Understanding ISO for Astrophotography

Does increasing the ISO on your DSLR make it more sensitive? No! Yes! Depends!

Let’s start with what ISO is and means. It’s not an acronym and is pronounced eye – sew, not eye – es - oh. If I had a dollar for every time I’ve been told that ISO stands for International Standards Organization, I could buy a really nice lens. Not only is this not true, but there’s actually no such thing as the International Standards Organization.

Go ahead, I’ll wait here while you Google that.

Okay, maybe I’m splitting hairs because there is actually something called the International Organization for Standardization, and maybe they just rearranged the letters – right? Nope, not even. An international organization would probably avoid using English-lettered acronyms, don’t you think?

Double up

Doubling the ISO is like doubling the exposure (for out of the camera results).
Richard S. Wright Jr.

Instead, ISO is short for the Greek word isos, which means equal (see the bottom of this page for more on this). This term is incredibly useful in photography. Not that long ago, film was the medium of photography, and ISO 200 film was twice as sensitive to light as an ISO 100 film. ISO 400 was twice as sensitive again, etc. These sensitivity standards allowed you to know that for X amount of light, and a focal ratio of f/x and such-and-such shutter speed, ISO 800 would give you a good exposure. This worked no matter whose camera or lenses  (or brand of film) you used, and it made photography comprehensible, repeatable, and predictable. For a dimmer scene, all other things being equal, you could increase the sensitivity of your camera by, for example, using ISO 1600 film. ISO films really were truly, chemically more or less sensitive to light exposure.

No more film! Silicon now reigns in photography!
Richard S. Wright Jr.

Enter the era of digital cameras. Now you have an electronic sensor that receives light, turns photons into electrons (a little bit of an over simplification), and then reads out an image digitally. Digital sensors have a certain Quantum Efficiency (QE), which is a measure of its ability to convert light to electronic signals. A QE of 50% for example, would convert half the light falling on it to an electronic signal.

Let’s say you have a given image sensor that's as sensitive to light as ISO 800 film, and you build a camera around that sensor. Imagine now that you want to sell that camera to the photographers of the world. There is a lot of photographic freedom associated with varying ISO’s, so selling an ISO 800-only camera would be a quick route to bankruptcy.

Instead, digital cameras mimic traditional cameras by allowing you to set your own ISO. To make the camera appear more sensitive, the camera would amplify the signal from the chip, and to make the camera less sensitive, it would dampen (darken) the image from the chip. The camera would do both actions in a standardized way, matching the ISO ratings of film. This is nothing more than sensor gain, which CCD imagers knew about well before the CMOS revolution.

Correlating gain to ISO is far from an anachronism, because now, everything you know about photography stays the same. As a bonus, you can adjust ISO on the fly without changing film to get the exposure you want. Plus, you don’t have the added expense of buying and developing different types of film. Not hard to see how digital photography was able to replace film as the mainstream tool of choice so quickly.

In a sense, a digital camera has an inherent fixed sensitivity to light. Upping the ISO does nothing but brighten the image in the camera before saving it to your memory card. You'd think you could accomplish the same thing with Adobe Photoshop's levels and curves and it wouldn't be any different . . .  except that it is!

A couple of months back, we talked about noise. Less noise makes your camera more sensitive in the sense that fainter signals become detectable. One of the sources of noise is readout noise, which is the uncertainty that pollutes your image as it’s read out electronically from the camera. The faintest signal in a single exposure is limited by the noise in your image, and a significant source of that noise in very low light situations can be the electronic read noise. (In most sensors, there is also a very faint dark fixed-pattern noise that shows up in low-light images too.)

What happens if you read out the image, signal and noise together, and then “brighten it” with Photoshop? Well, you increase everything, both the image you want, and all the noise too. Brightening an image in Photoshop is NOT the same as increasing sensitivity.

Noise hides fainter stars and features. Decreasing noise actually increases sensitivity!
Richard S. Wright Jr.

However, what happens if you amplify the signal on-chip, before the read-out noise can be added? Now, your faintest signal can be boosted above the noise floor where the read noise is hiding. Fainter signal and details now are brighter than the background noise . . .  you’ve technically made the camera more sensitive!

Which of these two methods does a DSLR use when increasing ISO? Well, both.

