Controlling Ambient With Shutter Speed

[ciaran]

Another tutorial from my website, which I thought may be of value here? it's a direct copy and paste.




When shooting Monika at the weekend, I was very kindly assisted by Lola Gavin, a member of my camera club. Lola was also one of the participants at the most recent workshop I gave on flash photography. During the shoot I wanted to demonstrate the effect that shutter speed had on exposure when mixing flash with ambient light. I know I’ve covered this many times before in my various tutorials and workshops on flash (flash 101, mixing flash with ambient etc.), but with that said, now that I have some images to back up the theory I thought it was worth posting them here.

First of all let me acknowledge that these aren’t the strongest images in themselves, but for demonstration purposes I think they serve the purpose very well. When dealing with a pure flash exposure, where there is no ambient light at all, we have various methods of controlling the exposure:

• ISO
• Aperture
• Flash power/duration
• Distance of flash to subject

NOTE: Shutter speed has no effect on the exposure when using flash

When we’re using ambient light, we have the traditional methods of exposing an image:

• ISO
• Aperture
• Shutter speed

If we’re in a situation where we have both ambient light and flash, we’re now mixing flash with ambient and all 5 controls come into play (as shown below).

Shutter speed is the most interesting factor because it is by controlling the shutter speed, that we can mix in more or indeed less ambient light into the shot. Changing any other variable will either effect both the flash exposure and the ambient or change the exposure for the subject.

The three shots above were all taken such that the aperture, ISO, flash power and distance between the subject and model are all constant. For each shot I simply changed the shutter speed. Decreasing it, allowed more light into the shot, brightening the sky (ambient part of the exposure) as is shown on the left image, which is taken @ 1/60. Increasing the shutter speed, to 1/125 for the middle image, I decrease the amount of ambient in the shot, turning an other wise bright evening, that little bit darker. Finally increasing the shutter speed by another stop to 1/250 I can kill most of the ambient light in the shot. However in all 3 cases, the subject is exposed the same as it is the flash that is lighting the subject. Click here to see a larger resolution version of the image.

This technique can be a very creative tool for the photographer. Indeed given a powerful enough shot you can make the brightest day look like night. A system which allows you avail of high speed sync is also advantageous, because it allows you increase your shutter speed to levels which cut out virtually all ambient light, but still allow you expose the subject with flash.

[glcook1]

Ciaran,

Thank you for the tutorial. I do have a couple of questions about it. What was your flash power set for and how far where you from your model?

Thanks,

-lc

[ciaran]

Originally Posted by glcook1

Thank you for the tutorial. I do have a couple of questions about it. What was your flash power set for and how far where you from your model?

I'll gladly answer the questions, but it's important to note that the answers have nothing what so ever to do with the technique above. This technique can be applied using a flash of any power and at any distance from the subject. Obviously the caveat being that the flash is powerful enough to light the subject and overpower ambient.

In the case of these shots, I was actually using 3 flashes (2xSB800 and 1xSB900) on a single tri-flash bracket, fired into an 86" umbrella.



Distance wise, the light source was probably anything between 7 and 10 feet from the model. From recollection, for the 3 images above, it was closer to 10 feet. The flashes were each turned down to about 1/16 power.



This is another shot from this setup. It was taken at just after 5pm in the evening and the flash was used in this case to overpower ambient, making it look darker than it really was.

[sdsanta]

Would this work if the flash was in TTL mode? Or would the flash try to compensate for the change in shutter speed and adjust it's power setting.

[Songman45]

Crazy question: Why three flash units? If they were shooting at 1/16th power each that's roughly half of any one flash unit's power. Did you fire three to completely fill in the umbrella?

[ciaran]

Originally Posted by sdsanta

Would this work if the flash was in TTL mode? Or would the flash try to compensate for the change in shutter speed and adjust it's power setting.

Nope. This most certainly would not work with TTL. TTL tries to give a balanced exposure between ambient and flash. The purpose of this is to deliberately change this balance/ratio. I'm sure playing with exposure and/or flash compensation you could force the camera and flash into producing results like this - but if you're going down the compensation route, why not just do it manually in the first place?

Originally Posted by Songman45

Crazy question: Why three flash units? If they were shooting at 1/16th power each that's roughly half of any one flash unit's power. Did you fire three to completely fill in the umbrella?

Good question There are a few reasons for using 3 flashes.

• More power. For the first 3 shots above, I had the flashes down at 1/16 but for other shots on the day, I needed more light.
• With more heads, you can dial down the power of each flash. The combined power maybe the same as a single flash could give you, but you now have significantly faster recycle times and the battery life for each unit is also prolonged.
• Better spread! The tri-flash head gives a far more even spread in comparison to a single flash unit, particularly when using an umbrella of this size.

[patterfr]

Ciaran
Thanks for the good examples.
Very clear.
( Now, slightly off topic, with the triflash, isn't there anyway to use a single pocketwizard? Is there some sort of "splitter" out there?)

