Measuring exposure levels on an image

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Mark
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I’m sure I should already know the answer to this, but currently I can’t think what it is.

I’m trying to measure the flash output of flash units combined with different shutter speeds with a leaf shutter, so using a meter is not an option as the vagaries of the shutter system will affect the exposure too.

So I want to measure the level of over and underexposure on the RAW test image on the computer, Is there a way to do that using Photoshop or similar software? Do the exposure slider numbers in Lightroom equate to stops? Any ideas folks please?
 
Colour picker tool and comparing RGB values? But off the top of my head I wouldn't know how to convert that into stops.
 
The best why to see how an exposure change is by photographing a true grey this should be in the centre of the histogram. if its left or right you can see how much under or over it is.
Metering is always for mid-grey.

Oh shutter speed should not have any effect of a flash as the flash speed is much faster than your shutter. The shutter will effect ambient light.
 
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That Tri-Balance is just the job, though actually the black and white areas won't help much. If you take a shot of that, you'll get three spikes on the histogram and the grey one should be exactly in the middle for technically correct exposure. No need for computer - just check the camera's LCD. Very accurate method - you could calibrate your meter against it ;)

Be interesting to see how you get on at fast speeds with a leaf shutter. What camera and flash unit are you using? Things will move around according to power setting and flash duration.
 
Colour picker tool and comparing RGB values? But off the top of my head I wouldn't know how to convert that into stops.

Yep I was hoping for something similar but for exposure.

Oh shutter speed should not have any effect of a flash as the flash speed is much faster than your shutter. The shutter will effect ambient light.

Its a leaf shutter so capable of syncing faster than the flash exposure.

That Tri-Balance is just the job, though actually the black and white areas won't help much. If you take a shot of that, you'll get three spikes on the histogram and the grey one should be exactly in the middle for technically correct exposure. No need for computer - just check the camera's LCD. Very accurate method - you could calibrate your meter against it ;)

Be interesting to see how you get on at fast speeds with a leaf shutter. What camera and flash unit are you using? Things will move around according to power setting and flash duration.

So use my light meter to see how much the histogram changes when over/under 1 stop, then do the testing?

Its the Hasselblad X1-D and I'll try it with Profoto B1 and B3.

My un-scientific test yesterday in the garden suggested that with the B3 on full power, the difference in flash exposure between shooting at 250th and 2000th, was a 1½ stop underexposure. So at 850th sec you still have all the flash power, which I think is about right for the flash duration at full power for that head which is around 800th.

Its quite possible dropping the power on the head down 1 stop will increase the flash speed enough that you can sync at a faster shutter speed, with a possible net gain, but that's what I need to test, and remember to write it down!
 
You do understand how a leaf shutter works?

Mike

Because of the way a leaf shutter works if the flash duration is longer than shutter opening then only part of the power is recorded, Many battery powered lights that do high power HSS like the Godox AD600 which has a t0.1 at full power of 1/220 second, now many of the fuji cameras with a leaf shutter can sync all the way to 1/1600 which is approximately 1/8 of the duration i.e. 7/8 of the shutter pulse is not recorded so shutter speed does make a difference on flash.

Mike
 
Thanks Mike, can't disagree with any of that, and his test method is worth a try too.

Obviously since that article the Fuji GFX has come out, and with a shocking sync speed max of 1/125th!

Yes it was the test method that interested me and made we realise how easy it is to throw away flash power with a leaf shutter

Mike
 
<snip>

Its the Hasselblad X1-D and I'll try it with Profoto B1 and B3.
<snip>

Sweet (y)

You have one IGBT-controlled flash unit and one voltage-regulated, they may behave slightly differently at apparently similar settings. Another thing with leaf shutters at high speeds and low f/numbers, is that the shutter can act as a kind of lens aperture for part of the exposure time that does strange things to depth of field. More something to be aware of than to worry about I think, but you'll soon find out. This is kinda new territory you're exploring here. Take notes! :)
 
Its quite possible dropping the power on the head down 1 stop will increase the flash speed enough that you can sync at a faster shutter speed, with a possible net gain, but that's what I need to test, and remember to write it down!
I don't think you'll see any net gains... quite possibly the opposite. The main issue is the timing of the flash peak (which will remain ~ constant at different power settings) as SS increases. Does Profoto allow for offset timing?
 
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You think so do you?

If the flash duration is, say, 1/500sec or longer, and the shutter speed is 1/1000sec or less, then something's got to give.

ONLY for the converted and processed JPG that the camera makes for the screen disply, It is not the RAW histogram which is the true exposuer.

Mid-grey is the centre of the histogram, and it will stay in the centre from Raw to JPEG. It's the left/right ends that might move.
 
This was slightly dirty test, but still tells the story, apologies for the shadow on the white bit, a combination of me and the camera.

The target was lit with a 1200w/s pack on half power, which is my preferred method as it cuts recycling time down to under a second and gives me more battery life.

Top left was as read from the light meter 250th sec F16, then in sequence staying at f16, 500th sec, 750th sec, 1000th sec, 1500th sec, and ending bottom right 2000th sec.

By my eye, I'd say no more than ½ stop loss in the final image.

If this proves the same, out in the field, I'll be very happy.

XiDflashtest.jpg
 
You think so do you?

