When is the moment actually captured? Flash duration or shutter speed?

I want to better understand how the camera's shutter sync speed and the flash duration of a strobe capture the image.

My understanding is that unless the subject is perfectly still, the flash duration must be fast enough to freeze the moment. Considering that many cameras can't sync at speeds faster than 1/125 and some at 1/250 then it would follow that it's not the camera's shutter speed that is actually freezing the action but rather, a fast flash duration from the strobe(s).

Correct so far? I think so. I hope so.

Ok, now.... I think I've also recently learned something very interesting that comes in to play when you're outside the studio and not in control over ambient light.

On my light meter (most meters actually), when I trigger a flash the meter displays the percentage of flash for the exposure. When I work in the studio that number is always 100% and is never influenced by any other light sources. So this means that when I fire the strobes and take a picture that the only light affecting or contributing to exposure is that from the strobes and other than the duration of the flash, the remaining time that the shutter is open is in darkness and movement or action has been frozen.

Now if I shoot outside and start taking readings from my strobes I can see that even though I might meter f/8 I'm also seeing 30% and 40% as the percentage of flash in the exposure. This tells me that 60% to 70% of the exposure is from ambient light and that means that instead of a short burst of light there is a burst of light with a constant ambient contributing to the exposure, which completely changes the way the image is captured if there is movement.

If I'm correct, this leads to the much talked about 'competing with the sun' topic. With a more powerful strobe, the percentage of flash will go up which means that the percentage of ambient contribution goes down, thus allowing for better captures when there is movement. It also follows that as there is less ambient light then the problem is lessened and the flash duration is capturing the moment with little to no ambient contribution during the remaining time the shutter is open. This why many photographers will shoot in the later part of the day or at sunset and they don't have to compete with the sun. It would seem that to truly compete with the sun you really need a ridiculous amount of power, especially if you're shooting full length or group and the lights can't be positioned 2 feet from the subject(s).

Is my logic right here?

Robert,

OK, this is a rather complex topic, but i am going to attempt to help here.

1. You are correct that the exposure is really the flash duration and not the shutter speed. This is why you get the same exposure at 1/60th as you do at 1/250th in studio conditions.

Now, shooting outdoors throws this logic completely out the window because you also have ambient exposure to consider, which does rely on shutter speed too.

Shooting in the later part of the day etc only assists you to not require so much power from your strobes to accurately balance daylight vs ambient. You still need to run the same balance for a good looking photo.

If for instance you were to have only a 10% light contribution from ambient, then your background is going to be really dark. So while your moving subjects might be nice and crisp, the rest of the image will probably look crap.

This is the compromise that we have to make shooting flash outdoors.

Now flash durations do actually change with flash output too, but i won't go into that.

Does that help at all?

Rob

As far as I can tell your logic is correct. I'm not sure about the amount of power needed.

I guess I'm confused by this and other recent discussions on competing with the sun. I read a while ago (and actually tried) cross lighting. Is that not competing with the sun? I used one Sunpak 555 optically triggered by a 550EX on camera. It worked well. I followed the instructions from Strobist. Samples from that Strobist article here: http://strobist.blogsp​ot.com …-31-balance-flashsun.html

Surely I'm missing something.

I agree with your logic. Couple thoughts from different angles: "Competing with the sun" topic got more to do with over come the camera's sync speed. Shooting in golden hours gives photogs more control in apertures.

Robert,
Your logic is correct, but I don't think I would agree with your conclusions.

TMR Design wrote in post #5603042
This why many photographers will shoot in the later part of the day or at sunset and they don't have to compete with the sun. It would seem that to truly compete with the sun you really need a ridiculous amount of power, especially if you're shooting full length or group and the lights can't be positioned 2 feet from the subject(s).

Early morning and late afternoon are better times to shoot because of the quality of the light, not just the quantity. With the sun low in the sky, it travels through more atmosphere before hitting the surface. Models can face into the sun without squinting. Shadows are not as harsh. The color temperature of the light is lower and gives images a nice warm glow. The lower light level, which allows use of less flash power and also allows the use of larger apertures, it just one of many benefits.

