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Incident light meters  Rate Topic 
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Posted: Fri Mar 26th, 2021 18:38
 
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chrisbet



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Excuse my ignorance on this - I have been reading elsewhere about the use of incident light meters measuring the light falling ON the subject to set exposure - to my simple mind this seems innately wrong - surely you need to expose for the light REACHING the sensor, which for a dark subject will be considerably less than the light falling on it.

Can one of our professionals cast some light on the subject?

How do you get the subject best exposed - especially if back or side lit?



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Posted: Fri Mar 26th, 2021 18:51
 
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jk



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Using incident light metering you just point the invercone (white bobbly cover) back towards where you are taking the picture from and this gives you the notional equivalent of using a grey card with incident light metering.

Does that make sense now?
I can probably dig out a good tutorial on it or maybe Graham has one from his training notes.
I will try to find a url.



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Posted: Fri Mar 26th, 2021 18:56
 
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Yes I understand HOW to do it, my question is WHY do it that way ? Advantages / disadvantages?



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Posted: Fri Mar 26th, 2021 18:56
 
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Here you go.

https://www.digital-photo-secrets.com/tip/3369/incident-vs-reflected-light-and-which-type-gives-you-better-photos/

https://blog.pond5.com/7066-perfecting-exposure-how-and-when-to-use-a-light-meter/

https://expertphotography.com/incident-light-reflected-light/



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Posted: Fri Mar 26th, 2021 19:04
 
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jk



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The why is down to experience using any technique.

Remember there isnt a correct exposure only an exposure!
An image that is rendered as a high key image is exposed differently to one that is rendered as a low key image. In the high key image you will be shifting (increasing) your exposure of mid-grey towards white and the reverse if you are exposing to give a low key rendering.

The light falling on the subject as measured by incident light metering will allow you render whatever is there as mid-grey, white or black at whatever is its true tone.  
With reflected light metering a dark subject will fool the meter and give you more exposure and conversely a light object will give you a suggested reading that is underexposing the subject.

Try reading a tutorial on the Zone System.  
Sometimes it is easier to experiment and see than try to explain this stuff.

https://photography.tutsplus.com/tutorials/understanding-using-ansel-adams-zone-system--photo-5607



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Posted: Fri Mar 26th, 2021 19:18
 
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chrisbet



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That is where digital photography scores over film - you can see the results quickly and adjust as necessary, but that isn't always possible because you are capturing the moment.

I tend to use spot metering with the horses as they are often backlit, that results in overexposed backgrounds - it was interesting to read in one of those articles that grass was close to 18% grey - I guess you could use the grass to measure exposure, lock that and compose - I wonder if the grey sand in the manege is close to 18% grey?



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Posted: Fri Mar 26th, 2021 19:57
 
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OK so you want to shoot a white object and a grey object on a black background.

If you use a reflected meter reading e.g. TTL then your camera will see a large black area with two small areas that are lighter.  If you use Nikon matrix metering it does some clever metering so we will assume instead it is a dumb meter.  
So the meter sees the black ground mostly and thinks that correct exposure for this is for a mid-grey (Kodak Grey card or 18% grey card) so it gives say 1/15 at f8 for your ISO200 setting.   However in reality it is a black background which is about 3-4 stops less exposure!  So your exposure should be (3stops less) so 1/125 @f8 with ISO200.

Now swap the black background for a white one.  The meter sees it as a mid-grey again, not white (3-4stops more exposure is required), so gives you an reading of 1/1000 @f8 with ISO200. But the correct reading is 1/125@f8 with ISO200.

Now swap the background for a mid-grey one.  The meter sees a mid-grey again, so gives you a reading of 1/125@f8 with ISO200.

If you had used incident metering then since the light falling on the subject is constant the correct exposure 1/125@f8 with ISO200 is always indicated, (for a mid-grey exposure) is shown whether or not the background is white, black or grey!  

That is the simple story.
Then it is complicated by shiny or matt black, white grey.



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Posted: Fri Mar 26th, 2021 20:05
 
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Yes UK lush green grass is about 18% grey equivalent.

Spanish green grass is paler (less green) so you need an adjustment, similar in Italy I would guess.

The sand looks lighter than 18% grey to me but it could be that you need to EV+0.3 or even +0.7 to get it right.
If you measure the difference between incident and reflective readings you should see the adjustment that you should or could apply.



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Posted: Sat Mar 27th, 2021 04:33
 
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chrisbet



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Ah - light bulb moment - so it is the meter in the camera being too damn clever for its own good!

Yes, I use +0.7 in Italy - now I know why rather than fiddling till it looks right!

But am I right in thinking that using spot metering on the part of the scene you want correctly exposed is the right thing to do?

Or is HDR a better option?

Or shooting in RAW and post processing?

Just starting to get my head round the way the metering works - it was so much simpler in the old days of a box brownie with a fixed shutter speed and a lever for sunny & cloudy!



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Posted: Sat Mar 27th, 2021 05:56
 
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Click here to comment on this image.

chrisbet wrote:
Ah - light bulb moment - so it is the meter in the camera being too damn clever for its own good!

Yes, I use +0.7 in Italy - now I know why rather than fiddling till it looks right!

But am I right in thinking that using spot metering on the part of the scene you want correctly exposed is the right thing to do?
Yes as long as it is mid-grey (Kodak 18% grey card).  That is why some professionals carry a grey card.  
Like you have discovered grass in UK is mid-grey! 
But in sunny Italy and Spain it is lighter in the summer and darker in the winter.  

Or is HDR a better option?

Or shooting in RAW and post processing?

Just starting to get my head round the way the metering works - it was so much simpler in the old days of a box brownie with a fixed shutter speed and a lever for sunny & cloudy! HDR is a lot of fiddling and you actually need a lot of practice to get it right so it isnt garish or with bright bright highlights and dark grey shadows.

RAW is my preference but if you are good at assessment the SOOC (Straight Out Of Camera) JPGs are Ok.  It is about your end needs.

No comment on the sunny/cloudy lever.  Was it better that Nikon Matrix metering?  I can remember a lot of duff over/under exposed images in the past!


You can use Sunny16 rule.



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