Recently I have been experimenting with a polarizing filter and a 4 stop ND filter mounted together in the same mount. I have found that this combination works very well on some shots but then completely wrecks other images with a rather dull flattish look.
I was wondering if any NPN members have come across this at all and does anyone no the reason why this happens.
I've noticed that if the sun enters the back of the viewfinder during a long exposure, the result is a dull shot like you describe. Just one idea.
Posted by Lance Warley on April 10, 2012 at 6:31 AM
I've used a polarizer and a ND many times with no problem, so I'm thinking Phil may have nailed it.
Posted by Alan Olander on April 10, 2012 at 9:29 AM
How would light entering the viewfinder during an exposure have any effect? Light from the viewfinder cannot enter the mirror box with the mirror up during an exposure unless there's a light leak somewhere. Light entering the viewfinder can affect metering prior to tripping the shutter, however.
I would say that it has more to do with the polarizer and the position of the scene to the sun.
Posted by David Bostedo on April 10, 2012 at 11:12 AM
I'd agree with Alan... it's also possible that you're getting internal reflections between the polarizer and the ND depending on light angles. Are these multi-coated filters? That would help minimize any possible reflections issues.
Well, I don't know the "how" of it, but I can tell you that the experience Dennis describes is exactly what I experienced during a trip to the coast a while back.
I set up a shot with an strong (10-stop, if I recall correctly) ND and polarizer. I took several images looking in one direction, all of which came out just fine. I then rotated the camera 180-degrees and took one in the opposite direction. It was mid-morning and the rising sun was hitting the back of the camera hard...including the viewfinder. The shot was properly exposed, more or less, but very low in contrast...effectively ruined.
I then shielded the viewfinder from the sun with my hand and tried the shot again, making no other adjustments. Problem solved. The next several shots came out just fine (aside from the fact that my composition was crap).
My decision to shield the viewfinder wasn't some brilliant analysis of cause and effect that I came up with on the spot. It was based on a vague recollection that I'd previously read some advice cautioning against the effect of light hitting the back of the viewfinder during exposure. I believe this advice appears in my camera manual (Canon 5DII) and that the stock camara strap even includes some little rubber thing that you're supposed to use to shield the back of the viewfinder to prevent this effect.
Anyway, hope this helps. I could be 100% off base here and the cause of what I observed in the field was something different than I concluded at the time. But I don't think so...
EDIT: Just pulled up the Canon 5DII manual online. This phenomenon is mentioned briefly at p. 100, which suggests use of the "eyepiece cover" included on the strap.
Posted by Alan Olander on April 10, 2012 at 6:36 PM
"some little rubber thing" = eyepiece cover
This is from the 5DII manual:
"If you take a picture without looking at the viewfinder, light entering the eyepiece can throw off the exposure. To prevent this, use the eyepiece cover (p.23) attached to the camera strap."
This is talking about light entering the viewfinder and influencing the exposure meter. Light entering the viewfinder during the exposure cannot reach the sensor unless you have something seriously wrong with your camera.
I agree Alan...the manual does seem to refer to an effect on the exposure meter. But as I generally shoot on manual that was not the cause of the problem I experienced in the field. I suspect that whatever we might want to believe about the mirror blocking all light, the sun shining into the back of a viewfinder for 30 seconds is powerful enough to cause some effect on the final image. Or maybe David is correct regarding internal reflections between the filters...all I really know is that when I shielded the viewfinder from the sun, all of the subsequent exposures were fine.
If the mirror is partially silvered and the viewfinder is uncovered with strong light entering it during the course of a long exposure, some of this light will transmit through the mirror and into the mirror box. This extraneous light would be captured by the sensor and would render as reduced contrast.
It is also possible that there may be some flare induced by the air/glass surfaces of the stacked filters.
Thanks so much everyone for the excellent reply's. I have an idea that this low contrast problem has been caused by one of the above. From memory I have only had this problem when there is strong light from behind or the side. I think that all of my shots into the light have been fine. I will do some tests soon and report back with my findings, if and when I nail it.
Posted by Ian Cameron on April 14, 2012 at 7:01 PM
Hi all
I haven't read all the comments back so my answer is just one of personal experience based on lower value ND filters together with polarisers.
My Pentax 67II is often used with a screw in 95mm linear polariser attached directly to the lens followed by a Hitech 4 stop ND filter attached to the other end of the polariser. There is an air gap between the combination of a few millimetres. Under nearly all circumstances the image quality remains high and contrasty throughout. The only time I found it to be compromised was when sunlight struck the back of the ND filter and apparently reflected light back up the lens via the air gap. Shielding the back of the ND filter from direct light seemed to cure the lowering of contrast. The effect apparently increases with the heavier ND filters like 6, 8, 10 stop NDs but the manufacturers fit a rubber gasket to seal the generated airgap. The problem seems to be exacerbated when the Hitech / Lee ND filter is attached to the lens followed by the 105mm circular polariser which generates increased airgaps and more rear surfaces to reflect light back up the lens and thereby creating flare and lowering contrast. Whether it is different for digital users I don't know.
I shoot very long daytime exposures using a combination of ND filters and sometimes also stacked with a polarizer. These may be several minutes or sometimes even much longer. I can relate to the contrast and lighting issues seen here.
