Post by Admin on Mar 24, 2021 15:01:50 GMT
If you are looking for a specific suggested ISO to use for your camera you can check it here:
Suggested ISO values for Nikon cameras.
Suggested ISO values for Canon cameras.
Suggested ISO values for Sony cameras.
The ISO on DSLR cameras is probably one of the most misunderstood settings and people don’t seem to agree on the best values to be used for astrophotography. I’ve seen articles and people recommending anything between ISO100 and ISO3200 for exactly the same camera. This is very unfortunate, especially since choosing the best setting can make quite a difference in your end result.
Much of the confusion comes from the fact that the reality about ISO for digital cameras is very counter intuitive and people have trouble ‘accepting the truth’. However, I do want to stress that I think we can all agree on the ‘best ISO’ for your model. Time to have a detailed look!
ISO misunderstood
In daytime photography we are taught about the ‘Exposure Triangle’; ISO, aperture and exposure time. The three are interconnected we learn, and to get the same amount of light, you need to double your exposure time if you go from F2.8 to F4. Alternatively you could double your ISO and leave your exposure time as is. If you increase one of the three, you can decrease one of the others in order to get the same exposure in terms of light captured. Or so we are told at least.
In almost every article that is explaining this, ISO is defined as the light sensitivity of the sensor. So a higher ISO value means a more sensitive sensor. hmmm. hold on. Does this make sense?
Sure, in the analog days of film you would have film that used larger grain to be more sensitive, so the film really was more sensitive for a higher ISO value. But how would that work for a digital sensor? Are the pixel sizes suddenly increasing when you increase the ISO? No, of course not. So how does the ISO setting increase the sensor’s sensitivity to light then?
It doesn’t.
It really doesn’t.
Increasing the ISO setting is NOT making your camera pick up more light. It is NOT changing your sensor and it is NOT increasing sensitivity.
Ok, so what does this setting actually control then?
The ISO setting is determining the amplification factor that the camera applies to the signal that represents the amount of photons captured in a pixel. Using a higher ISO just means you are amplifying the light you captured on the sensor. So your pixels never collect more photons, it is just that the signal is amplified. In a sensor this amplification can be done both analog and digitally. And this is the really important part of choosing the best ISO setting for your camera for astrophotography.
ISO and Read Noise
If you increase the ISO, you’ll increase the noise. This has been true for years, even though the sensors are getting better and better. It is important to really understand what this noise is that is increasing; the read noise. The read noise is quite complicated but can simply be understood as the noise that is inherent to the electronics of the camera. This means this noise is independent of exposure time. So a longer exposure will have the same read noise as a very short exposure, and this is (partly) why exposing longer will increase your Signal to Noise Ratio (SNR).
Many reviews and reports on camera performance and sensor statistics will state that the read noise is depending on the ISO. For our understanding it is necessary to look one step further.
Read noise consists of two different parts; the so called upstream and downstream noise. The upstream noise is all the noise that occurs in the electronics before the actual Analog-to-Digital conversion in the ADC. The downstream noise is anything from that point onward, which of course includes the noise of the ADC itself.
Now remember that the ISO setting controls the amplification of the output signal of the sensor. This means that all the upstream noise will be amplified with it, while the downstream noise remains consistent. In other words; we gain SNR! And this of course is a good thing right? So should we choose very high ISOs then? Well, not so fast. As always, things aren’t quite that easy;). There is a limit to the gain you will get, which is determined by the downstream noise. Once the upstream noise gets high enough OR the downstream noise gets close to zero, the upstream noise will simply swamp out the downstream noise, making it irrelevant. So once we move past this point, the read noise will be scaling linearly with the signal, gaining us nothing anymore in terms of SNR; If you double the signal, you’ll double the noise. It is this situation what people tend to call ‘ISO-less’, as there is no real gain and you can do this amplification also in post processing. This might even be better in terms of Dynamic Range (DR). I’ll explain why.
Please note this is an oversimplification, but for our understanding it will do for now. For the exact math of it all I recommend this paper.
ISO and dynamic range
There is another, very important thing to keep in mind when we are talking about the ISO setting. This is the influence of ISO on the dynamic range. Simply put, the dynamic range determines how well a camera can distinguish and record the light levels between the faintest (black) area and brightest (white) area in a picture. The dynamic range is determined for the most part by the ability of a pixel to hold a certain amount of electrons, the full well capacity. Think of our analogy of the buckets of rain again; if a bucket can hold 1000 drops, it will hold more information than if it can only contain 100 drops. This is what we call the dynamic range.
Furthermore, the dynamic range is limited by the ADCs capacity to represent the different charges in numbers, determined by the amount of bits the ADC can use.
