Understanding nikon CLS

[brian_hooks]

<p>In an effort to continue advancing my photography I have been working on off camera flash. I feel I've been pretty sucessful using the CLS system and have recently purchased a Yognuo flash trigger to be able to control my flash in more demanding situations (ie outdoors daylight). This has forced me to begin using my flash in manual mode and I'm finding it dificult to "know" what power setting is right for each situation. In the past I've taught myself how to adjust my apature/shutter speed by leaving the camera in apature/shutter priority then seeing what it chooses and then puting the camera in manual and making adjustments from there. I'm having more problems doing this with my flash as I have yet to see what my flash power has been in any of the exif information plus I'm unsure how to factor in other variables such as distance from subject or light modifiers (ie softbox). Is there any type of chart/graph that will help with setting my flash power. I realize that in manual mode the flash "tells" you the effective range it will be firing in but again with modifiers in place things seem to get a little trickier.<br>

Brian</p>

 

[ilkka_nissila]

<p>No, there is no "chart". But with experience you can learn a good starting point for the flash power in situations where you typically photograph in. This works well for formal portraits, for example. What I do is just set the exposure parameters for ambient, then subtract a little of that exposure, and turn on the flash and fire a test shot e.g. at M1/16 (my starting point depends on ambient conditions and is just a guess based on prior experience). If it lifted the shadows a bit then I know I'm close. A couple of test shots is usually enough and then I can begin shooting. In event situations usually I just use TTL and compensate (if I'm using flash at all). I'm quite used to both TTL and manual modes in situations which I often use flash in. If I have to go to a situation where I've not been then I guess results will be more variable and require more testing.</p>

<p>But back to the question, many modifiers come with some kind of stop rating which gives a ball park of how much the light is attenuated but it just a rough estimate and depends on the reflectivity of the environment in which you use it, distances to walls and ceilings etc. </p>

<p>Oh, I forgot. In the old days, when even test shots (using Polaroids) cost a lot of money, people used <strong>flash meters</strong>. A flash meter will tell you what aperture to use given the (manual) power you set in your flashes. I often use it in the studio to save time. </p>

[Matt Laur]

<p>I'll agree with Ilkka. This usually turns into a seat-of-the-pants thing, unless you're using a flash meter. Which is why I went old school and bought a flash meter! I don't use it often, but when time's short, it's indespensible.</p>

[nigel_farmer]

<p>I would recommend taking a look at the tutorials on Neil van Niekerk's website, and can also recommend his books.</p>

[pete_s.]

<p>Flash power depends on what aperture and iso you are shooting at, what mixture of ambient and flash you want, the distance from the flash to the subject, the zoom setting on the flash head and any light modifiers and gels attached.</p>

<p>There are ways to calculate it in your head but it's a bit convoluted. What you can do to speed up your learning process is to try out different modifiers and see what power you need to shoot at a certain aperture and iso setting and at a fixed distance. Then make a sheet with your settings and you can use that to extrapolate what you need.</p>

<p>Distance follows the inverse square law, flash power/aperture/iso changes in stops, etc etc.</p>

<p>BTW, one way to check what the camera does in auto is to notice how long it takes for the flash to recycle. That will give you a rough idea at what power it is firing.</p>

 

[pete_s.]

<p>Just to elaborate more for you.<br /> Flash power:<br /> 1/1 = full power<br /> 1/2 = full power - 1 stop<br /> 1/4 = full power - 2 stop<br /> 1/8 = full power - 3 stop<br /> 1/16 = full power - 4 stop<br /> 1/32 = full power - 5 stop</p>

<p>Distance example<br /> 10 ft = 1/8 power<br /> 14 ft = 1/4 power (+1 stop)<br /> 20 ft = 1/2 power (+2 stop)<br /> 28 ft = 1/1 power (+3 stop)</p>

<p>Aperture scale in full stops<br /> f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16</p>

<p>ISO scale in full stops<br /> ISO100, ISO200, ISO400, ISO800, ISO1600, ISO3200</p>

<p>Shutter speed scale in full stops<br /> 1/4s, 1/8s, 1/16s, 1/30s, 1/60s, 1/120s, 1/250s.</p>

<p>For example:<br /> If you move the flash and increase the flash to subject distance 40% you need one more stop of light. That means you increase the flash power from 1/8 to 1/4 or change the aperture from f/4 to f/2.8 or increase the iso from 200 to 400. All of these thing will mean that you have the same exposure from the flash as before you moved it.</p>

<p>However both aperture and iso will change the ambient exposure as well. If you are just using flash it's not a problem of course. If you however are using ambient light as well you could change the shutter speed to combat your changes to iso or aperture. In the example above where in increased the iso or opened up the aperture one stop you would need to change the shutter speed one stop so if it were at 1/30s you need to go to 1/60 to keep the ambient exposure the same.</p>

<p> </p>

[pete_s.]

