Maximum ISO for flash work

GlennG · 02-26-2018, 06:18 PM

I did a quick Google query and found no good guidance for the maximum iso that I should use for flash work. Do you all have a maximum Iso that you will never exceed for flash work?

stevebrot · 02-26-2018, 06:40 PM

In practice, if no combination of aperture or flash attenuation are able to provide a proper exposure, the ISO is either too high or too low.

Steve

swanlefitte · 02-26-2018, 06:44 PM

What conditions do you mean? I have never had them.

Adam · 02-26-2018, 07:05 PM

If you push the ISO too high, you might get close to exceeding the sync speed, so I guess that would probably be the biggest limiting factor. When there's plenty of light, image quality is otherwise quite good, even at high ISOs.

Adam
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Angel Perez · 02-26-2018, 08:59 PM

Originally posted by GlennG

I did a quick Google query and found no good guidance for the maximum iso that I should use for flash work. Do you all have a maximum Iso that you will never exceed for flash work?

There is no good guidance because it really all depends on the situation. Different situations require different solutions. As you see some have put there pic and how and why they shot it.

So I shoot with a K-3 and in general during an event I try to keep my ISO at 800 and 1600 with flash. I play with my aperture and my shutter speed to get the ambient light and to play with my DOF if I am shoot groups I try to keep my Aperture at about 5.6 and take it to 8.0 if the group gets larger.

If I am shooting in my studio I shoot at 100 ISO about 90% of the time and go to ISO 200 if I think my speedlights or strobes are getting too hot and slow.

Again these are just basic guidelines and numbers I found that work for me. You can start with any of these and then play with your settings to get to your basic solutions.

If you can tell us what kind of shooting you are planning to do.

Good luck

**clackers** · 02-26-2018, 09:28 PM

Originally posted by GlennG

I did a quick Google query and found no good guidance for the maximum iso that I should use for flash work. Do you all have a maximum Iso that you will never exceed for flash work?

When you know the ambient light is not important, there's no reason to go above 100.

When you do want the background to matter, make sure that looks good in Live View (Manual Mode) first, then afterwards flash your subject based on those same settings. Rotate the dial for Flash Exposure Compensation to suit.

BrianR · 02-27-2018, 06:31 AM

No flash-specific upper limit for me, just the usual practical ones. Regarding the flash and iso:

The iso is too high if I can't get the exposure I want (can happen with flash if their power can't go low enough and I left my ND gels at home) or if I find the noise unacceptable for the end use of the photo.

The iso is too low if I can't set my flash to a high enough power to get the exposure I want, which could be because it's at max power already or because I have reason to keep the power low (for shorter recycle times, shorter t.1 times, or possibly just lower power to keep from blinding my ~~victims~~ subjects).

The exposure I want from the ambient portion may also put limits on the iso, too high and you may be bumping into sync speed limits (out in full sun, I'm almost always at iso 80 when combining ambient & flash), too low and I may end up needing too wide an aperture or too long a shutter speed to get the contribution from the ambient I want.

aleonx3 · 02-27-2018, 07:01 AM

There is no such thing as 'too high' when it comes to iso setting with flash assuming that you are keeping it within the proper exposure of the entire scene using PTTL flash. It all depends on what you want the resulting image to be in terms of flash and ambient light mix. A good example would be shooting a subject with a night scene, how much of background you want to incorporate with the subject into the image.

photoptimist · 02-27-2018, 07:28 AM

The only true upper limit that I've heard of (besides the scene-specific practical limits or trade-offs in attenuation, ambient light, and the exposure triangle) is caused by atmospheric haze.

If you try to use a flash with an ISO 100 Guide Number of 54 meters at say ISO 25600 to get a GN of 54 * √(25600/100) = 54 * 16 = 864 m (almost 1 kilometer!) to light up the landscape, there's a good chance of getting a fogged image if the flash is on the camera. It's just like the problem of driving in fog with headlights on. The fogging effect can be reduced by moving the flash far off to one side but that can pose other issues with uneven lighting of the foreground.

