Question regarding on-camera flash

[Manfred M]

In an indoor setting using iTTL setting on the flash, the ISO setting will "mainly" influence the flash exposure: however I am not saying that that the ISO setting does not have some influence on the ambient light. Just not as much as the SS

Bruce - Doubling the ISO while keeping the same shutter speed and aperture will have a similar effect as doubling the shutter speed. The flash will reduce its contribution to the image, but the ambient light will increase its contribution. So the net effect is actually going to be the opposite of what you are thinking.

[Digital]

Bruce - Doubling the ISO while keeping the same shutter speed and aperture will have a similar effect as doubling the shutter speed. The flash will reduce its contribution to the image, but the ambient light will increase its contribution. So the net effect is actually going to be the opposite of what you are thinking.

Manfred, in your example, the flash "contributes" less to the overall exposure. If the SS, and aperture are a constant in your example, then the only thing you have changed is the ISO (i.e. you have increased the sensitivity of the camera to light). As a result if the flash's output is reduced then the ambient light influence is increased. This makes sense.
ISO influences flash output. Your point about doubling the ISO would influence (double) the SS since by increasing the sensitivity of the camera to light, you would need less time for the shutter to be open (decrease the time that light hits the sensor).
It appears to me that (using your example) that the ISO influences both the flash output, AND the amount of time (SS) that the sensor is exposed to light.
What I am trying to ask is that I was under the impression that aperture influences flash output; that SS influences ambient light. I was wondering what side of the coin ISO fell on. It appears that it falls on both sides equally. I know that the exposure triangle is interrelated even in flash photography.


Bruce

[Manfred M]

Bruce - you are starting to "get it". The issue is that all of these statements involve a level of simplification, so they are not 100% correct, but close enough for you to understand the relationship a bit better.

At that high level, I would suggest that you look at things from an exposure triangle standpoint and look at the contribution of the two main light sources - the amount of light to get a correct exposure will remain the same for a given ISO, shutter speed and aperture. What changes is the mix of the two light sources, and that is definitely a more complicated relationship. In the view you have, I would tend to suggest that the link between ISO and shutter speed is more similar than ISO and aperture when it comes to using a flash as the impact of the ambient light will be more apparent with both.

A higher ISO is going to give you a faster flash recycle time, so if you are shooting in bursts, that becomes an important parameter as well.

[Digital]

Manfred, thank you. You have been very helpful in resolving (at least as far as I am concerned) my questions. As you can tell I have never delved into this very deeply. As a reference this will definitely assist me in using flash in an indoor setting.


Bruce

[inkista]

Dunno if this will help, but what the hey.

... I was under the impression that aperture influences flash output; that SS influences ambient light. I was wondering what side of the coin ISO fell on. It appears that it falls on both sides equally. I know that the exposure triangle is interrelated even in flash photography...

Ambient is controlled by iso, aperture, and shutter speed.

Flash is controlled by iso, aperture, flash power output, and flash-to-subject distance.

So, the idea that aperture influences flash output is partly incorrect; like ISO, aperture also controls both flash and ambient together. Shutter speed is the only independent setting in the triangle, because if the camera is at or below sync speed, then the flash burst is going to be much much faster than the shutter speed, so leaving the shutter open for longer won't gather any more light from the flash.

If you want to balance your exposure more towards the ambient without affecting the flash exposure, you can use a slower shutter speed.

If you want to balance more towards the flash without changing the ambient exposure, you can increase the iso or aperture, and then compensate for the change with the increased flash exposure by making the shutter speed faster by the same amount of stops. Or you can just increase the flash's power output, or move the flash closer to the subject: these two factors only affect flash, not ambient. TTL, because it adjusts the flash's power output automatically, can let you drag not just the shutter, but also the aperture and ISO to increase ambient.

See also: The Strobist's "Lighting 102: Assignment - Balance" and Tangent's "Dragging the Shutter" and "Dragging the Shutter, Revisited".

[Dave Humphries]

Ambient is controlled by iso, aperture, and shutter speed.

Flash is controlled by iso, aperture, flash power output, and flash-to-subject distance.

So, the idea that aperture influences flash output is partly incorrect; like ISO, aperture also controls both flash and ambient together. Shutter speed is the only independent setting in the triangle, because if the camera is at or below sync speed, then the flash burst is going to be much much faster than the shutter speed, so leaving the shutter open for longer won't gather any more light from the flash.

Thanks Kathy, that has saved me some typing.

Bruce, please erase the term "mainly" from your mind, this is not helping your understanding.

There is no "mainly" about it, please see Kathy's first two sentences above, they are absolutes.

