Rotating front element and polariser

[ChrisR]

I have a very nice 35-70 Pentax-M zoom lens, 2.8-3.5. The front element rotates when focussing. I've read elsewhere comments about difficulties using polarisers with such lenses, but I was thinking to myself that the polariser is either crossed or aligned, regardless of rotation. So, what's the truth? Is it worth paying to get a polariser for this lens? It has a larger filter size than any of my other lenses.

 

[Nomad Z]

I don't see why it should matter if you focus and then rotate the polariser to give the effect you want. On any lens, and on any filter, the start of the filter thread is essentially random, so, once fitted, the orientation of the non-rotating part of the polariser will never be predictable. If it did need to be predictable, it would have a bayonet fit to make sure of it. Even if it did, the orientation would be lost if you switched the camera from portrait to landscape (or any angle that looks right, if you're Garry Winogrand).

 

[rob-nikon]

I think the key issue is focusing can turn the filter. Every time you focus the polariser would turn changing its effect unless you turned the polariser to the desired effect. That would be fine if you are taking say a landscape on a tripod as you can focus then set the polariser for the effect you want as the focal point isn't moving. Whereas with a moving subject the change of focus will be turning the polariser changing the effect every time the camera alters focus.

 

[Woodsy]

Provided that you focus, hold that focus, and then adjust the polariser by visual inspection through the view finder, it makes absolutely no difference what so ever.

I'm not sure what you mean by 'crossed or aligned, regardless of rotation'?

 

[ChrisR]

"Crossed or aligned" probably describes the state of my eyes when trying to think (apparently not very clearly) about polarisation!

I managed to convinced myself that, given there are two elements in a polariser, they worked like two pairs of polarised specs would work... if you hold them parallel (aligned), they let light through, but if you hold one pair vertical and one pair horizontal (crossed), they don't. If something like that were the case, then the orientation of the polariser itself would be irrelevant. But I was clearly wrong, and have just demonstrated so to myself by holding up a naked polariser, turning the front element until the polarising effect is seen, then turning the whole polariser through 90 degrees when the effect disappears.

So, d'oh!

Back to the particulars of the case, the focus ring on this lens is quite large and the focus is a little stiff, so if I focus and then adjust the polariser I'm unlikely to affect the focus. Could be annoying though, as I do find myself fiddling with the focus several times in a shot, particularly with the LX. And it's obviously a bit more hassle than the normal case of setting a polariser where you want it, and leaving it in position for several shots with merely a little tweaking to check.

Anyway, overall I think buying a CPL for this lens is worth while. A filter holder for grads OTOH would be a bit more of a pain, I think. I'll stick with primes in those situations! (Almost all my primes have a 49mm thread, thank you Asahi-1978!)

Thanks @Nomad Z @rob-nikon and @Woodsy

 

[Peter B]

How big is the filter thread Chris? remember you'll only need a linear polariser for manual focus lenses, and they should be a bit cheaper maybe.

 

[RaglanSurf]

"Crossed or aligned" probably describes the state of my eyes when trying to think (apparently not very clearly) about polarisation!

I managed to convinced myself that, given there are two elements in a polariser, they worked like two pairs of polarised specs would work... if you hold them parallel (aligned), they let light through, but if you hold one pair vertical and one pair horizontal (crossed), they don't. If something like that were the case, then the orientation of the polariser itself would be irrelevant. But I was clearly wrong, and have just demonstrated so to myself by holding up a naked polariser, turning the front element until the polarising effect is seen, then turning the whole polariser through 90 degrees when the effect disappears.

So, d'oh!

Back to the particulars of the case, the focus ring on this lens is quite large and the focus is a little stiff, so if I focus and then adjust the polariser I'm unlikely to affect the focus. Could be annoying though, as I do find myself fiddling with the focus several times in a shot, particularly with the LX. And it's obviously a bit more hassle than the normal case of setting a polariser where you want it, and leaving it in position for several shots with merely a little tweaking to check.

Anyway, overall I think buying a CPL for this lens is worth while. A filter holder for grads OTOH would be a bit more of a pain, I think. I'll stick with primes in those situations! (Almost all my primes have a 49mm thread, thank you Asahi-1978!)

Thanks @Nomad Z @rob-nikon and @Woodsy

Have you considered getting a similar focal length lens without a rotating front element?

 

[StephenM]

remember you'll only need a linear polariser for manual focus lenses, and they should be a bit cheaper maybe.

I was under the impression that, at least with some cameras, TTL metering could be affected by a linear polariser.

 

[Peter B]

I was under the impression that, at least with some cameras, TTL metering could be affected by a linear polariser.

There might be issues on some fancy modern metering systems, but I'm pretty sure it shouldn't be an issue on older systems. I've used linear polarisers on AF cameras without any problem, but that's on digital where you can chimp the effect straight afterwards.

 

[StephenM]

The Pentax LX was mentioned, and according to this post on Pentax Forums, it can be affected depending on how you use the camera. Depending on how you define "fancy modern metering systems" in terms of date, the LX came in in 1980.

- A special note should be taken when using polarizing filters on the LX. A special half mirror was added to the LX to make the viewfinder brighter and easier to focus. This mirror is multi coated and acts like a polarizing filter, so if you use a linear polarizer and shoot in the manual mode you could get a messed up meter reading. However in auto (aperture priority) mode you can use either a CPL or LPL, as the half mirror flips up when the shutter button is pressed and the exposure metering is done off the film plane. Since I only use LPL’s this is a big relief and another reason to use the LX in Auto mode only.

 

[ChrisR]

How big is the filter thread Chris? remember you'll only need a linear polariser for manual focus lenses, and they should be a bit cheaper maybe.

It's 58mm. I did find some fleabay sales of LPLs, only marginally cheaper and raised the issues that @StephenM mentioned (although not specifically mentioning the LX).

Have you considered getting a similar focal length lens without a rotating front element?

Quite a few lenses of this vintage have the problem, but the strict answer to your question is yes, I've considered and rejected this for the moment. I REALLY like this lens!

In fact I've ordered a small set of 58mm filters off the bay, we'll have to see how it turns out! Thanks again, folks.