Exposure when using a Circular Polariser

zedbra

zedbra

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Name
Geoff
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I've just come back from taken 91 photos, and I'm about the most pleased I've been after a major photo-taking session! So I'm all smiles about that. :D:D

However, for many of the shots I used my circular polariser, and they have come out too dark (they are recoverable but definitely not correctly exposed). I've used this filter several times in the past and haven't noticed a problem, but that may be because I haven't compared it with similar photos taken without the polariser before. The pics I took with my nifty 50 looked great, and then I reached the pics using my Nikkor 18-70 (80?) with polariser attached. Well they look good but are all underexposed by about 1 stop. I have a Nikon D70s so the metering is TTL, so surely then I don't need to make manual adjustments? Or does the way the polariser works somehow fool the sensor?

Any help much appreciated,
Geoff.
 
Whenever I've used a polariser I've not noticed any significant change in exposure. There are some effects such as skies going darker and some colours being more saturated. This can sometimes give the impression of underexposure, but I wouldn't have thought this was the case with your images. OK Polariser do cut down the light but I would have thought your TTL system would have taken care of that.

Try some experiments, take a shot with the Fifty, and a similar one with with the 18-70. Keep the image size the same. Now repeat with the Polariser. Any difference. Also check the "Stupid" things such is the camera on manual, have you dialed in some exposure comp and forgotten to take it off. May seem stupid but I've done it. ( Fortunately without taking too many pictures).

Also take a look at the exif data to see if there is any clue there
 
it is a circular polariser isnt it!!! linear polarisers dont work on digital cameras
 
jerry12953 said:
A polariser will reduce exposure by 1.5 to 2 stops, but your TTL meter should be able to deal with it. You shouldn't need to compensate manually for it.

Look for another explanation!:)
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Sounds alright to me!
 
Thanks Chappers for your tips, and to the others for your help. I'm glad you generally agree that TTL should expose it correctly. I have nothing set incorrectly, and have tried similar shots both with and without the polariser, straight after one another, so changing nothing else at all. Well I'm at a loss to explain but they are definitely underexposed. All I can imagine is that the way the polariser works somehow makes the sensor get things wrong. I don't know why, but then I'm not an expert on how polarisers work.
 
A circular polariser certainly shouldn't cause shots to be underexposed, as already mentioned the camera meter registers the fall in light due to the polariser and should compensate accordingly by reducing shutter speed/opening the aperture (depending on mode)...if you were in manual mode then you could have underexposure if you set the exposure before "fitting" the polariser & didn't compensate afterwards, this is the only thing I can think of.....have you checked the settings between those taken with & without to see if there's a difference ?

simon
 
Thanks Simon. I was in fixed aperture mode the whole time. I've just looked back at my shots to check the shutter speed on those with and without the polariser, but it isn't too clear when I had it on and off with the same lens. Hmmm...I'll do a proper test when I next have my camera out in sunlight, as this is bugging me now.
 
DiddyDave said:
Any auto setting will compensate perfectly for the filter m8, as would a manual setting set via the camera's own meter even in manual mode

You did something wrong, it wasn't the camera's fault

Shoot the operator !!!:lol:

DD
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:lol: I really didn't, I couldn't have done. All I did was change lenses and fit the polariser, and nothing else. I took many photos after changing the lens and of course made a few changes of settings (usually just aperture) with each shot, and let the camera sort out exposure. I'll do a proper test, maybe tomorrow, and if something still isn't right I'll post the results here without any editing. :)
 
zedbra said:
:lol: I really didn't, I couldn't have done. All I did was change lenses and fit the polariser, and nothing else. I took many photos after changing the lens and of course made a few changes of settings (usually just aperture) with each shot, and let the camera sort out exposure. I'll do a proper test, maybe tomorrow, and if something still isn't right I'll post the results here without any editing. :)
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In the nicest possible way m8 - I think you're wrong - you did it !!!

If you're shooting a scene and a cloud comes over your camera (on any auto setting) will alter the exposure accordingly - exactly the same with a filter

If it didn't it's either 'broke' or it's you!

If it gets it right most of the time then it couldn't get it wrong just because a cloud came over and reduced the exposure required; in the same way it couldn't suddenly be wrong because you stuck a filter on it - hence it's YOU :lol:

DD
 
zedbra said:
but it isn't too clear when I had it on and off with the same lens....
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I'm confused by this statement....

You were saying that the CPL shots were under exposed but then say that you don't really know which shots were taken with the CPL.

Is this;
1) a general exposure problem irrespective of whether the CPL was used?
or
2) a problem that occurs with the CPL but not every time the CPL is used?

