The new club season got off to a good start with an informal meet at Devils Bridge, Kirkby
Lonsdale were it was great to meet a number of new members, plus see some of the old regulars. The theme of the evening seemed to be centred around using big stopper filters down by the river for the majority, while a few wandered up into Kirkby Lonsdale for to capture a few photographs around the streets.
For those new members, who may not be familiar with the term, a "big stopper" is designed to block light from reaching the sensor allowing for a much longer exposure time for a given aperture. Very useful for allowing photographs to show motion of objects in situations where the light levels are bright and beyond the cameras capability to produce a long enough exposure time.
They are simply darkened pieces of glass or perspex, they are supposed to neutral in colour so they don't alter the colour balance of the image. However, this is not always the case as my big stopper makes everything look distinctly greenish, requiring alteration of the colour balance in post processing, more about that later.
They come in varying strengths, anything from 2 stops little stopper, to 10 stops big stopper, and beyond. Remember, each stop represents a doubling of the exposure. Assuming we are shooting at a fixed aperture, it will be the shutter speed that gets longer to allow sufficient light to reach the sensor.
Example: Normal exposure of a sunny scene at ISO 100 is f11 @ 1/250th of a second.
With a 2 stop filter exposure becomes..........................................f11 @ 1/60th of a second.
With a 10 stop filter exposure becomes.......................................f11 @ 4seconds.
Each shutter speed in the table below is twice as long as the shutter speed preceding it.
250th -125th -60th -30th -15th -8th -1/4 -1/2 -1 -2sec -4sec-8sec-15sec-30sec-1min-2min
Starting for 1/250th of a second two stops, equals two jumps to the left. six stops, six jumps to the left, ten stops equals 10 jumps to the left and so on.
So we can see, stoppers can make a huge difference to the exposure time. They allow us to
capture the motion of objects in our images in situations where it may not have been otherwise possible. Flowing water and rippled water surfaces are the usual favourite subjects, as are clouds when in motion. They can also be used in situations where people milling around a scene can be made to disappear by using a long time exposure. They can also be used in conjunction with flash units to darken a background whilst lighting the foreground subject with light from the flash.
Key points to remember while using them;
1. Ensure the "stoppers" are fitted hard up to the filter holder with no airspace in between. They usually have a little foam gasket which blocks the space between filter and holder preventing extraneous light from creeping in around the edge ruining the image. When using multiple filters, ensure the stopper is the one nearest the camera, and use a piece of card or cloth draped over the top of the filter holder to block any light getting into the gaps between each filter, for the same reason. Use your hands as a shade if necessary.
2. It is easier to compose and focus your image without the stopper fitted. Frame your composition with the camera on a tripod, ensuring you focus the image correctly. Manual focus is best, if not essential, most camera auto focus system are not able to operate correctly through such a dark filter.
3. Meter the scene with the cameras exposure meter without the filter fitted and calculate the correct exposure using the Lee Big Stopper app, or Photopills app. Camera exposure meter might work correctly but it could also be confused so it pays to do a check. Set the camera to Manual setting for aperture and shutter speed. Otherwise, set aperture priority but be aware, the longest your camera could expose may only be 30 seconds in automatic mode. Exposures longer than 30 seconds may mean the camera needs to be set in the B setting for shutter activated manually using a remote trigger or cable release.for the require exposure time. In fact using a remote trigger or cable release helps prevent camera shake when triggering the shutter. If you don't have one, use the cameras self timer.
4. Not all stoppers are created equal. My 10 stop LEE big stopper is more like 11 stops, not 10. Its colour is not neutral either. It gives the images a greenish cast. Get to know your stoppers actual strength by doing some trial exposures of varying lengths around the exposure you calculated it should give. Only needs doing once, and you can apply your correction each time you use the filter in future. Regarding colour cast, take one exposure with something white or grey in shot with which you can adjust your white balance. Again, it only has to be done once for a given location. I carry a grey card in my bag which allows me to create a white balance each time I shoot in a new location. The white balance corrected in this image can then be applied to all subsequent images taken at that location during post processing. (The advantage of shooting RAW files).
I think that covers about everything, if you need to ask a question or think I've missed something, leave a comment on the blog.
Looking forward to the next meeting, Friday the 21st September, at the Memorial Hall, Burton in Kendal, don't forget to bring some images on a USB stick.
See you there.