Each DSLR model, based on the image sensor it uses, will have a sensitivity equivalent to some intrinsic ISO. ISO settings slower than this will be scaled down (which also reduces the noise contribution). Images shot at a higher ISO will be scaled up on the chip, boosting the signal before the read noise gets added in, and thus giving us an effective increase in sensitivity to faint signal. However, most DSLR’s can only use so much on-chip gain; they'll then further boost the signal after the readout. At this point, you are no longer getting any effective sensitivity gain, you're just amplifying the signal and the noise together. (Super-high ISO’s also reduce your dynamic range, but describing why will take a whole other blog post!)

For now, my advice is start astro-imaging with your DSLR at ISO 1600 and experiment. Shoot the same scene at various ISOs and compare the results. On a newer DSLR model, you might be surprised with what you get.

12 thoughts on “Understanding ISO for Astrophotography

  1. Alain Maury

    You appear way too young to know much about photographic films and things like that. When I was young, the sensitivity of films was measured in… ASA, which stood for “American Standard Association” (see https://en.wikipedia.org/wiki/Film_speed#ASA and while you are there look two paragraphs below ). ASA became later ANSI ( see https://en.wikipedia.org/wiki/American_National_Standards_Institute ), and later the ISO defined a norm, based on the ASA definition (i.e. a film of 400 ASA was also 400 ISO), see here : https://www.iso.org/home.html . There is such a thing called an international organisation for standards. For example it uses, like all countries in the world, but Liberia, Myanmar and the US, the metric system…
    At the time there was a science called sensitometry, and for a black and white emulsion, the ASA or ISO speed of a film was defined 1/E, where E was the quantity of light (lux.seconds) necessary to obtain a density of 0.3 above the density of base+fog of the emulsion… Using a sensitometer to expose the film and a densitometer to measure it, we could measure the characteristic curve, which was a plot of Log E versus density. To measure the sensitivity the film had to be processed in such a way that the point 1.3 (in log) further away than the point 0.3 of density would be with a density of 0.8, etc… so no, ISO does not come from a Greek word.
    The digital photography measure of ISO was related first to the film sensitivity. So that a digital camera set at 400 ISO would give the same “correct” exposure in the same conditions as taken with a film of 400 ISO, or ASA :).
    Alain

    1. Donald-Sime

      An international organization would probably avoid using English-lettered acronyms, don’t you think?
      Like for instance why would the “International Consultative Committee for Telephone and Telegraph” be CCITT?
      Because they were based in Geneva, Switzerland and the language used was French so “Comité Consultatif International Téléphonique et Télégraphique” can be easily seen as CCITT

      That said given the choice of one syllable or three, … well, just call me lazy.

    2. Andy-de la PlazaAndy-de la Plaza

      Perfect Alain! I totally agree! The definitions for film sensibility are old. As an amateur astronomer an avid reader of the history of astrophotography, I recall Scheiner degrees (german, Scheinergrade) in astroplates. Then with the popularization of portable film cameras, the scales based on DIN, Weston, ASA and GOST scales. And yes, we’re talking about associations or instutes of technical norms in this particular case regarding film sensibility. Therefore DIN stands for Deutsches Institut für Normung, ASA for American Standards Association, Gost for государственный стандарт the russian norms dated back to 1925! Then came DIN+ASA and the International Organization for Standarization created the ISO norms for film sensibility around mid 70s.
      ISO short form for the greek “isos”? No way!!!

      1. Richard S. Wright Jr.Richard S. Wright Jr. Post author

        Hey guys, thanks for chiming in with more of the history about film sensitivity standards. I do recall ASA myself (my first film camera was in the 70’s actually – but I was never serious about it until the late 80’s), but ISO was still the “most recent” standard, and the spring board from which we have the current system for DSLR’s.

        The main point is that ISO gives us a good useful framework for photography, even with digital cameras. However, some of the tradition of actual “sensitivity” is actually no longer in effect. I’ll be exploring this more in future blogs.

  2. RDXJON

    Although ISO in your reference has nothing to do with photography, there actually is an International Standards Organization. It is sometimes referred to as International Organization Standards but the acronym is still listed as ISO or more accurately as ISO 9000. It is a family of quality management systems used by many organizations (including Nikon, Canon and Sony) which is probably why some people mistake the photography term for the quality management acronym.