Bob
 
Dave - There's no need to keep SHOUTING. :lol: I think I would know if I changed any settings. Maybe you'd disagree with me that they have come out underexposed, but please accept the fact that I know how to set my camera and nothing has suddenly changed on it's own or by my own mistake. I have checked that I didn't have anything weird set without realising. :)
 
Bob - Good point. What happened is that I was using one lens, and the results were fine. I changed to another lens and attached the polariser straight away, and from the results can see that those pictures are underexposed. I then took the polariser off later. I'm now wondering whether maybe it's the lens that is somehow causing this. When I changed lenses before it was while taking many photos of a similar view in the same direction. Then later I was elsewhere, and I'm not sure what effect the polariser had then. I'm in bed now but tomorrow I'll look at my photos again and possibly do some tests with both lenses. I still stick by what I said that when I changed lenses and fitted the polariser, with the same settings and taking similar photos, the results were underexposed. I guess it could be a problem with the lens. Very unlikely, but all possibilities are very unlikely.
 
totally normal. under certain conditions the CP will make the camera underexpose but the handyness of digital means you can look at the histogram and correct with +EC. just be more careful next time lol

zedbra said:
I've just come back from taken 91 photos, and I'm about the most pleased I've been after a major photo-taking session! So I'm all smiles about that. :D:D

However, for many of the shots I used my circular polariser, and they have come out too dark (they are recoverable but definitely not correctly exposed). I've used this filter several times in the past and haven't noticed a problem, but that may be because I haven't compared it with similar photos taken without the polariser before. The pics I took with my nifty 50 looked great, and then I reached the pics using my Nikkor 18-70 (80?) with polariser attached. Well they look good but are all underexposed by about 1 stop. I have a Nikon D70s so the metering is TTL, so surely then I don't need to make manual adjustments? Or does the way the polariser works somehow fool the sensor?

Any help much appreciated,
Geoff.
Click to expand...
 
POAH said:
totally normal. under certain conditions the CP will make the camera underexpose but the handyness of digital means you can look at the histogram and correct with +EC. just be more careful next time lol
Click to expand...

I've never taken much notice of the histogram, so that's something I'll take some time to learn about. It's a shame the LCD display is useless for checking how accurate the exposure is.

It's interesting that you say the CP can sometimes make the camera underexpose, as it clearly has in my case (although most people don't believe me! LOL). I've read that elsewhere since starting this thread too. When the sun comes out I plan to test this fully myself.
 
CaptainPenguin said:
Is this a bit of a red herring,I have a Linear Polariser a a hangover from Film days and it appears to work fine both on my EOS 350D and Finepix 6500FD
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i use a linear on a d50 and it seems to work as well ,im glad you posted the above ,
 
zedbra said:
I've never taken much notice of the histogram, so that's something I'll take some time to learn about. It's a shame the LCD display is useless for checking how accurate the exposure is.

It's interesting that you say the CP will make the camera underexpose, even though it clearly has in my case (although most people don't believe me! LOL). I've read that elsewhere since starting this thread too. When the sun comes out I plan to test this fully myself.
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Okay then - point taken - summats different your end, so long as you can work round it that's all that matters

In situations where there is likely to be blown areas throughout the image and possibly on your subject too, the histogram isn't of much use - sure you can see some clipping but not what's clipped

Better to review the image using the 'Blinkies' as then you can see if & what's blown and decide if it matters; if it does, you can alter your exposure and shoot again

:)

DD
 
DD - Okie kokie mate. I've just re-written that last paragraph of my post by the way as it didn't make any sense when I read it back! I think you knew what I meant anyway. :)

I am already reading up on the histogram and how to interpret the thing. :)
 
In what way do you think the picture is underexposed?

The reason I ask is that a polariser will INCREASE THE CONTRAST, that is one of the things it does. Shadow areas will go much deeper, but the highlights will remain clean. Polarisers do need ot be used carefully. You can actually use this effect to make a highlight stand out against a shadow area - the shadow can be made to go inky black without underexposing the highlight area, simply by introducing the polarisation effect.....spot meter for the highlight area (human face?) and the shadow areas will deepen, whilst the human remains properly exposed.

The angle of the light will also effect how the polariser works - best is with the light at about 30 degrees to the lens axis (the direction of view). This 30 degrees does NOT have to be over your shoulder....it can be 30 degrees downward, or upward or indeed contr-jour. Polarisers canincrease highlight detail dramatically when the light is coming towards the lens a bit.

Just put the polariser on and go and point the lens at different things and rotate the filter and see the effect - if you use the stop down button you will get an idea of how things would turn out. This way you will be able to "read" a situation and instinctively know when a polariser will benefit a picture - and when it will ruin it!