Now, if we look at amplification we can see what will happen to the dynamic range; the signal values get amplified, thus raising the ‘floor’ of the signal. BUT, at the same time the upper limit of the dynamic range remains fixed, determined by the ADC. So, the difference between the highest measurable/recordable value and the lowest value is decreasing as we are amplifying, and thus raising, the lowest value while the upper value remains fixed. In other words, we loose dynamic range if we increase the ISO. And this is very bad for astrophotography, where we have to deal with dynamic range challenges in all our pictures because of faint versus bright stars for
ISO and Unity Gain
Unity gain is a concept that is introduced often when discussing the choice of best ISO to use for astrophotography. However, unity gain would only be relevant to consider if you are trying to capture very faint signal AND you want to do this by using only a few exposures. In all other cases unity gain really doesn’t matter, and you certainly don’t want to choose it if it means loosing Dynamic Range.
You can read more details on unity gain and why it doesn’t matter here.
1/3 and 2/3 ISO stops
Most cameras offer steps of 1/3 to change the ISO setting. Don’t use those settings for astrophotography, always use ‘full’ ISO stops!. The camera is just scaling your images to mimic these ISO settings. For 1/3 stops, like ISO 125, the camera will simply use the analog amplification of ISO 100 and scale it digitally to match ISO 125. On the other hand, for a 2/3 stop like ISO 160, the camera will use the next ISO setting and scale it back down. You’ll encounter some articles talking about using 2/3 ISO settings since it has a better SNR, but for astrophotography you want to be in the range where you already have the best SNR from your ISO setting and don’t want any digital scaling as you loose information.
Just stick to using the ‘real’ ISO stops; 100, 200, 400, 800, 1600, 3200, etc.
Summary; ISO considerations
Ok, lets take a step back and look at what we have established;
ISO has nothing to do with the sensor’s sensitivity
Instead, ISO is an amplification of output signal
this amplification will be done either by an analog amplifier or digitally, depending on the brand/model and the ISO setting
Increasing the ISO decreases dynamic range
Increasing the ISO increases SNR until the upstream read noise swamps out the downstream read noise
Point 3 and point 5 determine the best setting for your camera.
So even though this is really counter intuitive for most of you, stop thinking about ISO as a way to increase sensitivity or a way to make your image brighter. It really really doesn’t!
The best ISO for your DSLR
The best ISO for your dslr will of course depend on your particular model, but in general it will be defined by the following;
The best ISO for astrophotography for any DSLR is the lowest ISO level from which either a.) the upstream noise swamps out the downstream noise OR b.) the amplification will be done digitally in camera, whatever value of both is the lowest.
Starting from that value the read noise will be (more or less) consistent and we can call the sensor ‘ISO-less’ and we gain nothing by increasing the ISO, while only hurting ourselves by diminishing the dynamic range.
Defining the best ISO for your model
At this point you probably still are confused as to what ISO value is the best for your specific DSLR. It’s nice that we have a clear definition of what the best value is, but how do you determine what this means for you in real life?
Unfortunately there is no direct answer to this question, as the ISO levels from which a dslr starts amplifying digitally is generally not listed anywhere. However, the read noise levels for your camera at different ISO settings can be found, which uses data from DxO to calculate their curves. Digital amplification will be visible in these results as an (dramatic) increase of read noise for the highest ISO settings, since you’ll start amplifying the downstream noise as well if you do it digitally. So in general you would be fine at checking the read noise curves and see when they start to even out at the lowest values and use this as your ISO for your model. In the graph below I’ve plotted the read noise for the Nikon D7000 and the Canon 6D, not to do any qualitative comparison, but just to show you that the best ISO settings can and will greatly differ per model!
This graph makes it very clear why there can be and remain so much debate and confusion about this matter! Based on this graph I would guess that ISO1600 would be best for the Canon 6D, while ISO100 (!) or 200 would be best for the D7000. I recommend to test it out for your self to be sure, but at least these graphs give guidance on what to test; you don’t have to try the 6D at ISO100 and testing the D7000 at ISO1600 is also a waste of your time.
Concluding the best ISO discussion
This matter is really, really complicated which is increased even more by the fact that we have to work with facts and numbers that are not generally made public by the manufacturers. I’ve simplified things (a lot) in some places to try to give you an explanation that can be understood without diving in to the math or doing lots of side research to even understand what I’m talking about here.
I hope I succeeded in doing so, and would love to hear your further questions or remarks in the comments below.
Original Article
Suggested ISO values for Nikon cameras.
Suggested ISO values for Canon cameras.
