Canon 40D Live View Artefact

[citivolus (Ric)]

So while testing my new field flattener today (more on that later), I ran across this interesting behaviour. I was shooting using live view to manually focus, and with very fast shutter speeds.

The attached were shot at 1/5000 second. One was with live view active, and one was normal shutter by deactivating live view before exposing.

I can only assume it is due to the way the shutter closes, and the fact that it is still exposing one end of the sensor while the other is covered. As a normal shutter setting would expose in an open/closed wiping pattern, and live view exposes in a half wipe, it isn't tough to figure out the source of the problem. I found it was visible from around 1/1600 of a second on up. I guess flat frames could partially compensate for it.

Notice how one side is over exposed, and the other under?

Has anyone else run across this?

Eric

[Dennis]

Hi Eric

I haven’t yet tried the experiment at those relatively fast shutter speeds. IIRC, the 40D shutter is a focal plane shutter with two blinds producing a slit as they travel across the sensor area?

At the highest speeds, the slit would be very narrow; at slower speeds, one blind would have completely opened before the 2nd blind began to close?

Cheers

Dennis

[citivolus (Ric)]

As I understand it when shooting with live view, one blind is open and the other closes to end the exposure. The non-instant nature of the movement of the blind, as well as its acceleration, results in less exposure at one side, and more at the other, producing the exposure problem shown in the picture. This is of concern to us from an astronomical perspective mostly when used for solar imaging in an ND3 filter; I can't think of much else that would be bright enough to result in these lengths of exposures, short of terrestrial daytime use.

Regards,
Eric

[RB (Andrew)]

Originally Posted by citivolus

This is of concern to us from an astronomical perspective mostly when used for solar imaging in an ND3 filter; I can't think of much else that would be bright enough to result in these lengths of exposures, short of terrestrial daytime use.

Regards,
Eric

hmmm interesting.

One thing though, you'll only need to use live focus once to pre-focus when solar imaging.
If you then switch to manual focus and leave focus as is, you can just take the shot.

Does that sound right?

It would be interesting to find out though, why it's doing this.

[citivolus (Ric)]

You may also want to use live view when composing a shot of a sunspot for example, if using an ND4 (accidentally referred to as ND3 above) filter it might be a bit bright to look through your viewfinder. The Thousand Oaks 3+ filter I have lets through 10x the light of a typical white light ND5.

Other times live view may come into play in similar scenarios would be when photographing an eclipse while simultaneously visually observing. It would be disappointing to find a gradient in your images after the fact.

To get around the problem, all you need to do is switch out of live view once the scene is composed, but it would be nice if a firmware update would provide the option to do a "normal" shot automatically above say 1/1000 second. At those speeds you don't need mirror lock-up for anything other than lower noise.

[citivolus (Ric)]

I had a chance to experiment a bit today, and it turns out that the camera already has the capability built in to change the live view shooting behaviour. Just select "Mode 2" or "Disabled" in the silent shooting options for live view, and it will change how the curtains move. I believe "Disabled" does a standard shot.

Eric