Try thinking about it this way:
1) Any and all light making the exposure passes through the lens, therefore is affected by the aperture (no matter how brief it is).
2) Any and all light making the exposure obviously impacts the sensor; therefore it is affected by ISO (no matter how brief it is).

Hence settings 1) and 2) affect both flash and ambient in equal amounts - that's just physics.
However, (and this is where it gets confusing) if you are using TTL flash, the camera will compensate (within limits of maximum and minimum flash output) for changes to aperture and ISO. This makes understanding what is going on, and how to vary the relevant control, in the right direction, so tricky.

3) As the flash is so brief, below the sync speed; changing the shutter speed only affects the ambient exposure.
4) The flash power and distance is also relevant, but when shooting with it on camera, TTL takes care of this.

Understanding is not helped by the fact that there are several well known (and otherwise sane) internet photography "experts" who peddle the untruth* that "ambient exposure is controlled by shutter speed and flash exposure is controlled by aperture".
This is not true; it is an over simplification (ignores ISO) and assumes that TTL is being used, but doesn't explain how the camera achieves this, leading to much confusion. Especially when people try to take control, as you are. The over simplification is only helpful to the people that are "Auto only" shooters with no intention of trying to improve. Unfortunately, to anyone with a bit of knowledge, they can really muddy the waters.

I am quite riled about this topic (as you may detect) because in learning myself, I have watched a lot of YouTube and other videos, read a lot of books, shot test exposures myself in TTL and manual and observed the effects of changing the controls - and as a result, I believe I now have a thorough understanding - and you'll find me shouting at my screen when I see/hear those untruths/over simplifications being taught to people. /rant


This is why I think people new to flash are far better off NOT using TTL flash (at first), they should use manually set flash power for a few days/weeks until they fully understand what is happening.

As it happens, that's exactly how I started (manual only), having only recently got a TTL capable flash, which I can now use when appropriate (e.g. if flash is on camera). Most of the time, I use flash off camera (which gives better light angles) and therefore in manual, because it makes life simpler.

Cheers, Dave

[Rent]

If one puts the camera in manual, then sets the shutter speed to 1000th, then opens the built in flash, the shutter speed will automatically set to the sync speed ( usually ) There is not usually any reason to use longer Roy

[Manfred M]

There is not usually any reason to use longer

For indoor shots, I would generally agree, Roy. For outdoor, definitely not.

Also your comment about the shutter speed setting back automatically is correct only if you use an integrated flash that communicates with the camera AND the HSS function is disabled.

[Rent]

Manfred I did say oen the built in flash
Roy

[Manfred M]

Manfred I did say oen the built in flash
Roy

Sorry I missed that. Let me add my recommendation on the built in flash. Depending on the camera model, the HSS constraints are there for the built-in flash too.

Don't ever use it for anything other than triggering off-camera slave flashes. It has too many downsides; positioned in such a way that it can easily cause red-eye, it is totally fixed and can't be swiveled into a position where it can be bounced off the ceiling or walls and compared to decent hot shoe flashes, it has fairly low power.

I'm not even particularly happy when using it as a fill flash, as it is so close to the camera lenses, they act as flags and prevent the light from going where you want it.

[Digital]

Dave, and Kathy - thank you for your response. After studying your threads, I think I am beginning to see the light (no pun intended).
I know when using flash you are dealing with two sources of light (flash and ambient). Based on my earlier "education", I was misinformed, and as a result I was attempting to oversimplify what part of the exposure triangle influenced these sources of light. The way you stated makes a whole lot of sense: the entire exposure triangle influences ambient light. This is true whether you are using flash or not. Adding a flash to the equation complicates the equation some what because due to the short burst of light from the flash, and the relative slowness of the SS (in relation to the flash), the SS has little or nothing to do with the light from the flash. The other parts of the exposure triangle (aperture, and ISO) do have influence on the flash output, based on the subject to flash distance. In iTTL the flash output will be influenced by the aperture, and ISO settings as well as the flash to subject distance. As I well know, this requires very little thinking.
Dave, based on your comments, I am going to experiment with manual setting for the flash. In this way I cam manually manipulate the flash output without worrying about compensation from the flash regarding what settings the aperture, ISO, and flash to subject distance are. This way I can experiment with various settings, distances.
Please feel free to correct me if I am still missing the point. I am trying to understand these concepts in my own terms.

Bruce

[Dave Humphries]

Dave, based on your comments, I am going to experiment with manual setting for the flash. In this way I cam manually manipulate the flash output without worrying about compensation from the flash regarding what settings the aperture, ISO, and flash to subject distance are. This way I can experiment with various settings, distances.
Please feel free to correct me if I am still missing the point. I am trying to understand these concepts in my own terms.