Suggested ISO values for Sony cameras.
The ISO on DSLR cameras is probably one of the most misunderstood settings and people don’t seem to agree on the best values to be used for astrophotography. I’ve seen articles and people recommending anything between ISO100 and ISO3200 for exactly the same camera. This is very unfortunate, especially since choosing the best setting can make quite a difference in your end result.
Much of the confusion comes from the fact that the reality about ISO for digital cameras is very counter intuitive and people have trouble ‘accepting the truth’. However, I do want to stress that I think we can all agree on the ‘best ISO’ for your model. Time to have a detailed look!
The ISO setting is NOT influencing your cameras sensitivity to light!
ISO misunderstood
In daytime photography we are taught about the ‘Exposure Triangle’; ISO, aperture and exposure time. The three are interconnected we learn, and to get the same amount of light, you need to double your exposure time if you go from F2.8 to F4. Alternatively you could double your ISO and leave your exposure time as is. If you increase one of the three, you can decrease one of the others in order to get the same exposure in terms of light captured. Or so we are told at least.
In almost every article that is explaining this, ISO is defined as the light sensitivity of the sensor. So a higher ISO value means a more sensitive sensor. hmmm. hold on. Does this make sense?
Sure, in the analog days of film you would have film that used larger grain to be more sensitive, so the film really was more sensitive for a higher ISO value. But how would that work for a digital sensor? Are the pixel sizes suddenly increasing when you increase the ISO? No, of course not. So how does the ISO setting increase the sensor’s sensitivity to light then?
It doesn’t.
It really doesn’t.
Increasing the ISO setting is NOT making your camera pick up more light. It is NOT changing your sensor and it is NOT increasing sensitivity.
Ok, so what does this setting actually control then?
The ISO setting is determining the amplification factor that the camera applies to the signal that represents the amount of photons captured in a pixel. Using a higher ISO just means you are amplifying the light you captured on the sensor. So your pixels never collect more photons, it is just that the signal is amplified. In a sensor this amplification can be done both analog and digitally. And this is the really important part of choosing the best ISO setting for your camera for astrophotography.
ISO and Read Noise
If you increase the ISO, you’ll increase the noise. This has been true for years, even though the sensors are getting better and better. It is important to really understand what this noise is that is increasing; the read noise. The read noise is quite complicated but can simply be understood as the noise that is inherent to the electronics of the camera. This means this noise is independent of exposure time. So a longer exposure will have the same read noise as a very short exposure, and this is (partly) why exposing longer will increase your Signal to Noise Ratio (SNR).
Many reviews and reports on camera performance and sensor statistics will state that the read noise is depending on the ISO. For our understanding it is necessary to look one step further.
Read noise consists of two different parts; the so called upstream and downstream noise. The upstream noise is all the noise that occurs in the electronics before the actual Analog-to-Digital conversion in the ADC. The downstream noise is anything from that point onward, which of course includes the noise of the ADC itself.
Now remember that the ISO setting controls the amplification of the output signal of the sensor. This means that all the upstream noise will be amplified with it, while the downstream noise remains consistent. In other words; we gain SNR! And this of course is a good thing right? So should we choose very high ISOs then? Well, not so fast. As always, things aren’t quite that easy;). There is a limit to the gain you will get, which is determined by the downstream noise. Once the upstream noise gets high enough OR the downstream noise gets close to zero, the upstream noise will simply swamp out the downstream noise, making it irrelevant. So once we move past this point, the read noise will be scaling linearly with the signal, gaining us nothing anymore in terms of SNR; If you double the signal, you’ll double the noise. It is this situation what people tend to call ‘ISO-less’, as there is no real gain and you can do this amplification also in post processing. This might even be better in terms of Dynamic Range (DR). I’ll explain why.
Please note this is an oversimplification, but for our understanding it will do for now. For the exact math of it all I recommend this paper.
ISO and dynamic range
There is another, very important thing to keep in mind when we are talking about the ISO setting. This is the influence of ISO on the dynamic range. Simply put, the dynamic range determines how well a camera can distinguish and record the light levels between the faintest (black) area and brightest (white) area in a picture. The dynamic range is determined for the most part by the ability of a pixel to hold a certain amount of electrons, the full well capacity. Think of our analogy of the buckets of rain again; if a bucket can hold 1000 drops, it will hold more information than if it can only contain 100 drops. This is what we call the dynamic range.
Furthermore, the dynamic range is limited by the ADCs capacity to represent the different charges in numbers, determined by the amount of bits the ADC can use.