<p>Also check the actual numbers from the flash power scale and the iso scale and shutter speed and you'll find 1, 2, 4, 8, 16, 32, 64 etc.<br /> This is actually 2⁰,2¹,2²,2³,2⁴ etc.<br /> In the case of the shutter speed it has been rounded off, so 1/30 of a second is actually 1/32 of a second.</p>

<p>If you check the aperture scale and the distance examples you will find that it's the same numbers as the other scale except with a tighter spacing. So instead of 1,2,4,8,16 if goes <strong>1</strong>, 1.4, <strong>2</strong>, 2.8, <strong>4</strong>, 5.6, <strong>8</strong>, 11, <strong>16</strong> etc.<br /> This is actually 2^0x0.5,2^1x0.5,2^2x0.5,2^3x0.5,2^4x0.5 etc.</p>

<p>Everything has to do with the fact that one stop is twice the amount of light. Shutter speed, aperture, iso and flash power is all in stops.<br /> An increase in the aperture (diameter of a hole) to double the size means that the four times the amount of light passes through because the area is now four times bigger. That's why the aperture has to change in smaller increments for each stop (actually the square of 2).<br /> Twice the distance means that the light area will increase four times. So the same as aperture changes.</p>

<p>If this is not clear to you, try to find a book that explains how this is all interconnected to each other. It's much easier to use flash in manual, when you know what kind of effect changing one parameter will have. Also aperture, iso and shutter speed is just normal photography and forms the basics so get started on those first.</p>

[rodeo_joe1]

<p>Not sure what this question has to do with CLS, since the Yongnuo trigger doesn't support CLS but....<br>

Back in the day before TTL flash metering, i-TTL or any other such refinement, we had to rely on things called "Guide Numbers" to calculate the aperture to be used for a flash exposure. This system worked reasonably well for many years, since the output of a flash is consistent and you only need to know how far away from the subject the flash is to find the appropriate exposure.</p>

<p>The system is fairly easy to understand and to use: Divide the Guide Number (GN) by the flash-to-subject distance to get the aperture number for the correct exposure. GNs are given in either feet or metres, and for a base sensitivity of 100 ISO. For example a speedlight might have a GN of 28 in metres; so if the subject is 10 metres away, that gives you an aperture of f/2.8 (= 28 / 10).</p>

<p>The only fly in the ointment is that flash equipment manufacturers are all habitual liars when it comes to giving out their Guide Numbers. If a maker advertises a GN of 36, then the <em>real </em>GN is going to be more like 24. I've yet to measure a hotshoe speedlight that exceeds a true GN of 28 when measured with a flashmeter and verified by an actual exposure backed up by the histogram on the camera. Be that as it may, the Guide Number system is a good guide to the correct exposure provided you make allowance for the makers' false and inflated claims.</p>

<p>I would start with the assumption that your flash will realistically have a GN no higher than 28 in metres or 92 in feet. Divide that by the subject distance to get the aperture number to be used. If the flash is bounced or reflected, use the <em>total distance</em>, from flash to bounce surface + bounce surface to subject, and after working out the aperture subtract another stop. Same with using a white umbrella or softbox, although a softbox may need more than one stop of compensation.</p>

<p>Things get a bit more complicated when you change the ISO speed or reduce the power of the flash. For each doubling or halving of flash power or ISO you need to multiply or divide the GN by 1.4. So starting with your GN of 28 (metres/ISO 100) and increasing the ISO to 200 would give you a new GN of 40. Conversely, setting the flash to 1/2 power while keeping the ISO at 100 would give a GN of 20, etc.</p>

[ilkka_nissila]

<p>I just set my SB-900 to M1/1, 35mm coverage, pointed it at the flash meter at 4m distance (inside my living room), fired the test button and the flash meter read f/10.6 at ISO 100. Prediction based on GN: 40/4 = 10. </p>

[rodeo_joe1]

<p>Check your settings again Ilkka, not even my Metz 402, with a maker's stated GN of 60 (m/100 ISO) gives a real GN that high. Sure you haven't got the meter set at 200 ISO, which is Nikon's base sensitivity? And what does an actual picture look like if you give that exposure?</p>

<p>According to their repair manuals, both the old SB-25 and the SB-900 use the exact same value of storage capacitor. I have two samples of SB-25, and both give me a true GN of 28 at the 35mm zoom setting. Same for two samples of Canon 540 EZ. That's measured with 3 separate flashmeters and, as above, verified with actual pictures and the histogram on my D700.</p>

