IS THE 500 RULE for astrophotography completely WRONG?!?

Hi everyone this is Nico Carver. My website is at nebulaphotos.com and today we’re gonna find out does the rule 500 work or is it completely wrong? First off what is the rule of 500? well it’s a rule of thumb for determining the maximum exposure time in seconds of an untracked photo before the Earth’s rotation causes your stars to trail. So that’s a lot to take in let’s unpack that everything we’re talking about in this video is assuming you just have your camera set up on a fixed tripod we don’t have a astrophotography mount or a tracking device like a star tracker it’s just the tripod and the camera and then we take a photo of the night sky typically we’d want to do this with a shutter release or an intervalometer or the timer function on your camera cuz you don’t want to accidentally get some motion blur by hitting that shutter so use one of those devices the easiest one if you don’t have anything is just set your camera up to be on a on a timer so like a two second or three second time or after you hit the shutter and that’s it so what we’re talking about is fixed tripod no no mount or tracking device how long can we expose for untracked and sometimes when we have this kind of setup we may want to see what we call star trails as that can be an interesting type of photo like this one I took in Glacier National Park in Montana this is just a single long exposure but if you want to do Milkyway photography or wide field deep sky astir photography we typically want the stars to be little pinpoints of light little round dots not trail arcs not lines not egg-shaped we want them to be round but there is a dilemma because while we want to get round stars we also want to expose for as long as possible the reason is so we can get enough to make the photo dynamic because we want to actually be able to see the Milky Way or see the Northern Lights or whatever or the nebula or whatever it is that we’re really trying to bring out in that photo other than the stars as an example I can take a very short picture so here’s an example of a ¹⁄₁₀₀ of a second picture of the night sky and you don’t see much of anything it’s business lead arc we might see a couple little star cores and those stars are actually round in this photo because it’s a very short exposure but we just don’t have enough signal to see anything interesting other than those very little bright star wars so the question is again how do we calculate the maximum exposure time with no star trailing well the rule of 500 is the most famous answer to this question and it’s a very simple math equation which makes it compelling let’s say you’re using a lens and printed on the side of the lens is the focal length so this is the Canon 50 millimeter also called the Canon nifty 50 it is a inexpensive lens if you don’t have much money but you want to get started in Astro photography this might be a good choice because it’s not very expensive and 50 millimeter is also a nice one to use because it’s very easy to divide by 50 so let’s go ahead and plug in the numbers here and we actually only have to plug in one number 500 divided by 50 millimeters gives us 10 and that’s it the rule claims you can expose for 10 seconds before your stars will be visibly trailed and before I talk about does this work which we will get to I mean I’d actually do the tests and we’ll see I want to talk a little bit about the history as far as I know I’m not sure I’ve done some research and I’m not sure exactly where this rule came for I am a librarian I have access to a lot of older books on astrophotography and in all of these books I could not find the rule of 500 or any rule as simple as this one many of these books did have rules for trying to come up with exposure time untracked but none of them had something like this internet research tells me that this came from the film era and it used to be the rule of 600 so you read that we divide 600 by the focal length and it was only meant for printing photos enlarged up to 8 by 10 inches about this big so anything over that this rule acknowledged that you might be able to see star trails but if you were using film and you were printing you know fairly medium sized print not too big than it was supposed to work but then at some point people caught on to this rule and it became mantra it became the rule - that - no and it was repeated in countless articles videos forums all over the internet and that sort of surprises me because with digital cameras this rule doesn’t actually seem to work but let’s go back does it work I would argue that it all depends on your expectations what it doesn’t do is it does not reliably result in pinpoint round stars with any of the modern digital sensors that I tried it with at any focal length so if your expects poq tation is to see no star trails then no this does not work doesn’t work though for very wide field pictures for instance you know shooting the Milky Way with a 14 millimeter lens with a nice landscape in front then sharing that on Instagram or even printing it out you know about this big sure it does work for that I I think you can get a very nice photograph following the rule of 500 and you can even win contests with the rule 500 I know this for a fact because before I got really into deep sky Astro photography with telescopes and mounts and all that I was using the rule of 500 back in 2014 and I won opt is best night sky award for this shot of the aurora borealis or the Northern Lights but if we zoom in here you can see the stars are clearly trail their lines not round points so is the rule completely wrong as I asked in my title again I’d say it depends on what you expect from the rule if you just want a basic rule of thumb for your nightscape where the focus is really the whole scene and the emphasis is more on the landscape and other aspects like in this photo there Aurora then I think the rule of 500 is not bad at least as a starting point I’d always recommend though experimenting a bit you might find that a rule with especially with modern digital sensors a rule of 400 might work a little bit better rule 400 would be more conservative and it would result in a shorter exposure time for instance if we go back to this 50 millimeter lens and we solve the equation that would give us a exposure time of 8 seconds rather than 10 so so far I’ve been started to thinking about like you know single shots that kind of thing but but what about us who really want to do untracked Astro photography and we want to be sure we have round stars pinpoint stars even when zoomed in to a hundred percent on a