Interesting observation with cheap CPL filter

[fatdeeman]

I have used cheap UV filters as basic protection for a long time and have never seen any change in image quality, I have used the green.l brand multicoated filters from ebay with no problems.

I have also been using some cheap Sakar CPL filters, in particular I have been using one on a sigma 18-50mm lens for over a year with great results but I noticed something strange the other day.

When I put the same filter on some lenses the image quality is greatly effected.

I have noticed this so far on a Canon AF 55-250mm lens and a few manual focus 135mm lenses.

On various 28mm and 50mm MF lenses everything works fine and the images are tack sharp at the pixel level, no difference at all can be detected but on these longer lenses the image turns to mush!

It almost looks like motion blur which I thought it might be at first because it was happening on longer lenses but the blur rotates with the filter! I have tried shutter speeds over 1/800 with no change, you can tell something is wrong just through the viewfinder.

I put the filter back on the sigma lens and everything was fine again!

There seems to be some relationship between focal length and how much a cheap filter effects image quality!

I know at the end of the day you get what you pay for but I found this a strange effect.


I'm going to sample a few other brands of these dirt cheap filters to see if any work better on long lenses.

It makes more sense now when I read conflicting opinions online about cheap filters.

[cooltouch]

Interesting. In general terms, the longer the focal length of a lens, the less effect will be seen by any sort of imperfection set before the lens. So it seems curious to me that you seem to be experiencing more aberration issues at longer focal lengths. Whatever the explanation is, I'm quite interested in learning what it would be.

For the sake of accuracy, are you setting up the camera in identical fashions? E.g., using a tripod or faster shutter speeds with the longer lenses? Same ISO as with the wider lenses? Have you tried other filters -- like your cheap UVs -- to see if these have a similar effect?

Best to eliminate as many variables as possible in order to determine the actual culprit.

[fatdeeman]

The cheap green.l UV filters work fine on the longer lenses, I used a tripod for all the tests.

It is very strange, you can just hold the filter in front of the lens and see the blur appear and then take the filter away and it's fine that's through the viewfinder!

The filter is fine on the Af Sigma lens as well as a 28mm Soligor, a 28mm Tokina and several 50mm Pentax lenses. So far it has shown blur on the Canon 55-250mm, a "Helios" branded 135mm, a Pentacon 135mm, and a 200mm Vivitar. I haven't tried it on any other lenses so far but what I have done is try two other Sakar filters with the help of step up/down rings on some of the same lenses and the same thing happens! I also tried a fourth Cheap CPL that was unbranded and the same thing happens again, it's beautifully sharp on a 28mm lens and horribly soft on a 200mm lens.

I'm very intrigued, it doesn't seem to be an issue of resolving power as they work fine with wider lenses on both my aps-c canon and the m4/3 Panny G1 which technically needs much greater resolution because it's pixels are tiny compared to the Canon

[calvin83]

I doubt that the thickness of the glass in the filter will be quiet uneven when compared to some high quality. On the other hands, the relatively poor precision of the filter rim and the relatively low standard in quality control will make the filter glass misalign to the optical centre of the lens.

When the glass of the filter has uneven thickness or misalign to the optical centre of the lens, the glass will bend the light (it acts just like a small prism). In wide lenses, the angle of view is large. A small change in the patch of the light may just cause a small shift of the image in the camera sensor which may not be noticable. In tele lenses, the angle of view is small. Any small uneven of the filter will result a huge shift of the image in the camera sensor.

[DigiChromeEd]

[Orio]

that pun is fun

[cooltouch]

I had a chuckle over the malapropism myself.

And what calvin83 wrote makes sense besides. I think one way to test his assertion would be to position the filter differently -- partially unscrew it, in other words -- and see if the distortions follow along with the filter's new orientation.

[fatdeeman]

Well it's the best explanation by default so far lol

I know that when the actual filter is turned the distortion turns as well so it makes sense so far.

Maybe by holding the filter in front of a 50mm lens and moving it around I can cause the defect to occur by exaggerating the deviance in the light path, I will experiment more tomorrow.

I have bought on ebay a green.l and a Tian ya CPL, they were ridiculously cheap so no harm done if they are poor too but I have had good results with green.l uv filters and Tian ya cokin "p" copies so we shall see.

So far the cheapest CPL I have got that seems to work on any lens was a Kood branded Cokin "p" fit but I do find using square filters a bit of a pain when you're switching between lenses a lot.

[calvin83]

[fatdeeman]

Fascinating Calvin, this sums it up perfectly.

What I found most interesting in the article is that even high quality filters can cause problems with longer focal length lenses because even minute imperfections are magnified as the lens gets longer. Perhaps this is why a lot of super tele lenses have drop in filter systems at the rear. It seems that the poor image on longer lenses is only partly down to the filters being cheap, they are perfectly good for use on wider angle lenses.

