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Author Topic: 5DIII Exposure difference from Lens to Lens with Speedlite  (Read 2878 times)

gilkeyb

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5DIII Exposure difference from Lens to Lens with Speedlite
« on: November 01, 2012, 09:23:50 PM »
Recently I upgraded from a 7D to a 5D mark III.  With the new body I also purchased a 70-200 f2.8L IS II.  This combo has worked quite well for me through a couple events so far.  But, as I am preparing for a (family) wedding this weekend, I noticed a weird trend.  Attached you will find two pictures, no time was spent on them, they are out of camera, and they were just to show the exposure difference.  Both pictures were taken at 1/100, f2.8, FC:0, iso 400. The first was taken with the 70-200.  The second, with my Sigma 85 f1.4.  As can be seen, there is a large difference in the exposure from shot to shot.  I should also note that the speedlite used was a Yongnuo YN-568 (batteries were fully charged in both instances)

This is the first zoom that I have spent much time using, especially with a speedlite.  Before I relied predominantly on my 50/1.4, 85/1.4, and 100 Macro. 

Can anyone lend any guidance as to why I am having such variance in exposure?  I though it may be due to the length of the lens, my first attempt was to take the lens hood off.  This seemed to have no significant effect.

Perhaps it is something easy that I am missing.  Forgive me if this is the case.  I am still relatively new with photography.  If its just a trait of using the long zoom with a speedlite then that is something I can adjust to.  By increasing the FC on my speelight to ~+3 I can get a similar exposure to the prime lens. 

DB

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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #1 on: November 02, 2012, 12:30:12 AM »
The issue is the distance from the flash to the subject. I use a similar flash unit - the Yongnuo 565 - which has a similar guide number (GN), and is a very powerful flash. Each flashgun has a minimum power output and if your aperture is too big (in this case f2,8) and your ISO too high (ISO=400 in your shots, I would've used ISO=160 in your 2nd shot), then the flash unit cannot reduce the power output enough to match the chosen aperture, so your 2nd shot is over-exposed. Personally, I would have chosen ISO=640 for the zoom lens and ISO=160 for the prime, or alternatively, you could have stopped down your 85mm to f4 and that would have halved the reflected light.

You could also try taking the flash off-camera and/or bouncing the flash off the ceiling - by the looks of your 2nd shot the flash head was directed at your subject's forehead - the skin above his right eye is blown-out, almost no detail, which is why I think it looks overexposed. Trigger the flash remotely and lens choice will no longer adversely affect your exposure.
« Last Edit: November 02, 2012, 12:41:42 AM by DB »

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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #2 on: November 02, 2012, 03:07:43 AM »
What mode were you shooting in? Manual? Aperture?

The lens hood should make no difference - I always shoot with mine on...

Have you enabled the external speedlite in the menu system?

I am not familiar with Yongnuo - but my 430EXII is VERY finicky with my 5DIII - when the flash is the primary light source the exposures are perfect - but fill-flash is all over the place...

Yes, you definitely want to nail this down before you shoot a wedding!
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gilkeyb

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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #3 on: November 02, 2012, 08:19:53 AM »
The issue is the distance from the flash to the subject. I use a similar flash unit - the Yongnuo 565 - which has a similar guide number (GN), and is a very powerful flash. Each flashgun has a minimum power output and if your aperture is too big (in this case f2,8) and your ISO too high (ISO=400 in your shots, I would've used ISO=160 in your 2nd shot), then the flash unit cannot reduce the power output enough to match the chosen aperture, so your 2nd shot is over-exposed. Personally, I would have chosen ISO=640 for the zoom lens and ISO=160 for the prime, or alternatively, you could have stopped down your 85mm to f4 and that would have halved the reflected light.

You could also try taking the flash off-camera and/or bouncing the flash off the ceiling - by the looks of your 2nd shot the flash head was directed at your subject's forehead - the skin above his right eye is blown-out, almost no detail, which is why I think it looks overexposed. Trigger the flash remotely and lens choice will no longer adversely affect your exposure.

Both images were taken the same distance from the subject, and I attempted to set the zoom at 85mm (this obviously didn't happen exactly, as the framing is slightly tighter with the zoom).  The flash was bounced directly off my ceiling which is ~12' tall and a just off-white color.  You seem to get to the point of my question in that: The lens can make a difference, even when the aperture is the same in both cases? Shooting the zoom wide open at f2.8 and stopping down the prime to f2.8 can still result in different exposures?

What mode were you shooting in? Manual? Aperture?

The lens hood should make no difference - I always shoot with mine on...