computer monitor is there a rule for us and I can say yes but the downside is it has to be a bit more complicated because the rule of 500 or 400 or whatever it is only accounts for one thing focal length it doesn’t account for a number of factors that could have an impact on whether or not you get star trails in your photograph and the most important being pixel pitch I can tell you for a fact that the size of the pixel pitch in your sensor does make an impact okay I made a little crude illustration here if you imagine one sensor has large six micron pixels this is fairly common size for a full-frame sensor there are six microns across and six microns tall so this in this grid this is four pixels now imagine a sensor a more common modern sensor which would have three micron pixels so for the four pixels here that we have in the six micron pixel array we have 16 3 micron pixels you’ll notice all these pixels are square that’s the most common thing in modern sensors almost any modern sensor will have square pixels so that makes a little easier to draw something out like this what you’ll find is that with the sensor that has the smaller pixels if we use the same focal length you’re going to get start railing much quicker on this finer pixel pitch than you would on the with the camera with the larger pixels and this is because we’ve dramatically changed the image scale let me try to explain this here with an example this example is not going to be perfectly sound but hopefully you at least get the idea here let’s imagine that a star falls right here on both sensors if we do a short enough exposure that star will look round on both sensors in the the camera with the smaller pixels it’s just going to fall onto that pixel right there and it will make a fairly round pinpoint exposure I mean pinpoint star if if it falls on that pixel in the six micron one then it would fill that whole pixel right and you’d still have a nice pinpoint star but then let’s imagine we do a longer exposure and this star starts to spread across two pixels as the Earth rotates okay what do you have then you have star trailing so you have a line instead of a point well on the camera sensor with the bigger pixels it’s still falling within that bigger pixel so you still have a pinpoint of light now this is a simplified example I’m not getting into all the sensor characteristics but hopefully it made some sense why pixel pitch does matter okay another factor that the rule of 500 ignores it’s not quite as important as pixel pitch is the declination of the object you’re shooting so an easy way to think of this is the closer your object is to one of the celestial poles either the North celestial pole or the South celestial Pole the longer you can expose for without star trails and this is quite obvious when you look at a photo like the one I showed earlier that’s just a single long exposure and the North celestial pole is roughly centered and you can see that close to the celestial Pole the trails are shorter and the further that we move away from the pole they get longer so I was looking around for a rule that would take into account some of these factors that I mentioned and I came across someone who had already created a really nice formula his name is Frederick Michaud of the societe astronomique du Havre. I’m sorry if I butchered the French there but anyways he created what he calls the NPM rule and the MPF rule is a lot more conservative than the rule of 500 and it takes into account all the things that I want it to the formula looks like this bit complex but basically it looks at the focal length it looks at the pixel pitch it looks at the focal ratio it divides all that by the focal length and the cosine of the declination and that’s how it arrives at the time in seconds it’s not a rule that most people could figure out in their head but a nice thing that frederique has done is that he’s put together a very useful web page here on the Society’s website and I’ll link to this below and I also just sort of want to show you how to use it right now just to show you it might look like a lot of information at first but it’s actually not too hard once you get used to it and then you can use this very useful npf rule and fully understand everything that it’s doing for you so let’s jump into that okay looking at the website here if you do read French then you can use it in the original French but if you don’t then I’d recommend using Google Chrome because built into Google Chrome browser is the Google Translate so if I go up to the address bar in the upper right there’s the translate button and I can click translate this page and choose English and it does its best job to translate to from the French to the English so that should help a bit understand the rule and learn something from the page I’m not going to go through this all because I want you to read this on your own one things a little confusing is it changes from the npf rule to the MFN rule not sure why anyways we just go through here and all the things that we need to fill out are highlighted in this blue and then when we’re done filling that out the results will be down here in the yellow so we start with picking our camera brand and then pick the model so I’m using the 5d Mark 3 today then we’re gonna put in the information about the lens so I’m just going to use the example of the lens that we’ve been talking about which is the nifty 50 so it’s 50 millimeter focal length and an aperture of 4 is what I’m gonna use then this is nice that you don’t really have to know the declination of your object you can just tell it which direction you’re shooting so I’m going to choose southeast I’m going to choose my latitude of my location here which is about 42 degrees and my target location is about 40 degrees above the horizon if you don’t know these things ahead of time you can look them up in Stellarium or one of the many planetarium apps last thing to choose here is whether we’re gonna shoot it horizontally or vertically and then we just click calculate the exposure time button and all of the information is down here in the yellow box and we can see with a rule 500 we would have 10 second which agrees with what we’ve been talking about with a simplified MPF rule we have 6 and ¹⁄₂ seconds we could round that down to 6 and with the complex npf rule we have some difference here across the sensor but basically 3 seconds we’re gonna round down to 3 if you keep scrolling down in this page it explains exactly how he calculated the MPF rule and how he came to the formula and it explains it very well so I would definitely recommend reading that all