Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

submit your photo


Hall of Fame
View past winners from this year

Please participate in Meta
and help us grow.

Take the 2-minute tour ×
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It's 100% free, no registration required.

Suppose there are two photos taken in the following scenario:

Scenario 1 - Photo taken using ISO of 3200.

Scenario 2 - Photo taken using ISO of 100.

Both photos are taken in a medium light scenario. The photo in Scenario 2 is quite dark, while the photo taken in Scenario 1 looks good.

From my understanding, higher ISO gives a more grainy photo. So could a photo in scenario 2 (which appears quite dark) be modified in software (e.g. Photoshop) to increase the brightness, and be better than the photo taken with a higher ISO?

share|improve this question
    
I believe Scenario 1 will yield the better image with less noise - see here: photo.stackexchange.com/questions/6615/… –  MikeW Feb 25 '13 at 9:57
add comment

9 Answers

up vote 28 down vote accepted

From my understanding, higher ISO gives a more grainy photo

I'm afraid your understanding is incorrect. High ISO doesn't necessarily give a more grainy photo as there are other factors involved. In some circumstances it can be the case that lowering ISO increases noise. I did an experiment a while back to prove this:

What you're seeing is exactly what you describe, one image shot at ISO 1600 unedited, versus the same photo shot at ISO 100, brightened in post to match the first image.

As you can see the ISO 100 image is considerably nosier.

What's going on is that images contain shot noise and read noise. Shot noise occurs because photons are emitted randomly by lightsources which gives rise to variations in the light hitting each part of the sensor. Read noise occurs as the analogue signal is transferred from the sensor to the ADC.

What the ISO setting on the camera does is amplify the analogue signal before readout and digitization. When amplifying the signal the shot noise gets amplified too, thus the signal to noise ratio is the same. However the read does does not get amplified as it happens after amplification. When you shoot at low ISO and brighten the image in software, the photon noise, and the read noise both get amplified, giving a higher level of noise.


So why is ISO deemed to determine image noise?

It is true that the lowest possible level of noise is achieved at the lowest possible ISO with as much light as possible hitting the sensor.

It is also true that the greatest speed for my car is achieved in fifth gear. But this doesn't mean putting it into fifth with the engine idling will increase my speed. The biggest influence on speed in the throttle position. Gearing just prevents stalling or overrevving the engine.

In the same way the biggest influence on noise is the total amount of light falling on the sensor. But you have to set the ISO to avoid over or under exposing the image.

ISO does not have a direct relationship with noise, if you have a very dim scene with little light falling on the sensor then you will have noise regardless of the ISO setting. Likewise if you have tons of light falling on the sensor you will have very little noise regardless of the ISO setting (though you might have overexposure!)

Another problem with perceptions of noise and ISO is that in any of the automatic shooting modes P/Tv/Av increasing the ISO will cause the camera to alter the shutter speed and/or aperture which results in less light hitting the sensor which means more noise. However in manual mode for a fixed shutter speed and aperture, increasing the ISO will not result in more noise being present in the image.

So why does any of this matter? After all if you want the lowest noise your camera can offer you just set it on a tripod, pick ISO 100 and leave the shutter open as long as possible before the image becomes overexposed.

The problem is that thinking about ISO first can lead to misunderstandings. For example when using the above approach in dim light with the aperture wide open, the shutter might max out at 1/30s. People remember the ISO 100 = lowest noise part, forget about the need to get a correct exposure (or are mislead by the image on the back of the LCD, which can look well exposed when you're viewing the screen in the dark) and end up underxposing the image, giving more noise than if they'd shot at say ISO 400.

It is just as correct to say the lowest possible level of noise is achieved when as much light as possible is hitting the sensor (without overexposing the image) and the ISO is as high as possible (without overexposing the image). In most cases the highest ISO possible will be 100.

Thinking about the level of light first, then the ISO avoids pitfalls when there is some limit to how much light you can get onto the sensor during the exposure.


There are other minsunderstandings that originate from thinking about ISO as the primary factor that determines noise. One such misunderstanding relates to a camera's base (minimum native) ISO. Someone with a camera Y whose base ISO is 200 might think "since ISO 200 gets me the cleanest images, wouldn't it be great to have ISO 50 like camera X". Now it may be the case that camera Y has a sensor with fantastic quantum efficiency, and very good microlenses meaning it is very efficient at capturing light, hence images become overexposed quickly, leading to a high base ISO. Camera X might have a much older sensor with poor QE, no microlenses and low fill factor. It wastes a lot of light and thus requires longer exposures. It also produces images with the same level of noise at ISO 50 as Y does at ISO 200. Just because the number is lower doesn't mean it's better.

Finally thinking about light first helps explain "ISO less" sensors, such as the latest batch of Sony sensors found in cameras like the Pentax K5 or Nikon D800. The read noise is so low that it doesn't make any difference if you amplify the signal prior to readout, meaning that you can get comparable results at many different ISO settings, proving that ISO isn't responsible for image noise.


