Beginner ISO Question

VT50 · May 17, 2009

I have a question about ISO, why do people shoot at 200 ISO in regular day light? I'm newto photography, but isn't the rule of thumb is to keep the ISO as low as possible when shooting in regular lighting situations? Especially when you have a entry level Camera (Canon 40D)

pathfinder · May 17, 2009

What's wrong with shooting at ISO 200 with a 40D?

There may be slightly more noise, but not very much with a 40D.

Are these images destined for a newspaper, a web image, or a snapshot? You will not be able to tell any difference.

If you are planning a very large print with critical sharpness and quality, then yes, ISO 100, MLU, tripod, cable release, the best glass you can afford. But not every image has to meet these specifications for many of us.

I shoot my 40D at ISO 800, and ISO 1600 rather freely. Proper exposure, and a bit of denoising, and they look splendid.

Rules are meant to be broken!!

VT50 · May 17, 2009

Thank you, I appreciate your comment. That's a good point about the printing thing. I saw your galleries and WOW!!!, defiantly blew me away.

pathfinder · May 17, 2009

Pictures are free today, feel free to experiment a bit.

While you are correct that generally lower ISO is better, there are many times when the light demands higher ISO to allow the shutter speed and aperture needed.

This is ISO 1600 with a 40D as a Christmas snapshot Is the grain that offensive?

Richard · May 17, 2009

VT50 wrote:

I have a question about ISO, why do people shoot at 200 ISO in regular day light? I'm newto photography, but isn't the rule of thumb is to keep the ISO as low as possible when shooting in regular lighting situations? Especially when you have a entry level Camera (Canon 40D)

I wouldn't call a 40D entry level. It's a highly capable camera with very low noise. In many situations, you would have to resort to pixel peeping to see any difference between 100 and 200. That being the case, a higher ISO permits faster shutter speeds at any given aperture, which means less chance for motion blur.

Tim Kamppinen · May 17, 2009

Another thing that may not be obvious to you Canon guys is that 200 is the base ISO on most Nikon cameras. ISO 100 is available but it has less dynamic range from what I've read. So most Nikon shooters default to ISO 200 in daylight or other well lit situations rather than 100. Of course as others have said, sometimes you need higher ISO's. Even in broad daylight, you might want to use a higher ISO if you're using a long lens or shooting a moving subject and you need a higher shutter speed than you can get otherwise.

Art Scott · May 17, 2009

VT50 wrote:

I have a question about ISO, why do people shoot at 200 ISO in regular day light? I'm newto photography, but isn't the rule of thumb is to keep the ISO as low as possible when shooting in regular lighting situations? Especially when you have a entry level Camera (Canon 40D)

I used to shy away from any camera that had its lowest ISO at 200....... but for some reason this is getting to be the norm......I have a konica minolta prosumer P/S (KM A2) and shoot it regularly at iso64....and my 2 KM 7D's (DSLR's) I shoot anywhere from iso 100-3200.........

But my own rule is too shoot as low an iso as i can and still maintain sharp focus handheld............I do tri or mono pod when I have too

pathfinder · May 17, 2009

Richard wrote:

I wouldn't call a 40D entry level. It's a highly capable camera with very low noise. In many situations, you would have to resort to pixel peeping to see any difference between 100 and 200. That being the case, a higher ISO permits faster shutter speeds at any given aperture, which means less chance for motion blur.

I wouldn't either Richard, and I have owned a series of full frame DSLRs, so I do have something to compare to. My daily walk around, carry on my way to work, drag along in my tank bag on my motorcycle, is still my trusty 40D. A large percentage of my Popular Shots on my smugmug gallery were shot with a 40D.

For that matter, I don't consider a G9 or a G10 an 'entry level' camera either, as I have 16x20 framed prints from them as well.

Thunder Rabbit · May 18, 2009

Howdy.

Hi, VT50. As usual, Pathfinder, Richard, and the rest are right on the money.

But, not content to leave well enough alone, I thought I might serve up some additional food for thought.

To truly understand ISO, it helps me to think about film. ISO (International Standards Organization) ratings were originally developed way back when as a standard way to compare the relative light sensitivity of different film emulsions. When applied to digital cameras, the standard is somewhat artificial, but none the less vital.

Although film and digital cameras both use ISO standards, and have ISO settings, there is a significant difference in how these settings actually operate in each system.