again the author is frederique Mashhad and I really like that he took the time to put this together and figure out this formula so I hope other people find it as useful as I did so another way to calculate both the MPF rule and the rule of 500 is with an app I found called photo pills I’ve actually had this app for a while but didn’t realize it did all of this you can get this app from the App Store it’s available both on Android and iOS devices and it’s currently $9.99 on either App Store so it’s a little bit expensive for an app but it does a lot more than just help you calculate exposure time it also has a virtual AR planetarium and all these different cool features built in for planning your astrophotography so right here on the home screen for photo pills if I scroll down a little bit one of the tools is called spot stars so I’m just going to click on that it has a camera database so I’m going to search for my camera I’ve already chosen the candidate 5d Mark 3 but I’m just gonna choose it again here we’re gonna put in the focal length of the lens so I’m just gonna use 50 millimeter again the aperture I’m gonna use F for the declination of the object and it also has an AR mode which is pretty cool it puts a planetarium on top of your camera and then you can just find the object you’re looking for in the night sky and set it that way then the last thing is it says default or accurate and what this is is the default is the simplified MPF roll and the accurate is the full complex MPF rule I’ve looked at this app across many different focal lengths and apertures the declination z’ and all of that and these results for both the complex and simple nvf roll do match up with Frederick michaud’s website so I feel that you can use photo pills instead of the website it’s very handy to have this sort of mobile app when you’re out in the field for calculating an NPF rule not having to do all that math and then you can also compare it right below to the rule of 500 I like that photo pills says right here that it fails with most cameras the rule 500 and after watching this video I think most of you would maybe agree with that so that’s it for photo pills let’s move on to testing in the field okay so so far we’ve just sort of been talking about the theory we’ve looked at the website for this new NFP rule but I really want to get out under the stars and test these things for myself so tonight we’re going to be testing the rule of 500 versus both the simplified NFP rule and the more complex NFP rule and just for completeness sake also do the rule of 400 and we’re not going to just be testing it with at one focal length we’re gonna do it with a variety of different lenses to see how these different rules perform at different focal lengths so I’m gonna go all the way from an eight millimeter fisheye we have a 24 millimeter 40 50 85 all the way up to 135 normally I wouldn’t do untracked astrophotography with a 135 millimeter lens I just think that’s too long it results in the fact you have to do very very short exposure and then the noise swamps any signal you get so typically the the longest lens that I would do untracked astrophotography is my 85 millimeter but just for completeness sake I’m going to do the gamut here of different focal lengths and so I’m gonna be just using the Frederick’s calculator using my laptop here and we’ll try all of this out I’m going to be using my 5d Canon 5d but I almost might also use my Canon RA a little bit here so that we can try different pixel pitch to to make see if that makes a difference so onto testing one last thing I’ll say about testing is that I always pick a either a night with some moon white because I typically don’t do as much astrophotography during the full moon phase or a night like tonight where it’s actually a semi cloudy there’s a lot of passing clouds but for testing purposes that’s okay we’ll just pick a patch of sky that doesn’t have clouds and take a few shots all right okay and here are the results with the let’s start at the bottom actually so the rule of 500 I will say that to me these are clearly lines the the stars are clearly trailed rule of 400 oh and I should mention these are all with our sample lens here the 50 millimeter lens I did do this same test on all kinds of different lenses these results were basically the same across all the different focal lengths I tried it on didn’t really matter the focal length too much I got basically the same results so rule of five hundred I’m gonna say does not really work rule of 400 it’s slightly better you might call that sort of an elongated egg-shaped star but for me that’s still not acceptable okay simplified npf it’s really sort of midway between the complex MPF and the rule of 400 I would say for some people especially if this was a wide field shot with landscape and that kind of thing this one might work but it did not result in round stars so we’re going to go ahead and cross it out to okay npf it works I can tell you that I tried it lots of different situations and in every situation I threw at it the npf formula did in around stars now some people may feel that it’s still too conservative because they’re trying to take a picture of the Milky Way and the Milky Way doesn’t really come out with npf formula so again this is going to come back to your expectations if your goal is around stars npf is gonna give you that maximum exposure time you can expose for you still may be disappointed though so what I would suggest is if you’re planning to stack your untracked photography and I’m gonna go into that in the next video so if you’re interested in stacked untracked Astro photography keep watching the MPF formula complex npf formula is what I’d use if you are are really more of just you’re interested in a single great shot and you’re more of like a landscape nightscape person then I would be looking probably as the simplified npf as a starting point wait hopefully you’ve learned something here if there’s any takeaway it would be figuring figure out what style of extra photography you do and how much star trailing is offensive to you maybe you find the rule of five hundred level of star trailing is just fine for your style of Astro photography and I have no problem with that I just wanted to sort of show you the options and what each rule of thumb can do I’ll just give you a little preview here my next video is going to be a Ryan nebula without any kind of star tracker so just a fixed tripod and we’re gonna be seeing what we can do with hundreds of exposures of the Orion Nebula but without tracking using the NPF rule all right till next time. Clear skies! .