As for a "ridiculous amount" of light to compete with midday sun, I guess it's a matter of opinion. I often shoot at f/11 indoors with two B1600 units at 1/4 power. So if I took them outdoors I could crank them up to full power, shoot at f/22 (or use a 2-stop ND filter) and underexpose the ambient by more than 2 stops (sunny 16 would put me at 1/250 and f/10). If I used softboxes instead of umbrellas I could get even more light from the strobes.

Also keep in mind that most outdoor portrait shooters like to get their subjects out of direct sun whenever possible. By putting the subject in the shade (or creating your own shade) you only need to provide enough light to match a sunlit background.

I don't know whether this will help your understanding but have a look at posts 14 and 15 in this thread - https://photography-on-the.net/forum/showthre​ad.php?t=499526. The examples are nothing to do with freezing motion but are to do with balancing ambient lighting with subject lighting and independently altering one or the other.

Here are some nice examples as well.

The answer to your subject title question is, "The moment captured is always determined by the shutter speed".

If you were in a dark room with no lights at all and held the shutter open for 10 seconds, you would capture 10 seconds of darkness. This is easily shown by having a firefly in the room and in the field of view of the camera. You'd see a 10 second trail of the firefly flashing on and off (assuming the firefly is cooperative enough to do its "thing"). You could use a flash to light up the firefly itself for a spit second but you'd still have the 10 second on-off light trail.

the sun is just another light source, with a definite direction and intensity. it is modified by the exact situation - time of year, time of day, atmospheric conditions, diffusion as it falls on the objects of the earth, etc.

you are choosing to use this light by shooting outdoors. use it like you would any other light. determine your ratio based on the role the ambient light is to play - main, fill, accent, etc.

yes if you want to use direct sun as a kicker, you are talking f/16, f/22, max sync. to get those apertures at any decent distance with diffusion you are going to need 1200-2400ws or more.

you are typically not going to have a problem 'freezing' your subject. if, for instance, you are using shade as fill, and shooting let's say f/5.6 at iso 100, then the shade light will need to read, for instance, 2.8 at x shutter. use normal guidelines to determine if x is too slow. if so, use iso to get the speed you need. adjust the flash in inverse proportion.

this is really only relevant early or late in the day or in extreme shade, where ambient levels drop to result in questionable shutter speeds. otherwise you are typically going to have all the light you need.

Thank you everyone for contributing. I appreciate the explanations and links.

I wanted to clarify some of these things before asking a followup question and now that I am clear, here it is.

If I'm shooting with a shutter sync speed of 1/125, ISO 100, f/8, with strobes at full power (flash duration 1/1200s) and I'm only registering 40% flash when I meter, and my subject moves, will I get a blurred shot? Will this actually freeze the moment or will the contribution of ambient light being recorded during the remainder of the time that the shutter is open allow for that movement to be recorded, giving me some motion blur?

Robert,

If there is an ambient light and ss is low, like 1/125 like you mentioned, then there is goshting from movement. I have not done the experiment myslef but just look at the sports shots where there is mix of ambient and strobes. That's why I think cameras with sync speed higher than 1/250 are so deriable for freezing action.

TMR Design wrote in post #5603042
I want to better understand how the camera's shutter sync speed and the flash duration of a strobe capture the image.

My understanding is that unless the subject is perfectly still, the flash duration must be fast enough to freeze the moment. Considering that many cameras can't sync at speeds faster than 1/125 and some at 1/250 then it would follow that it's not the camera's shutter speed that is actually freezing the action but rather, a fast flash duration from the strobe(s).

Correct so far? I think so. I hope so.

Ok, now.... I think I've also recently learned something very interesting that comes in to play when you're outside the studio and not in control over ambient light.

