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09-19-2009, 06:21 PM   #1
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Polarizer Question

I have a DA 40 mm with a Hoya CPL, and when I used it today in bright sunlight at f5.6 and Av mode, the shutter speed was set at 1/40 sec.

Are some CPLs darker than others? That seems like an awfully slow shutter speed for broad daylight. I have a cheap Quantaray CPL on another lens, and I have gotten very good results.

Or is all this normal and to be expected?

09-20-2009, 06:28 AM   #2
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Usually a couple of stops is "normal", but not all glass is created equal.

There are other threads on this forum that I've seen recently that will help you out as well. Try a search and take advantage of all the great advice found here in this forum. For example, https://www.pentaxforums.com/forums/general-photography-techniques-styles/594...questions.html

Last edited by singlespeed; 09-20-2009 at 06:38 AM. Reason: added link from forum search results
09-20-2009, 02:26 PM   #3
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You forgot to mention the other important factor......What was your ISO?
09-20-2009, 03:39 PM   #4
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Well, I was off on my f-stop; it was actually f11, and my ISO was 100.

I know the filter by nature has to slow the shutter speed; I just thought it was odd that I lost so much speed. I took the same shot without the filter and the shutter was much faster (I deleted the picture, so I don't know the exact number).

I guess my main question was whether some CPLs are "darker" than others. If I can't get a faster shutter than 1/40 or so, my life might be difficult.

I'll obviously have to keep experimenting as well.

09-20-2009, 04:19 PM   #5
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In theory a perfect polarize will reduce the light by one stop. As there is no such thing as a perfect polarize most real world polarizes will be between 1.1-1.5 T /stops with the better one closer to the 1. Having said this what is this really saying? First if half the light is one pole and half the other pole then you get 1 stop but this is not the case in most real world situations. The light coming to the camera can have more of one pole then the other so when you turn the polarizer you can get more then 1 stop. Next not all polarizerís isolate as well as others. So with the lesser polarizer you may get less light and see less change as you turn it.

So yes there is a difference between polarizes but from what I see you shouldnít see more then 1-2 stop with some of it being the leakage from bad isolation. You may want to get a better polarizer but I donít think it is going to get your shutter speed up a lot. You need to manage your f/stop for your DOF and add your T/stop to get your shutter speed. In this case you are indicating you need more shutter speed so do you need all the DOF and trade that for shutter speed or do you let the ISO slip up with the little more noise with the possible loss of detail?

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09-20-2009, 04:57 PM   #6
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"Bright daylight" lets us directly apply the Sunny 16 rule (if you're not familiar with this, go learn it).

At ISO 100 and f16 your shutter speed should be (roughly) 1/100.

So at f11 your shutter speed should be (roughly) 1/50.

That means either your polarizer was cutting no light at all, or it wasn't really as bright outside as you thought it was. The latter is more likely, as most people not familiar with doing eyeball-metering aren't likely to notice the difference between a full Sunny 16 broad daylight level of light and one or two stops less than that.

Sounds to me like everything was working just fine. The problem is that you're using a light-reducing filter on a low ISO setting with the aperture shut way down and expecting high shutter speeds. In other words, it ain't the camera, it's the operator. If you expect/need higher shutter speeds then either:

1. Bump up your ISO
2. Open up your aperture
3. Remove the filter
4. Some combination of the above
09-20-2009, 05:03 PM   #7
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Makes sense to me. I figured it was user error.

Thanks for the replies.
09-20-2009, 07:10 PM   #8
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QuoteOriginally posted by Mike Cash Quote
"Bright daylight" lets us directly apply the Sunny 16 rule (if you're not familiar with this, go learn it).

At ISO 100 and f16 your shutter speed should be (roughly) 1/100.

So at f11 your shutter speed should be (roughly) 1/50.

If 1/100 at f/16 is a correct exposure, the equivalent would be 1/200 at f/11, not 1/50. So, if his camera is metering at 1/40, he's losing about two stops due to his polarizer. Since the term "bright sunlight" is somewhat subjective, there is room for variance. The Sunny-16 rule applies to EV 16 light level, which is not subjective.

09-20-2009, 08:04 PM   #9
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QuoteOriginally posted by noblepa Quote
If 1/100 at f/16 is a correct exposure, the equivalent would be 1/200 at f/11, not 1/50. So, if his camera is metering at 1/40, he's losing about two stops due to his polarizer. Since the term "bright sunlight" is somewhat subjective, there is room for variance. The Sunny-16 rule applies to EV 16 light level, which is not subjective.
You're quite right; I slipped on the shutter speed.

Allowing a stop for the polarizer and up to another stop possibly split between the polarizer and a not-as-bright-as-you-think day certainly clears things up, though.

It was my understanding that "Sunny 16" is a guide for estimating exposures using a sunny day as a baseline, and that it is intended to be a "close enough" guide for times when one doesn't have a meter. When did EV 16 get thrown in the mix? Doesn't the Sunny 16 method predate the EV system?
09-20-2009, 09:04 PM   #10
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An Ev and a stop are the same thing. One is the newer more correct way of saying it and to other the older short hand way.

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09-21-2009, 12:56 AM   #11
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DennisH: Most polarizing filters are also neutral density filters with a value of 3 stops. The amount of light transmitted by a polarizing filter also depends on the angle of the filter to the light source (the sun) so it may actually have an effective value of 4 stops. This is in line with your experience.
It is worth noting that super-wide angle lenses do not make good partners to polarizing filters as the effective angle of the light changes from one side of the image to the other.
09-21-2009, 06:34 AM   #12
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EV and "Sunny 16" Rule

QuoteOriginally posted by Mike Cash Quote
You're quite right; I slipped on the shutter speed.

Allowing a stop for the polarizer and up to another stop possibly split between the polarizer and a not-as-bright-as-you-think day certainly clears things up, though.

It was my understanding that "Sunny 16" is a guide for estimating exposures using a sunny day as a baseline, and that it is intended to be a "close enough" guide for times when one doesn't have a meter. When did EV 16 get thrown in the mix? Doesn't the Sunny 16 method predate the EV system?
You're right. Sunny 16 is just a rule of thumb, to make it possible to guesstimate the proper exposure without a meter.

However, if you have an old meter that shows EV numbers, you will find that EV 16 is the value at which the rule works. I did not mean to imply that there is anything "official" about EV 16 in the definition of the rule. IOW, if the meter is reading EV 16, then the exposure indicated will be f/16 at 1/ISO. I was merely trying to make the point (apparently not very artfully), that the light might not have been quite as bright as the OP thought. A bright, sunny day to someone like me, here in Cleveland, is probably not the same as a bright, sunny day to someone in Miami.
09-22-2009, 08:02 AM   #13
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OP, make sure you heed what photog has written (although I do not think most cir cpl's have a -3 value) but the amount of light fall off IS directly related to the direction of the incoming light. This needs to be taken into account.
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