# Thread: How to determine distance of subject from the camera?

1. ## How to determine distance of subject from the camera?

Hi! all. I'm trying to learn to take photos with a external flash. Read that when shooting with flash there is this Guide Number that i must set & the formula to achieve the number requires me to know the distance & the f/stop number. but problem is how can i figure out the distance of the subject from the camera? I'm using a Canon EOS 650D with Sigma 18-250mm HSM DC Macro lens.

2. ## Re: How to determine distance of subject from the camera?

distance scale, else a measuring tape.

3. ## Re: How to determine distance of subject from the camera?

Mental math helps.

Do note though that most modern cameras use the distance reported to them by the lens, or the use of a pre-flash, to determine the flash output. But it seems like you want to discard all the helpful technology and go full manual?

4. i'm new to flash photography. what i need to know is if i were out on outings with friends or at an event & i wish to take pix with the external flash how can i determine in order to use the appropriate amount of flash. i see those ppl who take pictures at events, weddings with their flash they dun even need to use any tool to measure distance. but results are great. how do they do that?

5. use inverse sq law.

6. ## Re: How to determine distance of subject from the camera?

Originally Posted by wolvie77
i'm new to flash photography. what i need to know is if i were out on outings with friends or at an event & i wish to take pix with the external flash how can i determine in order to use the appropriate amount of flash. i see those ppl who take pictures at events, weddings with their flash they dun even need to use any tool to measure distance. but results are great. how do they do that?
ettl/ ittl (auto flash in C and N systems) in AV mode ...? shoot RAW, a little bit of lightroom, PS, etc...

anyway if you really insist on Manual Flash (hopefully in a static lighting situation),

to make sense of your GN you will need to compare ISO rating as well...GN is nominally based on ISO 100 (do correct me if i am wrong).
eg
at ISO 800 you are 1x2x2x2 = 3 stops faster hence your real GN is effectively 3x nominal ISO 100 (the quoted) GN.

to find the distance you may wish to use distance scale if you have distance marking on your lens. legacy lenses such as M42, etc etc will have it.

Otherwise, you could use a laser rangefinder (if want very precise), or use a measuring tape . Sometimes it may not be worth the trouble as nominal GN may not be the real effective GN you have in your hands... check reviews for your flashgun to see ...

7. Originally Posted by wolvie77
i'm new to flash photography. what i need to know is if i were out on outings with friends or at an event & i wish to take pix with the external flash how can i determine in order to use the appropriate amount of flash. i see those ppl who take pictures at events, weddings with their flash they dun even need to use any tool to measure distance. but results are great. how do they do that?
It comes with experience.

Measuring of distance will work on paper but in reality when you are on location and shooting your photos, such theories can only be used as general guideline. Why? Because it is not practical to keep measuring distance when in a dynamic environment.

If you increase your distance by 2x, you need to up flash power by 4x, and the list goes on.

For weddings some photographers may be using TTL, which negates the need to keep adjusting the flash power when the distance changes. Others may be so experienced enough to dial in the right amount of flash power which is needed for the shots.

In short, just read up on the basics of flash photography, then go out and shoot your photos. Practice makes perfect. Of cause' you can bring your measuring tape along to reinforce the theories but I bet you will keep the tape in your bag after a few use.

8. Erm. so if i buy a auto mode flash unit, does that mean i do not have to worry myself with the guide number?

9. Originally Posted by wolvie77
Erm. so if i buy a auto mode flash unit, does that mean i do not have to worry myself with the guide number?
Partly true. It only means the flash power will be calculated automatically. If you think you need more or less of the power, then just dial in the flash exposure compensation accordingly.

10. ## Re: How to determine distance of subject from the camera?

Originally Posted by wolvie77
Erm. so if i buy a auto mode flash unit, does that mean i do not have to worry myself with the guide number?
basically yes, you can depend on the camera/flash TTL flash most of the time

but at some situations, you need to use some flash compensation, eg you shoot a wedding couple in white gown white suits, cutting a huge white dummy cake at the ballroom stage, the camera meter will be fool by all the white in the scene, so you need set flash compensation to +2

or you shoot wedding couple at outdoor under shade with flash fill in, to give natural fill in effects, you will set flash compensation to -2.

these are not written in books, is by experiences.

11. ## Re: How to determine distance of subject from the camera?

Please take some time to read your camera manual and this link: Flash Photography with Canon EOS Cameras - Part I.
It's all there and for a beginner I would strongly recommend using the ETTL technology that you bought already with your camera.
For balancing ambient and flash you can follow up here: Strobist: Lighting 101: Balancing Flash and Ambient, Pt 1

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