tutorials

General terms Long exposure Photography is closely related to Neutral Density filters which are semi-transparent pieces of glass that get placed in front of your lens. What makes them special, however, is that they obstruct a precisely controlled fraction of incoming light, and do so uniformly. The obstruction also aims to be equal across the visible spectrum, thereby not introducing a color cast (although this isn’t always the case). This last characteristic also happens to be why it’s called a neutral density filter. Even though they might appear gray or even opaque to our eyes, this isn’t how your photo will appear; the camera’s metering automatically compensates by letting in more light. However, the viewfinder will still appear very dark, so photographers often compose their image prior to placing the filter in front of the lens. Unfortunately for consumers, manufacturers do not use a standardized classification system to describe the strength of their filters. For example, B+W designate its 1000x grey filter after the f-stop reduction:  110,  while others like Formatt Hitech, Lee  designate their filters using a density factor. So, a B+W filter of 110 would correspond to a Formatt Hitech filter of ND 3.0. Hoya designates its 9 f-stop grey filter by x400 based on its filter factor which is somehow lower than the absolute value of x512. Lets see an overview of the different descriptions concerning ND filters as a function of f-stop reduction and optical density but adding also a new parameter the filter factor. What is the filter factor? Each “stop” of light-reduction corresponds with a halving of light. A given filter strength therefore passes only 1/2 strength of the initial incoming light, where “strength” is the filter strength in stops. For example, a 3-stop ND filter (ND 0.3) therefore only passes 1/8th the incoming light (since 1/2^3= 1/(2*2*2) = 1/8).  So for this filter the filter factor is 8, and the transmittance (allowing light to pass in %) is the inverse number in % , that is  12.5%.  In the below table, you can clarify the properties of each filter. What happens if we stack ND filters? It is very common to stack ND filters to increase their light-reducing strength, but adding multiple filters to the end of a lens can cause vignetting. Keep in mind that no matter how many ND filters are used, the densities of the filters were added and the filter factors need to be multiplied. For example, if you start with an ND filter with a density of 0.6 and add another filter of 0.9 density, the total density of those two filters would be 1.5. This filter combination achieves an f-stop reduction of -5, while the total filter factor of this combination is 32  as illustrated in the table below. Basic steps to perform a Long exposure shot First you have to find out the unfiltered exposure time. Measure this with your camera in aperture priority mode using your desired f-stop. Switch to Bulb mode If you used AUTOFOCUS in aperture priority mode DO NOT forget to switch to MANUAL when you go to Bulb mode In Bulb mode: set the same aperture you selected in step one, and choose the appropriate exposure time (always have with you a stopwatch to time the exposure) Those of you who are fully confident working in manual mode will be able to measure the unfiltered exposure time in manual mode and then adjust the exposure length as required. In a very bright day when probably the sun is behind you,  you may need to think about covering the viewfinder before engaging the shutter. There are several ways of doing this: Cover the camera with a dark cloth. Use some thick masking tape (electrician’s tape) over the viewfinder. Use a small cap which is designed for this purpose. Is there a mathematical formula to calculate the exposure times? Yes, there is a simple mathematical formula and it is described in the following figure with a simple example. Useful timetables I have compiled some useful tables where you can find the appropriate exposure time when you have already chosen the unfiltered time and the type of ND filter you use. All the values are in seconds except it is declared otherwise. I believe this table below is maybe more useful since it deals with darker ND filters and longer exposure times. How can we read these tables? Check first the unfiltered shutter speed you want (this is the one you determine form the aperture priority settings..) and then go all the way following the row to the intersection with the appropriate column where you see your ND filter. And that is your new exposure time. Lets say for example you read in your camera settings a shutter speed of 1 sec (very low-light scene) then (see table below) if you are using a ND 3.0 filter , that is a 10 f-stop reduction, you need to time 15 minutes with your stopwatch when you take your photo in Bulb mode. Is that all? I am afraid no.. and i will give the reasons in the next section. Just to finish with the tables i will give you a most useful table where we can have exposure times for stacking filters. Be aware and use the minimum number of filters to achieve your required number of f-stop reduction. What i mean? If you want to achieve a 16 f-stops reduction use a 10- and a 6- filter and not combinations like -6, -6, -4. Increasing the number of ND filters can cause serious vignetting problems. You can work both ways with this chart (and every chart in this post). You want to make an exposure of approximately 1 hour (maybe  a traffic scene where you do not want to have the cars movement or people moving around..). You check first the setting of your photo without filters and lets say you have a value of your Shutter speed close to 1/15 secs. Then looking at this table below you see that for an hour exposure you need 16 f-stops. You put two -8 or -10 and -6 and that’s it! You have two -10 stops filters (that is two ND 3.0’s)

