Distance scale on Af 540 FGZ

vorstem2 · 05-28-2012, 11:42 PM

When I put the AF540FGZ in a mode I get an effective distance range, how do I know where to put the flash within that range to get a proper exposure

calsan · 05-28-2012, 11:50 PM

If it's showing say, 10m, that means that the beam of light will effectively illuminate 10m from the front of the flash unit for your given aperture.

vorstem2 · 05-29-2012, 12:03 AM

Calsan it for instance gives a range, at say F8 it would typically show 0.7 to 8m. I see on a Nikon flash for instance it gives a fix distance, but on Pentax it gives a range. Does this mean I can place the flash at 0.7 m and get the same exposure as when I place it at 8m. that cannot be true because what about the inverse of square law.

stanislav · 05-29-2012, 12:10 AM

The distance scale gives you maximum distance that flash can illuminate for a certain combination of ISO, aperture and focal length of the lens. The flash than calculates the power required to illuminate subject itself. If the subject is further away than the maximum shown at the scale, then the subject is going to be underexposed since the maximum flash power is not going to be enough to give propper illumination. If the subject is closer, the flesh should cut the power itself when it detects that certain amount of light has bounced back to the detector in the flash unit.

The focal length is used to "zoom" the flash beam so that it does not illuminate parts of scene which are not going to be captured in the frame. The difference between "35mm" and "digital" settings of the zoom are due to APS crop factor, which effectivelly givies narrower field of view, thus the focal length showed varies between the two.

vorstem2 · 05-29-2012, 12:21 AM

Stanislav, so if I put the flash closer to the subject, less power would be used by the the flash than if I put it at say 6m.

stanislav · 05-29-2012, 12:29 AM

Correct! Moreover, it will need less time to recharge as well.

vorstem2 · 05-29-2012, 12:44 AM

Thanks this helps me a lot already as I am struggling with the flash off camera

calsan · 05-29-2012, 01:43 AM

Inverse of square law?? You need to watch this video and get rid of the excess unnecessary information!

Youtu.be

**Lowell Goudge** · 05-29-2012, 05:56 AM

Also note that A mode is being used most likely because the lens you are using does not have aperture contacts required to support P-TTL. This means you are using the flash's internal sensor to control the intensity, not the camera. The distance range is the distance from Minimum power at the short end, to Maximum or full power at the long end) for the fixed settings of your camera and lens ISO (which is fed automatically via the camera, but you need to check this because they do not always update correctly) and aperture which you are responsible to coordinate between flash and camera

The other thing to remember, with the flash is to consider ALL of what is in frame, with respect to distance and illumination.

Looking at the simple sketch in the video posted by calsan, if your subject is at 2 meters and the flash correctly exposes the subject, a background at 4 meters will be under exposed by 2 stops, i.e. the difference between F16 to F8

Positioning your subject relative to the background is important in terms of using this difference in lighting to establish depth in the image.

stanislav · 05-29-2012, 07:07 AM

Oh well, I tried to keep it simple but....

Let's put it this way: A mode on flash will work well if:
1. You keep flash at the camera or very close to it (so that distance from flash to subject is similar to distance from camera to subject)
2. You point the flash straight ahaed (i.e. no tilt, no swivel, no bounce).

In all other cases you'll have to manually compensate for the effect you are interrested in, so I guess that the manual mode could actually work better where you control the actual output power from the flash.

Now to make it more complicated and to mention inverse square law, it applies as it is in open areas. For instance if you are in a small room with white blank walls (a toilet for instance), then more or less all the light from flash will eventually get back to the camera/lens regardless of where you point the flash. However if you are in the open or huge rooms with non-reflective surfaces (dome of a church, football stadion) than only directly lit subject is going to be visable with everything else left dark.

Now to complicate even more: inverse square law is an approximation. It says basically that the measured intensity of light from a point source (which illuminates evenly in all directions) is inverse proportional to the square of the distance from that light source. "Law" as such is rather intuitive if you think of a box wide a side of say 1 meter (or foot), and put the light source in the middle of it. Then light comming to one of the sides is covering 1 square meter. Which means thatall available light is spread on the surface of the box of 6 square meters. Now you let the box "grow" so that the side is 2 meters instead of one, than one side i 4 square meters which means that the same amount of light illuminating 1 squeare meter now has to illuminate 4 square meters (total of the whole box is 24 square meters). So just by doubling the distance the measured intensity is divided by 4. (Actually instead of the box you should think of the "sphere" which gives different numbers, but general relation still holds). Now a point light source evenly spreading on all sides is just an approximation, but it holds good in many cases being it a candle, a light bulb, moon, sun, stars or galaxies.