That true max ISO depends on atmospheric conditions (humidity, pollution, dust, altitude, etc.) but seems to be up their in the nether regions of ISO (e.g., 25600 or 51200, and higher).

Class A · 02-27-2018, 07:46 AM

I guess another "high ISO limit" is introduced for optical (aka "wireless") triggering of off-camera slave flashes by the fact that the weak optical control flash starts to have a visual impact at high ISO settings. Typically some reflective surfaces in the frame and nearby the on-camera flash need to be present, but there is always some residual contribution of the master flash (on-board or not), even when it is set to "control mode", which can become disturbing if the camera settings allow it.

mcgregni · 02-28-2018, 07:14 AM - **1 Like**

Yes, in a previous discussion I was able to conclude that from an EV setting of F2.8 ISO 800 this could potentially be a concern .... Any settings less than this should not present a problem.

For me, the main reason I would want to increase ISO with flash is in order to in increase the effective guide number of my flash. This is likely to be needed indoors in large rooms when bounce angles are extreme and the distance the light has to travel to bounce back is high. This will bring more power, faster recycling and longer battery life, all great benefits in an event situation.

The actual ISO limit depends mainly on the camera sensor and your required level of IQ .... For example, with my K7 I won't go past ISO 800 for this, but I imagine with a K1 I might be happy with 6400, a further 3 stops of GN. This will turn an on camera flash into a real power house.

The other point about higher ISOs is to record more ambient light to mix with your flash lit subjects. Obviously, as mentioned before, the max sync speed wil be the limiting factor really .... There's hardly any point in moving into HSS territory just to get more ambient light recorded. Indoors this will rarely be an issue though, unless you insist on shooting at widest apertures always.

The other real limit is of course your flash power .....and I mean having too much (this was mentioned earlier by another poster also). We think with flash we can never have too much power .... Well, we can at very high ISOs! The ambient light levels make no difference here, it's all about the flash to subject distance, and very high ISOs will quickly bring that GN up so high that you will not be able to power the flash down enough. So it's really all about the size of the space and the flash to subject distances.

stevebrot · 02-28-2018, 10:19 AM

Originally posted by mcgregni

The ambient light levels make no difference here, it's all about the flash to subject distance, and very high ISOs will quickly bring that GN up so high that you will not be able to power the flash down enough.

Yep! That is the limit.

There are also implications for P-TTL if the preflash attenuation limits* have been exceeded resulting in unpredictable outcome.

Steve

* Pre-flash duration is calculated in advance based on distance (reported by lens) and EV.

mcgregni · 02-28-2018, 11:16 AM

Yes, and then I guess at the last micro-second an adjustment is made based on the actual amount of flash light returned through the lens to the metering sensor ... (this is to take into account the reflective properties of the subject). I would add to Steve's point above that the known ISO and aperture values would also be used, as well as the distance, in calculating the initial pre-flash level before it is sent out.

Class A · 02-28-2018, 06:16 PM

Originally posted by stevebrot

* Pre-flash duration is calculated in advance based on distance (reported by lens) and EV.

Where did you find this information?

swanlefitte · 02-28-2018, 07:08 PM

Some interesting stuff pointed out in this thread but I find it funny that a flash minimum is too much when any light modifier kills soo much light. Against ambient one almost has to use a gel anyway for balance. Is there something i am not thinking about which makes sucking a strobes light a hard thing to do?

02-26-2018, 06:18 PM	#1
GlennG Veteran Member Join Date: Aug 2010 Location: Houston, Texas Posts: 824	Maximum ISO for flash work I did a quick Google query and found no good guidance for the maximum iso that I should use for flash work. Do you all have a maximum Iso that you will never exceed for flash work?

