# Understanding Manual Flash

#### patrick76

##### New Member
I am new to flash photography or rather photography as I just took up this hobby few months ago.

I have read articles from the web and books and came across terms like Guide Number, Aperture, Flash to Subject distance etc. I am trying to understand how these terms are related to one and another.

I know that Guide Number = Aperture X Flash to Subject distance. For example, if I am using a Flash with Guide Number 58m and I need to select f/4 then my Flash to Subject distance becomes 14.5m. I may need to power down my flash to arrive at a practical distance for shooting, say 2m from the subject.

Am I correct to say that if I power down my flash then my Guide Number will be reduced accordingly as below?

Full power = 58m
1/2 power = 41m
1/4 power = 29m
1/8 power = 20.5m
1/16 power = 14.5m
1/32 power = 10.3m
1/64 power = 7.3m
1/128 power = 5.1m

Based on the above example, I should be selecting a power range from 1/64 to 1/32.

#### Andrew Ng

##### New Member
If you have a high end flashes, set to manual mode. At 90degree head position, adjusting power, aperture and iso and zoom setting on the flash will show you the distance flash can cover. Else, just shoot ttl.

#### ricohflex

##### Senior Member
Use this

GN Calculator | DPanswers

At f/4
1/64 Distance = 1.8m
1/32 Distance = 2.6m
Provided your ISO remains at ISO100.

To be exact for 2m, set to 1/32 and f/5.1

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#### patrick76

##### New Member
So more or less what I have done is correct?

Use this

GN Calculator | DPanswers

At f/4
1/64 Distance = 1.8m
1/32 Distance = 2.6m
Provided your ISO remains at ISO100.

To be exact for 2m, set to 1/32 and f/5.1

#### Edwin Francis

##### Senior Member
So more or less what I have done is correct?
Yes, but take 2 things into account:
- guide numbers are often inflated
- for modern hotshoe flashes, GNs are at maximum zoom setting.

#### patrick76

##### New Member
Yes, but take 2 things into account:
- guide numbers are often inflated
- for modern hotshoe flashes, GNs are at maximum zoom setting.
Thanks for highlighting!

How about for bounce flash? How do you guys normally go about setting your flash?

#### eleveninth

##### Senior Member
shoot by feel or get a meter. dun need remember all these numbers lol

#### Shizuma

##### Senior Member
theoretically a bounced flash will have to travel the distance between your flash , the bouncy surface and the subject. use that as guide reference.

note the light will be softer quality due to the reflected flash appearing to be from larger source.

#### ricohflex

##### Senior Member
So more or less what I have done is correct?
Yes.

Flash meters are quite reasonably priced. Get one. You can find a used flash meter at a fraction of the new cost.

#### silvermoon1407

##### New Member
Just shoot, review image, adjust, shoot again. You not using film right? Just take ur time and experiment and get the feel.

Of course, don't go cover weddings or events with manual flash haha

#### catchlights

##### Moderator
Staff member
Thanks for highlighting!

How about for bounce flash? How do you guys normally go about setting your flash?
depends on how high is the ceiling and what color of the ceiling.

you are in digital age now, and (flash) light meter is very cheap, get yourself a light meter or use gray card with your camera histogram.

btw, are you shooting with flash on camera? if yes, than use TTL or Auto mode on flash, it is very reliable and convenient.

#### eleveninth

##### Senior Member
amd dont bounce into the sky

#### daredevil123

##### Moderator
Staff member
Yes, but take 2 things into account:
- guide numbers are often inflated
- for modern hotshoe flashes, GNs are at maximum zoom setting.
Not necessarily. Some speedlights' GN are stated at 35mm zoom setting.

#### patrick76

##### New Member

I am learning so as far as possible I try to use manual flash. I know that I need to use E-TTL if the flash-to-subject distance is ever changing.

How about using a diffuser? Is it the same as using bounce flash since the intensity of light is reduced as well?

#### CorneliusK

##### Senior Member
The concept of GN is useful for comparing approximate power between flashes, and it is also useful for understanding the relationship between flash power and the distance between the flash and the subject.

But in real life, there are many other factors which can affect the final amount of light on your subject - i.e. the zoom setting of the flash, whether you are putting the flash through a modifier, the amount of ambient light already falling on your subject etc.

The most reliable method to figure out the flash power setting would be (like some have already mentioned) to use a light meter, but the trial and error method also works well. I used to use a light meter but I have gotten so good at trial and error method that I have stopped using it entirely.

#### CorneliusK

##### Senior Member

I am learning so as far as possible I try to use manual flash. I know that I need to use E-TTL if the flash-to-subject distance is ever changing.

How about using a diffuser? Is it the same as using bounce flash since the intensity of light is reduced as well?
You need to read up a lot more on flash usage.

The basic principle behind bounced flash is to increase the size of the light source to the subject. Instead of lighting the subject up directly with the flash head, which is a very small light source and thus will cause harsh shadows on the subject, you direct the flash on a wall or ceiling and light it up. The light from the ceiling will then bounce of it and some of it will fall on your subject. As the light source is now much larger, the light falling on your subject will look less harsh and usually more pleasing looking.

A diffuser cap on the flash will scatter its light in all directions. If you are in a small room, then it will give an effect similar to bounce flash by bouncing the light off all the walls and ceiling. But if you are not in a room, or near any surface suitable for bounce flash, then it will have very little effect besides wasting your battery power.

#### pentlynk

##### Senior Member
Hi TS, to add to this discussion, all that has been mentioned refers to the proper exposure for the subject, which is usually in the foreground. I think you may need to consider separately for the background (ie how much exposure you want for the background independent of the subject). My method is mostly trial and error, but my baseline is usually a setting (selected ISO and shutter speed) that illuminates the background the way I want (and I will adjust aperture depending on subject distance).