Have you enabled the external speedlite in the menu system?

I am not familiar with Yongnuo - but my 430EXII is VERY finicky with my 5DIII - when the flash is the primary light source the exposures are perfect - but fill-flash is all over the place...

Yes, you definitely want to nail this down before you shoot a wedding!

These shots were done in manual so that I could be certain the shutter speed/aperture/iso/etc were all the same from one picture to the next.  Yes, I have enabled the speedlite in the menu system.

Fortunately i'll be just doing this wedding for fun, as i'll also be participating.  But it would be nice to know for the next event/wedding that roles around.

Thank you both for the quick replys.  Hopefully I can make sense of this soon.

Brian

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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #4 on: November 02, 2012, 10:44:52 AM »
Just changing lenses, even at the same F number and focal length, CAN change exposure.  Camera lenses are not calibrted to have the same T-stops, only the same F-stops.  I have seen in other threads that this can make an obvious difference in an exposure.  How far off are those?  If its around 1 stop, it COULD be just the difference between the lenses.

Was the flash also in manual?  If not, the meter could also be to blame.  Something could have fooled it.  Try this - set the camera and flash in manual.  Same F stop, Same Tv, same ISO, and try both lenses on a target an equal distance away.  See what happens.  The camera's meter will have been taken out of the equation there, and you should be able to see how much, if any, the equipment affected the exposure. 

In my experience with all my canon bodies, ETTL metering is great when it works, but occasionally it goes off and does its own thing.  Manual flash is not too hard to manage after a couple of test shots, and provides more consistent results.


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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #5 on: November 02, 2012, 11:42:17 AM »
i wonder about the Sigma's ability to relate TTL info to the flash if you were in fact in TTL mode on your flash. i am not positive about how TTL info is gathered and communicated but because of this i would do 2 additional tests:

1. shoot a canon 85mm (if you can get your hands on one) with the sigma 85mm just as you did with the 70-200 and see if the same problem occurs
2. shoot the same test again in manual flash mode with the sigma 85mm and canon 70-200mm (this should yield the exact same results...if it doesnt something is terribly wrong)

also, have you repeated the original test and gotten similar results? if not i would do it again just to see if it was a fluke. sometimes there are connection issues with my canon flashes on canon bodies that cause the flash to go out of TTL mode and default to manual flash power at full power. if i take the flash off and put it back on the camera it will then recognize TTL communication again.
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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #6 on: November 02, 2012, 11:47:16 AM »
i wonder about the Sigma's ability to relate TTL info to the flash if you were in fact in TTL mode on your flash

I think it's unlikely that Sigma lenses transmit subject distance data to the camera to support E-TTL II flash metering, which is something that Canon lenses do (the newer ones, anyway - for example, the original 85L does not, that was added with the MkII).

But in this case, it's not relevant - the distance data are ignored when the flash is bounced, which the OP stated was the case here.
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gilkeyb

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Re: 5DIII Exposure difference from Lens to Lens with Speedlite
« Reply #7 on: November 02, 2012, 12:08:52 PM »
i wonder about the Sigma's ability to relate TTL info to the flash if you were in fact in TTL mode on your flash. i am not positive about how TTL info is gathered and communicated but because of this i would do 2 additional tests:

1. shoot a canon 85mm (if you can get your hands on one) with the sigma 85mm just as you did with the 70-200 and see if the same problem occurs
2. shoot the same test again in manual flash mode with the sigma 85mm and canon 70-200mm (this should yield the exact same results...if it doesnt something is terribly wrong)

also, have you repeated the original test and gotten similar results? if not i would do it again just to see if it was a fluke. sometimes there are connection issues with my canon flashes on canon bodies that cause the flash to go out of TTL mode and default to manual flash power at full power. if i take the flash off and put it back on the camera it will then recognize TTL communication again.

This is certainly a repeatable event.  I would say I switched back and forth between lenses 5-6 times and had the same situation happen each time.

I took a brief gap at lunch to try setting the *flash* (originally said "camera") in manual mode.  I can't be certain that this would have fixed the issue last night, as the lighting in my living room is now drastically different.  But it did result in similar exposures, lens-to-lens, with the same settings.  This seems to indicate to me that the flash is firing with less power when the zoom is attached compared to when the prime is attached when in E-TTL mode.

I did note that my settings in the camera were "evaluative" metering for flash, as opposed to average.  Not sure if this will make a substantial difference. 

Thanks all for weighing in.  I will continue to experiment with different settings.  If need be, the flash can just be operated in manual mode

Brian
« Last Edit: November 02, 2012, 12:55:34 PM by gilkeyb »