M-Mode and Flash really works, hunh?

[LexLuther]

So this past weekend I was messing around with the camera and flash (20d&580EX)while visiting the in-laws and decided to throw it into 'M' mode. Picked f8 and 1/125 based on what I think I saw someone state here in the forums. Ofcourse the camera was telling me that it would be underexposed, and comparing it to the exposure settings 'P' mode was telling me (without flash) I thought the shot would pretty much be useless. Funny thing is, it came out perfect, and it's bothering me because I can't seem to wrap my head around the concept. Up until that night, I was strictly a 'P' mode type of person, but this might sway me.

My biggest problem to date with flash was always being stuck at f4 and having people in the front row of a shot being in focus, while people a little further behind went out of focus.

[Ogrt48]

I think it was saying it'd be under exposed because it's metering for the ambient (sp) light and not the light from the flash in the room.

[robertwgross]

Yes, before the shot, the camera metering always shows you what would happen with ambient light. Then, at least in some modes, in a tiny fraction of a second the pre-flash pops, the pre-flash reflected light is metered in the camera, then the main pop happens, and then the sensor captures the light. So, the only time the camera really adjusts for the flash exposure is in a very tiny fraction of a second. That is what throws off many users.

However, go back and look at the actually exposure settings after the shot. See what it really used, not what it told you in advance.

---Bob Gross---

[OviV]

Bob,

I do not believe the exposure settings are changed at all. If I understand it correctly, when you have your camera in M mode and your flash in ETTL mode the camera will try to compensate for your exposure settings with flash to supplement the ambient light in order to get propper exposure.

[LexLuther]

Well, I know I can shoot in my basement all night long in P-Mode where it always defaults to f4 @ 1/60, and when I process the RAW files that is what shows in the exif data.

Av mode doesn't work very well for me in dark conditions, because it seems to always slow down the shutter speed a great deal as if there wasn't going to be a flash. I can understand this, because in Av mode the flash is switching into "Fill Flash" mode, and I think it'll be very interesting to see how this works when I'm on the beach in Cuba for my brothers wedding.

Tv mode has me a little confused, but I haven't used it much.

M mode is what has me really scratching my head. I really hate just shrugging it off and thinking "It works, why try to figure it out" because I'm sure there'll be a time when I NEED it to work and it ends up not working.

(I'm getting so tried of shooting in my basement as well, bleh!)

[robertwgross]

OviV, these results change as we change from M to P and back.

---Bob Gross---

[OviV]

Bob,

Understood but they do not change if you stay in M mode. Only the flash power is adjusted by the camera.

Lex,
It works in M mode and not in Av because in Av the camera adjusts your shutter speed for ambient light whereas in M mode, you set the apperture and speed and the camera compensates with the flash. I guess it is still trying to fill flash. I'm sure Scott will correct me if I am wrong.

Ovi

[PacAce]

Well, I know I can shoot in my basement all night long in P-Mode where it always defaults to f4 @ 1/60, and when I process the RAW files that is what shows in the exif data.

In P mode, if you turn off the flash, you will find that the camera will set the aperture to f/4 (if that's the fastest available) and the shutter to the same speed that would be selected if you were to set the camera to Av and use f/4. However, if you turn on the flash, the camera will automatically set the shutter to 1/60 (or a higher shutter speed if ambient light is very bright) because it assumes that what you want is a flash picture and don't care about the ambient lighting.

Av mode doesn't work very well for me in dark conditions, because it seems to always slow down the shutter speed a great deal as if there wasn't going to be a flash. I can understand this, because in Av mode the flash is switching into "Fill Flash" mode, and I think it'll be very interesting to see how this works when I'm on the beach in Cuba for my brothers wedding.


In Av mode, the camera assumes that you are interested in using the ambient light as your main light source so it meters as if the flash isn't being used. With aperture set the f/4, the shutter speed selected should be the same as that selected when the camera was set the P mode assuming we're still in the basement, as above. The difference is that the shutter remains the same even if you turn on the flash. When you take the shot, just before the shutter opens, the camera fires a preflash to determin how much flash output is needed to provide fill lighting. Then the main flash is fired when the shutter opens. The amount of flash power used is determined by the camera. Since the main source of the light is assumed to be the ambient lighting, the flash will only only output enough light to provide fill lighting.


Tv mode has me a little confused, but I haven't used it much.

In Tv mode, the camera works just as it would in Av mode, except that you are selecting the shutter speed and letting the camera select the aperture, whereas in Av mode, it is reversed.

If the aperture available on the lens is not wide enough for proper exposure at the shutter speed selected, the aperture will blink to let you know that. However, if you turn on the flash, the camera will take a picture and use the flash output to get the proper exposure to compensate for the lack of ambient lighting.


M mode is what has me really scratching my head. I really hate just shrugging it off and thinking "It works, why try to figure it out" because I'm sure there'll be a time when I NEED it to work and it ends up not working.

(I'm getting so tried of shooting in my basement as well, bleh!)

In M mode, the camera works basically the same as in Tv mode except that now you are selecting both the aperture and the shutter speed. If the combination of the aperture and shutter speed does not provide for the correct exposure with ambient lighting, the camera will let you know that. However, if the flash is attached and turned on the camera will again compensate for the lack of ambient light by using the light from the flash and you should get a well exposed picture.

Note: The above scenario assumes that the subject is within the distance range of the flash at the aperture used. If not, then the picture will come out underexposed.

[Tom W]

Like Leo says, the flash will adjust its brightness to make proper (or close to proper) exposure, regardless of your settings. Well, unless your settings are outside the range of what the flash can produce.

[scottbergerphoto]

It's all in the EOS Flash Sticky ;)
Regards,
Scott

[toddb]

I almost always shoot in "M" mode when using the flash, but watch the histogram because I know I lose allot of the flashes power when I bounce it, so I'm usually adjusting the ISO manually allot depending on my distance. I've done it enough now where I don't really even think about it when shooting. By far my favorite (and best photos I think) when shooting like this.

Thanks for all those great links and comments about theses flash guns Scott! Good to know stuff.

[LexLuther]

Thanks PacAce and rest. I was slowly coming to this conclusion myself. Scott, I am awayre of the EOS Flash Sticky, and trust me when I say I've read through it quite a few times and each time gained a bit more knowledge.

Last night here in my shop, where I have a lot of low hanging Fluroescent Lights and a 25' high ceiling the flash was working wonderfully in the different modes, and being far away from objects/machinery showed clearly what I could gain going from one mode to another. Ofcourse, most of that was thrown out the window when trying the 45° bounce flash + Cue Card. There was no way in the world it would light up enough of the scene. When my wife showed up though, the cue card started working again and she was well exposed, and I greatly noticed how raising ISO would brighten up the scene behind her.

Thanks again, and sorry for bringing up something which people have obviously tied to put to rest with the huge EOS Flash Sticky.

[scottbergerphoto]

.......I greatly noticed how raising ISO would brighten up the scene behind her.


Good point on how to use the ISO to not only increase the effective distance of your flash, but also to contol the background exposure.
Scott