International Photographers Association of Los Angeles 洛杉磯國際攝影家協會

在用5DM2和580EX2時，我發現以下奇怪的現象。因對Canon機還是新手， 希望大家能代為查證一下， 謝了。

When 5DM2 in APERTURE priority mode (24-105 lens at F4 max opening) with the latest 580EX2 flash turned ON, the camera does NOT pull the proper amount of light power from the flash, instead, it seems to give priority to shutter by slowing it down to 1/10 or even 1/8 of seconds to draw in more light. Consequently, the resulted photo becomes blurry and useless photo. I normally use shutter speed control when using flash. In one incident, I forgot and left the setting at Aperature priority mode and ran/discover this issue.

I had later reproduce this scenario (Aperture priority + flash) and verify that the camera does ignore the presence of the flash and tried to adjust the shutter speed first.

My question is: Is this normal in the Canon machines (or specific to 5DM2?). It does not make sens for the camera to ignore the power of the flash and UNDER UTILIZE it. It does not make sense to give primary consideration to the shutter speed instead of pumping up the flash power. When examining the photos, I can see that the flash DID CONTRIBUTE some light to the exposure but NOT STRONG enough so that the camera shutter speed WAS ADJUSTED to very SLOW setting (I also verify that it was not caused by probably weak battery because when I change the camera setting to shutter priority, the exposure became normal).

I have never seen this in Nikon camera. This behavior is really against normal expectation. Could this be caused by the fact that the 24-105 has IS feature and the camera was trying to take advantage of it. Even if that is the case, it's silly to trust the IS as it does not guarantee 100% result at all. I feel like something is wrong with the camera/flash in Aperture priority mode. But when setting the camera to SHUTTER priority mode, then the exposure is normal...

Could someone explain if this is normal and if it's so, what is reason for this kind of control approach?


I appreciate very much your time and help in this matter.

_________________
Paul Cao
http://www.flickr.com/pcaophotography
Nikon D70 + Kit Lens

That's how Canon EOS flash TTL works. It's different from Nikon. Av (aperture priority) and Tv (shutter speed priority) modes set the shutter speed or aperture to expose for the existing light conditions correctly. They then fill in the foreground using flash. If light levels are low you will need a tripod to avoid blur.

EOS flash photography confusion.

The main area of confusion in EOS flash photography is the fact that P, Tv, Av and M modes handle flash illumination differently, especially when ambient light levels are not bright. Here’s a summary of how the modes basically work when you have a flash unit turned on. This summary assumes that you do not have high speed sync flash enabled if that option is available to your particular camera and flash unit combination.

Mode Shutter speed Lens aperture
P Automatically set from 1/60 sec to the camera’s maximum X-sync speed. Automatically set according to the camera’s built-in program.
Tv You can set any shutter speed between 30 seconds and the camera’s maximum X-sync speed. Automatically set to match the shutter speed you have set.
Av Automatically set between 30 seconds and the camera’s maximum X-sync speed to match the lens aperture you have set. You can set any lens aperture you like.
M You can set any shutter speed between 30 seconds and the camera’s maximum X-sync speed. You can set any lens aperture you like.

And here are the details:

Program (P) mode flash.

The overriding principle of Program (P) mode in flash photography is that the camera tries to set a high shutter speed so that you can hold your camera by hand and not rely on a tripod. If that means the background is dark, so be it.

Program mode operates in one of two modes, depending on the ambient (existing) light levels.

1) If ambient light levels are fairly bright (above 13 EV) then P mode assumes you want to fill-flash your foreground subject. It meters for ambient light and uses flash, usually at a low-power setting, to fill in the foreground.

2) If ambient light levels are not bright (below 10 EV) then P mode assumes that you want to illuminate the foreground subject with the flash. It sets a shutter speed between 1/60 sec and the fastest X-sync speed (see above) your camera can attain. The aperture is determined by the camera’s built-in program.

