A77 + HSS flash = Stripey pictures

[mikeyp2000]

Hi,

By accident, I think I have discovered a problem with the A77 when using HSS flash.  I can get the same effect with my HVL-f58AM and HVL-F42AM.  I posted some sample pictures on Dyxum here:

http://www.dyxum.com/dforum/a77-hss-flash-stripy-backgrounds_topic83417_post972028.html#972028

Problem:

   Using high shutter speeds (1/500s to 1/8000s) and a HSS enabled flash gives stripey pictures

Workaround:  Disable Electronic Front Curtain Shutter in the Setup menu.

The workaround isn't particularly satisfactory - I don't want to keep delving into the menus to enable or disable EFCS.  I want to leave EFCS on as it gives that great A77 responsiveness.

Is Sony aware of this problem?  Can it be cured in firmware?  At the very least, an option to automatically disable EFCS when using flash and shutter speeds in excess of 1/500 would be very helpful!  The camera could then  auto-switch back to EFCS when the shutter speed drops into 'normal' non-HSS ranges or the flash is switched off.

Cheers

(PS - sorry to bring another A77 problem to the forum!)

[blaireau_photo]

Hello mikeyp2000

All I can assure you is that development staff within Sony do monitor these boards and look for the opinions of users like yourself. While I cannot guarantee/promise either way about firmware updates, it is always worthwhile to bring these to the attention of the online community.

I have shared this question with a contact of mine to see if they have any thoughts on the matter - if they have any ideas they will post them here.

Thanks,

Simon

[Mick2011]

There does seem to be a timing issue with the electronic first curtain (EFCS) and certain flash modes.

You'll get a variety of anomalies, including underexposure, if you have EFCS on when using TTL fill flash, for example. I suspect it's partly due to the close proximity of very high shutter speeds to flash duration, and partly a result of the nature of EFCS functionality. It's a truly amazing facility, having super-responsive 12fps shooting at your fingertips, but there do appear to be a couple of trade-offs.

In your case it's producing a kind of electronic moiré effect. When you consider the way EFCS works –
manipulating sensor voltage row-by-row, exposing the chip as the 'wave' of charge moves across it – combined with the pulsed output of HSS flash, it kind of makes sense, and explains the decreasing intervals of stripes in your tests, as shutter speeds are increased to maximum while flash output remains the same. It's also possible that the second (mechanical) shutter is where the synch problem lies, but I confess I'm out of my depth in explaining why.

It may well be that the implementation of both super-high frame rates and super-high HSS synch speeds is not yet a well-refined partnership, which is of course a great shame. The A77 user manual is quite vague on the EFCS issue, warning users to disable it when using certain (older) lenses; I take this to be a recognition that some apertures were not designed to rapidly open/close at rates in excess of 1/10000s. I'm afraid I have no idea whether the flash issue is a recognised one or not, but not dissimilar optical anomalies have been linked to shooting wide-open with very bright lenses, supposedly as a result of the difference in sensor proximity between the two shutter curtains.

My personal conclusion is that EFCS may need another generation to become seamlessly compatible with some 'legacy' functionality, or (more likely) that legacy gear will become less compatible as HSS technology progresses to properly mesh with advanced shutter systems. It may be that future alpha generations incorporate an 'EFCS auto-off' function that detects incompatible flash and lens combinations; I really don't know if this is anything that would be addressed in a firmware update. I'll alert the technical team to this thread and (as ever) get back to you here if anything comes of it.

The very bright backlighting example is just about the only time I could see the conflict you discovered being an issue, but I'm sure you could find others It may be that the majority of your shooting will be absolutely fine with EFCS enabled... I sincerely hope so! Knowing there are situations in which it needs to be disabled is surely preferable to keeping the camera at the EFCS-off default. That's certainly how I'd approach the problem, for what it's worth.

Cheers
Mick

[mikeyp2000]

Mick,

Thanks for your thoughtful and candid response.    I'll watch for further developments with interest.   The stripes don't affect most of my shooting and as summer begins to return, i will be using HSS flash for fill flash on harsh sunny days.    I tend to use quite a lot of negative flash exposure compensation which I expect will go some way towards mitigating the problem.   If the sun pops out, I'll see what I can see in this sort of situation.

Cheers

[Mick2011]

I'd be interested to see how you get on.

A suggestion would be to shoot at ISO50 (rather than ISO200) in mid-day sun and perhaps stop down one or two from wide-open so you can synch at slightly lower speeds. With older, slower-synching cameras it was something I got used to doing quickly in fill-flash situations & might be preferable to reaching for that menu button.

Have fun

Mick

[mikeyp2000]

....Just as a follow up.  I was previously a bit concerned that using  HSS fill flash on sunny days might show the stripes in the filled  shadows.

Happily, with EFCS switched on (as usual), I took some  pics of my youngest son playing in the garden this morning.  (1/2000s to  1/1000s,  f3.5 ISO100) and HSS fill flash from my f58.  On fairly close  inspection, I can't see any stripes in filled shadows.

So, I'm  not concerned about this using HSS fill flash with EFCS after all.  I  had flash exposure compensation set to -2 or -2.3 to get a satisfying  balance between the ambient and filled flash areas without looking too  "flashed".  The shadows are nicely subtly lifted and no stripes.  I  suppose the -2 FEC allows the stripes to be to slight as to be  unnoticeable in 'normal' pics.

[Mick2011]

Hey, that's good to hear Mikey

I imagine the majority of your photos will be absolutely fine with EFCS enabled at high shutter speeds, as long as your subject is within the 'typical' situations for that sort of setting – daylight sports, very bright backlighting, etc.

The same (or very similar) is true of sensor dust, easily visible by taking a defocussed shot of a plain white wall. I know all of my shots have dust specks on them, I just can't see them in everyday use.

Hope it continues to work out well for you.

Mick