Hi Bruce,

I think you're getting there now and if you shoot some manual flash power tests, also using the camera set on manual for ambient exposure, this should help you 'get your head round it'. I say that because if you have everything under your direct control, shoot a logical set of variations with a consistent scene, then examine the differences in images with the benefit of the EXIF data to refer to, you should succeed.

Good luck, Dave

[Digital]

Hi Bruce,

I think you're getting there now and if you shoot some manual flash power tests, also using the camera set on manual for ambient exposure, this should help you 'get your head round it'. I say that because if you have everything under your direct control, shoot a logical set of variations with a consistent scene, then examine the differences in images with the benefit of the EXIF data to refer to, you should succeed.

Good luck, Dave

Thanks Dave. It is good to know that I am on the right track. I am already using manual mode on my camera. I have experimented controlling the ambient light by manipulating the SS on my camera.

Bruce

[inkista]

... Adding a flash to the equation complicates the equation some what because due to the short burst of light from the flash, and the relative slowness of the SS (in relation to the flash), the SS has little or nothing to do with the light from the flash.

Sidenote: this is correct as long as you're not using HSS or tail-syncing (aka hypersync). Both of these are technical workarounds to get past the camera body's sync speed limit. This will become far more important to you when you start doing off-camera flash. The skinny on sync speed and HSS is in van Niekerk's high-speed sync tutorial, and on PocketWizard's page on hypersyncing. Just bookmark 'em for now, and after you go Strobist outside, and get ticked off that you can't get shallow depth of field, look at 'em.

The other parts of the exposure triangle (aperture, and ISO) do have influence on the flash output, based on the subject to flash distance.

NO. Not based on subject-to-flash distance. Subject-to-flash distance and flash power setting are independent factors of the iso and aperture. Think of ambient as having the triangle. And flash as having a square. And that the triangle and square share two corners. That's how complex your light-think gets when you throw flash into the picture. You have two separate sources of light, and some of the controls are independent (shutter speed, flash power, flash-to-subject distance), and some are inter-related (iso, aperture). So, achieving the desired balance between ambient and flash can be done by a variety of different ways.

One other thing that will no doubt confuse you early on is that ambient and flash mix. While the sources are separate, a subject/background can have both flash and ambient on them. Think of it like two layers in Photoshop combining in a single image: there are going to be points where the light from both sources overlap. You can try flagging off the flash or distancing/shading a subject from ambient light sources, but you may also have to consider that controlling just flash or just ambient may still not have exactly the effect you're envisioning if you're strictly equating background to ambient and subject to flash.

I always tend to recommend being really really comfortable shooting in M for ambient-only photography and swapping stops comfortably in your head between iso, aperture, and shutter speed before getting into flash. Because when you get into flash photography, your metering doesn't do what you think it's gonna do. Just putting the needle on "0" isn't going to give you a great exposure any more, because the meter can only read the light that's in the scene. And the flash isn't there yet. So you will need to be able to tell the camera not to put the needle on "0", but to compensate for the flash that's about to enter the scene.

Or you can use TTL.

In iTTL the flash output will be influenced by the aperture, and ISO settings as well as the flash to subject distance. As I well know, this requires very little thinking.

Just to get into the technical nitty-gritty, what's actually happening with TTL is that your camera is telling your flash to send out a small low-powered "pre-burst" of light at a known power/brightness level--in essence putting the flash into the scene before metering. The camera then meters that pre-burst, and, based on the camera settings, as well as how you set the metering/focus (distance), adjusts the flash's power output to what the auto-exposure thinks of as a "good exposure" level (within the limits of the flash's power output). It's only using iso, aperture, and focus distance data to aid in where to put the numbers, but the exposure is mostly based on the metering information. Just as ambient-only photography auto-exposure puts your iso/aperture/shutter speed (depending on the shooting mode) based on the metering information it's getting.

This is why we love TTL: it's fast, convenient, and it's metering-based, so it adjusts quickly. It's also why we hate TTL--it's metering-based, so it's situationally unaware, and can screw up and bias exposure in a simplistic way (i.e., putting your average value at middle-grey) that can be very wrong. Put someone in a white shirt and shoot with TTL and you may get underexposure. Put that same person in a black shirt and shoot with TTL, and you may get overexposure. Nature of the beast and metering-based exposure algorithms.

Think of TTL like A in the camera shooting modes. Fast, convenient, in-the-ballpark, great for run'n'gun shooting in changing light conditions, but you may still have to ride exposure compensation now and again. Think of M like M in the camera shooting modes: Slower, but more precise, and utterly consistent--a way to "lock in" the flash power setting.