Despite people's protestations to the contrary, light definitely DOES enter and affect the exposure via the viewfinder although the impact of that is heavily dependent upon the length of the exposure. It doesn't matter that the mirror is up because just about every DSLR is porous to light to some degree or another if you expose for long enough. You should also be aware that there are other potential sources for light leaks including, and not limited to, the DOF indicator on the top of the lens, even the LCD display on the top of the camera and in the case of MF digital the mount of the MFDB also. The other things to consider are glare from sidelighting across the front filter which may reduce the contrast of the image plus also induce internal reflections between the filters.
If you add all these factors up you'll understand why most very long exposure shooters (1) cover the rear viewfinder - always, (2) cover the lens DOF scale, and (3) use either a lens hood or flag to shield against glare across the front filter element. If you stack filters such as with the LEE or Cokin systems then you'll also have to tape around the edges of the filters to prevent light coming in from the sides.
Here are a couple of extreme cases of what I'm talking about from when I shoot with my Alpa & Phase One back:
http://forum.getdpi.com/gallery/files/1/1/7/img_0617.jpg
http://forum.getdpi.com/gallery/files/1/1/7/img_0619.jpg
Discussion
12 repliesI was wondering if any NPN members have come across this at all and does anyone no the reason why this happens.
I would say that it has more to do with the polarizer and the position of the scene to the sun.
I set up a shot with an strong (10-stop, if I recall correctly) ND and polarizer. I took several images looking in one direction, all of which came out just fine. I then rotated the camera 180-degrees and took one in the opposite direction. It was mid-morning and the rising sun was hitting the back of the camera hard...including the viewfinder. The shot was properly exposed, more or less, but very low in contrast...effectively ruined.
I then shielded the viewfinder from the sun with my hand and tried the shot again, making no other adjustments. Problem solved. The next several shots came out just fine (aside from the fact that my composition was crap).
My decision to shield the viewfinder wasn't some brilliant analysis of cause and effect that I came up with on the spot. It was based on a vague recollection that I'd previously read some advice cautioning against the effect of light hitting the back of the viewfinder during exposure. I believe this advice appears in my camera manual (Canon 5DII) and that the stock camara strap even includes some little rubber thing that you're supposed to use to shield the back of the viewfinder to prevent this effect.
Anyway, hope this helps. I could be 100% off base here and the cause of what I observed in the field was something different than I concluded at the time. But I don't think so...
EDIT: Just pulled up the Canon 5DII manual online. This phenomenon is mentioned briefly at p. 100, which suggests use of the "eyepiece cover" included on the strap.
This is from the 5DII manual:
"If you take a picture without looking at the viewfinder, light entering the eyepiece can throw off the exposure. To prevent this, use the eyepiece cover (p.23) attached to the camera strap."
This is talking about light entering the viewfinder and influencing the exposure meter. Light entering the viewfinder during the exposure cannot reach the sensor unless you have something seriously wrong with your camera.
It is also possible that there may be some flare induced by the air/glass surfaces of the stacked filters.
--P
I haven't read all the comments back so my answer is just one of personal experience based on lower value ND filters together with polarisers.
My Pentax 67II is often used with a screw in 95mm linear polariser attached directly to the lens followed by a Hitech 4 stop ND filter attached to the other end of the polariser. There is an air gap between the combination of a few millimetres. Under nearly all circumstances the image quality remains high and contrasty throughout. The only time I found it to be compromised was when sunlight struck the back of the ND filter and apparently reflected light back up the lens via the air gap. Shielding the back of the ND filter from direct light seemed to cure the lowering of contrast. The effect apparently increases with the heavier ND filters like 6, 8, 10 stop NDs but the manufacturers fit a rubber gasket to seal the generated airgap. The problem seems to be exacerbated when the Hitech / Lee ND filter is attached to the lens followed by the 105mm circular polariser which generates increased airgaps and more rear surfaces to reflect light back up the lens and thereby creating flare and lowering contrast. Whether it is different for digital users I don't know.
Despite people's protestations to the contrary, light definitely DOES enter and affect the exposure via the viewfinder although the impact of that is heavily dependent upon the length of the exposure. It doesn't matter that the mirror is up because just about every DSLR is porous to light to some degree or another if you expose for long enough. You should also be aware that there are other potential sources for light leaks including, and not limited to, the DOF indicator on the top of the lens, even the LCD display on the top of the camera and in the case of MF digital the mount of the MFDB also. The other things to consider are glare from sidelighting across the front filter which may reduce the contrast of the image plus also induce internal reflections between the filters.
If you add all these factors up you'll understand why most very long exposure shooters (1) cover the rear viewfinder - always, (2) cover the lens DOF scale, and (3) use either a lens hood or flag to shield against glare across the front filter element. If you stack filters such as with the LEE or Cokin systems then you'll also have to tape around the edges of the filters to prevent light coming in from the sides.
Here are a couple of extreme cases of what I'm talking about from when I shoot with my Alpa & Phase One back:
http://forum.getdpi.com/gallery/files/1/1/7/img_0617.jpg
http://forum.getdpi.com/gallery/files/1/1/7/img_0619.jpg