You can use a polariser on dull days too - but introdce a good bit of warmth, otherwise the blue of the polariser plus the blue cast from the cloud will make everything lifeless. An old favourite trick was to use an 81B warm up with a polariser. For a digital this equates to putting your white balance on "cloudy". The warmth is the finishing touch to a polarised picture. In bright sinshine with bright blue skies you don't need to warm it, but sometimes it works a treat - puts a good tan on flesh!

One of the problems with polarisers and people with film was velvia and even the newer provia tranny films was that flesh tones turned into burnt lobsters! You often got very red faces......you don't get that with digital, or I haven't come across it with mine.
 
Another possibility... the shots taken with the filter featured a lot of sky/bright light that fooled the meter into underexposing. The filter being present was just a conicedence, the under exposure would have happened anyway as some EC was needed to correctly expose the shot.
 
Hi Lensflare, thanks for that info. I've used my polariser a little and am aware of how it can change the photo including increased contrast, but the pictures in general still seem too dark, and it isn't just due to extra contrast. This morning I read up on how to use my histogram, which is something I wasn't too sure of before, and I've been going through my photos from yesterday to see exactly what's going on, and I've adjusted the exposure setting of the RAW photos to see how the histogram was affected. From what I've seen, my opinion is that the camera has set the exposure such that if it had made it any brighter, the histogram would have been too bunched up on the high side (even though I think it needs to be set that way for the photo to look right). I am easily able to manually adjust the RAW settings to make all the photos look fine. So I think it's just one of those times when the camera got it wrong - Although it didn't when I used the other lens. Hmmm.
 
pxl8 said:
Another possibility... the shots taken with the filter featured a lot of sky/bright light that fooled the meter into underexposing. The filter being present was just a conicedence, the under exposure would have happened anyway as some EC was needed to correctly expose the shot.
Click to expand...

The shots did contain a lot of sky, but photos I took with my other lens without a polariser fitted were fine. So it could be the fact that the polariser increased the contrast, making the clouds brighter and therefore giving an incorrect reading. I am aware of that happening though so usually I set the exposure on land before composing the actual photo I want. Maybe I simply didn't do that after changing lenses and fitting the polariser? I'd be surprised but....Well who knows.
 
The polariser wouldn't increase the brightness of the clouds. The increase in contrast comes from the sky being darker - the clouds look brighter but they're not really. A filter that could increase the available light would be worth it's weight in gold ;)

If you were shooting aperture priority how did you set the exposure on land before taking the shot?
 
pxl8 said:
The polariser wouldn't increase the brightness of the clouds. The increase in contrast comes from the sky being darker - the clouds look brighter but they're not really. A filter that could increase the available light would be worth it's weight in gold ;)

If you were shooting aperture priority how did you set the exposure on land before taking the shot?
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Yes I feel a bit stupid suggesting that the polariser would increase brightness as obviously it wouldn't. :D

I set the exposure by moving the camera down a little so the sky wasn't in the shot, locking the exposure, then composing the shot. That's always how I set the exposure when I want to make sure the main part of the shot is correctly exposed.
 
I think seeing one of the shots with the exif data present would help at this point.
 
PXl8 - Yes I agree, and I was just preparing that when I noticed something. So just to confuse everyone (including myself) further...

While preparing a similar shot with one lens and the polariser, and the other lens without polariser, I have noticed that when opening the RAW file in Photoshop Elements and looking at both photos, my camera has set them both to look about the same (under-exposed but I could put that down to the reflection of sky in the water). The reason I thought the ones without the polariser were brighter is because when I open those in Windows as a preview, they are definitely brighter, even though they are darker in PSE. I hadn't yet attempted to edit those non-polariser photos you see, as I preferred the polarised shots, so I didn't spot this before.

So, SOMEhow, my software is displaying polarised images darker than non-polarised! Obviously that's nonsense that it wouldn't do that when they are both exposed roughly the same, but....Well at this point I'm going to forget this because I keep running into more and more "ifs and buts".

When I'm next out shooting I'll do some tests just for my own sanity! For now, I'll just assume that all my photos were actually under-exposed, most likely because a large chunk of each photo contained water that reflected the sky. So I blame my software for displaying some of the photos differently from how they actually were!

Sorry if this has driven anyone else crazy, and thanks for all the help and comments. :)
 
If the image is opening in ACR it's probably doing the default of auto adjusting the settings to what it thinks is right.
 
pxl8 said:
If the image is opening in ACR it's probably doing the default of auto adjusting the settings to what it thinks is right.
Click to expand...

I'm not using that, it's just a free ad-on to Vista so it displays the preview of RAW files just like it would a jpeg instead of opening other software. It's just odd that it displays different photos differently as I thought it displayed them as-is, but maybe not, and the difference in contrast is confusing it.
 
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