Note now that flas does NOT enter that definition since the light is focused in one direction and/or one area. Same holds even for scene lightning or any other source of focused light (laser for instance). So the question which arizes is what does inverse square law have to do with flashes when those are sources of focused illumination. The answer to that qustion is that an object (all objects) illuminated spread light in the same fashtion the point light sources do (of course appart from mirrors or objects with mirror-like surfaces).

So inverse square law refers actually to the intensity of the light reflected from the object that comes back to camera.

If you would use a point light source (say a flash with a diffuser) than the law would actually be inverse 4'th power law since the light intensity is inverse squared as a function of distance between the light source (diffused flash) and the object, and then inversed squared again as a function of distance between the object and the camera. This is actually a simplified (or oversimplified) example of radar equation (radar from a single source with single detector) which is another example for usage of same principles as in photography (both radar and light are electromagnetic waves).

**Lowell Goudge** · 05-29-2012, 08:21 AM

stanislav your second point is not quite right, for 2 reasons.

First , Many Auto flashes, including the AF540 have a swivel and tilt head, so as long as the sensor faces the subject, and the sensor is the same distance away from the subject as the flash is, then the metering will work correctly. the only thing to be considered is that the total light path of the flash respect as an approximation 2 times the direct sensor to subject maximum distance.

Second, and I would not use the AF540 as an off camera flash for this reason, you can get for some auto flashes, a remote sensing head so that the sensor is on the camera shoe, and the flash on a long leash. I have one for my sunpak autozoom 3000, (which has a GN=30 meters) and this allows quite a separation between camera and flash, but you could still use this with a remote trigger, as long as the trigger and sensor head respect the subject to camera distance.

stanislav · 05-29-2012, 08:35 AM

Lowell, I totaly agree with you! It does not matter how specific I am trying to be, there is still some misinterpretation possible ...

Well to try to be more specific here I go once again....

AF540 is a zoom flash, and zoom as such has function only (well in most cases anyway) if you point it straight to the subject. Moreover the distance shown on the flash has also no meaning of you "bounce" flash of some surface. In that case yes, the flesh is going to work fine provided it is powerfull enough to make enough light (depending on the geometry, size and reflectiv caracteristics of the room). There is also an option to have a part of scene illuminated much more than the rest and so on. It is useful if you want to experiment and if you know what you want to obtain.

Remote sensing is nice option, it certanly gives more options in lighting the scene, I totaly missed that option. On the other hand if you have all controlled in such detail, you can as well go all manual, perhaps with several "cheap" flash slave units or some combination of different light sources. Anyway knowing the "theory" behind it will not make you worse artist or photographer

**Lowell Goudge** · 05-29-2012, 09:04 AM

Originally posted by stanislav

Anyway knowing the "theory" behind it will not make you worse artist or photographer

How true...

vorstem2 · 05-31-2012, 04:22 AM

Gosh now I am totally confused. It seems off camera flash is fairly complicated with Pentax

**Lowell Goudge** · 05-31-2012, 04:54 AM

off camera flash in any mode other than P-TTL is always difficult because there is no way for the camera to know what is going on and make adjustments.

if you were doing off camera flash in P-TTL mode, the camera and flashes would sort this out for you, but in A mode, you need to have a lot of things set correctly including the retention of subject to flash and subject to camera distance, so that the flash's sensor can attempt to meter as if it were still on the camera.

It is not all that hard, and digital helps because you can see your mistakes at zero cost, but it does take some basic calculations or estimations for each flash, based upon the distances you use.