Essentially this is the way tail flash and hSS work and cameras like the Hassleblad X1D are capable of syncing at 1/2000 second and there are very many flash durations longer than that depending on flash used and power setting, hence why I linked to the article I did so that you can test and find the setting where are are throwing away the least power. I repeat my question, "you do understand what a leaf shutter is ?"

Mike
 
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This was slightly dirty test, but still tells the story, apologies for the shadow on the white bit, a combination of me and the camera.

The target was lit with a 1200w/s pack on half power, which is my preferred method as it cuts recycling time down to under a second and gives me more battery life.

Top left was as read from the light meter 250th sec F16, then in sequence staying at f16, 500th sec, 750th sec, 1000th sec, 1500th sec, and ending bottom right 2000th sec.

By my eye, I'd say no more than ½ stop loss in the final image.

If this proves the same, out in the field, I'll be very happy.

XiDflashtest.jpg

Result! It looks like that, while the flash duration is quoted as t.5-whatever, at half power most of the brightness is concentrated into a much shorter time and that's what you're getting (y)

I’m sure I should already know the answer to this, but currently I can’t think what it is.

I’m trying to measure the flash output of flash units combined with different shutter speeds with a leaf shutter, so using a meter is not an option as the vagaries of the shutter system will affect the exposure too.

So I want to measure the level of over and underexposure on the RAW test image on the computer, Is there a way to do that using Photoshop or similar software? Do the exposure slider numbers in Lightroom equate to stops? Any ideas folks please?

Yes, LR's exposure slider works in stops - kind of. It's not linear though but concentrated around the mid-tones area (see the lighter background zone on the LR histogram). In other words, if you're just wanting to measure the mid-grey with relatively modest shifts of maybe up to two stops, LR should give an accurate figure. I also found this similar question https://forums.adobe.com/thread/1420597 - see the last answer that reads: "In P.V. 2012 (LR4 and LR5) you cannot equate Exposure slider movements to in-camera exposure changes because the latter is a linear shift of luminosity and LR Exposure is applied on a curve, affecting primarily mid-tones and tapering off at the high end. Moreover, it is not even a fixed and predictable curve, but rather "auto-adaptive", altering the roll-off according to the value of the original white point. In P.V. 2010 (and 2003), however, Exposure was linear and Brightness applied the curve and the option to revert to P.V. 2010 processing does exist in any 2012 edition." You can easily change the Process Version in the Camera Calibration panel of the Develop module.

Also, it's handy to know that the default background tone of the LR working window is exactly neutral mid-grey. So... switch the image to Black & White (to remove any cast) and adjust exposure so the grey panel matches that background tone and you have mid-grey. Histogram will show a spike exactly in the middle.
 
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[QUOTE="
Mid-grey is the centre of the histogram, and it will stay in the centre from Raw to JPEG. It's the left/right ends that might move.[/QUOTE]
I have to say you are wrong If the camera setting are set to alter the contrast or colour saturation then the Mid-Grey will move in the JPG so will be not a true reading
 
The best why to see how an exposure change is by photographing a true grey this should be in the centre of the histogram. if its left or right you can see how much under or over it is.
Metering is always for mid-grey.

I have to say you are wrong If the camera setting are set to alter the contrast or colour saturation then the Mid-Grey will move in the JPG so will be not a true reading

Really interesting that you also seem to argue with yourself :thinking:

Mike
 
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[QUOTE="Mid-grey is the centre of the histogram, and it will stay in the centre from Raw to JPEG. It's the left/right ends that might move."

I have to say you are wrong If the camera setting are set to alter the contrast or colour saturation then the Mid-Grey will move in the JPG so will be not a true reading

No, altering the colour doesn't effect exposure, and changing contrast doesn't alter the position of mid-grey - what that does is stretch or compress the histogram equally from the left and right, leaving the middle where it is.
 
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No, altering the colour doesn't effect exposure, and changing contrast doesn't alter the position of mid-grey - what that does is stretch or compress the histogram equally from the left and right, leaving the middle where it is.

OK think that if you believe it, I know your wrong and I have tested it out as well. ANY adjustments will alter the histogram full stop. The reason is because you have three channels and any adjustment will be different for each channel you cannot rely on the in-camera jpg histogram It's just a guide, in the same why it can show blown out highlight when in fact they are not in the RAW file. I will not say any more on this subject. END
 
OK think that if you believe it, I know your wrong and I have tested it out as well. ANY adjustments will alter the histogram full stop. The reason is because you have three channels and any adjustment will be different for each channel you cannot rely on the in-camera jpg histogram It's just a guide, in the same why it can show blown out highlight when in fact they are not in the RAW file. I will not say any more on this subject. END
So what histogram are you relying on? If it's not something like RawDigger, then it's still "just a guide"...
 
[QUOTE="
Mid-grey is the centre of the histogram, and it will stay in the centre from Raw to JPEG. It's the left/right ends that might move."

I have to say you are wrong If the camera setting are set to alter the contrast or colour saturation then the Mid-Grey will move in the JPG so will be not a true reading

Chaz, I am only talking about mid-grey, but changing saturation and contrast do not effect it (LightRoom/ACR).

Try this, just for your own interest. Take any image and look only at the centre of the histogram. Change contrast and the left/right ends will move, but the centre will not. Adjust saturation and the values will change vertically, but not move from the centre. Maybe switch to black & white to see more clearly.
 
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