TV Setting with flash

[HighStile]

Sorry if I am asking a dumb question but I have just become confused as to how the TV setting works with flash. I assumed that with TV the speed is fixed and everything else adjusts to set the correct exposure, but I appear to get some anomalies. I have only just noticed this when playing around with a new Speedlight - although that has nothing to do with it because I have put it back in the box for the time being. The camera is a 40D with 17-85 lens. So, using the camera's flash I select TV mode and select 1/60 (which shows the largest aperture at 4.5). When I take the picture the speed adjusts to 2 seconds, which obviously can lead to some blurring if I move. This is the sort of result I would expect if I selected AV mode. If I select Auto mode the camera readily takes the picture at 1/60 4.5. The question is, why does the camera override my speed setting and why is it so different from the Auto setting?

[Dermit]

I am not that familiar with the 40D but on my 5D I know that I can set a custom setting that will override a setting if it determines that there will be an underexposure/overexposure. Like in your example with a Tv setting of 1/60 the camera first went to the aperture and opened it up all the way to f/4.5. It saw that this was not enough to make the exposure so with the override switch turned on it lets the camera now go after the shutter speed to try and get the right exposure... this is why it dialed it in to 2 seconds.

With this override enabled it would also do something similar in Av mode. If you set and Av of say f/16 and it picked a shutter of 30 seconds and it was still not enough it may go to the aperture and start opening it up to make the proper exposure.

It's like tellling the camera, look, I want this shutter speed (or aperture), but if you really can't get me the right exposure I give you permission to dial in a different setting than the one I picked.

Look in your manual, there should be a custom function to turn this off and on. The reason it may pick, like in your example, 1/60 and f/4.5 in Auto mode is that in Auto mode (with flash on) it will depend on the flash for the exposure and not the camera. In the Tv or Av modes it does not care if you have a flash on or not it will always try to make the proper exposure with the shutter/aperture.... and on some cameras it will even adjust the ISO automatically to try and get the right exposure. With the override function off when you try to shoot in Tv or Av and it can't get the right exposure this is where you will see the min or max shutter or aperture show up and it will be blinking. It's telling you that it is at it's limit and still can't make the right exposure.

Because of this most photographers when shooting in low light and when depending on flash for the main light source will almost always shoot in manual mode. In manual the camera will not try to do anything other than shoot what you have set so it becomes much more predictable as to what exposure you will get. You may not care that the ambient light is underexposed by 2 stops because your flash will do the work on your subject. Make sense? ( I think the custom function is called a Safety Shift)

[HighStile]

Hi Dermit

You hit it on the button. I found my Safety Shift, which for some reason was changed from the default settting "0" to "1" ("Enable"). Acccording to the manual this works in TV and AV mode and reads "If the subject's brightness changes suddenly and the current shutter speed or aperture becomes unsuitable, the shutter speed or aperture is shifted automatically to obtain a suitable exposure". I have now disabled it! Still a little baffled though, because there was no sudden change in the subject brightness.

I presume that Safety Shift overrides the Flash Lock Sync speed in AV mode, where you have the option to lock the flash speed to 1/250 sec ( If both these functions are enabled there appears to be a conflict - or am I being dumb again).

Many thanks for solving my question so accurately and promptly.

Kind regards

[SYS]

It's not the subject. It's the background that the TV mode was trying to expose correctly. The background in your case must have been dark enough that the lens's widest opening at 4.5 wasn't going to do the job it requires to expose it properly except to go to 2 sec. In TV mode, you can set any shutter speed between 30 seconds to the camera's max X-sync speed which is 1/250 with 40D. The camera will then automatically set the aperture value to match the shutter speed chosen, and it did except even the max aperture value of the lens wasn't enough to properly expose for the dimly lit background.