Because the camera tries to keep the shutter speed at a reasonable speed for handholding the camera you will end up with dark or black backgrounds if you take a flash photo in P mode when ambient light levels are not bright.

On most if not all EOS cameras, P mode is not shiftable when flash (internal or shoe-mounted Speedlite) is used. Note also that DEP mode cannot work correctly with flash - its metering settings basically revert to P mode if you try it.

Tv (shutter priority) mode flash.

In this mode the camera lets you change the shutter speed. It then automatically chooses an aperture setting to expose the background correctly. Flash duration (flash output) is determined by the flash metering system. In other words, the camera always works in fill flash mode when it’s in Tv mode - it always tries to expose the background adequately, unlike P mode.

If the maximum aperture value of your lens starts flashing in the viewfinder it means the background of the scene you’re shooting is too dimly lit. If you want to try and expose the background then you should decrease the shutter speed to compensate. Otherwise the camera will just try and expose the foreground with flash and the background will come out dark. Naturally at slower shutter speeds you’ll need to use a tripod to avoid blurring caused by camera shake.

As always, the camera will prevent you from exceeding its built in X-sync speed unless high speed sync is available to you and engaged. If the minimum aperture value of your lens starts flashing then your scene is too brightly lit. You must then either engage high speed sync if it’s available or perhaps put a neutral density filter on the camera or use slower film or a lower digital ISO. Or turn off flash altogether and simply use a reflector of some type to bounce ambient light onto the subject.

The 420EZ and 430EZ flash units will operate in A-TTL mode in Tv mode, but the 540EZ works only in TTL mode. Note that in this mode type A camera bodies underexpose the background by up to a stop when light levels are low and an E-TTL flash unit is engaged. If this is the case try testing by comparing the aperture setting with M mode, which does not do this. You may need to apply exposure compensation if this effect exists on your camera and is undesirable.

Av (aperture priority) mode flash.

Av mode lets you set the depth of field by specifying the lens aperture. The camera then chooses a shutter speed ranging from 30 seconds to the camera’s X-sync speed, in order to expose the background correctly. If that means the shutter speed is some really low value so that you need to use a tripod to avoid camera-shake blur, so be it. In dark conditions, therefore, Av mode works in slow sync mode.

Flash duration (flash output) is determined by the flash metering system. Like Tv mode the camera always works in fill flash mode when in Av mode.

There is one exception to this. A number of EOS cameras have a custom function you can set to ensure that the shutter speed in Av mode when using flash is locked to the X-sync speed. The EOS 10/10s and Elan II/EOS 50, for example, have such a custom function, which lets your camera behave more like P mode when in Av mode. However this custom function will only lock the camera to X-sync in Av mode and will not choose a shutter speed from 1/60 sec to X-sync, the way P mode does.

As always, the camera will prevent you from exceeding its built in X-sync speed unless high speed sync is available to you and engaged. If the shutter speed value of 30" flashes in the viewfinder then there isn’t enough light to expose the background correctly and you’ll need a larger aperture or faster film or higher ISO. If the camera’s X-sync flashes in the viewfinder then you’ll need to decrease the lens aperture, engage high speed sync if it’s available or use slower film or a lower ISO.

The 420EZ and 430EZ flash units will operate in A-TTL mode in Av mode, but the 540EZ works only in TTL mode. Note also that some people have reported that in this mode their type A camera bodies underexpose the background by up to a stop when light levels are low and an E-TTL flash unit is engaged. If this is the case try testing by comparing the shutter speed setting with M mode, which does not do this. You may need to apply exposure compensation if this effect exists on your camera and is undesirable.

Manual (M) exposure mode flash.

In manual exposure mode you specify both the aperture and shutter speed, and your exposure settings will determine how the background (ambient lighting) is exposed. The subject, however, can still be illuminated by the automatic flash metering system since the flash can automatically calculate flash output levels for you. This is a marked contrast to the olden days, when photographers would carry around little flash exposure tables with them in order to work out manual flash settings.

