How to let 580EX II determine the strength of flash required when in bounced position?

[csphua]

Hello,


I just bought my 580EX II not long ago. I mounted it to my 50D, with 18-200mm, f3.5-5.6 kit lens. I found that the auto zoom function is not available when I set it to bounced position. I have no choice but to set flash exposure compensation manually, and sometimes need few attempts to get it right.

Is there a function for 580EX II to determine the strength of flash required based on the distance to objects when it is in bounced position, like the auto zoom function when the flash is put to forward position?


If no, then what is the best way to set flash exposure compensation / the strength of flash when in bounced position? How do I use bounced flash for object far away?

[peety3]

Bounce flash may not be successful for an object far away - you're likely illuminating a lot of the near area and not getting much light to the distant subject. You may need to aim the flash directly towards the subject at those distances. Do you have some samples to show us?

[csphua]

I had the same result like what you mentioned. A lot of bright area from near area and my subject underexposed at the far end.


If aiming flash directly to distant subject, it would be the same as using built-in flash, can see harsh shadow. Any tips on how to avoid harsh shadow for distant subject in this case?

[peety3]

Sadly, I don't think there's much of a solution with on-camera flash. A remote flash, bounced upwards but from a spot much closer to the subject, might be the best way.

[Mark Elberson]

Any tips on how to avoid harsh shadow for distant subject in this case?

You may want to try using a Sto-Fen Omni-Bounce. I have one more my 430EX and I rarely shoot without it. The light is less harsh and much more pleasing.

[George Slusher]

Before I get started, have you read the manuals for the flash and the camera? They will answer a lot of your questions. Most of what I'll post here is in those manuals.


It sounds like you've confused auto-zoom with the E-TTL exposure system. They have essentially nothing to do with each other. The 580EX II will determine correct exposure even in a bounced situation--wedding photographers use that all the time. It will also work with the zoom set to manual--I've done that many times, especially when using a diffuser or softbox.


What the zoom does is to set the angle of the flash a bit larger than the angle of the lens. That doesn't affect autoexposure, but it does affect the "efficiency" and coverage of the flash. If the flash were set at 24mm and you were using 105mm, then much of the light from the flash would illuminate areas not in the frame. Similarly, if the flash were set at 105mm and you were using 24mm, the flash wouldn't illuminate the entire frame.


Before going into this particular situation, a bit about flash and exposure. Flash exposure is based upon two things that don't affect ambient light exposure: flash output ("power," though it's really the duration of the light pulse) and distance. The flash output is often represented as a "Guide Number" (GN) at a particular ISO and zoom setting. The GN is the product of aperture and distance. (That's why the GN has dimensions of feet or meters.) If you know two of the three factors--GN, aperture, distance, you can determine the third. With manual flash, we usually know the GN and distance, so we can determine the aperture. Conversely, if we know the distance and the aperture we want to use, we can determine the GN required. In effect, that's what the E-TTL II system does, though it does it differently. (Your 50D can pass distance information from some lenses to the flash, but that's not necessary.)


Autoexposure in flashes can be done in several ways. The first such flashes were self-contained, like my Vivitar 285HV. The flash has a sensor that reads the reflected light from the flash. When the sensor determines that enough light has been received, it shuts off the flash. You can often set the sensor for various "levels"--it usually puts a neutral density filter over the sensor. A mechanical "calculator" (like a slide rule) on the flash body lets you determine the range of distances that the flash can handle for each setting.


The other method is "TTL"--through the lens. TTL systems have gone through several generations. The first used a sensor in the camera that measured light reflected from the film surface. That doesn't work very well with digital cameras, so the current Canon E-TTL II uses a low-power (very short duration) pre-flash to determine the exposure. In most cases, that is how your 580EX II and 50D will work.


That should explain why the autozoom doesn't have anything to do with the E-TTL II autoexposure. If there is enough light available from the flash, the E-TTL system will give the correct exposure. (See below, however.)


What the zoom setting does do is set the angle of coverage, the angle (cone) in which the light is concentrated. The zoom does affect the GN. At a particular distance, a longer zoom setting will concentrate the light into a smaller area, illuminating that area more brightly. (You can easily see this by manually setting the zoom to the maximum--105mm and set your lens to 18mm. Take a flash photo of a wall maybe 30 ft away. You should see a bright spot in the middle of the photo.) The longer zoom settings on the flash will give a higher GN, but that applies only to the area illuminated.


Now, why is the autozoom turned off when the flash head isn't facing directly forward? Simple: the light will spread out more because it will travel a longer distance, so the lens zoom setting is not the same as the flash coverage angle. The distance information from the lens & camera doesn't mean anything, either. What counts is the actual distance the light travels and how it gets there. However, the E-TTL system does take this into account, as it measures the light that actually arrives at the lens. (Be thankful that you don't have to manually calculate bounced flash exposure, as I once did.)


