Finding the Optimum Camera ISO / Gain for Astrophotography

[Music] hi again everyone this tutorial is about iso and game and finding out what settings are the best to use for astrophotography i'm going to start with a little bit of theory and then i'm going to demonstrate how you can do a simple test on your setup to find out what the best iso and gain settings are for you hope you find it useful so we'll start with the terms signal and noise signal is the term we use for the measure we make of the actual light so as the photons are gathering is the stuff we want noise on the other hand is a random fluctuation in the measurement generated for each pixel by the sensor itself in the camera and we certainly don't want it it's not related to the scene that we're imaging in any way and it makes faint features hard not impossible to see in our images because they're down close to where the noise is so all camera sensors generate noise unfortunately that's a fact of life but some do so more than others some sensors are better than others and noise generated by the sensor is always higher at higher temperatures and so it's often referred to as thermal noise now just as an example here's a shot taken with the monochrome camera with the lens cap on so there's no light coming in at all no photons and i've stretched it uh in photoshop so you can actually see uh the noise because obviously it's a very low level it would just look black to the eye but it is there and it's when you do that stretching of your of your final image in the post-processing you see this anyway so i've stretched it so you can see it um the hot pixels would always be the same if there are hot tools on your sensor but the noise is a randomness so if i were to take this again and again it was the randomness that's different every time which is what we call the noise so dark frames as you may have seen for calibration are taking out the systematic part but the random part dark frames don't help at all and that's what we're interested in here is that random laws this is the same thing from a color sensor you'll notice it's pretty green and that's because of the biomatrix uh which on this particular sensor is two greens for every one red and one blue pixel so it's dominated by green so that's normal uh now signal to noise ratio is really the thing we're interested in and uh now that we understand signal and noise we now need to be thinking about that ratio we need that ratio to be as high as we can so that faint objects that would otherwise be buried in the noise are actually at a higher level than the noise so we want a high signal-to-noise ratio and there are different ways to maximize your signal to noise ratio in your imagery and the first is to use a wider aperture whether that's a lower f number on your camera lens or a telescope with a bigger aperture for the same focal length uh if it's got a lower lower f number then it'll gather more light the light faster so uh the other next technique is to use a longer total exposure time so that either means uh stacking more images of the same duration or stacking the same number of images of a longer duration total total time of the exposures as that goes up your signal adds your noise averages so that ratio between the signal and the noise in your final image increases you could also cool your sensor and get that thermal noise down generate less noise in the first place doesn't change the level of your signal because the noise has got smaller that signal to noise ratio has increased so just uh after a visual representation so you can actually see signal to noise ratio at work this is a single 60 second shot taken apart the orion nebula with a color camera and you can see noise in the background and then taken 86 more shots and stacked them with the first one and this was the result so you can see the signal level is about the same in these two images but the noise level is dramatically reduced and in the background we see a much smoother background and there are also faint features such as this area here and up here where there are details we can now see which couldn't see at all with a single shot so it's a really nice example of a signal to noise ratio at work the next consideration is dynamic range completely different to signal to noise ratio dynamic range is about the range of brightness values that your uh setup can record and why is this important well in a typical scene and here's the whirlpool galaxy as an example there are bright features and faint features and we want to be able to see the detail ideally in both we want to see that our stars are nice and bright and sharp we want to be able to see the detail in the bright core of this galaxy and but we also want to be able to see those really faint dust lanes in the outer reaches of this uh this galaxy so so we want to be able to see detail in both the bright and the faint and that needs a high dynamic range if we have a low dynamic range and we use a long exposure then those bright objects will saturate or click and sometimes known as being blown out the stars are blown out and in fact in this example the stars are a bit blown out and the core of the galaxy is a bit blown out it's just basically a disc they look like discs instead of points i can't see the real detail in there in the core of uh the whirlpool galaxy here because it has blown out uh