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.

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Readers' comments

#1: Comment posted by Kiril on January 28th, 2012 - 09:48:44 PM:
Could it be that Sony lens may be having more info on their chips that allow correction of artifacts caused by electronic shutter or correcting the operation of electronic shutter. It is not inconcievable that Sony had electronic shutter technology back in 2006, but did not feel it is worth releasng until it had to show 10 or 12 frames per second.

#2: Comment posted by Greg Beetham on August 5th, 2012 - 04:58:32 PM:

Yes that does seem to plausibly explain the variation in exposure when using the EFC but the situation doesn?t appear to change by very much (in my mental picture) when one imagines the EFC being replaced with a mechanical first curtain (reverting back to normal operation) keeping the distance relationships the same as in the diagram. The exposure variation would seem to get progressively worse the more one moves the shutter away from the sensor and the least when mounting the shutter as close as possible to the surface of the sensor, (which in turn would probably enable more tiny dust motes generated by the shutter blades settling on the sensor).
And one would expect that the effect would be at its worst on a FF camera if the shutter is not mounted with the minimum of clearance in the design. One wonders why not have a shutter with both front and rear electronic curtains surely there must be a way of doing that?
Greg

Michael Hohner answers:
If you look closely at your camera, you can see that the shutter blades are very close together, about 1 mm within each other. But the sensor is much deeper inside the camera, about 4 to 5 mm away from the shutter. So there is a big difference in position of the mechanical shutter and the electronic shutter.

#3: Comment posted by Greg Beetham on August 6th, 2012 - 03:13:12 AM:

Yes agreed 4 to 5 mm is a long way in front and responsible for the worst of the effect, perhaps SSS is preventing them mounting the shutter closer to the surface of the sensor? Also in the first part of the diagram above if the EFC is transformed into the equivalent mechanical shutter in the same position the light ray as depicted would still be able to reach a similar angle at the top regardless. But yes the big difference is at the bottom where the EFC is in effect being back-shaded by the mechanical shutter blade by virtue of it being at a distance out in front and the effect is at its worst at the fastest shutter speeds, whereas if they were both mechanical blades the light would penetrate through the slit by a very similar amount both top and bottom, (it would be much better if both parts of the shutter were electronic of course).
Greg

#4: Comment posted by Rob on January 19th, 2013 - 10:27:55 AM:
I've been shooting with my old Minolta lenses (all beercan generation) on the A77 with the electronic front curtain shutter option set to 'ON'. Up till now I've not been able to finf one shot that had uneven exposure. Waht would be the circumstances during shooting that would make the uneven exposure problem show up? Does anyone have any example pictures of the supossed problem?

#5: Comment posted by tomtom50 on October 4th, 2013 - 03:19:47 AM:
The error can be eliminated with a coefficient that describes the level of telecentricity of the design. Then the line readout scan can vary slightly in speed. This bit of data may only be included in newer lenses after EFC was envisioned or developed.

#6: Comment posted by Daniel Michael Russell on July 9th, 2014 - 09:41:33 PM:

Hi there.

Can anyone explain why I am getting crazy motion blur at 250th of a second using flash and a stopped down aperture? I had EFCS switched on (I think... I am confused by the A77 menu which says, "First Curtain Shutter ON," and does not specify "Electronic", but when I switched it "Off", I now get a double audio noise when taking a photo, so I *think* switching it to "Off" means, "Turn OFF electronic and turn ON mechanical"). When I had EFCS turned on, I would get inexplicable motion blur even from moderate motion in my subjects. I have not gone out to test it with the setting changed yet.

Example: a human posing and standing still: their face would be blurred far too much. I took shots of geese the other day at 250th with flash and in bright sunlight, stopped down, and with focus on their eyes, yet motion blur was present to a ridiculous degree in their faces just from them pecking at their own bodies (fluffing their feathers).

In fact, I don't think it seems to be matter what my shutter speed is. My camera seems highly prone to motion blur *all the time*, no matter what I do, so all my non-still-life photos are softer than they should be.. it's frustrating to no end!

Most of my lenses are Minolta standards, but it seems like the 70-210 f4.0 is particularly prone to this.

Michael Hohner answers:
Yes, “First Courtain ON” means “Electronic First Courtain ON” in the menu. Without looking at pictures it's hard to tell what's going on with our camera.