This is how flash works in manual mode. Note that we’re talking about the manual exposure mode setting only, which can use automatic TTL flash metering (it will not use A-TTL metering in manual exposure mode). Also, we aren’t talking about setting the output of the flash manually - that’s manual flash and a different topic altogether.

* Set your camera to M for manual exposure mode.

* Set the aperture and shutter speed to expose the background correctly.

* Press the shutter button down halfway if your flash has a rear-panel LCD (liquid crystal display). The flash coupling range will appear in the flash unit’s LCD. This range is the distance that can safely be covered by the flash.

* If your lens has a distance scale you can check the current focussing distance to ensure that the distance to your subject falls within this range. Otherwise you’ll have to estimate.

* If the “flash ready” lightning bolt symbol appears in the viewfinder you can press the shutter all the way to take the photo. The flash’s TTL or E-TTL system will determine the flash exposure level of the subject.

If your flash lacks a rear-panel LCD you won’t have a preview of the flash coupling range, of course. Also, LCD-equipped flash units will not calculate the flash coupling range if you’re using bounce flash, and the coupling range will not necessarily be correct if you have a diffuser on the flash head.

Some Speedlite flashes, such as the 540EZ and 580EX, can display the coupling range in either feet or metres, depending on which measurement system has been set by the small switch in the battery compartment. Others, such as the 430EZ, are hardwired to one measurement system or the other, depending on where the flash was sold. US market flashes used feet and all other countries on the planet* had only metre flashes available to them. And the 580EX II can set the measurement system through a custom function.

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Live life by design, not by default.

Hi James,

Wow... you are really a great guy around here. Your knowledge, your help, your enthusiasm are sincerely appreciated.... I thought you are more of a Nikon guy but definitely I was wrong.

Since posting the question yesterday after noon, I couldn't wait but went online and did some search on that... I've found an excellent article about the Canon flash, its functionality and design approach. What you wrote is exactly right.... bravo

Like you've said, Canon's flash emphasizes on "FILL-flash" when it's set in A and T mode. In order to have full control or not to have FILL FLASH priority, one needs to use the M or P mode. If one does not want to allow the A mode to have FILL-FLASH priority, one could go into a special user function mode in the camera to change it from auto to 1/200 (I have not looked into this yet but basically this seems to allow you to have the max sync-speed while controlling the depth-of-field, which in a way is the same as using M set at A=x, and T=1/200 anyways).

I was afraid the camera would not communicate with the flash properly when it's set in M mode and that is why it's easier to use it in T mode (but you lose control of the depth of field in this case). Besides, T mode has one of the most important and critical use, that is also to control the flash-to-ambient light mixing ratio which is very critical when shooting under bright ambient light (like outdoor under the sun. It's a totally different and critical subject to discuss). Now that it's clear (and I have just verified this on my camera) that even in the M mode, the flash will adjust the power correspondingly, I will free ease to use the M mode more often. Of course, the flash can only do this within its "capability range and sync speed". That is, it will work within a fixed range of aperture and shutter settings. Outside of this range, the user would have to change the setting such as bumping up the iso or widen the aperture settings.... In a way, the M mode will operate in semi-automatic mode.

Actually, it's good to have the fill-flash mode somewhere (that is the A mode). Before posting yesterday, I've realized that it's great to use the A mode to shoot some indoor still subject as the lighting is very pleasant due to the weak fill flash, however, shooting human subjects in door (with normal indoor light and one window light), the A mode (fill-flash driven and very dominant) is pretty much helpless as humans just can't (learn to ) stay still even you shout "damn it, don't move" (haha).

Since we are on the subject, may I add the link of the article I've found yesterday below to help broaden our understanding on this subject.

http://dpanswers.com/canon_flash.html

Again, James, you are a great guy!

_________________
Paul Cao
http://www.flickr.com/pcaophotography
Nikon D70 + Kit Lens