so there's a dynamic range issue with this shot so it's a good example if your dynamic range is too low and you shorten the exposure well that will prevent uh having a blown out signal in the bright areas of the image but those fainter features they'll be weaker and they'll be going down towards the noise and be harder to see above the noise level so dynamic range very important here's another good example and the main part of the great orion nebula and there's this particularly bright area here where in this example uh it's really hard to see the detail it's being it's starting to blow out up here and we're just really struggling to see what's going on in this area but there are really faint in their velocity and dust areas around the edges of orion where you really need lots of exposure time and to be able to see them at all and get them up above the noise but when you do that you often find that this bright area will blow out and so it's particularly difficult target to image and actually the the technique that most people would use to resolve that would be to actually have two separate stacks of data one with a shorter exposure time in order to get the detail on the bright areas and another with a long exposure time to get the faint stuff and then blend them together somehow in photoshop so that's a very challenging target from the point of view of dynamic range so for good astrophotography images we need a high signal to noise ratio and a high dynamic range so now to iso and gain which is what this tutorial is really all about what are iso and game settings all about well they control the amount of amplification in your sensor that's a simplistic overview because in fact all sensors are designed differently in the detail of what's going on on the actual sensor itself are very complicated but iso and gain are used in all photography in conjunction with the exposure time and the aperture to achieve what we would call a good exposure so their purpose is to adjust the signal level in the sensor output so that we see the thing we're trying to take a photograph of at a nice level so why don't we just use the maximum iso or gain setting on our camera well firstly adjusting the iso again may change the noise level as well as the signal but all sensors are different they include different amplifier stages and some some of them switch those stages on and off depending on the isos that you select so it's not obvious what setting to use for iso and gain to give the best signal to noise ratio not obvious at all and it depends very much on what sensor you have but it is fair to say that a higher iso gain setting will almost always result in a reduced dynamic range so we've got this conundrum we've got a high iso gives us more signal great but it gives us less dynamic range not great so how do we know the best item going to use and what about signal to noise ratio as well so it's complicated it is complicated um so how do we get around that well thankfully there's a fairly easy test that we can do ourselves which i'm going to demonstrate in this video to help us figure out what's the best iso and game setting to use on our particular camera and you can do this test yourself and find out and go then when you're going out to take your pictures you'll know you're using the optimum iso game and before i dive into the demonstration video just to say there's a couple of useful references the first is that if you're using dslrs there's this excellent website photons to photos which is bringing together of data from many people's cameras around the world and there are graphs on there that you can look at and see how the dynamic range varies with iso on your particular camera and also how other things like shadow noise reduction other various effects go on it's quite a complex and technical website but you may find it useful to take a look but uh there's an excellent article which actually led me to making this tutorial uh on all about iso on petapixel if you follow the link here it's well worth a read it's not too long it's very easy to understand and uh this is actually where the test i'm going to do and demonstrate now came from and i think it's a really effective test and hopefully you will agree with that so i hope you find it helpful i hope you follow and that this gives you the answer you've wanted of what i say what game you should use so let's get into it so we'll start with the lights on nice and brightly switch the camera on and put it into live view okay so i'm going to switch the lens image stabilizer off and put it into automatic focus and then choose the subject and focus it then switch the lens to manual focus now i'm going to go to manual mode on the camera i'm going to set the iso to 3200 and the aperture wide open which in this case is f4 i'm going to go into the menus so first i'm going to turn off the long exposure noise reduction and then i'm going to turn off the high iso speed noise reduction and then i'm going to go to the image quality and select raw with no jpeg and then come out of the menus and now i'm going to half depress the shutter so i can use the exposure meter at the bottom and adjust the exposure time first i need to drop the lights though so we'll just dim the lighting now [Music] so with the lights dimmed now i'm going to half press the shutter and increase the exposure time until i get a good