[rodeo_joe1]

<p>PS. I think even Nikon themselves would be surprised if your Guide Number was correct Ilkka, since their stated GN for the SB-900 at 100 ISO and 35mm zoom setting is only 34!</p>

[ilkka_nissila]

<p>My settings are correct. The base sensitivity of the camera I use (D3X) is ISO 100 (as is that of the D7000, D800, and D4) , not 200. My two flash meters both read correctly and usually within .1 stops of each other.</p>

<p>The guide number is intended to guide aperture choice in typical usage scenarios (at the time when the GN system was put into use). To measure it you need a small room with light or white colored walls and ceiling that affect how much of the light gets to the subject or flash meter (most people live in small rooms where reflective surfaces are close and they are most commonly light colored because they do not like to live in dark caves - of course, exceptions exist). To measure the GN correctly, you need to move the flash sufficiently far from the flash meter to correspond to a typical shooting distance (in my case, 4m). At a really close distance (1m) the reflections from the surfaces in the room do not contribute much to the exposure and this leads to underestimation of the GN.</p>

<p>This is the idea behind how the manufacturers to determine guide numbers for their flashes (of course they will have a reference space or instrument for testing it). They do not put the flash in a black chamber or cave and then measure the GN that way because it's not the most common use scenario and would lead people to overexpose their indoor shots if they followed such a guide number. A certain contribution from reflected light is assumed.</p>

<p><em>I think even Nikon themselves would be surprised if your Guide Number was correct Ilkka</em></p>

<p>Marketing people aside, I am pretty sure the people who designed the flashes and cameras know how this is done correctly and they report the GN according to their standardized measurement protocol. Nikon's rated GN matches closely the one that I got (my room may be a little more reflective than the normative room that Nikon uses, leading to the small difference).</p>

<p>DXO find the ISO ratings of the Nikon DSLRs to be slightly inflated (1/3 stop, so ISO 100 is really 80) compared to what they would expect. If you use correct exposure rather than flash meter reading to validate the GN then this discrepancy could come into play.</p>

[brian_hooks]

<p>Thanks to all for the information. It looks like I will have to use a combination of mathematics,repetition, and luck to get the lighting perfect.<br>

@rodeo joe sorry for the confusion with the title. What I was wanting to know was what/how cls was choosing flash power so that I could manually set power with my remote trigger. A best case scenerio would have been to be able to see flash power in exif data so that I could reverse engineer the lighting. </p>

 

[pete_s.]

<p>Back in the days when a flash would always fire at full power and the reflector was fixed and the film was what it was, it made sense to use guide numbers as it's based on distance and f-stops. That was the two main variables you had to deal with.</p>

<p>But when you have to account for flash power, iso and flash zoom as well as gels and light modifiers it doesn't makes as much sense anymore to base flash exposure on guide number.</p>

<p>So I came up with a system similar to apex that is based on stops instead. It was probably nothing new, but I hadn't read or seen anyone use it that way before.</p>

<p>The principle is that you convert the guide number to stops first. Then all variables are calculated in stops as well including distance and aperture. That way you can use simple add and subtract to find out whatever variable you are seeking - usually the flash power setting.</p>

<p>Read at you own peril!</p>

<p><br /> ..........</p>

<p><br /> An example in metric:<br /> SB900 at 35mm zoom, full power, iso 100 has a guide number of 34 meter. So at 1 meter distance the aperture would be f/34. f/34 is <a href="http://www.google.com/search?q=ln%2834%29%2Fln%282%29*2">10.2 stops</a> down from f/1. Let's say 10 stops to make it a bit easier. That would be our new guide number expressed in stops.</p>

<p>So let's say we are trying to expose a subject where the flash is 4 meter from the subject and we want f/5.6 and iso 100 if possible. What flash power should we use?</p>

<p>Well, 4 meter is 4 stops down from 1 meter, f/5.6 is 5 stops down from f/1 and iso 100 is iso 100. Compared to shooting at 1 meter and f/1 this means that we have lost 9 stops of light (4+5). Since the flash will provide 10 stops at full power it means that the flash is 1 stop too hot (10-9). So we need to drop the flash power 1 stop which means 1/2 power.</p>

<p>..........</p>

<p>If we are using feet I suggest using 10 ft as the base. SB900 at 35mm zoom, full power, iso 100 has a guide number of 111.5 ft. At 10 ft we get 111.5/10=f/11.15. In stops that is <a href="http://www.google.com/search?q=ln%2811.15%29%2Fln%282%29*2">6.96 stop</a>, so let's make our new guide number 7 stops.</p>

