CPL, how wide is too wide?

I always thought the funky sky with a CPL was only a problem at ultrawides, but lately I've been finding that even up to 24mm on APS-C I can get some pretty uneven skies.

Like this example here, which is at 18mm and I am not sure I Like the sky:

Is the uneven sky more of a function of where the sky is vs cpl?

No focal length is too wide.

You just need to use a little care when using a CPL at wider angles. If the sun is 90 degrees to your position, you can't crank up the CPL strength to full power and expect to get an even sky. Try a slightly different composition without such an extreme angle to the sun and/or back off the CPL strength a bit. Then, if you still have issues with un-even skies, they should at least be less significant and you can fix them in post.

i used a CPL on a 10-22, no issues

Okay, so I just need to dial it back when shooting with the sun at shallow angles? It's so hard to see in the viewfinder, though! Thanks for the advice.

Yeah, even on 24mm (which I shoot a lot of nowdays) the CPL can do this. You just have to watch through the VF or on the LCD if using LV before tripping the shutter. By dialing back the strength of the CPL you can mitigate this to some degree, but mostly it requires moving to a different angle to the sun.

Or you can switch from a CPL to a GND. A GND is not affected by the angle to the sun and will produce it's effect evenly across the frame.

tkbslc wrote in post #15064760
Okay, so I just need to dial it back when shooting with the sun at shallow angles? It's so hard to see in the viewfinder, though! Thanks for the advice.

You got it and you're right, it is tough to see through the viewfinder - it's a bit easier with live view.

Because the strength of the effect is directly related to your angle to the sun/CPL strength you dial in, you can use this information to limit your risk of an un-even sky but sometimes your composition needs to have that shallow angle to the sun and the rest of your scene requires a strong degree of polarization to enhance colors in foliage or minimize reflections. If this is the case, multiple exposures with different CPL strengths (or with the CPL removed in one exposure) or some adjustments in PP might still be necessary.

Good luck - and nice shot BTW.

gjl711 wrote in post #15064798
Or you can switch from a CPL to a GND. A GND is not affected by the angle to the sun and will produce it's effect evenly across the frame.

Unfortunately, they aren't the same tool and have completely different uses. A GND is for balancing the exposure in a high contrast scene where a polarizer is used to block polarized light from hitting the sensor to enhance colors and/or reduce reflections.

True, they are not the same tool but the effect to the sky will be the same. Reducing the exposure in the sky will render a nice blue sky uniformly across the frame.

There isn't a black/white answer to your original question of how wide. It is a function of the focal length yes, but also the angle of the sun above the horizon. The higher the angle, the less likely the problem at any given focal length.

Depends on the sun position and such more than anything. I have used a CPL on a 10-22, 17-40 on FF, etc... With no issues at all. The other weekend I used one at 24mm and had pretty bad banding in the skies and near black skies at full power with the CPL.

Just depends on a few factors.

gjl711 wrote in post #15064849
True, they are not the same tool but the effect to the sky will be the same. Reducing the exposure in the sky will render a nice blue sky uniformly across the frame.

Not necessarily - the features of the sky will look different with a CPL because it will not bring down the exposure in clouds and colors in the atmosphere the way a GND would.

A GND will allow your camera to capture detail in a sky that would normally be blown because of great dynamic range, the CPL merely enhances the data that is received - it can't recover data outside of the dynamic range your camera can capture without a GND.

They are seperate tools altogether and one is not a substitute for the other.

I've had to dial polarization back at 70mm on FF! Yes, I was a little surprised and had to check it wasn't something else, but for that shot the difference in the sky from one side of the frame to the other was too much. I've also used it plenty at 10mm on crop/17mm on FF.

Depends on the sky, conditions, sun position, angle, moisture etc.. etc...

Also you can get quite uneven skies without a CPL - it also occurs naturally. If you dialed back the amount of polarization on the posted shot and cropped in a bit from the right, it might look just fine.

rick_reno wrote in post #15064751
i used a CPL on a 10-22, no issues

Ditto. It depends on which angle the sun is at to your photograph/lens axis.