Blurry images - 70-300mm lens or polarizing filter causing the problem?

[mikeB]

Hi there,

I recently picked up a polarizing filter for one of my lenses as I knew I would be doing some shooting out at the lake. The kids were in the water and I had my canon 70-300 with the filter on. When I got back to the cottage I noticed that the photos were not as sharp as usual. Would a polarizing filter cause this? The colours were perfect but the images are not good for printing.

I will admit that the filter was not an expensive one. But will that really make the difference?

Any comments would be great! Thanks

[cicopo]

Yes a filter can cause it & it's an easy thing to test. Shoot a set of photos (same subject,  same light) with & without at a few focal lengths & compare. Good polarizers generally cost over $100.

[Carnac]

Could be the filter, but more likely it is due to slower shutter speed (you loose from 1.5 to 2.5 stops using a polarizing filter).

With a telephoto lens you need to keep the shutter speed up for sharp pictures.

As stated above - you can test your filter out by shooting some test images from a tripod.

If you decide that the filter is the problem, look into quality filters from either B+W or Hoya.

Jim

[hcbph]

You also didn't mention whether you got a circular or linear polarizer.  I'm no expert but a circular is a linear with an extra layer on the back to deal with the digital aspects of AF.  As a test, I tried a linear polarizer on a T4I in bright light, it did work.  I tried it in a low light setting (same filter and lens) and got poor results.  In addition to the tests noted, try one where you manually focus on something at 'infinity' with the polarizer on and see if it's in focus there.  You could be reducing the light enough that AF isn't working correctly (already noted).

[TCampbell]

The polarizer _must_ be a "circular" polarizer... not a "linear" polarizer or "top" polarizer (which is another name for linear polarizer.)

A circular polarizer is a linear polarizer with a quarter-wave plate on the back.  Without this, a linear polarizer makes it very difficult for a camera to achieve correct focus (it also makes it difficult for the metering system to get correct results.) 

[ebiggs1]

"Would a polarizing filter cause this?"

Any or all the factors above can account for OOF or blurry (not the same!) photos. Your lens is slow to start with and sacrificing 2+/- stops is not good.
If you normally take pretty nice sharp shots, you have probably already answered your own question.

[David151]

This is an old thread and not related to a Canon camera but it might be of interest. I found this thread when searching the issue. I have recently had exactly the same problem with every Sony camera I have owned with a long zoom fitted. I mainly shoot video. Circular or Linear filter made no difference. Auto or manual focus made no difference High end or budget filters, standard or wide band polar filters made no difference. Good light made no difference. Every time the subject looks sharp in the viewfinder, but when you view the recording the subject is out of focus.

Trolling through all the files on all the cameras I have owned only two cameras showed sharp polarised images at the long end of the zoom. A Panasonic GH5 micro four thirds camera fitted with the G-Vario 45-175mm lens and a Sony rx100 VII.

I'm still at a loss to figure out why focus works on only those two cameras with a polar filter attached.

[kvbarkley]

They probably use contrast detect and not phase detect focus. Phase detect is sensitive to polarization.

[David151]

That explanation has been stated before but it doesn't tally with my experience. On all my Sony cameras except my pocket rx100 I use manual focus. The little 200mm zoom Sony RX100 is phase detect outo focus and works fine with a filter. Nice sharp images. There is something else going on........

[TCampbell]

There can be many reasons why an image is not tack sharp.  It always helps to see the image and the shooting information (EXIF data) as well as any other relevant info (was the camera hand-held vs. on a tripod). 

But when it comes to filters … filter quality can make a difference.  I use numerous filters in astronomy including wide-band filters (e.g. “red”, “green”, “blue”) and narrowband filters (specific wavelengths such as “hydrogen alpha” or “oxygen III”), solar filters (visible light blocking filters that block most of the Sun’s light) and I’ve learned that (a) when I change filters (some of these filters are installed on a motorized “filter wheel” and I can rotate to the next filter) that I DO need to re-focus.    But also … the optimal focus that I can achieve WITH a filter may not be as good as the optimal focus I can achieve WITHOUT a filter (or with a higher quality filter) — depending on the type of filter.

It is not always intuitive.  For example, I’ve used solar filters that are glass — so you have this nice flat mirror-like quality glass (very smooth) … vs. a solar filter made out of Mylar film (so it’s wrinkly and not stretched tight like a drum because they fear that temperature changes on a tightly stretched filter may tear it).   You would think the wrinkles in the Mylar would hurt the focus and the glass filter would be better.  But my experience showed the opposite … I could never get things to be quite tack-sharp with the glass filter … but I could get it sharper with the Mylar filter.

The usual suspects for images not being tack sharp are, of course, focus … but also things like: was there subject movement?  Was there camera movement?  Was the camera on a tripod with image stabilization turned on (which can create a feedback effect that actually hurts focus)?   Is the lens damaged (optical aberrations)?  

But if all things being equal … and the ONLY difference is whether the filter is present … you might consider whether the quality of the filter is an issue.  A linear polarizer can have a detrimental impact on phase-detect auto-focus (it wont impact contrast-detect focus systems).  But a “circular polarizer” (adds a quarter-wave plate behind the linear polarizer) _helps_ the phase-detect focus systems (just make sure it is a circular polarizer).

Apart from that … what’s the filter brand?  Possibly the optical quality of the filter itself is at issue.

Once upon a time, it was artistic to deliberately allow part of the filter to have poor optical quality and you did this by taking a clear glass filter (often it was just the UV “sky” filter) and smear a thin coat of petroleum jelly only around the periphery of the filter but leave the center clear.  This scattered only a percentage of the light but allowed part of the light to pass through unchanged.  The result is a subject that looks sharp … and yet looks as if they are in a foggy mist.  There were commercially made filters to achieve the same effect often named “spot diffusion” filters.  I don’t see people doing this so much anymore … but it is an example to point out that harming the optical quality of the filter will result in a soft-focus.