exposure here so you'll see that a one second exposure at this iso 3200 looks sufficient so we'll now take that shot and there's our shot so now i'm going to just change iso with nothing else so i'm going to drop right down to 100 iso select that and take the shot again i'm being careful not to move around the room i don't want to change any shadows i don't want the lighting conditions to change at all i just want to change the iso and then retake the shot so i'm doubling the iso each time so this time is iso 200 and take the shot and then i'll select iso 400 and take the shot and then choose iso 800 and take the shot and now 1600 take the shot i will redo 3200 although i've obviously already taken that one i'll just do it as part of my sequence so i'm confident the lighting conditions are exactly the same and now double again to 6400 and then the final setting of my camera release of 12800 you can keep going until you get to the highest side so your camera can do okay so that's my data i'm now going to switch the camera off and take that data up to the pc okay i've copied those eight images onto my pc and i've opened up lightroom and i've imported them so i'm now going to go to the develop tab and i'm going to take one picture at a time and i'm going to adjust the exposure so that all of them match the exposure for the iso 3200 shot so i won't make an adjustment to that one so we start with the iso 100 image which is pretty dark and uh let's just select that image and go to exposure and rather than using the slider i'm just going to go to the number at the side double click it and then to 5 and hit return so that's now lifted the iso 100 exposure by 5 stops i'll now go to the iso 200 and i'll do the same thing except i'll lift that by four stops i'll go to the iso 400 and i'll lift that by three stops the iso 800 i'll lift by two stops the iso 1600 i'll lift my one stop 3200 would leave alone the 6400 i'm going to put minus one so i'm going to go down one stop and then the 12 800 i'm going to go down two stops and minus two if we now go through those images this is 12 800 6400 3200 1600 800 400 200 and 100 so it's pretty convincing that the signal level in all eight images are now identical so what we'll do now that we've matched the signal level in all eight images is to zoom in on one area zoom in on this area just here and i'm going to switch between the different images and compare the noise level we know the signal's the same but what's the noise doing of course where the noise is lowest is where we've got the best sequence of noise ratio because the signal levels are all matched so i'm actually going to take a screenshot of each of the images in the same portion and i'm going to put them in powerpoint so that i can flick easily between them i'll be back in a second okay by the magic of youtube that's done and uh now i've got eight powerpoint slides each one with the same segment of image for each iso and i've labeled each one of the top so there's no confusion about which one is which so we'll start with the iso 100 and we compare that with the iso 200 it's very easy to see that the 200 has a much lower noise level than the iso 100 now we can go from 200 to 400 and you can still see that there is an improvement it's not as big as a step as it was from 100 to 200 but definitely the noise has dropped so now we'll go to 800 and even with a 400 to 800 i would say there is a slight improvement in the noise it's not huge anymore and then from 800 to 1600 i'm really struggling to see any difference at all in the noise and then sixteen hundred thirty two hundred there's no change thirty two hundred to sixty four hundred there's no change from sixty four hundred to 12 800 there's no change so what this means is that i get for the same signal level remember we're going to stretch our astrophotography images to see the signal and when we do so we want the noise to be as low as possible so if i take iso 800 or 1600 or 3200 or 6400 or 12800 and i take my astrophotography images with any of those and i stretch them my noise is going to look the same and so because of the dynamic range issued i want to use the lowest dynamic range that is going to the the lowest iso which gives me the widest dynamic range without paying a penalty in signals to noise and that is iso 800 for my sensor which is the canon eos 5ds yours may well be different i do not want to go below iso 800 because then i'm seeing a definite degradation in the noise so with that simple test i now know that russia photography with this camera iso 800 is the right iso to use now i'm aware that the youtube video compression might make it difficult to see these noise levels due to compression artifacts so i've prepared this mosaic which is a static image and hopefully that will be easier for you to see the changes in noise levels between the different iso settings i hope you do the same thing on your sensor if you like this please subscribe to my channel helps grow the channel encourages me to make more videos i really appreciate it also means you get to see when i've released another tutorial as well thanks very much for watching see you next time you

Channel: Martin’s Astrophotography

Views: 11,806

Keywords: astrophotography, ISO, Gain, Best ISO, Optimum ISO, Optimum Gain, tutorial

Id: ysYAQtIc9H8

Length: 18min 43sec (1123 seconds)

Published: Sat Sep 26 2020