I also highly recommend doing the flash-in-M thing so you can suss out how it all works. Flash, unlike the ambient exposure settings, leaves almost no evidence behind in EXIF, so unless you keep notes, it's pretty tough to look back at a picture you took a while back and see what your flash settings were.

One last note. Flash distance-to-subject isn't a linear thing. It's inverse-square. (1/x^2) That means, if you half the distance, you quadruple the amount of light from flash. Double the distance, and you quarter the light. Triple it, and your light goes down to 1/9 what you had. Falloff is extremely rapid.

And don't worry. It does take a while to wrap your head around all this stuff. Nobody gets it right away. And everybody feels like their head's gonna explode. But practice and experimentation more than anything will help nail the principles down. And when you do get it, and you start pulling your flash(es) off your camera and onto stands, that's when the fun really begins.

Ok, really really my last note. Everything I ever learned about on-camera flash, I learned from Neil van Niekerk's Tangents website. I'd recommend starting with his Flash Photography Techniques section. If you're more comfortable with books instead of webpages, he's published this in dead tree format, as well. If you begin to get interested in studio-style lighting with off-camera flash, van Niekerk's decent for that, too, but the motherlode of information that started it all is David Hobby's Strobist website. I always make a very bad Matrix joke that Tangents is the blue pill and Strobist is the red pill.

[Digital]

Kathy, thank you for the informative thread. It will take some time to get this information into my thick head, However I am confident that I will eventually get it. I have learned so much from you guys in the last few days.
Thanks again.

Bruce

[Manfred M]

Bruce - just to add to Kathy's comment. The only time I use iTTL flash is when I shoot in "Run & Gun" mode, i.e. I shoot in a rapidly changing shooting environment where the shooting position and lighting changes significantly from shot to shot.

Under any other circumstances, I shoot on manual. This is partially related to my history with flash. When I first started working with it, there was no flash-to-camera integration and the automation, if it existed at all was very, very basic. The problem with the iTTL flash is that there are too many variables happening all at once and I cannot predict how minor changes to what I am shooting will affect the outcome. If I shot on manual, I am 100% in control and I get extremely consistent images from shot to shot.

I think for the non-flash shooters, an example they might relate to is Auto White Balance (AWB) . While it really doesn't make any difference (in theory at least) when shooting raw, I prefer setting a manual WB. That way all may shots look the same when I open them in post and there aren't any minor differences between shots, so evaluating them from a common base that is free from minor changes in the white balance makes this a lot easier. There is one less variable to worry about.

The same thing goes for shooting manual flash, there are no surprises!

[Cantab]

Manfred, when I used to shoot manual flash in the days of film, the flash gun had a dial/chart on the back so that it would give the aperture setting to use for a particular flash to subject distance, based on the guide number for the flash gun (as well as the ASA if I remember correctly).

When I converted to digital and first acquired an iTTL flash, I realized that the guide number was not easily discovered. When you shoot with manual flash, do you do the guide number arithmetic or is it all studio work where you use a flash meter?

[Manfred M]

Manfred, when I used to shoot manual flash in the days of film, the flash gun had a dial/chart on the back so that it would give the aperture setting to use for a particular flash to subject distance, based on the guide number for the flash gun (as well as the ASA if I remember correctly).

When I converted to digital and first acquired an iTTL flash, I realized that the guide number was not easily discovered. When you shoot with manual flash, do you do the guide number arithmetic or is it all studio work where you use a flash meter?

For studio work I use a flash meter 100% of the time and try to have my meter along at other times too. When I don't have one around, the histogram is a good guide, but it can take a few test shots to nail the setup. I find it's close, but I have more trust in the meter, especially in tricky lighting situations. The one thing about the meter is I get a reading that gives me the contribution of both the flash and the ambient light sources.

Both of these techniques are a lot better than the dial on the flash. The dial was fine for direct flash, but totally useless for bounce flash and that's where the mental arithmetic came in, estimating how far the light had to bounce and how much of it would actually hit the subject.

[Digital]

I went to Neil van Niekerk's Tangents website. Very informative. He gave an acronym for the flash exposure: PAID
P (flash power) A (aperture) I (ISO) D (distance from flash to subject)
Also, I read his post on manual flash in which I am going to try my hand.

Bruce

[Manfred M]

I went to Neil van Niekerk's Tangents website. Very informative. He gave an acronym for the flash exposure: PAID
P (flash power) A (aperture) I (ISO) D (distance from flash to subject)
Also, I read his post on manual flash in which I am going to try my hand.

Bruce

You will also notice that when van Niekerk discusses on-camera flash, he is bouncing the light, rather than using direct flash, whenever he can.