Film is both the sensor and storage medium built into one. With film cameras, you can actually change the sensitivity of your sensor, the film. You do this by selecting a film emulsion that is more sensitive or less sensitive to light. The sensitivity of the film to light is determined for the most part by the size of the photo reactive silver halide molecules. Larger molecules react more quickly to light. Slow film, ISO 64, has smaller molecules than ISO 400. And hence finer grain. Grain is to film photography what noise is to digital photography. Not exactly, but close enough for government work. When we see grain in film, we are seeing the molecular structure of the film emulsion. Faster films have more grain because the molecular structure is larger and therefore easier to see. With film we always try and shoot the lowest ISO we can get away with, because the slower film will always have finer grain. You can get film with ISOs of 8 or lower. I used to shoot a lot of Kodachrome 25. (Sadly, it has been discontinued.)

Changing the ISO setting on a film camera just changes the set point the camera metering system uses to compute exposures. Only inserting film with a higher or lower ISO changes the sensitivity of the sensor (the film). The film ISO rating and the camera ISO settings don’t have to be the same. When shooting slide film, it is common practice to underexpose by 1/3. Slide film, unlike digital sensors, usually suffers underexposure better than overexposure. So, when shooting ISO 64, this can be achieved by setting your ISO to 80. The metering system does the math and yields what it thinks is the best setting. So, even before digital came along, ISO was a popular point of confusion.

Then, along came digital photography. With digital cameras you do not change the sensitivity of your sensor. You can’t change the film. You achieve a similar effect by applying amplification to your capture data. When you increase the ISO, you are increasing the amplification of the capture data. Digital sensors are not perfect, and these imperfections are also amplified. We call it noise. More amplification means more noise. Changing your ISO settings does not affect the sensor. It does simultaneously change the set point for your metering system and give instructions to your camera processor to amplify the capture data by a certain amount. You delink the ISO of the metering system and the ISO of the sensor by applying exposure compensation settings.

This is what I meant when I said that ISO ratings are somewhat artificial when applied to digital cameras. But they are still necessary as a set point for calculating exposures. And provide a consistent bridge between film and digital photography. And you’ve got to call it something.

In a parallel universe where digital photography was invented but film wasn’t. And all we ever knew was digital photography, instead of “ISO 100, 200, 400”, and such on our cameras, the equivalent adjustment options would most likely be something like “-1, 0, +1, +2, +3 …”. Which is far more comprehensible to me. But I prefer numbers small enough that I personally have enough digits to keep track of.

We live in the universe where film came first. And digital photography inherited the potentially confusing terminology of film, and added a layer of its own. And all those extra zeros don’t make it any easier. At least not for me.

And Tim brings up a very important point.

Different sensors in different cameras have different native ISOs, or base sensitivity. Nikons (at least many of them) have their base sensitivity pegged at ISO 200. Meaning that its sensor is twice as sensitive as a camera that has its base sensitivity pegged at ISO 100. Apparently, digital sensors don’t necessarily get more noisy as their base sensitivity increases. Why this is, I don’t know. (Dammit, Jim, I’m a chemist, not an electrical engineer!) Whatever the case, the vast majority of the noise comes from the amplification of the capture data. Nikon must believe that their faster base sensitivity, ISO 200, suffers no significant disadvantage when compared to another makers slower ISO 100 base. If this were not the case, I’m sure one of us would have noticed, and Nikon wouldn’t be selling any cameras.

Shooting digital, there is no inherent advantage to a lower ISO. This concept is something of a holdover from film days. The lowest noise levels will be generated when you shoot at your cameras native ISO. Not the lowest ISO setting. The capture data will not be amplified, and noise will be minimized.

It is conceivable (even likely) that eventually, our digital technology wizards will offer us cameras with native ISOs of 400, or 800, or more. And the lowest noise level will be achieved when the camera is set at that number, even if it’s 800. Because it won’t be amplified. Seldom do photographers suffer from too much speed. It seems I almost always could use a little more. And if you do need to slow things down, there are ways to do it that don’t compromise image quality. It’s easier and better to slow down than speed up.

My Nikon has a native ISO of 200, and I shoot outdoors at ISO 200. Even on a tripod. Which is optimum. I could set it to ISO 100, but all that is doing is de-amplifying my capture data, and I don’t see the point in that. I think the main reason the ISO 100 setting is even there is to mollify folks like me who are used to thinking that lower ISO is better. But now that I’ve sorted things out, I never use it.

Anyway, this thing is getting a little long. Sorry about that. (I’ve finally convinced my wife that when I’m doing this I’m working.)

Hope it has been of some help.