The trick will be to find out how far you can go before there is a noticeable effect and decide which lenses are safe to use with these cheap filters.

I found this article very interesting and informative, it might even explain why some people dismiss even the most expensive filters, perhaps those people have had poor results using tele lenses.

To quote the article:

"Filters can and do affect image quality on modern cameras, even high quality filters. The larger the physical aperture on the lens, and the longer its focal length, the greater demand there is on filter quality. Thin filters are more likely to suffer from distortion as they are easier to bend and harder to manufacture with high optical quality. On shorter focal length, wide angle lenses, the apertures are small and the focal lengths shorter so that optical imperfections are not magnified like they are on telephoto lenses. Thus the same filter that may degrade the images from a telephoto lens may be just fine on a short focal length lens. "

[calvin83]

Here is a quote from the B+W Filters for Still Photography.(http://www.schneideroptics.com/info/faq/bw.htm#qu13)

13. Why are B+W filters more expensive than most other brands?

First, most of our filters are mounted in brass rings to prevent the filter from binding to the aluminum housing of the lens. Second, our filters are manufactured the same way fine optics are made, they are cut ground and polished on state-of-the-art computerized machines. Also, most of our filters are made from Schott glass where the color is dyed in the mass. B+W tests each filter for flatness and parallel thickness which assures you of the accuracy of our filters. In addition, we have an interferometer and a scanning spectrophotometer on our premises.

A two dollars Green L filter may works well on your wide angle lens but not your big guns.

[cooltouch]

Well, the evidence is obvious, isn't it. This is quite interesting to learn, though. I've never made it a habit to mount filters to the fronts of my lenses to protect them, for example, preferring hoods instead. But still I've known from personal experience for years that, the longer the focal length of a lens, the less an image is disturbed by imperfections on the front surface of the lens. For example, I owned for years a Nikkor 180mm f/2.8 ED that I bought heavily used (but for a cheap price) from a press photographer, which had a chip out of the front element. Yet it was one of the sharpest lenses I owned, and I got some incredible shots with it. Obviously the chip had not effect on photos. You can even hold a piece of string in front of a tele lens and it won't be visible through the viewfinder or on the image.

So, sitting here, trying to reconcile my experiences with what the experts are saying regarding filters, the only thing I can think of is that my Nikkor 180 had a localized problem whereas a filter's imperfections would be spread across much of the field of view. That's the only explanation that makes any sense to me so far.

By the way, that Clarkvision link has some interesting articles besides the one on filters. Just finished reading one comparing film emulsions to digital and found it to be very informative and pretty accurate as far as I can tell.

[fatdeeman]

I experimented a little more today, I now have 3 kinds of cheap CPL filter and they all give broadly similar effects, they work very well in terms of darkening skies and increasing saturation, there is very little colour cast also.

So far I have tried the unbranded ebay filter, a Sakar filter and a Tian Ya filter.

The first two seem to work fine up to a 35mm equivalent focal length of about 80mm (ie: a 50mm lens on aps-c or a 40mm lens on 4/3), The Tian Ya is somewhat superior giving no noticeable effect until a focal lengh of somewhere between 100-150mm. This is just a rough range, it really comes down to a combination of focal length and physical aperture size.

I am still awaiting the Green. L filter but I am not expecting miracles.

I have read a few different places now about how even the most expensive filters can give terrible results on certain lenses, one place was the reduser forum, Obviously the people there are making video but the principles are the same, one member is using expensive Schneider filters but still getting horrible results so on that basis I suppose these cheap filters still represent good value if used on wider lenses, they will fail you on longer lenses but it seems even the best filters have a point where they begin to effect image quality.

Here's the discussion on the reduser forum:

http://www.reduser.net/forum/showthread.php?t=39893

It also seems the reason many super tele lenses have rear slot in filters is to avoid this problem because it's easier to make smaller lenses optically flat.

On another note, I haven't noticed any issues with the Green.l MC UV filters, they have been fine regardless of focal length, perhaps UV filters are easier to make flat?

I think ultimately I will look at buying one expensive CPL for the longer lenses and just stick with the cheapies on the wide angles.

[cooltouch]

[calvin83]

When using step up ring on tele lens, is there any chance the filter will be misaligned because of the unevenness of the step up ring?

[fatdeeman]

[calvin83]

Most step up ring are made of aluminum alloy. It is difficult to take the set up ring off if it is tightly fitted to the lens. B+W and Heliopan made some brass step up which should more durable than the ordinary aluminum step up.

A few months before, I stacked a Brass B+W UV and a Brass B+W CPL to a lens. They do not bind to the lens but they do bind together. It took me some time to seperate them apart.

[cooltouch]

I can think of no reason why step-up rings need to be tightened so tight they bind. IF they've been tightened so tight they bind, then they've been tightened too tight. I've always found a light snug fit to be sufficient. Same with filters, far as that goes.