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Tara R. Swartzendruber
 

Help Me Understand: Flash in Theater


Tonight I was taking some test shots in a rather dark theater with my speedlight on camera. I wanted all in focus, so I chose f11 on aperture priority. The camera selected 1/60 shutter and the photo was quite underexposed. So, I switched to "P" mode and the camera chose 1/60 shutter and 5.6 aperture and the photo turned out pretty good. So, why didn't the camera choose a slower shutter speed when I set the aperture at f11 to get the correct exposure?


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July 07, 2010

 
- Gregory LaGrange

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  Assuming that the flash is dedicated to whatever type of camera you had, since you were on aperture priority, the camera read you had the flash on and selected its default sync speed. However, using f/11, the flash probably isn't strong enough to properly light something that's at a typical distance of a theater stage with that small of an aperture.
Also, when you use auto exposure with flashes (whether the camera is on auto or the flash is on auto), the sensors that read the scene can't know what it is that you want to light. So the stage in the distance may have been severely underlit, but perhaps the area directly in front of you was properly lit. Your flash may even have an infrared sensor that factors in object distance, and makes its calculations and then assumes that since you chose f/11, based on the brightness readings from the scene that you're trying to light just what's within 8ft in front of you.
On p mode, the camera chose a better combination of aperture/flash power. Really, just any big area or anything that's at a distance that's in a very dark place, f/11 isn't going to be good with just a hotshoe flash. If there's a distance scale on the back of your flash, you'll probably see that it indicates that if the aperture is f/11, then the distance that the stage was, is beyond the range of what the flash can do at that aperture.
Also note that on p mode, it's going to be set up to not go below a certain setting, like the sync speed of 1/60. It's like the microwave settings for people who really don't know how to cook. You have some meat but you have no idea how long to cook it, so you hope there's a "meat" button. Beginners usually aren't adept yet at holding a camera steady, so P mode will never go below 1/60 but will change the ISO up if needed.


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July 08, 2010

 

Tara R. Swartzendruber
  Thanks for a wonderful response. It has prompted a few more questions....

1. if I take a photo of the overall scene, how do I ensure that it is fairly sharp if I have to go down to say, an aperature of 5.6? With one person that's fine, but with more than that, I'm concerned with clarity.

2. does "P" mode typically change the iso setting if it needs to? I usually pre-choose my iso based on the situation and haven't noticed that it changes from that. (I have a Nikon D700).

3. In this situation, are there any tricks/tips to know what settings are best to use? I do have center weighted metering on, but I didn't zoom in to anything in particular, just "shot the stage" so to speak.

4. My flash is an SB800. I have it set to TTL. I haven't noticed if there is a distance meter.

Technically, I'm not a beginner. I have a home studio and do lots of work in my studio and outside with kids, babies, etc.... but haven't really ventured into other areas such as theater lighting, weddings, etc....so it's another area to learn/understand.....

Thanks again, this is very helpful!


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July 08, 2010

 
- Gregory LaGrange

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  Depth of field depends on focal length, distance, and aperture. If a small aperture isn't practical, then you have to use a workable compromise. Choose a good focal point. If this is an act that's in process on stage, then what can't be done can't be done. If people are in the back then you don't worry about that. Focus on the front, or some key element, and everything else will work itself out.
You can't use barely any depth of field for a good face shot if you get the eyes in focus. You get the focal point back at the ear, people won't say the ear's in focus. They'll say the picture, as in whole picture, is out of focus.
You're not a beginner, but that's what P mode was intended for. People who just want to grab and go. P mode tries to keep everything middle of the road. Doesn't pick the biggest aperture because of focusing/depth of field concerns. But it tries to keep the shutter high enough for motion blur. On film cameras, P mode would automatically activate the pop up flash. Or sometimes it would beep, or do something to tell you that you needed a flash. With a film camera, you're stuck with the iso of the film. A digital camera, you're not. So P mode in a digital may change the iso for if it needs. Depends. Could be yours doesn't, but on a more expensive Nikon it will.
Stage shots you have to do based on what's actually lit, or how reflective something is, not on what's seen in the entire viewfinder. Not just spotlights, but even general lighting of a scene, the curtain or the background may be dark to make everything stand out more. That's why people recommend to get away from using P mode. I don't know if this is just the stage and how it's set that you're intending on getting, or if it's some kind of show or play that's going to be going on.
I doubt you'll find a distance scale on any dedicated flashes. Although if your display is lcd, something may come up like it if you switch your flash to manual.


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July 09, 2010

 

Randy A. Myers
  Here is some basic info of flash as far as aperture and subject distance are concerned. GN is for the Guide Number of the flash in case you didn't know that already.

The basic formula is: GN = Aperture x Distance
or
Aperture = GN / Distance
or
Distance = GN / Aperture.