05-28-2012, 11:42 PM	#1
vorstem2 Inactive Account Join Date: Apr 2009 Location: Roodepoort Posts: 10	Distance scale on Af 540 FGZ When I put the AF540FGZ in a mode I get an effective distance range, how do I know where to put the flash within that range to get a proper exposure

05-29-2012, 08:35 AM	#12
stanislav Senior Member Join Date: Jan 2010 Location: Stockholm Photos: Albums Posts: 125	Lowell, I totaly agree with you! It does not matter how specific I am trying to be, there is still some misinterpretation possible ... Well to try to be more specific here I go once again.... AF540 is a zoom flash, and zoom as such has function only (well in most cases anyway) if you point it straight to the subject. Moreover the distance shown on the flash has also no meaning of you "bounce" flash of some surface. In that case yes, the flesh is going to work fine provided it is powerfull enough to make enough light (depending on the geometry, size and reflectiv caracteristics of the room). There is also an option to have a part of scene illuminated much more than the rest and so on. It is useful if you want to experiment and if you know what you want to obtain. Remote sensing is nice option, it certanly gives more options in lighting the scene, I totaly missed that option. On the other hand if you have all controlled in such detail, you can as well go all manual, perhaps with several "cheap" flash slave units or some combination of different light sources. Anyway knowing the "theory" behind it will not make you worse artist or photographer Last edited by stanislav; 05-29-2012 at 09:29 AM.

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05-28-2012, 11:50 PM	#2
calsan Pentaxian Join Date: Nov 2008 Location: Perth, Australia Photos: Gallery Posts: 1,548	If it's showing say, 10m, that means that the beam of light will effectively illuminate 10m from the front of the flash unit for your given aperture.

05-29-2012, 12:03 AM	#3
vorstem2 Inactive Account Join Date: Apr 2009 Location: Roodepoort Posts: 10 Original Poster	Calsan it for instance gives a range, at say F8 it would typically show 0.7 to 8m. I see on a Nikon flash for instance it gives a fix distance, but on Pentax it gives a range. Does this mean I can place the flash at 0.7 m and get the same exposure as when I place it at 8m. that cannot be true because what about the inverse of square law.

05-29-2012, 12:10 AM	#4
stanislav Senior Member Join Date: Jan 2010 Location: Stockholm Photos: Albums Posts: 125	The distance scale gives you maximum distance that flash can illuminate for a certain combination of ISO, aperture and focal length of the lens. The flash than calculates the power required to illuminate subject itself. If the subject is further away than the maximum shown at the scale, then the subject is going to be underexposed since the maximum flash power is not going to be enough to give propper illumination. If the subject is closer, the flesh should cut the power itself when it detects that certain amount of light has bounced back to the detector in the flash unit. The focal length is used to "zoom" the flash beam so that it does not illuminate parts of scene which are not going to be captured in the frame. The difference between "35mm" and "digital" settings of the zoom are due to APS crop factor, which effectivelly givies narrower field of view, thus the focal length showed varies between the two.

05-29-2012, 12:21 AM	#5
vorstem2 Inactive Account Join Date: Apr 2009 Location: Roodepoort Posts: 10 Original Poster	Stanislav, so if I put the flash closer to the subject, less power would be used by the the flash than if I put it at say 6m.

05-29-2012, 12:29 AM	#6
stanislav Senior Member Join Date: Jan 2010 Location: Stockholm Photos: Albums Posts: 125	Correct! Moreover, it will need less time to recharge as well.

05-29-2012, 12:44 AM	#7
vorstem2 Inactive Account Join Date: Apr 2009 Location: Roodepoort Posts: 10 Original Poster	Thanks this helps me a lot already as I am struggling with the flash off camera

05-29-2012, 05:56 AM	#9
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	Also note that A mode is being used most likely because the lens you are using does not have aperture contacts required to support P-TTL. This means you are using the flash's internal sensor to control the intensity, not the camera. The distance range is the distance from Minimum power at the short end, to Maximum or full power at the long end) for the fixed settings of your camera and lens ISO (which is fed automatically via the camera, but you need to check this because they do not always update correctly) and aperture which you are responsible to coordinate between flash and camera The other thing to remember, with the flash is to consider ALL of what is in frame, with respect to distance and illumination. Looking at the simple sketch in the video posted by calsan, if your subject is at 2 meters and the flash correctly exposes the subject, a background at 4 meters will be under exposed by 2 stops, i.e. the difference between F16 to F8 Positioning your subject relative to the background is important in terms of using this difference in lighting to establish depth in the image.

