Flash power vs f-stops equivalent?

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rayonline

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#148957 6-Jul-2014 19:46

Hi on the flash head if we go from 1/2 power to full power is that equivalent to 1 f-stop as in the aperture?

Also the other thing is I was just playing around with a single speedlight with my Nikon D600 on a white/black brolly. I found that it was consistantly underexposing - is that right? I tended to use +1.3 or +2.0. This was night time indoors with lights off or around 4pm with overcast day. The camera settings were set to the max flash-x speed 1/200 on this and F5.6 which without the flash would been severely underexposing. Are the TTL flash not able to work intelligently in these situations - are they only for mid tone day time type of scenes?

Has anyone used the Profoto B1's and how is the TTL like on them?

Thanks.

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#1082030 6-Jul-2014 19:50

Yes on the stops. You need to read up on how exposure works, camera assumes 18% grey, ec or fec tweak that number. Google around that, I can post more from a keyboard, on phone now.

rayonline

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#1082034 6-Jul-2014 19:55

Yes, I am aware of that 18% thing. I like to shoot slides and need to spot meter etc. But with this flash thing. When the speedlight was behind a brolly it was underexposing, just 15sec ago, I asked my brother to take a picture of me and it was correctly exposed. Dark room 7.50pm lights off - but the flash was on the hotshoe TTL 0.0.

timmmay

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#1082052 6-Jul-2014 20:32

Bit confused, I think I need more information to help. Can you post the underexposed image (with exif) and one that's properly exposed, taken with the same settings? The subject makes a difference to metering. Tell us more about scene, setup, subject, settings, etc.

Are you saying that when you take an image of something shooting through/bouncing off an umbrella and it's underexposed, but without the umbrella it's fine? Could just be the flash doesn't have as much power as you're asking. In which case raise your ISO.

Most things will underexpose with no EC, though Nikon is generally better than Canon in that regard.

rayonline

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#1082053 6-Jul-2014 20:36

Setting: 5pm NZT or night as of now 8pm. Bedroom lights off with a bouncing brolly. ISO 800, F5.6 at 1/200.

With the brolly it is underexposed.

If I take the flash and put it on the hotshoe and direct flash it - it is fine at TTL 0.0 adjustments at the same ISO 800, F5.6 1/200.

As with the brolly not enough power - nope. I think I was at 1/8 power b/c I was shooting it in TTL flash (with the + compensation dialled in) and I switched to manual flash to see what power it was up to.

Could it be that - when the flash is off camera and behind a brolly it cannot read the distance?

timmmay

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#1082055 6-Jul-2014 20:42

Take the shot with the umbrella, then turn it around and take it with direct off camera flash. Post the results - visually or describe the difference. I would prefer to see an image, sometimes things like reflective objects from one angle can mess with exposure. Doubt distance matters, flash works by low power preflash and seeing what comes back.

You've said TTL and 1/8th power - obviously not at the same time though. Your way of posting is quite confusing, and you're not answering the questions I ask, which I only ask to help work out what you're doing wrong. Suggest you start from scratch and describe it again, and start posting sample images.

rayonline

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#1082058 6-Jul-2014 20:45

Will post images. When I meant was I shot TTL it was underexposed. Then I dialled in +1.3 compensation. It was better. Then I tried to recreate it by switching the commander mode from TTL to Manual, I randomly chose different power outputs to get a look that was similar to the TTL +0.3. It was 1/8.

But I will do it again and post eg's.

rayonline

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#1082066 6-Jul-2014 20:56

2 images. One darker and one brighter. Original JPG out of camera without going thru a software, just dragged and dropped.

Darker - behind reflector brolly. Commander mode TTL 0.0
Brighter - hotshoe took out of commander mode. TTL 0.0.

Ok, here is the link
https://www.dropbox.com/sh/cemapsf598bw7pu/AACwIWWiun11XB57srT2iQdna

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timmmay

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#1082153 7-Jul-2014 07:22

The mirror in the background is the problem, it fools TTL flash. Do some research into how TTL flash works, with it's low power preflash then the main flash. Cover the mirror or move locations, do it again, and it'll work as you expect.

rayonline

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#1082488 7-Jul-2014 15:44

I had another go at it - 10mins ago. 3.30pm. I faced the other way away from the mirror and it underexposes again.

I added 2 images to the previous link.
Bright image - hotshoe TTL 0.0
Dark image - reflector TTL 0.0

I set my F5.6 and 1/200 which is the flash x-sync speed. In manual mode.

However if I set it to A mode - the shutter was 1/40 the image was brighter. Same reflector TTL 0.0.

timmmay

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#1082490 7-Jul-2014 15:46

As requested earlier please post images with full exif, not thumbnails with exif stripped.

Try again at 1/100th and 1/50th to negate sync issues. Off camera flash often only syncs at 1/125th, sometimes even slower. Check your camera manual.

rayonline

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#1082499 7-Jul-2014 16:08

Here are the 2 shots 1/100 and 1/50 in manual mode F5.6
Direct flash

https://www.dropbox.com/sh/xrj6k7skfdvvd9i/AABr39kYkMEK3XDSqT0fGgZda

Here are 2 - shot at 1/100 and 1/50 in manual mode F5.6.
Reflector

https://www.dropbox.com/sh/z3izdzxde1uigcv/AADEeWGmYG5u6ZN4znQVzNm1a

timmmay

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#1082533 7-Jul-2014 16:22

In the 1/100th direct flash exif reports "exposure adjustment" of -1.7. You're also using rear curtain sync which is pointless and is potentially harmful unless you're using a really slow shutter. Change from TTL-BL to regular TTL.

In your direct 1/50th shot FEC is reporting as -0.7, which explains why it's brighter. Your umbrella 1/100th shot is reporting exposure adjustment of -3.1.

You can check this yourself, using Irfanview. Load the jpeg, hit I for information, look at the exif near the bottom.

I don't know if you're changing it manually or it's just reporting some information about the image. You probably know that with Nikon effective FEC = FEC set on flash + FEC set on camera body + EC set on body. Set them all to zero for now.

I suspect this is something a bit odd you're doing, we're perhaps starting to narrow down exactly what it is. I'm probably one of the better professionals at lighting in the country, and you do have me slightly confused about quite why you're getting these results.

rayonline

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#1082539 7-Jul-2014 16:32

Apologies ..
I left the flash on TTL BL for direct flash.

Post again:
1/50 speeds, brighter is direct, darker is reflector

https://www.dropbox.com/sh/nj7ai9eqs00bk7r/AAAHheQuJH-p8Vf_9b5MAQGma

rayonline

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#1082540 7-Jul-2014 16:33

Here is also the 1/100 version.
https://www.dropbox.com/sh/xikjkv9z9n25uh2/AACfYfjpqiY22-Itv0Szb0D7a

StarBlazer

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#1082541 7-Jul-2014 16:34

Is there any way to turn off the TTL? If you can, put the flash in manual mode on full power and then bracket the shots using the aperture until you get the correct exposure.

It used to be a lot harder in the old days with film where you had to be precise because you didn't get a second chance - I always used a flash meter for proper readings.

There is of course a possibility that the flash is underpowered - sorry I haven't taken the time so look at the specs. Light intensity falls away very quickly based on the square root rule - shooting into a reflector or through a translucent diffuser compounds this very quickly.

Also as a bit of advise, I wouldn't shoot at ISO800 - the noise could be quite visible.

Good luck

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