02-28-2018, 07:14 AM - 1 Like	#11
mcgregni Veteran Member Join Date: Sep 2012 Location: Surrey, England Posts: 2,603	Yes, in a previous discussion I was able to conclude that from an EV setting of F2.8 ISO 800 this could potentially be a concern .... Any settings less than this should not present a problem. For me, the main reason I would want to increase ISO with flash is in order to in increase the effective guide number of my flash. This is likely to be needed indoors in large rooms when bounce angles are extreme and the distance the light has to travel to bounce back is high. This will bring more power, faster recycling and longer battery life, all great benefits in an event situation. The actual ISO limit depends mainly on the camera sensor and your required level of IQ .... For example, with my K7 I won't go past ISO 800 for this, but I imagine with a K1 I might be happy with 6400, a further 3 stops of GN. This will turn an on camera flash into a real power house. The other point about higher ISOs is to record more ambient light to mix with your flash lit subjects. Obviously, as mentioned before, the max sync speed wil be the limiting factor really .... There's hardly any point in moving into HSS territory just to get more ambient light recorded. Indoors this will rarely be an issue though, unless you insist on shooting at widest apertures always. The other real limit is of course your flash power .....and I mean having too much (this was mentioned earlier by another poster also). We think with flash we can never have too much power .... Well, we can at very high ISOs! The ambient light levels make no difference here, it's all about the flash to subject distance, and very high ISOs will quickly bring that GN up so high that you will not be able to power the flash down enough. So it's really all about the size of the space and the flash to subject distances. Last edited by mcgregni; 02-28-2018 at 07:23 AM.

02-28-2018, 10:19 AM	#12
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by mcgregni The ambient light levels make no difference here, it's all about the flash to subject distance, and very high ISOs will quickly bring that GN up so high that you will not be able to power the flash down enough. Yep! That is the limit. There are also implications for P-TTL if the preflash attenuation limits* have been exceeded resulting in unpredictable outcome. Steve * Pre-flash duration is calculated in advance based on distance (reported by lens) and EV. Last edited by stevebrot; 02-28-2018 at 10:48 AM.

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02-26-2018, 06:40 PM	#2
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	In practice, if no combination of aperture or flash attenuation are able to provide a proper exposure, the ISO is either too high or too low. Steve

02-26-2018, 06:44 PM	#3
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	What conditions do you mean? I have never had them.

02-26-2018, 07:05 PM	#4
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,594	If you push the ISO too high, you might get close to exceeding the sync speed, so I guess that would probably be the biggest limiting factor. When there's plenty of light, image quality is otherwise quite good, even at high ISOs. Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

02-26-2018, 08:59 PM	#5
Angel Perez Senior Member Join Date: May 2015 Posts: 158	Originally posted by GlennG I did a quick Google query and found no good guidance for the maximum iso that I should use for flash work. Do you all have a maximum Iso that you will never exceed for flash work? There is no good guidance because it really all depends on the situation. Different situations require different solutions. As you see some have put there pic and how and why they shot it. So I shoot with a K-3 and in general during an event I try to keep my ISO at 800 and 1600 with flash. I play with my aperture and my shutter speed to get the ambient light and to play with my DOF if I am shoot groups I try to keep my Aperture at about 5.6 and take it to 8.0 if the group gets larger. If I am shooting in my studio I shoot at 100 ISO about 90% of the time and go to ISO 200 if I think my speedlights or strobes are getting too hot and slow. Again these are just basic guidelines and numbers I found that work for me. You can start with any of these and then play with your settings to get to your basic solutions. If you can tell us what kind of shooting you are planning to do. Good luck

02-26-2018, 09:28 PM	#6
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by GlennG I did a quick Google query and found no good guidance for the maximum iso that I should use for flash work. Do you all have a maximum Iso that you will never exceed for flash work? When you know the ambient light is not important, there's no reason to go above 100. When you do want the background to matter, make sure that looks good in Live View (Manual Mode) first, then afterwards flash your subject based on those same settings. Rotate the dial for Flash Exposure Compensation to suit.

