Leica SF20 Compensation

[primeval_forest]

Folks,

 

This may be a novice question - May someone tell the meaning of

compensation +/- button. I learned that the (+) means increase output

and (-) means reduce output. If this is correct, why is the panel of

SF20 display a greater distance for (-) and reduce distance for (+)

when compensate.

 

I was confused by this feature, which sounds contradicting to the

original concept I thought. (I play with it today in auto mode as my

M6TTL is with Leica AG.)

 

Are all flash unit work the same way?

[xav]

When you use (-) you don't really decrease output, you decrease the amount of light which will reach your film: Because you want less light on your film, the flash can provide this reduced amount of light at a greater distance.

[john_r._fulton_jr.]

It does sound confusing. When you set the strobe to plus 1-stop

compensation the strobe in A or TTL will put out one additional f-stop of light

above the normal output. Therefore the maximum distance is decreased since

it can only put out so much light. Conversely if you set minus 1-stop then the

strobe puts out less light and therefore the maximum distance increases since

it doesn't have to put out as much light.

I doubt that explains it clearly but it's about as well as I can do.

BTW, when I'm in edgy light that needs a bit of strobe I expose normal for the

available light and set the SF20 to minus 1 2/3 stops.

[hq]

Xavier, isn't this is something like reducing the flash output of the amount of light? The last time I tried mine with a Flashmeter; the more (-) given, the lower the flash output hence longer exposure was needed. The amount of light reaching the film isn't it something to control from our camera expensure via shutter and aperture? Scratching my head... how the flash can produce less light and yet achieve greater distance?

[jeremyt__]

the (-) button serves two functions related to your question. 1: In manual mode, it decreases the output of the flash. 2: when shooting in TTL, it underexposes by the number of stops indicated. To underexpose the subject, less light needs to reach it. Thus, your subject is "allowed" to be further from the camera. Another note: many people, including myself, choose to underexpose when shooting in TTL with the SF20. When inside, i usually go - 2.

 

-jeremyT

www.lifeinblue.com

[lutz]

Hmmm, try to see it this way: The (-) reduces the critical amount

of reflected light (measured by the auto-cell) necessary to

interupt further flash output. The lower the amount, the further the

distance from which it can be reflected.

[lutz]

Oops, while formulating three other posters have chimed in...!

Okay, so to further clarify: In practical terms you would choose to

lower that critical amount of reflected light in order to... sure,

underexpose. So, underexposure allows for a wider reach. As

others have pointed out, underexposure can be mandatory,

especially in night exteriors with faces in the foreground which

would burn out completely if not underexposed by several stops.

[lutz]

i.e.: ...if you weren't underexposing *the whole scene*.

[lutz]

i.e.: ...if you weren't underexposing the whole scene - as

indiscriminately measured by the cell.

[primeval_forest]

I guess you guys are really very knowledgeable. Although, I am still confused but trying to do some thinking...

 

There is something common in all post I guess was -

 

Like Lutz said - "The lower the amount, the further the distance from which it can be reflected". ... emm... Before I really try to intrepret, let me ask - What do reflected lights referring? I mean which light it refers to? The flash light or the ambient light? What has it got to do with distance in terms of reflected light?

 

I always thought the more power the flash output, the further the distance can be reach, hence luminating the object we are shooting and vice-versa. If (-) meaning reducing lights (so called underexposure), logically it should be only able to reach only a shorter distance.

 

But seems like you guys were say that reduce light to reach a further distance...? Guys, hope I am not too slow to get it!. Thanks.

[andrew1]

Flash control can seem puzzling. Lutz and others have given an

acurate description, but let me try to to put it into different words.

 

Strobe units have a given amount of power they can create and

translate into light with their flash tubes. This is sometimes

refered to as the unit's guide number. (Guide numbers are more

important when using a flash in manual mode, so don't worry

about that, but for our purposes a guide number indicates how

much light a unit can produce- the higher the guide number, the

more light it can put out.)

 

When you use a flash unit in automatic mode, that amount of

light can be used in one of two ways- it can give more intensity,

and give a smaller exposure aperture, say f11, or it can put that

light out at a further distance, but at a larger working aperture,

say f4. (Remember not to get confused about larger aperture

numbers being smaller openings, and vice versa.)

 

An important thing to remember about how a flash works in auto

mode (and this explains why the point about reflected light is

important) is that the unit puts out light until it's sensor "sees"

enough light being reflected back from the subject to create the

proper exposure- if you have the auto mode set to f8, the unit

puts out light until the sensor detects enough to make the

subject f8, and then cuts off the flash.

 

So, you have a unit that is capable of producing a given amount

of power. When you increase the power with the "+" this results

in a smaller aperture, again, say f11, but it can only be

sustained for a shorter distance because of the amount of power

it takes to generate this much light. When you reduce the

amount of power with the "-" button, the unit can sustain this

larger aperture exposure, say f4, over a much greater distance.

 

Also, Houquan, remember that shutter speed has no effect on

flash exposure in regards to how much light reaches the film- as

long as your camera can synch at the chosen speed- 1/50th or

slower for M/LTM cameras. Yes, there is the point about shutter

speed effects with flash, and ambient as either fill or main

exposure with flash fill, and these kinds of exposures require

use of shutter speed for proper exposure control, but with pure

flash, shutter speed has no effect.

[primeval_forest]

Guys,

 

How can I express my gratitude to all of you who are so superly technical. Thanks for sharing with me your knowledge. I find myself benefit from this thread. You guys teach me a lot and really clear my doubts that would otherwise lost. I only blame myself for being a slow learner... However, I sure you guys accelerated my learning process.

 

A Big thank you to all!