Now, if we look at amplification we can see what will happen to the dynamic range; the signal values get amplified, thus raising the ‘floor’ of the signal. BUT, at the same time the upper limit of the dynamic range remains fixed, determined by the ADC. So, the difference between the highest measurable/recordable value and the lowest value is decreasing as we are amplifying, and thus raising, the lowest value while the upper value remains fixed. In other words, we loose dynamic range if we increase the ISO. And this is very bad for astrophotography, where we have to deal with dynamic range challenges in all our pictures because of faint versus bright stars for
We loose dynamic range if we increase the ISO, which is very bad for astrophotography
ISO and Unity Gain
Unity gain is a concept that is introduced often when discussing the choice of best ISO to use for astrophotography. However, unity gain would only be relevant to consider if you are trying to capture very faint signal AND you want to do this by using only a few exposures. In all other cases unity gain really doesn’t matter, and you certainly don’t want to choose it if it means loosing Dynamic Range.
You can read more details on unity gain and why it doesn’t matter here.
1/3 and 2/3 ISO stops
Most cameras offer steps of 1/3 to change the ISO setting. Don’t use those settings for astrophotography, always use ‘full’ ISO stops!. The camera is just scaling your images to mimic these ISO settings. For 1/3 stops, like ISO 125, the camera will simply use the analog amplification of ISO 100 and scale it digitally to match ISO 125. On the other hand, for a 2/3 stop like ISO 160, the camera will use the next ISO setting and scale it back down. You’ll encounter some articles talking about using 2/3 ISO settings since it has a better SNR, but for astrophotography you want to be in the range where you already have the best SNR from your ISO setting and don’t want any digital scaling as you loose information.
Just stick to using the ‘real’ ISO stops; 100, 200, 400, 800, 1600, 3200, etc.
Summary; ISO considerations
Ok, lets take a step back and look at what we have established;
ISO has nothing to do with the sensor’s sensitivity
Instead, ISO is an amplification of output signal
this amplification will be done either by an analog amplifier or digitally, depending on the brand/model and the ISO setting
Increasing the ISO decreases dynamic range
Increasing the ISO increases SNR until the upstream read noise swamps out the downstream read noise
Point 3 and point 5 determine the best setting for your camera.
So even though this is really counter intuitive for most of you, stop thinking about ISO as a way to increase sensitivity or a way to make your image brighter. It really really doesn’t!
The best ISO for your DSLR
The best ISO for your dslr will of course depend on your particular model, but in general it will be defined by the following;
The best ISO for astrophotography for any DSLR is the lowest ISO level from which either a.) the upstream noise swamps out the downstream noise OR b.) the amplification will be done digitally in camera, whatever value of both is the lowest.
Starting from that value the read noise will be (more or less) consistent and we can call the sensor ‘ISO-less’ and we gain nothing by increasing the ISO, while only hurting ourselves by diminishing the dynamic range.
The best ISO for astrophotography for any DSLR is the lowest ISO level from which either a.) the upstream noise swamps out the downstream noise OR b.) the amplification will be done digitally in camera, whatever value of both is the lowest.
Defining the best ISO for your model
At this point you probably still are confused as to what ISO value is the best for your specific DSLR. It’s nice that we have a clear definition of what the best value is, but how do you determine what this means for you in real life?
Unfortunately there is no direct answer to this question, as the ISO levels from which a dslr starts amplifying digitally is generally not listed anywhere. However, the read noise levels for your camera at different ISO settings can be found, which uses data from DxO to calculate their curves. Digital amplification will be visible in these results as an (dramatic) increase of read noise for the highest ISO settings, since you’ll start amplifying the downstream noise as well if you do it digitally. So in general you would be fine at checking the read noise curves and see when they start to even out at the lowest values and use this as your ISO for your model. In the graph below I’ve plotted the read noise for the Nikon D7000 and the Canon 6D, not to do any qualitative comparison, but just to show you that the best ISO settings can and will greatly differ per model!
This graph makes it very clear why there can be and remain so much debate and confusion about this matter! Based on this graph I would guess that ISO1600 would be best for the Canon 6D, while ISO100 (!) or 200 would be best for the D7000. I recommend to test it out for your self to be sure, but at least these graphs give guidance on what to test; you don’t have to try the 6D at ISO100 and testing the D7000 at ISO1600 is also a waste of your time.
Concluding the best ISO discussion
This matter is really, really complicated which is increased even more by the fact that we have to work with facts and numbers that are not generally made public by the manufacturers. I’ve simplified things (a lot) in some places to try to give you an explanation that can be understood without diving in to the math or doing lots of side research to even understand what I’m talking about here.
I hope I succeeded in doing so, and would love to hear your further questions or remarks in the comments below.
Original Article