Railfan Technology > Digital SLR's and Polarizers

I often used to shoot my 35mm SLR with Fujichrome or Kodachrome using a Polarizer, especially in the West. I am finding that with a digital SLR I get some less than desirable effects not readily visible in the viewfinder. Commentary?

An example of this problem can be seen in the first photo in this post (yesterday) on the Eastern Board under Florida CSX.

http://www.trainorders.com/discussion/read.php?2,2249331

Perhaps post an image?



Edited 1 time(s). Last edit at 08/06/10 18:01 by qnyla.

If you want to use a polarizer when shooting digital, you will have to turn off the Auto Exposure and set the exposure control to direct sunlight. This will give you the best results.

Here are a couple of polarized Digital photos I took.

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COFLZephyr -

It is not entirely clear from your post what undesirable effects you are seeing. Depending on the type of polarizing filter you have recycled from film days, there may be some expected problems with auto exposure and auto focus.

In the old film days, we often used a polarizer, and a polarizer was a polarizer with only brand choices. Today that filter is called a linear polarizer. With current DSLR cameras, we need to use a circular polarizer. If you want to geek-out, look up circular polarizer on Wikipedia, which explains the difference between linear and circular polarizer filters.

In a Canon or Nikon digital SLR, the primary reflex mirror has an area that receives partial silvering. This allows some of the light to pass through the primary mirror to a secondary mirror, then to the auto focus sensor. The percentage of light transmitted through the partially-silvered primary mirror to the focus sensors is sensitive to the polarization angle of incident light, because of the partial silvering on the mirror. Similarly, the reflected light up into the viewfinder and the auto exposure sensors is also sensitive to the polarization angle, but sensitive in an opposing angle to what is transmitted. A circular polarizer does a sort of "circular scramble" of the polarization angle so that the auto focus and auto exposure systems see what you see through the viewfinder when you rotate the filter and can perform as intended.

When I first switched from film (Olympus OM series, Hasselblad and 4x5 view cameras) to my first serious digital (a Canon 10D) I bought rather expensive B&W circular polarizers. Curiously, 99+% of the time they stay in the camera bag, because the sky in Northern Colorado where I live is generally so clear and deep blue without enhancement. Today with my Canon 1DS Mark 3, the same is true and the polarizers stay in my desk drawer.

Your mileage may vary. Wishing you all the very best.
-riogrande491



Edited 1 time(s). Last edit at 08/06/10 20:37 by riogrande491.

Some polarizers give a green light image that appears in the photo when you shoot headlights, more commonly when you shoot them straight on. Usually it is at a symmetrical point from the headlight, but I don't see that in the first picture at all. Try shooting something still on a tripod if you have still conditions, one shot with and one shot without the polarizer, and see if you notice any differences or prefer either result. Especially try to shoot a headlight as straight on as you can. I've never put on a filter on my DSLR and don't feel I've missed anything, but that may partly be because I saved $60 extra buy buying my main lens with a half-broken filter mount! Still never put one on any of my other lenses though.

I use a polarizer filter any time I'm doing photography in sunlight, outside of that, it's useless! You lose at least 1/2 an f-stop with the filter on the lens so unless its sunny, there's not much of a need for it. With night photography, I use no filters, learned my lesson that filters cause light to bounce around inside the lens and can cause wierd light spots in the image.

An example of this was on my post yesterday in the Eastern Board "Florida CSX"

http://www.trainorders.com/discussion/read.php?2,2249331.

The Rock train coming out of Tampa over the bridge is polarized and is noticeably blue in hue. The Tropicana Juice train at Plant City is without the filter.

Maybe the polarizer messes with your white balance? If you have it set to Auto, that looks like the problem to me. Nothing wrong with Auto WB, but it may not work well with the polarizer.

COFLZephyr Wrote:
-------------------------------------------------------
> An example of this was on my post yesterday in the
> Eastern Board "Florida CSX"
>
> http://www.trainorders.com/discussion/read.php?2,2
> 249331.
>
> The Rock train coming out of Tampa over the bridge
> is polarized and is noticeably blue in hue. The
> Tropicana Juice train at Plant City is without the
> filter.

The auto White balance might be the issue. Need to find something sitting in one place for awhile that would be goof polarizing situation and try several methods. Might not be worth it base on some of the feedback. Appreciate the Auto White Balance perspective.

I find a neutral density filter works best for me on my digital in sunny AZ.