Bruce f-2.8

After many years of successful use, my 580EXII set to ETTL is suddenly giving me about 2 1/2 to 3 f stops underexposure with a StoFen diffuser mounted over flash head - but near perfect exposure without diffuser.

At first, I speculated that problem was in camera, but same amount of severe underexposure occurs when flash (w/ diffuser in ETTL) is used with both Canon 40D and 7D cameras (in manual mode); so, have to assume problem is with flash.

Grasping for straws, I made sure all camera "Flash Control" Menu settings were at factory default, per Canon tech's suggestions. This did not correct problem.

Also, per Canon tech's suggestion, ran test with flash in ETTL and camera in "P Mode"; got same 2 1/2 stop underexposure with diffuser on flash - and proper exposure without diffuser.

Had lengthy conversations with 3 different Canon techs about problem. All 3 were baffled.


(Do not want to complicate the dilemma described above, but I would welcome any feedback on the following side-issue, which occurred during testing with flash meter (Sekonic L-358): Flash meter will not give me a proper reading when flash (in ETTL mode) without diffuser is fired with camera (in manual), yielding perfect in-camera exposure. For example, with camera on manual, set to f/11, - get perfect flash exposure with ETTL without diffuser. Should, therefore, get reading of f/11 on flash meter, since f/11 gives perfect exposure in camera. Instead, I get a completely erroneous reading of f/4.5 on the meter (repeatedly). Other than this meter glitch(?) with ETTL, meter always gives me readings that comport accurately with good exposure with flash on manual. I don't think this has any bearing on the actual "diffuser-flash-ETTL-underexposure" problem described above, but I mention it, nonetheless, with the hope of receiving feedback on this side-issue from members of the forum. If the meter were able to give me readings in ETTL, it might shed additional light on the problem. So far, have not been able to get through to a Sekonic tech rep.)


Relevant(?) observation: With flash in manual mode, - independent, off-camera, flash meter readings (with a Sekonic L-358) indicate that flash with StoFen diffuser produces about 1 1/2 stops less light than without diffuser, - not, however, the 2 1/2 stop underexposure when diffused flash is on camera in ETTL mode.

Had always used speedlite with StoFen diffuser and ETTL for years with no exposure problems - other than minor adjustments in Flash Exposure Compensation. Never 2 1/2 stops in FEC, however.

A diffuser mounted over flash head should have no bearing on ETTL exposure determination, even if the diffuser reduces light output on manual flash by 1 1/2 stops.

Any insights?


A Curiosity: Back in June of 2008, a photographer posted the same exact problem on the DPReview forum. Unfortunately, contact information given is no longer up-to-date or valid - and his question was never answered on that forum:

>> When I use my new Stofen diffuser on my 580ex and 1D Mark III, I need to add +3 FEC. Why?? I'm shooting with manual exposure with a significant degree of underexposure so that I can make the ETTL flash the only source of light (I'm doing bug macro). Without the diffuser, it gets the flash exposure right, but with the diffuser, it's underexposed by three stops. I don't get it. I thought ETTL meant that it flashed till it appeared exposed adequately, but now I'm wondering if it decides on the flash amount before the flash fires, so since it doesn't know there's a diffuser, then it underexposes.

What's going on?? Thanks for any info.
--
Wildlife galleries
http://www.pbase.com/zeiler/ <<

speters Loc: Grangeville/Idaho

Sorry I can't tell either, I use the Sto Fen as well with my 5D MIII ( both with my 580's and 600's, but I never had that issue!

sueyeisert Loc: New Jersey

Try a different stiffen diffuser. Maybe you were sent the wrong one. I don't use them anymore.

sueyeisert Loc: New Jersey

Try a different stiffen diffuser. Maybe you were sent the wrong one. I don't use them anymore.

