Sony 70-300G Auto-Focus with Polarizer

[deathvalleydave]

Hi David,

I have used the Sony 70-300G on the A900 with the Sony/Zeiss Circular Polarizer, and have noticed that when zoomed all the way to 300, the auto-focus cannot lock on and produce tack sharp results -it's always a slightly out of focus image - I haven't really checked the other focal lengths, but at 300 - this is what happens. Is it possible using different kinds of polarizers would eliminate this issue?

Thanks,
Dave

[pakodominguez]

Minolta/Sony AF system supposedly can handle Linear AND Circular Polarizers. But I think you problem is not because the kind of filter, but because the amount of light: at 300mm, your lens is a f5.6, and your Polarizer take about 2 stops of light... Usualy camera makers recommend f8 as minimum for the AF system working properly.

[deathvalleydave]

Okay, thanks - are you suggesting that I stop the lens down at 300mm, and if so how far would you suggest I stop it down? From 5.6 to F8? Is your theory that letting less light in will help the auto-focus? I'm sure I probably had the lens open to max aperture at 300mm at the time...

[Dr. Harout]

At 300mm your lens' widest aperture is f/5.6, so adding the polarizer will decrease the amount of light entering the lens thus making the lens hunt for focus.

[[SiC]]

No stopping down is quite the opposite thing to what would help, seems you would need a faster lens :-/
On another note, unless you use a manual aperture lens altering aperture setting won't do anything at all for AF (since the camera will use the lens maximum aperture for focusing).

Good luck!

Br,
Zeb!

Edit: the Doc beat me to it

[pakodominguez]

Okay, thanks - are you suggesting that I stop the lens down at 300mm, and if so how far would you suggest I stop it down? From 5.6 to F8? Is your theory that letting less light in will help the auto-focus? I'm sure I probably had the lens open to max aperture at 300mm at the time...

what you need is a Polarizer that does not take 2 f stops of light... DK mention a new Hoya that can help you...

[deathvalleydave]

Thanks - could someone point me to where I can take a look at this new Hoya polarizer?

[Dr. Harout]

Thanks - could someone point me to where I can take a look at this new Hoya polarizer?

I guess this is the one: http://www.ebay.com/itm/Hoya-HRT-62mm-C ... 4cf9703038

[pakodominguez]

Thanks - could someone point me to where I can take a look at this new Hoya polarizer?

This is the post:
http://www.photoclubalpha.com/forum/vie ... oya#p59791
And I think this is the one: http://www.adorama.com/HY62CPLHD.html

[deathvalleydave]

Okay - thanks, I'm going to try out the Hoya 62mm HRT circular polarizer / UV filter - and hope I can get tack sharp images at 300mm or at least the same sharpness as without using a filter. If not, will have to try for a refund.

[deathvalleydave]

Problem solved!

Thanks again everyone for helping me with this - now I'm loving my Sony 70-300G again! "Again", because I started getting down on it in terms of contrast, but now with the polarizer back in play - hopefully it can provide the images with more pop and contrast...

[David Kilpatrick]

Also, the quaity of the Hoya filter is very high. I found that even my Rodenstock UV on my 70-300mm was destroying sharpness at 300mm. I have just been using my Hoya for a week with perfect sharpness. On the last day, I totally forgot it was on the lens. I even shot indoors with it. It takes much less light away from an older type polariser.

David

[Dr. Harout]

David, are there any differences regarding quality of different types of Hoya cir-polarizers?
I mean quality of the filter and quality on the image.

[David Kilpatrick]

Huge difference. You get what you pay for. Hoya makes their own range with at least three identifiable polariser price points and types - the top one admits about half a stop more light than the standard one, has thinner glass, is far more accurately 'plane' and also more neutral in colour transmission. Hoya also make Marumi and Kenko brands, and in Kenko for example you can get an even lower grade standard polariser than the basic Hoya.

While it's OK to use a cheaper polarizer on a wide angle, on a long tele a poor filter can destroy fine detail sharpness just as effectively as heat haze (for example) will ruin it. I regret to say that my carefully-kept Minolta circular polarisers, maybe partly through age and use, are visibly inferior to the new Hoya which I am using now - HOYA HD FILTER CIR-PL DIGITAL.

David

[NorbertDupner]

Hey Death Valley Dave,

I'm heading out to Death Valley for three days in a couple of weeks. Any insider tips on places to see and shoot? I'll be renting a Jeep for one day to get off road. Also, any reason to lug my 70-400G lens out there with me? I hadn't planned on it, cuz it's a heavy mother to be schlepping through airports. Will be taking a 16-30 and 24-70 with me.

THX