Tiziano · May 18, 2009

As always Lee, informative and funny.

ziggy53 · May 18, 2009

Actually, the ISO sensitivities of digital cameras affect:

Sensor noise
Dynamic range
Color sensitivity
Color accuracy

On those cameras that allow "extended" ISO ratings, those ratings are usually beyond the "calibrated" ratings of the imager, image amplifiers and image processors, and all of the above generally suffer somewhat more than within the calibrated range.

On those Canon dSLRs that allow an ISO 50, dynamic range suffers compared to ISO 100, so less sensitivity is not always better.

Thunder Rabbit · May 19, 2009

Howdy.

Ziggy is right. Amplifying your capture data does more than create more noise. It can create color shifts. The same thing happens to film when you push it. Pushing film is chemical amplification. This is one place where digital and film merge again, and behave very much the same. And for the same reason.

Color slide film has three emulsion layers. One for each primary color, with a filter layer of that color above it. Each layer of emulsion records one color. Digital sensors do essentially the same thing, but in a much more complicated way. A way that I can barely understand, let alone explain. The color filters on digital sensors are arranged in what is called a “Bayer Array”. Rather than three emulsions, each with the appropriate filter above it, you have one sensor, with the filters arranged above it in a matrix. To me, a Bayer Array would be a handful of aspirin, but to a digital camera designer, it is a way to get three colors out of one sensor.

But in both, you end up with three sets of data, Red, Green, and Blue. The color shifts come in because, with the current state of technology, either film or digital, it is very difficult to make a sensor or film emulsion that is equally responsive to the entire visual spectrum. The response curves for the three color channels are different. The engineers get around this by fudging the film emulsion or digital conversion algorithm so that the negative effects of this are minimized when the film or sensor is used at its design ISO. But the fudging is not perfect. So, when you amplify the data, the imperfections are amplified too. And the colors shift. As Ziggy suggests, the further from the design ISO, the bigger the shift. With digital data, this can be corrected to a certain degree. With pushed (chemically amplified) slide film, you are stuck with the color shifts, so you call it art, and let it go. At least, that's what I do.

This is also the cause of the loss of dynamic range. Ideally the response curves for the three channels would be identical, and overlap perfectly, and would all clip at the same signal intensity. But they don’t. Even at design ISO. When amplified, the overlap is less perfect. The faster channel (I don’t remember which is which right now) moves to the right more than the middle or slowest channel. The three channels start to spread out, and the fast and slow channels begin to clip sooner. The same thing happens when you de-amplify, or attenuate your capture data by shooting at a lower than the native ISO. Except everything is moving to the left. But the result is the same. They move left at different rates, begin to spread out, and as a consequence, clip sooner.

So, theoretically, at least, when you shoot below the native ISO of a digital camera, you won't get more noise, but you will get reduced dynamic range and color shifts. Shooting at ISOs higher than the native ISO will give you reduced dynamic range, color shifts, and noise.

It's always something.

bgarland · May 19, 2009

As usual, great info from all the experienced folks on this forum. Thanks. How can you determine what the "native ISO rating" is for a particular camera? I have a Canon 50D but I have no idea what it's native ISO rating is.

Tiziano · May 19, 2009

Wouldn't it be the lowest ISO that the cam goes into without special steps?

pathfinder · May 19, 2009

That's how I think of it. For most Canon DSLRs that would be 100. Most Nikon DSLRs would be 200.

For many point and shoots - the Canon G9 and G10 for instance - it would be 80.

Tim Kamppinen · May 19, 2009

Yeah. On Nikons (not sure about any other brands) the 100 ISO setting is labeled as "Lo1" and any extended high ISO settings are labeled as "Hi1", "Hi2", etc. to indicate that these settings are beyond the normal range of the sensor.

HenryPeach · May 23, 2009

When they stop using the standard ISO numbers, and instead go with L, H, L1, H1, etc... that's an indication that the ISO change isn't done with a change at the sensor, but instead is a push or a pull using in-camera processing.

For instance on my Canon DSLRs L is ISO 100 overexposed a stop, and pulled back with the in-camera processing. H is ISO 1600 underexposed a stop, and pushed with the in-camera processing.

With film there was often a slight, but noticable difference between ISO 100 and 200. The real world image quality, noise, and dynamic range differences between ISO 100 and 200 with my Canon DSLRs is zilch in my experience. I often have a hard time telling the difference between ISO 100 and 400 in large prints. Today's APS and 35mm DSLRs are super clean, and according to dynamic range charts at dxomark.com there is at most about a 1/3rd stop drop in dynamic range between ISO 100 and ISO 400 for most modern DSLRs. I suppose there are times when I need that extra 1/3rd stop of DR, but most of the time the scene is either well within the DR of the camera, or well beyond it.

Beginner ISO Question

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