    Loved the article. Very informative for those of use still getting our feet wet with astrophotography.

  3. Richard S. Wright Jr.Richard S. Wright Jr. Post author

    I love being wrong, it means I get to learn something new. In this case though, I think I have to stand by my article about ISO.

    I did provide some links in my article to backup what I was saying. Here’s another useful one:

    http://www.digitizationguidelines.gov/term.php?term=isofilmspeed

    The government web site does call it the International Standards Organization, but the links are to the International Organization for Standardization, which is responsible for both the film standards (see ISO 2240:2003 & ISO 12232:2006 to be exact), and yes modern quality control standards such as ISO 9000. So, while even the editor for this government standards web site mis-called the name, the link they provided is to the appropriate organization, and I also provided a link to that organizations own web site which explains that ISO is short for Isos, and is not actually the acronym for their name. This argument/mistake/trap only makes sense to english speaking people 😉 It is very popular… but it is not true.

    I confess in the blog, and will confess again I am perhaps splitting hairs or being pedantic, but the name of the organization IS International Organization for Standardization… regardless of how many people might want to call it otherwise 😉

    Cheers and clear skies!
    Richard

  4. jmgoldba

    “that organization’s own web site which explains that ISO is short for Isos”

    International SOS? According to Wikipedia it’s short(?) for the Greek word ‘isos’ or equal. So to avoid having different acronyms in different languages they randomly truncate a Greek word AND, it seems, this meaning wasn’t made public until later. Well, they’re based in Geneva. ISO really stands for ISO’s Some Organization. You heard it here first! -Jesse

  5. Ralph Megna

    Richard, in my experience, using a variety of late-model Canon DSLRs, ISO 1600 is a pretty noisy setting. I try to keep it below ISO 800, and prefer 400 or below. For the very reasons you describe, it is often better to shoot at a low ISO and then stretch the dynamic range in post-processing.

    1. Richard S. Wright Jr.Richard S. Wright Jr. Post author

      This is a tricky tradeoff, and I used to always recommend never shooting over ISO 800 myself, and often would shoot at ISO 400 just exactly like you say. What I’ve found later though is while this works great on bright targets, for dimmer targets, the dark fixed pattern noise and read noise becomes a problem (this can also vary with temperature), unless the exposures are really long. If you can only do short exposures (nightscapes also in this category), a little bit more ISO can be advantageous.

      This advice varies a little bit too based on if your shooting nightscapes, or using a DSLR on a telescope. I plan to talk a little more about that next month.

  6. Martian-BachelorMartian-Bachelor

    > Each DSLR model, based on the image sensor it uses, will have a sensitivity
    > equivalent to some intrinsic ISO.

    This value can often be found in the “Maker Note” section of the EXIF data block included somewhere in the digital picture file, in a field called (understandably) “Base ISO”. For several Canon DSLRs I have it’s 400.

    Presumably, when the camera’s ISO is set to this value the amplifier gain is set to 1.

    Some picture handling programs are better than others at showing all the data in the EXIF block. For example, many cameras have an onboard temperature sensor somewhere not far from the light sensitive area, the value of which (in °C) is also saved out somewhere in this data block; it can be offset by +128, so negative values can be more easily stored in an unsigned byte field (0-255). Most photographers aren’t interested in whatever the temperature happened to be when they shot their photo, so this is one an astrophotographer might want to know that may not be seen in the typical image processing program’s ‘dump’ of EXIF data.

  7. Michael-Clark

    Even more than different films had different response curves that affected the highlights and shadows differently when two different emulsions had the same sensitivity in the mid-tones, the linear nature of digital sensors means the correspondence between an ISO 200 film and a digital camera set to ISO is only approximate. Film has a shoulder roll off in the highlights and a heel in the shadows whereas digital sensors have a more or less straight line response all the way from zero to full well capacity. While they can be very close in the midtones, what each does with highlights and shadows will differ.

    That’s before you take into account that many film manufacturers rounded their products’ ASA/DIN numbers down to the nearest “standard” sensitivity value. Digital camera makers, on the other hand, tend to round up ISO values. Then there’s Schwarzschild’s reciprocity failure with film exposures longer than about one second or shorter than anywhere from 1/1,000 to 1/10,000 depending on the particular film.

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