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.
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. Hoya designates its 9 f-stop grey filter by x based on its filter factor which is somehow lower than the absolute value of x 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.
For example, a 3-stop ND filter ND 0. In the below table, you can clarify the properties of each filter. 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. 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. 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 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.
I am afraid no.. 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. If you want to achieve a 16 f-stops reduction use a and a 6- filter and not combinations like -6, -6, 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. In theory these tables should give you accurate exposure times.
For the ND filters up to 5- or 6-stops these charts works perfectly but the darker the ND filter, the less reliable these tables get. If you expose at 2 minutes, the image will be very underexposed. So what is recommended is to go 2 or even 3 stops further, using 4 or 8 minutes of exposure time. There are two possible reason for this:. Still i found from my experience that these tables are quite accurate even for ND 3.
You can always use the simple but trusted method of trial and error. You take an underexposed image using exposure time of 60 seconds? Take another one at 90 secs. This is overexposed somehow? You should take into consideration the fact that if you shoot at, say, seconds, adjusting the next shot to seconds is not going to make any significant change.
You should try with changes of or even secs to see differences. There are always some convenient applications for android, i-phones that help to solve all these problems in calculations and i will add here some links. Concerning the i-phone users there are some applications i am sure you can find even more like:.
Every scene is unique concerning the light conditions and the dynamic changes of light. There is no rule, no application, no table, no mathematical formula to describe the best way to calculate the exposure time in a long exposure shot. Use your instinct, imagination and experience and you will soon know even before you take a shot what will be the ideal exposure time for your photograph.
Still there are some special light conditions close and afters sunset where you have to choose the right settings for your camera since you will have maybe only one or two shots to take before the absolute dark.
All these special conditions among others will be discussed in detail during my workshops. I also think it is good to print these charts out and laminate them to keep in your camera bag for reference. You are so kind to offer this to the public many would protect their skills. Congrats to you for being so caring to other by sharing your work and your skills. I faced the same situation with my Tiffen stop ND filter. Manual exposure with calculated values have resulted in utterly under-exposed results.
Thank you for putting this on the internet. I printed out the charts and laminated them to keep in my photo bag. Just wanted to let you know I appreciate your work. Thanks a lot, great page and info.. I would like to add a small addition is to use the histogram when we guess the experiment the right exposure..
There is a very simple way to determine correct exposure with an ND filter, without having to do lengthy computations and having tables handy by the way, there is a host of mobile apps out there to do this: Thank you for this article and all the work you put into it, and for sharing your passion with us. I just printed this and am laminating it for my camera bag also.
Your email address will not be published. Notify me of followup comments via e-mail. Notify me of follow-up comments by email. Notify me of new posts by email. Enter your email address to subscribe to this blog and receive notifications of new posts by email. Calculating exposure times Posted on: 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 is the filter factor? What happens if we stack ND filters? 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. There are several ways of doing this: Cover the camera with a dark cloth. Use a small cap which is designed for this purpose. Concluding remarks Every scene is unique concerning the light conditions and the dynamic changes of light. Hope you enjoyed it and wait for your comments! Mike April 16, at 1: Took the LE guide out with me and seriously reduced all the guess work. Subramanya September 16, at 1: Karen Muraskin January 23, at 9: Moataz February 10, at 3: Jurgen Lobert June 2, at 5: Mike August 24, at 6: Ed Pendrous August 29, at 5: This is my new favourite website.
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