[ciaran]

Originally Posted by patterfr

Ciaran
Thanks for the good examples.
Very clear.
( Now, slightly off topic, with the triflash, isn't there anyway to use a single pocketwizard? Is there some sort of "splitter" out there?)

Another great question

For sure, if you're using three flashes on one bracket it is very extreme, not to mention very expensive to use 3 pocket wizards. The main reason for me doing so was because I had 3 of them with me. But I have looked into different options since.

One option I have considered is breaking out a soldering iron and making my own 1xmini jack to 3xsync cable. Since then, someone pointed out to me that Belkin have what is called a Rock Star splitter which would do the same job (and saves you the hassle of having to make your own cable).

Of course another option would be to use 1 pocket wizard and put the other flashes in slave mode, which again would work fine.

[Songman45]

Thank you for answering my question. I would not have considered the fast recycle time, not that I do much rapid fire portrait work myself. Man, that's an expensive route to take. I know it all depeneds on what equipment you own. The top-end flash units and three pocket wizards plus the bracket to hold them would be prohibitively expensive for my situation. A portable power unit to fire the mono lights would be less espensive. It all comes down to what you have in the stable.

Steve

[ciaran]

Originally Posted by Songman45

Thank you for answering my question. I would not have considered the fast recycle time, not that I do much rapid fire portrait work myself.

I guess that all depends on what you call rapid fire? A lot of flashes on full power can take 3~5 seconds for a complete recycle, on fresh batteries. This can be quite prohibitive. I'm pretty slow and deliberate when I shoot, but I do need to be sure the light is there for when I want to press the shutter and not have to wait for a recycle.

Originally Posted by Songman45

Man, that's an expensive route to take. I know it all depeneds on what equipment you own. The top-end flash units and three pocket wizards plus the bracket to hold them would be prohibitively expensive for my situation. A portable power unit to fire the mono lights would be less espensive. It all comes down to what you have in the stable.
Steve

Don't get too hung up on the equipment I've shown here. You can buy 3 second hand manual (non-TTL) 3rd party flashes pretty cheaply; around the $100 mark. You can also use cactus/eBay triggers as opposed to pocket wizards or indeed just use slave mode. So this setup could be achieved for around the $300 mark.

[Songman45]

Exactly. I have triggers and monolights, but not small power flashes. A vagabond portable power unit runs $250. If I were to throw a picture of that sort of set-up someone would say "that's an expensive set-up" Though It would only cost me $250 additional to make it happen. And I'm sure there are many more people who have multiple flash units than who have multiple studio units. Approach has more to do with what you have in your bag than it does how much you have in your wallet.

Oh, and I think the multiple flash firing into an umbrella is a nifty idea for all the reasons you listed: Less strain and drain on each unit, quick recycle and longer battery life.

[transcape5]

This is just the topic I'm currently interested in. It is now bookmarked so I can access it and the links at a later time. Thanks, Ciaran.

[Rain Lily]

Definitely thank you, ciaran. I'm reading with interest too.

[Briodan]

Thank you for the tutorial. I myself have struggled shooting outside with a flash, and now I know why. I keep it set to TTL and now see that I need to take that to manual too.

I am assuming that the best way to set this type of shot is to use a light meter, but right now I don't have one. I do have an outdoor shoot coming up and I have to figure out how I can use the equipment that I have. In some cases, I have taken my Photogenic light set outside and that has worked well. This time though I need to be a little more portable.

The equipment that I will be using in addition to my D200 (and its flash) is a SB-900 and SB-800. So I figure (and based on what I learned here) that I will use the SBs under one 45" umbrella in commander mode, commanded from the camera flash. Without a light meter, do you have any suggestions?

And, yes a light meter is on my wish list.

Thanks,
Briodan

[deluco]

Originally Posted by ciaran

NOTE: Shutter speed has no effect on the exposure when using flash
.......

However in all 3 cases, the subject is exposed the same as it is the flash that is lighting the subject. Click here to see a larger resolution version of the image.

This is not exactly true as stated. When mixing ambient with strobe, the ambient does have an effect on the exposure. As you can see in your images...the left image has noticably more exposure than the one on the right. That's because the one on the left is getting more ambient light. Also the color will shift as the ambient creeps in. In my experience you can control the background exposure with the shutter speed, but after about two stops in change, you will start to see some exposure change and color shift.

[swampler]

Originally Posted by deluco

This is not exactly true as stated. When mixing ambient with strobe, the ambient does have an effect on the exposure.

It doesn't have any effect on flash exposure, but if it gets slow enough to get the ambient within a stop or two of the flash, then it will affect the subject as well as the background.

[deluco]

Originally Posted by swampler

It doesn't have any effect on flash exposure, but if it gets slow enough to get the ambient within a stop or two of the flash, then it will affect the subject as well as the background.

True that shutter speed won't affect the "Flash" exposure, however, what's important is the overall exposure, not just the flash. I think that when people hear something like, "flash is independent of shutter speed", they take that literally to mean that you can mix ambient and flash and that the exposure on the subject will always be the same regardless of the shutter speed. That's only true until your ambient is providing more than about 70% of the lighting (VERY approximate).