My preferred method of shooting is in manual mode with auto-ISO. This allows me to select the depth of field and amount of motion blur that I want/can tolerate in the image, and then have the camera minimise noise for me.

share|improve this answer
4  
Nothing like a simple test to disprove hours of speculation! +1! Also, now that I have auto-ISO in M, I hardly leave it. Greatest advancement in my recent body upgrade, along with silky ISO 1600 :) –  dpollitt Feb 25 '13 at 15:45
3  
"The reason for the association between high ISO and noise..." Matt, that's not exactly correct, and you know it. If the image can be correctly exposed at a lower ISO (and let's assume an integral multiple of the camera's base ISO to get poor implementations out of the way), it will have both lower noise (resulting from a more statistically valid sampling of the light—a probabilistic phenomenon—or reduced shot noise if you prefer) and a higher dynamic range. In your example, a still life, the ISO 100 exposure could easily have been 1/2s, resulting in less noise than the ISO 1600 exposure. –  user2719 Feb 25 '13 at 17:14
3  
(cont'd) I'm not taking issue with the overall statement—that exposing correctly will be better than "fixing in post"—it's just that you keep leaving out the part where you should say that correct exposures at lower ISO values (at least until the base ISO is hit) will result in lower noise than equally well-exposed shots at higher ISOs. –  user2719 Feb 25 '13 at 17:19
4  
Stan has nailed it. You get the lowest apparent noise with the highest possible signal to noise ratio and pixel saturation. Sometimes you have no option but to select an aperture and shutter speed, and push ISO as high as you can without clipping a that aperture and shutter speed...but generally speaking using the lowest ISO setting while still getting a proper exposure will result in the cleanest images. If you have the ability to expose for several seconds, or use an ultra wide aperture, while pushing the exposure to the right, at ISO 100...then do so. You'll have a cleaner image. –  jrista Feb 25 '13 at 23:50
1  
In the astrophotography world, people are using Canon DSLRs extensively and have split the hairs on the ISO question. Craig Stark has run the numbers on some Canon product and published a detailed analysis of linearity of data from these cameras: Profiling the Long-Exposure Performance of a Canon DSLR (July 2, 2012, Cloudy Nights) stark-labs.com/craig/resources/Articles-&-Reviews/… In a nutshell, he says that Canon is modifying the dark signal before it hits the raw file and that settings above ISO 400 are of limited value. –  smigol Feb 25 '13 at 23:54
show 7 more comments

If the iso 3200 is not overexposed then the iso 100 is very dark, not just quite dark. In 8bit range it would go between 0 and 15 pixel values, so very noisy, and the iso 3200 would be quite fine, except in dark regions.

These images are ISOs 200,400,800,1600 from left to right on an old Canon 400D where high iso was known to be bad. But still the ISO 1600 is the best.

iso 200-1600 Larger

Indeed, the best way to lower noise is by adding light to teh sensor, through settings or by adding light to the scene. These operations come with a cost: adding light changes the scene we want to capture, adding light through exposure time risks motion blur which can be worse than the noise, and opening the iris (if possible) decrease the DOF and makes the lens soft, and maybe even add some fringing.

Here I added flash, iso 200-800 (1600 would overexpose):

flash Larger

Here we see that now there is enough light to make a low noise image in all cases - especially notice that going up in iso still doesn't make it noisier. Lack of light makes it noisy, not high iso.

In conclusion, light > iso, but higher iso != your enemy; it is your friend when the light fails you.

share|improve this answer
add comment

Why edit photos in post? This is a forum for real photographers, not computer bugs, right?

If you have a good enough digital camera, I'd suggest you put it on auto ISO so that you can concentrate on other things in the post processing.

Meanwhile, you must never forget that no amount of postprocessing can replace bad lighting. So, as long as you got good lights, I think you're done with 95% of the photo at the time of clicking itself.

If you are a landscape photographer with a digital camera, I won't blame you for this question. I'd have suggested you shift to 4x5" format then. :P

For all the other sorts of photographers- wedding, fashion, news, sports, etc., they would put less time in post processing, hence they would put the camera on auto-ISO almost forever.

For the newer generation cameras, especially after Nikon D3 came out, low and high ISO settings have never made a difference. Pictures are always the same. It's because camera companies know that you're gonna enter a real field with these cameras and you won't have the time to jack around with camera settings.

So, my advice- forget about correcting the ISO in post. Just put your camera on auto ISO (and if you want, all the auto settings, like Nikon's "P" mode) and just go on and shoot!

Don't forget to check your lights though, if you're in weddings or fashion! :)

share|improve this answer
14  
"This is a forum for real photographers, not computer bugs, right?" Given the origin of the site as an offshoot of stackoverflow.com it's pretty much a forum for software engineers who also take photographs ;-) –  Matt Grum Feb 25 '13 at 16:47
add comment

To correct your statement a little,

"A higher ISO will produce a more grainy image at standard exposure."