02-27-2018, 06:31 AM	#7
BrianR Veteran Member Join Date: Dec 2010 Location: Ontario Photos: Gallery Posts: 3,332	No flash-specific upper limit for me, just the usual practical ones. Regarding the flash and iso: The iso is too high if I can't get the exposure I want (can happen with flash if their power can't go low enough and I left my ND gels at home) or if I find the noise unacceptable for the end use of the photo. The iso is too low if I can't set my flash to a high enough power to get the exposure I want, which could be because it's at max power already or because I have reason to keep the power low (for shorter recycle times, shorter t.1 times, or possibly just lower power to keep from blinding my ~~victims~~ subjects). The exposure I want from the ambient portion may also put limits on the iso, too high and you may be bumping into sync speed limits (out in full sun, I'm almost always at iso 80 when combining ambient & flash), too low and I may end up needing too wide an aperture or too long a shutter speed to get the contribution from the ambient I want.

02-27-2018, 07:01 AM	#8
aleonx3 Veteran Member Join Date: Jul 2007 Location: Brampton, Ontario Photos: Gallery \| Albums Posts: 3,996	There is no such thing as 'too high' when it comes to iso setting with flash assuming that you are keeping it within the proper exposure of the entire scene using PTTL flash. It all depends on what you want the resulting image to be in terms of flash and ambient light mix. A good example would be shooting a subject with a night scene, how much of background you want to incorporate with the subject into the image.

02-27-2018, 07:28 AM	#9
photoptimist Pentaxian Join Date: Jul 2016 Photos: Albums Posts: 5,122	The only true upper limit that I've heard of (besides the scene-specific practical limits or trade-offs in attenuation, ambient light, and the exposure triangle) is caused by atmospheric haze. If you try to use a flash with an ISO 100 Guide Number of 54 meters at say ISO 25600 to get a GN of 54 * √(25600/100) = 54 * 16 = 864 m (almost 1 kilometer!) to light up the landscape, there's a good chance of getting a fogged image if the flash is on the camera. It's just like the problem of driving in fog with headlights on. The fogging effect can be reduced by moving the flash far off to one side but that can pose other issues with uneven lighting of the foreground. That true max ISO depends on atmospheric conditions (humidity, pollution, dust, altitude, etc.) but seems to be up their in the nether regions of ISO (e.g., 25600 or 51200, and higher).

02-27-2018, 07:46 AM	#10
Class A Pentaxian Join Date: Aug 2008 Location: Wellington, New Zealand Posts: 11,251	I guess another "high ISO limit" is introduced for optical (aka "wireless") triggering of off-camera slave flashes by the fact that the weak optical control flash starts to have a visual impact at high ISO settings. Typically some reflective surfaces in the frame and nearby the on-camera flash need to be present, but there is always some residual contribution of the master flash (on-board or not), even when it is set to "control mode", which can become disturbing if the camera settings allow it.

02-28-2018, 11:16 AM	#13
mcgregni Veteran Member Join Date: Sep 2012 Location: Surrey, England Posts: 2,603	Yes, and then I guess at the last micro-second an adjustment is made based on the actual amount of flash light returned through the lens to the metering sensor ... (this is to take into account the reflective properties of the subject). I would add to Steve's point above that the known ISO and aperture values would also be used, as well as the distance, in calculating the initial pre-flash level before it is sent out.

02-28-2018, 06:16 PM	#14
Class A Pentaxian Join Date: Aug 2008 Location: Wellington, New Zealand Posts: 11,251	Originally posted by stevebrot * Pre-flash duration is calculated in advance based on distance (reported by lens) and EV. Where did you find this information?

02-28-2018, 07:08 PM	#15
swanlefitte Pentaxian Join Date: May 2015 Location: Minneapolis Photos: Gallery Posts: 4,068	Some interesting stuff pointed out in this thread but I find it funny that a flash minimum is too much when any light modifier kills soo much light. Against ambient one almost has to use a gel anyway for balance. Is there something i am not thinking about which makes sucking a strobes light a hard thing to do?