I often shoot at between 1/10th and 1/30th second with off-camera softbox strobe outdoors (with tripod). I try to attain about 50-60% strobe-provided exposure. Fortunately my meter provides the percentages for me.

[ciaran]

Some very interesting points and observations.

Originally Posted by deluco

This is not exactly true as stated. When mixing ambient with strobe, the ambient does have an effect on the exposure.

You obviously have a good understanding ox mixing strobe with ambient. However, as we're trying to be specific, let me be very pedantic and correct you on a few things.

I did not say that ambient does not have an effect on exposure. As you pointed out, of course it can. What I stated, and the purpose of the the thread was that shutter speed has no effect on the flash part of the exposure, so you can use shutter to balance/mix ambient.

Originally Posted by deluco

As you can see in your images...the left image has noticeably more exposure than the one on the right. That's because the one on the left is getting more ambient light.

You are getting mixed up between lighting ratios and exposure. The subject in both shots are actually exposed "identically" (as close to identical as is realistic). There is a difference in the tones, because the shot on the right has more fill from the ambient light, so the ratio between flash and ambient has changed. But the exposure value is identical. This was proved by metering at the time of the shot but can be displayed using the threshold adjustment layer in Photoshop.

The image below, shows all values/tones above 230 on the histogram. It shows an almost identical EV on all three images A lower threshold will obviously show differences as the amount of fill (mid-tones and shadows) varies. But the white points (exposure) are the same.

Originally Posted by deluco

Also the color will shift as the ambient creeps in. In my experience you can control the background exposure with the shutter speed, but after about two stops in change, you will start to see some exposure change and color shift.

Absolutely agreed on the colour shift. But the very nature of underexposing or overexposing means you're going to change the colour/saturation of the light source.

As for giving hard values. the idea of two stops is meaningless really. What's more important is ratio not absolute values. If flash is 10 stops brighter than ambient, then changing ambient by 2, 3 or even 4 stops will have little effect. If the original ratio is closer, then obviously a 2 stop change will have a greater effect.

Originally Posted by Briodan

Thank you for the tutorial. I myself have struggled shooting outside with a flash, and now I know why. I keep it set to TTL and now see that I need to take that to manual too.

Well be careful. I'm certainly not knocking TTL as it most definitely has it's uses. But for ultimate creative control, you really need to have all your equipment set to manual.

Originally Posted by Briodan

I am assuming that the best way to set this type of shot is to use a light meter, but right now I don't have one.

Yes, I do use a light meter, but it's not essential for this work, particularly if you're shooting digital. The joy of digital is that you can take an image and review it immediately, tweaking it where necessary. The joy of light meters is that you take this guess work and iterative process out of the equation.

Originally Posted by Briodan

The equipment that I will be using in addition to my D200 (and its flash) is a SB-900 and SB-800. So I figure (and based on what I learned here) that I will use the SBs under one 45" umbrella in commander mode, commanded from the camera flash. Without a light meter, do you have any suggestions?

Setup the lights in advance of the shoot. Use an assistant to stand where the subject will finally be standing. Place your light and take the shots. Make the necessary adjustments to camera/light as required. When everything is ready, all you need to do is ensure the subjects stand in the same position. And remember, if the ambient light changes between shooting your assistant and subjects, all you have to do is adjust your shutter speed

Originally Posted by swampler

It doesn't have any effect on flash exposure, but if it gets slow enough to get the ambient within a stop or two of the flash, then it will affect the subject as well as the background.

Precisely. And it's worth pointing out again that its down to ratios. If your flash is significantly higher than your ambient then 1 or 2 stops of ambient will not effect the overall exposure. As the ratio approaches 1:1, obviously a shift of either will change the overall exposure.

Originally Posted by deluco

True that shutter speed won't affect the "Flash" exposure, however, what's important is the overall exposure, not just the flash. I think that when people hear something like, "flash is independent of shutter speed", they take that literally to mean that you can mix ambient and flash and that the exposure on the subject will always be the same regardless of the shutter speed. That's only true until your ambient is providing more than about 70% of the lighting (VERY approximate).

Well to be precise again, flash is independent of shutter speed, assuming of course you're within the sync speed limitation of the system. Again the 70% is a very dangerous figure to quote exactly.

Originally Posted by deluco

I often shoot at between 1/10th and 1/30th second with off-camera softbox strobe outdoors (with tripod). I try to attain about 50-60% strobe-provided exposure. Fortunately my meter provides the percentages for me.

I use the Sekonic-758 (I presume thats the one you're talking about) and it is very useful for this kind of work, because it does provide percentages in terms of ambient to flash mix. So a lot of the guess work is taken out of the equation. But with that said, once you grasp the concept of ratios, this stuff becomes really straight forward.

[frannyk]

Thanks for the tutorial!

[bogiesbad]

Thank you for sharing this. I find your explanations here and the examples on your site are very helpful and easy to understand.