<p>Let's say that the flash is 10 ft from the subject and we want f/8 and iso 400 is OK. What's the flash power?<br /> 10 ft is 10 ft so no change here, f/8 is 6 stops down from f/1, iso 400 is 2 stops more light than iso 100. That means that we have lost 4 stops (6-2). Our guide number was 7 stops so the difference is 7-4=3 stops. We need to drop the flash power 3 stops. That means 1/8 power.</p>

<p>Depending on the recycling time of the flash maybe you decide that 1/4 power is fine as well. Then you can change the ISO to 200.</p>

<p>..........</p>

<p>PS.</p>

<p>When you think about different distances think how the f-stops series change for each stop:<br /> f/1, f/1.4, f/2, f/2.8, f/4 etc.<br /> If we look at distances in meter that would mean the following change for each stop:<br /> 1m, 1.4m, 2m, 2.8m, 4m etc.<br /> An if we look at distances in feet that would mean the following change for each stop:<br /> 10ft, 14ft, 20ft, 28ft, 40ft etc.</p>

<p>So if you know that f/5.6 is 5 stops down from f/1 then you also know that 56 ft is 5 stops down from 10ft and that 5.6 meter is 5 stops down from 1 meter.</p>

<p>Looking at the distance in feet it might also be interesting to note that if you go one stop wider than f/1 that is f/0.7. Then it's f/0.5 and f/0.3.<br /> So the distance in feet starting at 3ft would be 3ft, 5ft, 7ft, 10ft, 14ft, 20ft etc.<br /> If you shoot at 5 ft instead of 10 ft that's two stops more light.</p>

<p> </p>

[craig_supplee]

<p>I'm trying to emulate the test that Ilkka did today using my SB 600, SB 28, and Polaris SBD 100 flash meter and can't seem to figure this out. I stood the flash meter up on my couch, marked off the correct GN distance for ISO 100, set the flashes to ISO 100, 35mm zoom, aimed them at the meter, but am ending up with f stops of f45, f64, f32. I have used this flash meter before to correctly expose film shots in old cameras, but can't figure out what is going on with just firing the flash in hand. Ideas?</p>

[pete_s.]

<p>Craig, what distance did you use? Remember that the formula is GN=distance x aperture. Also make sure you set the shutter speed high on the meter or it will register a lot of ambient light as well.</p>

<p>Keep in mind that the flash doesn't really have an iso or aperture setting in manual mode. That's just so the flash can tell you the distance. The only setting that is needed is the flash power and zoom of the flash head.</p>

<p> </p>

[craig_supplee]

<p>Pete, I just reran my test. Flash meter leaning up against back of sofa set to highest shutter speed of 500, ISO 100, standing 10 ft away using SB 28 (GN 118') on M 1/1 got an f 64 reading. Same situation with SB 600 flash GN 98') gave me f 45. I don't understand these f stops, unless my meter is not working properly., or am I not understanding something here?</p>

[pete_s.]

<p>Craig, sorry but it seems that your meter is way off base.</p>

<p>I don't have a SB28 but I have a SB600 (actually two) and when I tested them both exactly the same as you tested yours the meter I used showed f/8 at exactly 10ft. Meter was Sekonic L308S and it was set to 1/500s and ISO100.</p>

<p>In theory if the guide numbers where correct it would show f/9.8. The difference is 0.58 stop, which is perfectly normal.</p>

<p>Your meter showed f/45 which is just not possible. That's more than four stops off any reasonable reading.</p>

[craig_supplee]

<p>Pete, I think I might get the stupid of the day award. After reading your last post I decided to replace the battery even though it was reading very high before. Still didn't work. Then I decided to try it with the dome for incident light slid in place. Voila! I was able to get reasonable(?) readings. It showed f 7.1 for the SB 600. This seemed low, so I then tried the flash on camera with both manual, and iTTL settings. The manual settings actually had the histogram more in the center, even though they looked too bright on my display. I do realize that the display is not too accurate. The manual for the Polaris doesn't mention anything about using the dome for flash, so that's what was confusing me. You learn something new every day. Thanks for your help.</p>

<p> </p>

[pete_s.]

<p>Hey, no problem, that wasn't stupid anyway but good you figured it out.</p>

<p>When I metered also got f/7.1 on some reading depending on where I held the meter but I picked the highest reading at f/8 assuming that would be in the center of the beam. The difference between f/8 and f/7.1 is only 1/3rd of a stop and if the meter is set to show only 1/3rd of a stop you don't know how close to f/8 you really where. I'd say it's perfectly normal.</p>

<p>Keep in mind that there are also some difference between meters as well as the calibration of them.</p>

<p> </p>