RobbieAB Loc: UK

Possibly a silly question, but are you getting the under exposure with the flash firing at full power?

aflundi Loc: Albuquerque, NM

I'd guess that the diffuser just eats up too much light. I'd suggest trying to shoot something much closer where less power is required and/or try raising your ISO by several stops. If you get the correct exposure under either of those conditions then you just don't have enough Watt-seconds to drive through that diffuser at that subject distance at that ISO.

rmalarz Loc: Tempe, Arizona

My guess is that your camera has no idea you have a diffuser in place. Thus, it's going by what it "knows" and that isn't working for you. As the photographer, you'll need to adjust for what the camera doesn't know.
--Bob

Bruce f-2.8

Thank you all for responding to my flash-diffuser problem.

At the heart of my dilemma now is whether to send the unit back to Canon for repair when 3 Canon techs have told me that they are completely baffled by the problem - or to simply buy a new speedlight, perhaps only to find that the source of the problem is really in each of the two cameras, although unlikely according to Canon techs.

Had photographed for years with the same StoFen diffuser over the flash-head, always achieving accurate exposure on ETTN, with maybe only an occasional tweak of FEC when necessary. Problem developed suddenly after not using the speedlight for about 3 months.

Did test the flash on manual at full power with Sekonic L-358 flashmeter. Got very good output every time. With diffuser over flash-head (on manual), output was reduced by only 1 1/2 stops.

Had hoped to run some tests with the flashmeter, with and without diffuser and on ETTL, but Sekonic confirmed yesterday what I had already noticed - that their flashmeter will not read flash output when the speedlight is set to ETTL. They were not able to give me clear explanation as to why that would be the case. I'm not sure they really knew why.

Although tests with flash meter indicated that diffuser only cut the flash output on manual by only 1 1/2 stops, nonetheless I increased ISO to 1,000, reduced camera/flash to subject distance to 6 or 7 feet. Same problem - on ETTL, got good exposure without diffuser but underexposure (2 1/2 stops) with diffuser.

I had speculated that, perhaps, a capacitor that controls the pre-flash output had gone bad, and consequently, the weakened pre-flash could not penetrate the diffuser to yield an accurate flash output that would comport with the f/stop set in the camera. Canon dismissed this idea, perhaps because I was getting consistent 2 1/2 stop underexposure at various f/stops, various ISO's, various camera/flash to subject distances.

In view of this conundrum, I will probably buy a new flash (and use the old 580EXII on manual when needed). The good news is that my wife readily accepted and encouraged a new equipment purchase without resistance. I figure I'm going to have some serious irreconcilable problems with my 40D and 7D in the near future. Anticipate full support from my wife for replacement with a new 5D MarkIV. ;)

Thanks again for all your suggestions and comments.

Bruce

rmalarz Loc: Tempe, Arizona

Bruce, running some very well thought out tests and providing the results to the Canon techs might assist in determining where the problem lies. Keep track of everything you do for these tests. Sometimes the most minimal cause and produce a major effect.
--Bob

Bruce f-2.8

Hi Bob,

Thank you for your additional suggestion.

I am at a loss as to what other variables (on camera or on flash) I can adjust to get the diffused flash to produce accurate exposure on ETTL.

Only additional observation I did not mention in my original post is the fact that I do get accurate exposure if I bounce the flash (with diffuser and in ETTL) off of a white ceiling, - even when varying the distance to the subject. Unfortunately, when I described this observation to Canon techs, they dismissed it, feeling that this bit of evidence did not provide any insight into resolving the problem.

Out of frustration, I sprung for a Flashpoint Zoom Li-on R2 TTL Speedlight as a replacement, which happened to be on sale from Adorama for $152. Even though it is less than 1/3 the price of a Canon 600EXII RT, the Flashpoint unit has gotten a lot of favorable reviews.

Hopefully, it will function well in ETTL with a diffuser.

Thank you again for your interest in the problem I was experiencing with my 580EXII and the StoFen diffuser.

Best regards.

Bruce