05-29-2012, 07:07 AM	#10
stanislav Senior Member Join Date: Jan 2010 Location: Stockholm Photos: Albums Posts: 125	Oh well, I tried to keep it simple but.... Let's put it this way: A mode on flash will work well if: 1. You keep flash at the camera or very close to it (so that distance from flash to subject is similar to distance from camera to subject) 2. You point the flash straight ahaed (i.e. no tilt, no swivel, no bounce). In all other cases you'll have to manually compensate for the effect you are interrested in, so I guess that the manual mode could actually work better where you control the actual output power from the flash. Now to make it more complicated and to mention inverse square law, it applies as it is in open areas. For instance if you are in a small room with white blank walls (a toilet for instance), then more or less all the light from flash will eventually get back to the camera/lens regardless of where you point the flash. However if you are in the open or huge rooms with non-reflective surfaces (dome of a church, football stadion) than only directly lit subject is going to be visable with everything else left dark. Now to complicate even more: inverse square law is an approximation. It says basically that the measured intensity of light from a point source (which illuminates evenly in all directions) is inverse proportional to the square of the distance from that light source. "Law" as such is rather intuitive if you think of a box wide a side of say 1 meter (or foot), and put the light source in the middle of it. Then light comming to one of the sides is covering 1 square meter. Which means thatall available light is spread on the surface of the box of 6 square meters. Now you let the box "grow" so that the side is 2 meters instead of one, than one side i 4 square meters which means that the same amount of light illuminating 1 squeare meter now has to illuminate 4 square meters (total of the whole box is 24 square meters). So just by doubling the distance the measured intensity is divided by 4. (Actually instead of the box you should think of the "sphere" which gives different numbers, but general relation still holds). Now a point light source evenly spreading on all sides is just an approximation, but it holds good in many cases being it a candle, a light bulb, moon, sun, stars or galaxies. Note now that flas does NOT enter that definition since the light is focused in one direction and/or one area. Same holds even for scene lightning or any other source of focused light (laser for instance). So the question which arizes is what does inverse square law have to do with flashes when those are sources of focused illumination. The answer to that qustion is that an object (all objects) illuminated spread light in the same fashtion the point light sources do (of course appart from mirrors or objects with mirror-like surfaces). So inverse square law refers actually to the intensity of the light reflected from the object that comes back to camera. If you would use a point light source (say a flash with a diffuser) than the law would actually be inverse 4'th power law since the light intensity is inverse squared as a function of distance between the light source (diffused flash) and the object, and then inversed squared again as a function of distance between the object and the camera. This is actually a simplified (or oversimplified) example of radar equation (radar from a single source with single detector) which is another example for usage of same principles as in photography (both radar and light are electromagnetic waves).

05-29-2012, 08:21 AM	#11
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	stanislav your second point is not quite right, for 2 reasons. First , Many Auto flashes, including the AF540 have a swivel and tilt head, so as long as the sensor faces the subject, and the sensor is the same distance away from the subject as the flash is, then the metering will work correctly. the only thing to be considered is that the total light path of the flash respect as an approximation 2 times the direct sensor to subject maximum distance. Second, and I would not use the AF540 as an off camera flash for this reason, you can get for some auto flashes, a remote sensing head so that the sensor is on the camera shoe, and the flash on a long leash. I have one for my sunpak autozoom 3000, (which has a GN=30 meters) and this allows quite a separation between camera and flash, but you could still use this with a remote trigger, as long as the trigger and sensor head respect the subject to camera distance.

05-29-2012, 09:04 AM	#13
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	Originally posted by stanislav Anyway knowing the "theory" behind it will not make you worse artist or photographer How true...

05-31-2012, 04:22 AM	#14
vorstem2 Inactive Account Join Date: Apr 2009 Location: Roodepoort Posts: 10 Original Poster	Gosh now I am totally confused. It seems off camera flash is fairly complicated with Pentax

05-31-2012, 04:54 AM	#15
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,886	off camera flash in any mode other than P-TTL is always difficult because there is no way for the camera to know what is going on and make adjustments. if you were doing off camera flash in P-TTL mode, the camera and flashes would sort this out for you, but in A mode, you need to have a lot of things set correctly including the retention of subject to flash and subject to camera distance, so that the flash's sensor can attempt to meter as if it were still on the camera. It is not all that hard, and digital helps because you can see your mistakes at zero cost, but it does take some basic calculations or estimations for each flash, based upon the distances you use.