Bouncing the flash will lower the effective GN because the light travels further than the straight-line distance to the subject and there will be some light loss at the surface it bounces off. (The light will also pick up the color of the surface.) Still, the E-TTL system will figure the correct exposure, as long as the flash has enough output. Just try it--set the 580EX II to E-TTL and shoot away.


A few other points:


- Flash exposure compensation works much like "regular" exposure compensation when the flash is the only or primary illumination that counts. In other words, setting the flash exposure compensation to +1 will overexpose the image by one stop. The primary uses for flash exposure compensation are when the E-TTL system doesn't give the results you want (see below, however) and when you want to use "fill" flash with ambient light. In the latter case, you'd set the flash exposure compensation to -1 (roughly) so that the ambient light will be the primary light and the flash will fill in the shadows.


- Your camera and flash work differently depending upon the camera's "mode" setting. In Program mode, the camera (probably--most Canon cameras do this) considers the flash to be the main illumination. It will set the shutter speed and aperture. (The actual settings may depend upon the lens' maximum aperture and the ambient light.) If you use aperture priority, the camera will set the shutter speed to give a correct exposure for the ambient light. This can be useful when you want to highlight a nearby subject with the flash but, at the same time, show a darker background in low light situations. The camera will be set for the general illumination with a slow shutter speed and the flash will correctly expose the nearby subject. (See your manual, probably under using the built-in flash--my 30D manual doesn't repeat this in the section on using an external flash.)


- If you use a direct flash as a primary light source, it will always result in nearby objects being brighter than objects further away. Remember that flash exposure depends upon distance. If your main subject is, say, 8 ft away and the background is 16 ft away (twice as far), then the background will receive only 1/4 the illumination of the subject (the inverse square law). Thus, it will be two stops less bright. (You can see the same thing with the GN. If the GN wee 88, at 8 ft, you'd use f/11. At 16 fit, you'd use f/5.6, two stops more open.


- When you ask about using bounced flash for an "object far away," how far away are you talking about? Remember that bouncing the flash reduces the effective range of the flash. This is especially true if the bouncing surface is also far away (e.g., a high ceiling). (I assume that you're talking about bouncing the flash indoors; it doesn't work very well outdoors.)


If you want even illumination, with little effect of distance, you'll have to be in an enclosed space and use some sort of general diffuser that spreads the light around all over the room. The light will bounce off the ceiling, walls, even floor and, if the flash is powerful enough, can give a general illumination. (You'll often read about "bare bulb" effects. That refers to some big strobes where you can take the reflector and lens off, leaving the flash tube exposed, usually on top of the strobe body.) The Sto-Fen diffusers work this way, though they lose a lot of light. Gary Fong's Lightspheres and WhaleTails lose a bit less light, but they're bulky and expensive.


However, these devices don't provide much control over directional light--e.g., you want the light coming from the right or left. For that, you can use bounced flash, though you wouldn't just tilt the flash head--you'd also swivel it. Bounced flash with the flash head pointing directly ahead and up can result in "raccoon eyes" in people, as the light is coming from above and casts dark shadows over their eyes. (One flash I had for my old Minolta camera had two bulbs--the big one that tilted and swiveled and a smaller one that pointed straight ahead to overcome that problem.)


There are lots of flash modifiers around. Some give that "bare bulb" effect (Sto-Fen, Lightsphere, WhaleTail, Lumiquest Ultrabounce, etc.), while others direct light forward by bouncing off something (Lumiquest Pocket Bouncer, et al, and many other brands) or by using a "softbox" to diffuse the light, softening shadows and giving a bit wider and more even coverage. In effect, they make the light source larger and thus less harsh. Most will work with E-TTL. There's no one "best" system for all situations. A lot depends upon technique, as well.

[George Slusher]

I should have been more clear & complete on how the flash works in Program mode. As I said, it depends upon the ambient light. With the flash turned on, the camera will determine a correct exposure for the ambient light within limitations, including a shutter speed of 1/60-1/250 and the available apertures on the lens, though my 30D seems not to open up wider than f/4, even with an f/2.8 lens. If it can do this, the flash becomes a "fill flash," rather than overpowering the ambient light. Thus, you'll get a normally-exposed picture with some "fill" in the shadows from the flash. If there isn't enough ambient light, the system will generally set the shutter speed to 1/60 (it may depend upon the camera) and the aperture to f/4 or the widest the lens has, if it's not as wide as f/4. Then, the flash will be the primary light. You can force the flash to be the primary light by using manual exposure and setting it to underexpose for ambient light. (E.g., set the shutter speed to 1/250 and use a smaller aperture than required for the ambient light.) The flash will still work with E-TTL--using manual exposure on the camera is not the same as setting the flash to manual mode.


For a LOT more, check this Photonotes article. Scroll down to the E-TTL section for more on how E-TTL works. The article is quite long and extensive--there are two more pages.