General remarks Everyone is interested in Long exposure photography needs to know about Neutral Density Filters. I started with my previous post to give the general description of how to calculate exposure times and i think now it is time to give you a guide about these filters. I thought about presenting my ideas as a regular post but then i decided that it is more pedagogical to show my report in the form of questions. So this is the complete FAQ about neutral density filters. Some points before i start: I narrowed my search to only some brands i know or are widely known. If you think that a company not mentioned here deserves a quote please mail me and i will update my post, or simply add a comment below. Also if you found some errors concerning the description i gave to the various products or maybe i have forgotten some important characteristics of them please comment and i will also update my report. I will try to be objective concerning the use of these filters based on my personal experience or the experience of photographers i admire. Of course these ideas may not reflect the general belief but what can i do about this? It is just my humble report and my ideas after all. Still anyone wants to add his personal experience is welcomed to add his/her comments. What are the types of ND Filters? The ND Filter Screw-in Type: This type gives you the ability to screw in the filter itself into the camera lens and leave it there as a part of the lens, whenever you want to change it or take it off, simply screw it backward and remove it. The ND Filter Slot-in Holder Type: This type consists of a holder that fixed (screwed) on the camera lens and provide the ability to slide in any rectangle ND filter into the holder’s dedicated slot. Variable ND-Filters Screw-in Type: Density can be varied just by rotating the indicator from Min to Max Brand: Formatt Hitech, JJ-Long exposure kit What is a Graduated Neutral Density Filter (GND)? Half of this filter is clear and the other half goes from Light to Dark to the top of the filter. In the middle of the filter, the amount of filtering increases from 0 stops to 1 – 4 stops usually, towards the top of the filter. You will find them in these 0.3, 0.6 0.9, and lately 1.2 density factors (that is 1, 2, 3, and 4 f-stops reduction). Also the second important characteristic of these filters is the type of gray-graduation (light to dark) from the middle to top. A Soft Edge filter makes a soft gray-graduation while in a Hard Edge filter this graduation happens abruptly. Lets say we have a 4×6″ filter GND 0.3 as an example which is a 6 inches high filter and the bottom 3 inches of it are clear or 0 stop. If it is Soft GND then at the horizon line, it will gradually get darker towards the top from 0 stops to 3 stops of light filtering while in the Hard GND case we will have 3 inches of clear and 3 inches of 3 stop filtering. Graduated ND filters are used where there is a large dynamic range between the brightest parts of a scene and the darkest part of a scene. For example a sunset or a scene taken towards the sun where the sky is very bright compared to the land. Without a graduated ND filter you would have to select either the sky or the land to be properly exposed and sacrifice the other to be blown out (pure white) or under exposed (black). So a graduated ND filter will allow to get a much better sunset shot in a single frame, the alternative being to take multiple exposures and blend them together afterwards in Photoshop or Lightroom. There is also the case of Reverse Graduated filter where the darkest part of the filter is right in the middle of the filter and then gets lighter towards the top of the frame. Most common use for this filter is shooting a sunset when the sun is right on the horizon. You will be reducing the amount of light coming in from the horizon (the sun) and letting the light in from the other parts of the scene. Technical info: More easy-handling are the Soft grads and so more popular whilst hard grads are really only suitable in certain scenarios such as oceanic scenes where you can align the transition line along the horizon. It is important to be aware that GNDs are a compromise, it is not so certain where the transition can occur and if it is exactly where you want in a frame. That is why you end up many times with trees that are darker on the top than the bottom. Someone can say that it is always a matter of post processing but things become almost impossible  when the alignment of the horizon with a hard edge GND is problematic . Although usually the GNDs are plate filters there are also some Screw-in types. The main problem using Screw-In GNDs is that you have no flexibility playing with the vertical position of their horizon line. And this is a BIG disadvantage.   What is a solid ND filter? The whole filter is dark and has a specific density factor. Usually these filters are from 0.3-3.0 density factors (until 10 f-stops reduction) both in Screw-In type and Slot-In Holder type. They made from glass or optical resin, with an index of refraction nearly identical to the optical glass  (pretty much as the GNDs). Recently we had a new entrance from Singh-Ray. A solid ND filter with 15 f-stops reduction! This will be available in both types (starts from 450 USD) Technical info: All filters produce some optical vignetting with wide lenses, because around the edges and corners the lens is looking through the filter at an angle, so it is effectively thicker and therefore more dense. Again you can correct