[Alpinus]

Hello

I've been trying to find out if I can use a linear polarizer on my Pentax 645. I understand its possible with some older cameras. Can anyone point me in right direction?

Thanks!

[lithedreamer]

Hello

I've been trying to find out if I can use a linear polarizer on my Pentax 645. I understand its possible with some older cameras. Can anyone point me in right direction?

Thanks!

As I understand it, linear polarisers only affect metering on AF cameras negatively, otherwise you should be safe.

[paulster]

As I understand it, linear polarisers only affect AF operation negatively, otherwise you should be safe.

Exactly. If you don't care about AF then you can save some money and use a linear polariser rather than a circular one.

[amoringello]

My opinion is its a myth perpetuated to sell CP.
Never seen AF affected in past 10+ years by a linear polarizer. At least not with any of my Pentax digital cameras.
Also, have not heard of anyone else showing factual evidence of it occurring, either.

[lithedreamer]

My opinion is its a myth perpetuated to sell CP.
Never seen AF affected in past 10+ years by a linear polarizer. At least not with any of my Pentax digital cameras.
Also, have not heard of anyone else showing factual evidence of it occurring, either.

I did some more research, and apparently LPs can also mess with the metering on AF SLRs/DSLRs. I'd like to see some actual side-by-side comparisons of a high quality LP and CP, but I couldn't find any online. Everyone just seems to take this for granted.

[photoptimist]

Actually, it's not the AF that's affected but the light meter. The half-silvered main mirror used in AF bodies does not reflect every direction of linear polarized light equally well. Thus the auto-exposure or light meter reading will be wrong for some settings of the polarizer. If you plan on using the polarizer at one angle (e.g., for reflections off of water), then EV compensation works. But if you are adjusting the polarizer for more complex situations (angled glass, shiny leaves, rainbows, or sky with the sun at an odd angle), then using a linear polarizer may require a bit of chimping to get the right exposure. Of course, if you are setting your own manual exposure with sunny f/16 or an external light meter, then linear polarizers are fine.

[CarlG]

My opinion is its a myth ...

Well put --- +1

[Digitalis]

A circular polarizer is just a linear polarzier with a quarter wave plate behind it, the image will look identical however the linear polarizer will transmit roughly 1/8th of a stop more light than the circular polarizer.

Never seen AF affected in past 10+ years by a linear polarizer. At least not with any of my Pentax digital cameras.

Not have I. To this day I use a mix of circular and linear polarizers.

[E-man]

I also use both linear and CP filters but I tend to like linear better because I can adjust the effect by rotating the filter. I don't seem to be able to get the same sort of control with a CP.

[Silent Street]

Use a circular polariser with cameras that have evaluative/matrix/3D/multipattern metering as transmitted light is skewed and this can effect the meter interpretation (over / underexposure).
Linear polariser is best for plain TTL meters.

Autofocus is not a factor.

[photoptimist]

A circular polarizer is just a linear polarzier with a quarter wave plate behind it, the image will look identical however the linear polarizer will transmit roughly 1/8th of a stop more light than the circular polarizer.


Not have I. To this day I use a mix of circular and linear polarizers.

You had me doubting what I wrote so I pulled out my K-1, a lens, linear polarizer, & circular polarizer to check the effects.

Effects with linear polarizer:
1. OVF visibly brightens at polarizer settings that reflect well off the main mirror
2. OVF meter reading brightens by 1/2 stop over-exposure at these settings
3. LV metering and viewing shows no change at any polarizer settings.
4. LV and OVF meter agree only if the OVF is dim.

Effects with circular polarizer:
1. OVF remains remains constant (but dim) at all polarizer settings
2. OVF meter remains constant at all polarizer settings
3. LV metering and viewing shows no change at any polarizer settings.
4. LV and OVF meter agree at all polarizer settings.

Conclusion: linear polarizers with OVF meter readings can be wrong as lithedreamer, Silent Street, and I state but the effect might be only about 1/2 stop.

NOTE: I did these tests on a non-shiny surface under diffuse-lighting. Subjects with naturally polarized light will have more complex effects.

[Alpinus]

Thank you all very much! A friend gave me his linear polarizer to use and it seems that it will work for me. I have a circular one for my digital camera but its too small for my 645 lenses.

[paulster]

There is some great information in this thread - thanks! I haven't used a linear polarizer in years because I got caught up in what appears to be an urban myth about AF, so it's time to go and try these again.

[photoptimist]

P.S. I just took a gander at the effects of linear polarizers on AF optics (using a dissected mirrorbox from an old Pentax ZX-50). At some settings, a linear polarizer does attenuate the light reaching the AF system but it's a very modest effect. It might affect the lowest EV the AF system can handle by maybe 1/2 stop and might also translate into slightly longer/slower AF at medium light levels.

Thus, the effect is there but probably not noticeable in routine use.

[Digitalis]

the effect might be only about 1/2 stop.

This depends upon the degree of polarization, but 1/8th to 1/2 stop difference can be observed.