On my light meter (most meters actually), when I trigger a flash the meter displays the percentage of flash for the exposure. When I work in the studio that number is always 100% and is never influenced by any other light sources. So this means that when I fire the strobes and take a picture that the only light affecting or contributing to exposure is that from the strobes and other than the duration of the flash, the remaining time that the shutter is open is in darkness and movement or action has been frozen.

Now if I shoot outside and start taking readings from my strobes I can see that even though I might meter f/8 I'm also seeing 30% and 40% as the percentage of flash in the exposure. This tells me that 60% to 70% of the exposure is from ambient light and that means that instead of a short burst of light there is a burst of light with a constant ambient contributing to the exposure, which completely changes the way the image is captured if there is movement.

If I'm correct, this leads to the much talked about 'competing with the sun' topic. With a more powerful strobe, the percentage of flash will go up which means that the percentage of ambient contribution goes down, thus allowing for better captures when there is movement. It also follows that as there is less ambient light then the problem is lessened and the flash duration is capturing the moment with little to no ambient contribution during the remaining time the shutter is open. This why many photographers will shoot in the later part of the day or at sunset and they don't have to compete with the sun. It would seem that to truly compete with the sun you really need a ridiculous amount of power, especially if you're shooting full length or group and the lights can't be positioned 2 feet from the subject(s).

Is my logic right here?

Robert, your observations are generally all true, but the highlighted text points to what I will take exception to...I choose late in the day or very early in the day, NOT because of 'compete with the sun' issues of flash intensity, but simply because the directionality and quality of light at that time is so much more desireable!

Also, at that time, you can put the sun behind the subjects, so that YOU control all of the light striking their faces, via placement of your lights, so that the ambient contribution in that situation is minimized. Mid-day gives you only the racoon eyes which you need to fill in.

With a fast moving subject that would be blurred at 1/125, yes the subject will have blur but it also will have that portion captured during the flash represented as a sharper edge...which is one reason for the use of 2nd curtain synch.

Wilt wrote in post #5604950
Robert, your observations are generally all true, but the highlighted text points to what I will take exception to...I choose late in the day or very early in the day, NOT because of 'compete with the sun' issues of flash intensity, but simply because the directionality and quality of light at that time is so much more desireable!

Also, at that time, you can put the sun behind the subjects, so that YOU control all of the light striking their faces, via placement of your lights, so that the ambient contribution in that situation is minimized. Mid-day gives you only the racoon eyes which you need to fill in.

With a fast moving subject that would be blurred at 1/125, yes the subject will have blur but it also will have that portion captured during the flash represented as a sharper edge...which is one reason for the use of 2nd curtain synch.

Thank you very much Wilt. This information is helping me as I do more location shooting under unusual and adverse conditions.

TMR Design wrote in post #5604759
If I'm shooting with a shutter sync speed of 1/125, ISO 100, f/8, with strobes at full power (flash duration 1/1200s) and I'm only registering 40% flash when I meter, and my subject moves, will I get a blurred shot?

This is typically the kind of situation that causes "ghosting". You'll get a sharp rendering of the subject (from the flash) but also some blurry edges in one direction or another (depending on direction of subject movement/camera movement and 1st or 2nd curtain sync).

TMR Design wrote in post #5604759
Thank you everyone for contributing. I appreciate the explanations and links.

I wanted to clarify some of these things before asking a followup question and now that I am clear, here it is.

If I'm shooting with a shutter sync speed of 1/125, ISO 100, f/8, with strobes at full power (flash duration 1/1200s) and I'm only registering 40% flash when I meter, and my subject moves, will I get a blurred shot? Will this actually freeze the moment or will the contribution of ambient light being recorded during the remainder of the time that the shutter is open allow for that movement to be recorded, giving me some motion blur?

Robert, with the flash contributing only 40% of the light, just over half the lighting will be from the ambient light so, yes, there will be motion blur visible if the subject moves fast enough to be blurred at a shutter speed of 1/125. The only time subject blur would not really be visible is if the ambient lighting is over 2 stop less than the flash or, in other words, the flash contributes greater than 80% of the light (for mid-toned subjects and higher if light-toned).