Gray ND
Neutral Density filters have several uses and offer the possibility to achieve otherwise unachievable results. ND filters appear gray and reduce the amount of light reaching the film. They have no effect on color balance. They have four main uses:
1 To enable slow shutter speeds to be used, especially with high speed films, to record movement in subjects such as waterfalls, clouds, or cars.
2 To decrease depth of field by allowing wider apertures to be used, which helps separate subjects from their background.
3 To decrease the effective ISO of high speed film (above ISO 400) and allow it to be used outdoors in bright situations.
4 To allow cine and video cameras (which have fixed shutter speeds) to film subjects such as snow, sand or other bright scenes which could cause overexposure.
Neutral Density factors:
5 By using a ND over the lens or flash, you can bring it much closer without it overpowering the image.
Cokin 152 (exposure adjustment = approx. 1 stop, reduces ISO 1/2)
Cokin 153 (exposure adjustment = approx. 2 stops, reduces ISO 1/4)
Cokin 154 ("P" series only; exposure adjustment = approx. 3 stops, reduces ISO 1/8)
Using Neutral Density Filters With Flash
by: Lionel Armstrong
Many times we would like to do some macro shots and this requires us to get real close to our subject. Most of the time the light is dim enough to where we have to shoot with maximum aperture and still have long exposure times. The problem here is a very small depth of field. The solution to getting more depth of field when shooting so close is to use flash. But what happens when you find out that the manual says that you have a minimum distance of about 5-6 feet? This is the time to use a neutral density filter on the lens or flash.
To get proper exposure you could leave the flash on automatic and use a 2 stop ND filter (Cokin 153) on the flash to cut the distances in half. If you want to get closer or use an f-stop other than what is available for automatic flash you need to use the manual setting and calculate the flash to subject distance using guide numbers.
In the manual for your flash there will be a small chart or a single listing with a guide number (in feet) for a particular ASA of film. An example would be the Vivitar 283 has a guide number of 120 (feet) with 100 ASA film. Now with that number and either the f-stop you want to use or the distance required you can find out the other number.
Here are the formulae:
GN / DISTANCE = F-STOP
GN / F-STOP = DISTANCE
To find an unknown guide number for your flash you will need to do some test exposures and use this formula for the image with the perfect exposure:
F-STOP X DISTANCE = GN
By looking at the info for the Vivitar 283, we find that with 100 ASA film the automatic operating ranges give the closest flash to subject distance of 2 feet in the PURPLE mode at an f-stop of 11. If we would like to use f 22 for maximum depth of field, we would need to use the manual setting (full power) and the flash at a distace of just under 5+1/2 feet.
120 / 22 = 5.4545454 ( 5.5 feet)
If we don't have the ability to keep the flash so far away, we need to bring it closer in by using a ND filter to cut down the intensity of the flash exposure. This can be done by putting the filter on the lens or on the flash. It would be better to put it on the flash so that you can have a brighter image in the viewfinder for focusing
Let's assume you have a flash bracket that allows you to have it about 1 foot away from your intended subject and your lens can focus on it that close. By now using the distance in the formula we find the required f-stop is 120. The nearest full f-stop to that is f 128 and we don't have that on the lens so we need to cut the flash exposure by 5 stops. We could do this with a P153 and P154 together on the flash or we could use a 2X teleconverter and a P154. This way would give the added magnification without changing your closest focus. Either way your effective f-stop is 128 and the flash to subject distance of 1 foot will give perfect exposure. In my case, I use an optional vari-power module to cut the flash exposure by 5 stops and not worry about a ND filter.
The image below is one that I did of an aphid crawling along a dead leaf. I had a 2X teleconverter and a reversed 28mm wide-angle lens with a P026 Warm filter taped to the rear of the lens to make the small color correction for the flash. I set the lens to f 22 and also needed to compensate 2 stops for the teleconverter and another 3 stops for the reverse adapter. My effective f-stop was f 128. I wanted to have a slightly shorter flash duration to totally freeze the subject and to also quicken the flash recycle time so I set my vari-power module to 1/4 power and compensated another 2 stops. I now needed the flash to subject distance for f 256 which came out to just over 5+1/2 INCHES!
120 / 256 = 0.46875 feet (5.625 inches)
I mounted my flash to a mini tripod and adjusted it's position so that it would be the proper distance from the subject and attached a PC cord to sychronize it with the camera shutter. I then needed to open the lens 1/3 stop to compensate for the P026 Warm filter. I needed to pre-lock the aperture control lever in place on the lens (before taping the P026 in place) and in order to see the dim image in the viewfinder, I had a bright lamp aimed at the subject and a black cloth over my head and the camera. The light from the lamp was not enough to show on the film but was needed for focusing. I waited for the aphid to get in the right spot and then pressed the cable release.
* Always remember to use the fastest shutter speed that allows flash syncronization.
Photo by Ken R Sheide
Cokin #21 (80B),Cokin #154(0.9ND).
"The sea crashing onto a rocky beach on Terceira. This was shot from above just after sunrise so I had to use a neutral density filter to slow the exposure enough." Ken R Sheide
The photo shown on this page is the ownership of Photographer Ken R Sheide and is not to be distributed in any way which may violate his copyright without permission from Ken R Sheide .