Guide Numbers and sync speed in flash.

[Don Hoey]

These are an indicator of power. High numbers = more power. More power = more reach.

With flash exposure is calculated based on flash to subject distance not camera to subject distance.

If the guide number is known ( usually stated in Metres at ASA / ISO 100 ) then aperture can be manually calculated by dividing the guide number by the distance in metres. You can therefore, as long as you know the guide number of your flash work out its reach within the limits of your lens.

Sync speed is the MAXIMUM speed that the shutter can run when using flash. At this speed or lower the shutter curtain on an SLR will be fully open when the flash fires. The actual duration of the flash is very short - thousanths of a second. Cameras with high sync speeds can cope with a wider range of subject matter when balancing daylight and flash.

I have attatched a graphic by way of illustration.

[Don Hoey]

You can quite cheaply arm yourself with a powerful second hand flash to add additional light or as a means of lighting within a tighter budget.

This is NOT a flash that will be directly attatched to your camera. The flash is to be remotely triggered by a camera system flash. Exposure will be a touch of trial and error.

There are now growing numbers of older flashguns on the secondhand market that are seriously good deals as they do not link into the modern high tech camera metering systems and people are trading in for those that do.

Trigger mechanisms are refered to as ' slaves '. When they register the light from a flash they trigger whatever flash they are attatched to. These come in various forms. When buying one the only real consideration is how in use it will register the ' master ' flash. If the camera with the master flash is to be some distance away then a long lead will be required to permit the slave to be situated near the master flash. These leads are low tech and therefore cheap.

For the remote flash get the highest Guide Number you can within your budget. When we come to the subject of diffusing the flash we are going to loose a fair bit of its light output.

I have attatched 2 pics of remote flashguns I use that also show a slave attatched. Both of these are well out of date by modern standards and are good buys. Cruising the net I have seen the 45CL for around £70. Mine was nearly £200 new.

[Canis Vulpes]

Don,

I cant thank you enough for the effort and information in these posts. Lots to consider but certainly not daunted :-)

Stephen Fox

[robski]

Good write up Don - I have been following it but I have been side tracked with a plumbing problem with my immersion heater - Any plumbing experts here !! Spent 2 days on it and the dam thing still not right !!

A couple of small points to add to this thread.

1) warning about putting old models of flash gun on digital cameras with a Hotshoe. Most of the older guns have a trigger voltage in excess of 200V which would fry the camera electronics.

Fortunately for me the Canon 20D has a Flash sync lead connection under the rubber flap rated at 250V.

2) As you have mentioned modern flash systems work on TTL metering basis. A number of these systems work by emitting a pre-flash (Canon E-TTL for example) to get some idea of the scene. Unfortunately the optical trigger units you illustrated attached to the remote flash are fired too soon by the pre-flash.

However there are a few new models of low power guns available for use as an optical slave which can be set to ignore the pre-flash.

Rob

[wolfie]

Rob I overcome all these problems by using flash only in manual mode, with the preflash switched of. Just as a matter of interest have a look at the photo I've uploaded, called watersplash. this gives some idea of the delay of the slave flash firing.

Not sure if a pasted link will work, but here goes http://www.worldphotographyforum.com...7&limit=recent

[robski]

Originally Posted by wolfie

Rob I overcome all these problems by using flash only in manual mode, with the preflash switched off.

Wolfie what model of flash gun ?

Rob

[wolfie]

Rob I have two Sigma guns, the Super and the ST, (ST is the basic one) The Super will act as a slave to the ST, but not the other way around.

Harry

[Don Hoey]

Originally Posted by robski

1) warning about putting old models of flash gun on digital cameras with a Hotshoe. Most of the older guns have a trigger voltage in excess of 200V which would fry the camera electronics.


Rob

I did mention this in post #3 but no harm in restating it.

This is NOT a flash that will be directly attatched to your camera. The flash is to be remotely triggered by a camera system flash. Exposure will be a touch of trial and error.

Harry your lighting technique is superb in your Christmas Cacti photo. The water droplets could so easily have been killed by flash. It was looking at glossy product photography that got me interested in using off camera flash in the first place. Your pictures will help to take the fear factor out of it, as some members are bound to want to have a go when they see this.

Don

[Saphire]

Don here are the photo's of the flashgun.

Christine

[Don Hoey]

Originally Posted by Saphire

Don here are the photo's of the flashgun.

Christine

Christine,

Thats super. All you need to have a play today is something to take a picture of. Not too shiney. Even a dull coin will do.

The biggest problem you have probably found is focusing as you can only really do that by moving the camera closer to the subject, and fine focus is hard to get. ( hence my comment on a fine focus rail ).

We can do it another way as an indoor job.

Set yourself up to take the picture with the subject on top of something you can easily slide on a table top.
Set your remote flash at an angle to the subject about 18 inches away. A second tripod helps but you may be able to tape it to something. Lay a piece of kitchen towel or other white tissue over the flash to diffuse it.

Set up and take a picture starting at f8. If a lot too bright try f16. If still too bright move the flash to 2 feet and go again. - Trial and error job here.
When you are happy with the exposure hold a piece of normal white printer paper on the other side of the subject to the flash angling it to bounce light back in and take your picture.

Two pics attached. ......... HAVE FUN


Don