Followup on Electronic Front Curtain Shutters

September 28th, 2011 - 07:32:02 PM:

Recently I've explained Electronic Front Curtain Shutters and how Sony recommends when not to use it. In the meantime, I've found the same warnings also in the A77's manual. I've also found another explanation of why not to use it with Konica Minolta lenses. It's related to the angle of incidence of light from the lens hitting the sensor.

Remember that the front curtain is in the sensor itself (i. e. at the image plane), but the rear curtain is positioned above the sensor, i. e. before the image plane. This does not matter when light hits the sensor perpendicular to the image plane. But it does matter when the angle of incidence is flatter. Then, depending on the position of the rear curtain, the light either shines a bit underneath the shutter, which means exposure begins too early, or the curtain throws a shadow, meaning exposure ends too early. So when the rear curtain moves across the frame, exposure of individual rows of pixels is first too long, then just right in between, and then too short at the end. So you get the uneven exposure that the manual is warning about. The effect will be most noticable with short exposure times. With long exposure times, the difference will be small compared to the total exposure and thus less visible.

The following illustration compares the situation at the beginning of the shot with that at the end of the shot. The red line on the image plane represents the area of the sensor exposed to light, corresponding with the length of exposure of corresponding pixels. The thick black line on the image plane is the electronic front curtain, the thick black line in front of the image plane is the mechanical rear curtain. The dashed line is the light ray from the bottom edge of the lens.

Left: beginning of shot; right: end of shot

When using a mechanical front curtain with such a lens, both curtains are in the same plane, the effect happens with both curtains and in opposite order, and the combined error in exposure is zero.

The question remains why the manual specifically warns against Konica Minolta lenses. This implies that Sony lenses are telecentric designs, with light from the lens hitting the sensor (almost) perpendicular (which is generally a good idea with digital sensors). But many current Sony lenses are just re-badged Minolta and Konica Minolta designs with no change of their optics. Certainly, just the name on the lens barrel won't make a lens telecentric. So the manual should actually warn against non-telecentric lenses, but of course this would leave the non-experts puzzled.

So the solution is: test it. Try using your lens with the electronic front curtain, and if you see uneven exposure from the top of the frame to the bottom, switch to the mechanical front curtain when using this lens. If the same lens causes stronger vignetting on a DSLR than on a film SLR, chances are that it will also exhibit a problem with the electronic front curtain.