says. Read more
It's nicknamed the 'Cycloptic Mustard Monster,' and is a 3D printed medium format camera. Read more
The new NanGuang LED lights are battery powered and come with accessories including filters and diffusers.
Have you been telling yourself, "Hey, I really need one of those 8K displays?" A video about Dell's new 8K monitor shows you what to expect. Is it really that much better?
Tamara Lackey, a Nikon ambassador USA and pro shooter, discusses embracing self-consciousness as a means of connecting with subjects.
There's a new Spiderman movie coming out and the poster been generating a lot of online chatter. Mostly about how it looks like the creation of a fevered teenager that just discovered Photoshop.
An honest defense of the system's merits, with photos as proof.
Copyright disputes are no fun at all. 'Binded' is a new startup that aims to simplify the process of registering - and enforcing - copyright for photographers. Read more
Not everyone wants to pay a premium for a long zoom camera. Thankfully, there are many reasonably priced cameras available, though they won't offer the same image quality as enthusiast models. In this updated roundup we look at big zoom cameras with more consumer-friendly price tags. Read more
Think Tank Photo has updated two of its popular bag lines with improvements to functionality. Read more
We’ve all seen Bob Jackson’s Pulitzer Prize winning photo, but there's another.
The sample footage looks good.
It will automatically pick the best camera settings depending on shooting conditions. It even promises enhanced functionality for your camera, like exposure and focus stacking. It already supports many cameras from Canon, Fuji, Nikon and Sony. Read more
As if $13,950 wasn’t enough to pay for a special edition lens, the Leica Store in San Francisco is offering a prototype of said lens for $24,995. Read more
Make those old photos disappear without deleting them forever.
Firmware updates enable 10 fps shooting with adapted A-mount lenses, and faster startup times and better compatibility for 20 fps shooting when using native lenses on the a9.
Fujifilm has released firmware updates for its camera models X-T2, X-Pro2, GFX 50s, X-T20, X100F and X-T1 and updates to three of its software products.
A 22 year-old Romanian photographer uses his DJI Phantom 4 drone to capture unique perspectives of the city where he now lives.
What's it like to ride the waves with champion surfer Kelly Slater? This VR video from Teton Gravity Research gives you a taste.
When Nikon released the full-frame D3 in 2007, it changed the professional photography industry. In this week's Throwback Thursday, Barney remembers a legend. Read more
The new stuff should have better red hues, improved sensitivity and finer grain - but don't worry - will still shift blues to green, greens to purple and yellows to pink.
Ricoh has introduced a new rugged compact camera with a 16MP CMOS sensor, 28-140mm lens, 2.7" LCD and built-in LED macro lights. Read more
This compact drone can shoot HD video using a 2-axis stabilized 12MP camera. Read more
The new Prynt Pocket can print a photo directly from their iPhone simply by inserting the phone into the printer, then snapping a photo. Each print will cost about 50 cents. Read more
Updates for Adobe Camera Raw and Lightroom CC bring support for the Sony A9 and Panasonic ZS70/TZ90, along with bug fixes.
The Triggertrap remote camera control system is no longer sold due to the company folding, but now users will be able to build their own. Read more
The Magic Format Converter comes with internal optics that expand the image circle of full-frame DSLR lenses for use on the Fuji medium format camera. Read more
The usually Apple-exclusive MacPhun software developer has announced that it will introduce PC versions of two of its most popular applications. Both Aurora HDR and Luminar should be available for the Windows operating system by the autumn of this year. Read more
Sony's newest G Master telephoto zoom, announced alongside the a9, is the first of the company's FE lenses to reach 400mm natively. We had one in California and photographed horses, portraits, and landscapes - check out how it did. Read more
Garmin has entered the 360-camera market with the VIRB, which captures 5.7K video at 30p as well as 15MP stills. Read more