The ISO used has no impact on the light entering the camera or the sensitivity of the sensor to light. A certain amount of signal must be registered to mute out the noise that is inherent in the system. At a high ISO, the sensor processes a smaller amount of light as more light and thus develops an image faster, but has more noise.

That same image, taken with a lower ISO, but at the same speed will still have the same amount of noise, but as Matt Grum points out, the digital conversion will likely amplify the noise and thus make a worse overall image as the levels going in will be so low when still in analog form.

The reason that there is less noise at standard exposure is that the additional time that light is collected by the sensor results in more signal compared to the relatively constant amount of noise.

Basically you get noise from both power fluctuations in the camera and in noise from the image sensor if you use a lower ISO and don't do a standard exposure.

share|improve this answer
add comment

Lots of factors here, it's not just a matter of a huge jump in ISO. You've a fast lens (F2.8) and a stationary object with light that can be shifted.

Why bother going all the way out to ISO 1600, if you can just turn up the ambient light, move to ISO 400 or so and remove noise as an issue?

If the light can't be shifted (not even with a small flashlight and piece of paper as a diffuser? seriously??), you don't have a faster lens, and you simply must have that picture of books in the dark...well, then at least take another few shots at ISO 800, 1000 and 1200 to see what you get.

Then, you'll be forced to (clutch the pearls!!) spend a whole MINUTE in Lightroom to balance the color, bump the exposure and add a bit of clarity before you're done.

share|improve this answer
add comment

Formally, it is better to go with higher ISO, as long as higher ISO is implemented the way it is supposed to be implemented: as higher gain setting for the internal analog signal amplifier in the camera, i.e. signal amplifier that works in the signal before it gets converted to discrete digital form.

From the purely mathematical point of view, internal amplifier also "multiplies" the signal, just like "Exposure" slider in some post-processing software does. However, the multiplication is performed on the analog side of the processing flow, before the signal is converted to digital form. This precludes any rounding errors that are invariably introduced when the multiplication (or virtually any other image value transformation) is performed on the discrete digitized values.

Of course, in order for all this to work, your camera must implement the ISO setting specifically by adjusting the gain of the internal analog amplifier. Cheaper digital cameras might implement "fake" ISO settings by keeping the amplifier gain constant and multiplying the image data after it was converted to digital form. Such implementation will offer absolutely no meaningful benefits over what you can do in post-processing. It is precisely the same thing as the "Exposure" slider in post-processing software.

Meanwhile, many modern SLR cameras implement majority of their ISO settings by adjusting the amplifier gain, while "simulating" the most extreme ISO settings (the highest and/or the lowest) by digital multiplication. There's no point in using such simulated ISO values for the reasons mentioned above. Some cameras might implement "standard" ISO values (lie 100, 200, 400, 800 etc) by adjusting the amplifier gain, while simulating fractional ISO values by digital multiplication from the nearest "standard" ISO value. Again, there's no point in using such simulated ISO values.

share|improve this answer
add comment

If you have a tripod and you can, use low ISO. If you cannot use a tripod, increase the ISO because grainy images are better than shaked ones. A very dark picture cannot be recovered (as a burned one) once it gets "full dark areas" because there is nothing to recover (no details). So, if you can see things through your viewfinder and after taking the photo you still have details, it's OK. Also, check the histogram.

share|improve this answer
    
This is not a good advice, see @Matt Grumm's answer for details. –  Håkon K. Olafsen Feb 25 '13 at 11:45
    
To clarify, the user was not asking if he should use a longer exposure on a higher ISO, he was asking if the same exposure time would be better artificially raised or if using a higher ISO would be better. Matt Grum's answer is the correct one for the question asked. –  AJ Henderson Feb 25 '13 at 14:31
    
If you can read the full answer: A very dark picture cannot be recovered (as a burned one) once it gets "full dark areas" because there is nothing to recover (no details) –  vectorialpx Feb 26 '13 at 11:09
add comment

"For all the other sorts of photographers- wedding, fashion, news, sports, etc., they would put less time in post processing, hence they would put the camera on auto-ISO almost forever."

You must be joking. Ever tried to shoot a rock concert with ISO on auto?

Furthermore, I think your "Why edit photos in post? This is a forum for real photographers, not computer bugs, right?" statement is unnecessary rude, to say the least.

share|improve this answer
add comment

It depends on the camera. Increasing the ISO boosts the signal from the sensor, and this is different than brightening the image in post. High ISO can be thought of as a hardware boost while brightening in post can be thought of as a software boost.

Increasing ISO might result in less noise, it might not, compared to post processing brightening. Most cameras boost the signal only up to a point (400 to 1000 ISO or so) and achieve effective ISO increases on top of that by software manipulation. Again, at what point this happens varies by camera model.

You'd have to test your specific camera model to know how the effects of high ISO and PP brightening differ.

In any case, you want a good exposure, that is you want raw data with the most useful information in it, and the expose to the right technique is usually a good way to achieve this.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.