I don't agree with one thing that was said in response to your question. P-mode does not select a higher ISO if needed. The ISO only changes automatically if AUTO-ISO is set. If Auto-ISO is not set, P-mode will not change the ISO. If Auto-ISO is set to be on, then it will change the ISO in all modes, even manual mode. You have to pick the slowest shutter allowed before Auto-ISO kicks in. You also have to pick the highest ISO you want Auto-ISO to use. Hope this helps.


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July 10, 2010

 

Tara R. Swartzendruber
  Thank you for clarifying the ISO issue. That's what I thought too since I never set the ISO to "AUTO-ISO", I didn't think it ever changed it unless I changed it.

Also, this whole flash thing has me confused. I generally set my flash to i-TTL and let it "do it's thing," but I assume that isn't probably the best thing to do as the flash doesn't know how far away my subject is. Any futher illumination you can do on this would be so appreciated.
Do you just guess at how far the subject is if you can't actually measure? Is this always measured in feet? What are the numbers on the back of my flash that are indicated in meters?

Thanks!!


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July 10, 2010

 

Randy A. Myers
  Not sure what you mean about the numbers indicated by meters unless you are referring to the lens mm displayed. That is the zoom of the lens if it is in range of the flash display. 28mm means the lens is zoomed to a focal length of 28 mm. It changes as you zoom the lens, the flash head zooms internally to match the focal length, up to a max of 85mm for my SB-600. After that it stays at the max 85mm zoom. Flash doesn't go very far. That's why event and wedding photographers use very fast lenses.


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July 10, 2010

 

Tara R. Swartzendruber
  but if a lens is fast (meaning very wide aperatures) then HOW do they keep everything in focus in a group, say standing in a "v" or in various levels?


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July 10, 2010

 
- Gregory LaGrange

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  If it says M, it's meters. If it says mm, it's millimeters for focal length. TTL is supposed to be better than just a regular sensor on the flash head because because the camera communicates it's info from the camera sensor and focal length to the flash.
Guide numbers are used to indicate a flashes power, and also so you can figure out settings without the use of a flash meter or any auto capabilities of the flash. And it's usually based on 10ft, at iso100. At 10ft, full power, you'd need an aperture of what the guide number is. So if you're shopping around for flashes, ones that have higher guide numbers have more power.
And if you need to figure out aperture, you start at the guide number(say it's 320). Full power at 10ft is f/32. So I'm using half power flash at 20ft. Half power takes you down an f/stop, f/22. A double of the distance takes away two more f/stops, so you're down to f/11.
It's based on 10ft because you can't do a straight division with changes in distance because light falls off exponentially as distance increases.
If you have to shoot a group at a wide aperture, then shallow depth of field is just what you have to deal with. Otherwise, you find a way to increase the aperture. Drop the shutter speed and tell everybody to stand still. Increase the iso. Or add some extra light so you can close down the aperture.


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July 10, 2010

 
- Gregory LaGrange

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  I want to also add that if you look around, you may not see the guide number expressed as a single number, but as the f/stop.


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July 10, 2010

 

Tara R. Swartzendruber
  thanks to both of you for such concise answers.
This whole flash math is complicated. I will need to study a bit more what you have said to see if I can make good sense of it. Is there a good book that breaks this down to the basics for a beginner on flash photography?
what you said about the group makes sense. I get confused sometimes when people talk about having a great fast lens for low light shots. While I understand the advantages, I also see many of the disadvantages of using the lens at it's "fastest" wide open aperature. I just wanted to be sure I wasn't missing something obvious here!


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July 10, 2010

 
- Gregory LaGrange

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  Don't know any books off hand. Most photo books mention it but go mostly into hard light vs. diffused light, color cast of artificial light, etc...
Try thinking about this. You have a flash that can light up an object 2ft away at f/16, iso100. And it doesn't even have to be at full power. For all practical purposes, you're lighting that object up just as bright as if you were out in sunlight.
But what happens if you use the same flash, same power, but light up something 4ft away. What you'll find is that what was f/16, will now be f/8. That's two f/stops lost. But with sunlight, why can you be at one point at f/16, but have something be 400 miles to the west and have the same f/16? Because the relative distance change so drastically with the flash. So because 2ft to 4ft is a double of the distance, two f/stops lost is 4 times the light. Now the sun is strong enough to travel millions of miles to the earth. So the increase of 400 miles isn't going to make a perceptible difference in it's brightness.
Something else, if I use the same flash at 2ft example, and then increase from 4ft, and make another increase to 6ft, I'll end up with 2ft=f/16
4ft=f/8
6ft=f/5.6
Compare the 2ft to 6ft, you'll see that's 3 f/stops lost all together. Now 2ft to 4ft(2ft increase) is 2 f/stops lost. But 4ft to 6ft(same 2ft increase) is a 1 f/stop loss.
So, if I have a flash that can reach out 10ft and light to f/16, if I add 2 more feet to make it 12ft will I lose a couple of f/stops? Probably won't even lose half and f/stop.


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July 12, 2010

 
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