Bokeh artifacts with Skylight filters

I'm posting this to the Nature forum because it is a problem that I've

encountered only when using a long telephoto lens, and I figured

nature shooters would be more likely to have encountered it. Some

pros (John Shaw for one, I think) don't routinely use UV skylight

filters on all of their lenses because any filter, regardless of

quality, will degrade image quality somewhat. Others feel that

the benefits of protection outweigh any slight image degradation.

I've never seen this problem discussed, though. When using a Sigma 400

APO lens with a UV skylight filter, I've noticed on many shots where

this a brightly lit, heterogeneous background, the out of focus

highlights sometimes take on the appearance of a series of parallel

diagonal lines. Sometimes it is quite apparent. You can see an

example (I hope) in this photo of a

<a href="http://www.stetson.edu/~pmay/moorhen.jpg">moorhen </a>,

particularly in the area above the bird's head. Recently, while

viewing a brightly lit background through the lens, I found that when

I took the filter off, these lines in the bokeh completely

disappeared, so apparently it is due to some sort of diffraction

effects originating with the filter (its a Tiffen). Questions: Can

anyone explain the physical basis of this phenomenon? Is it specific

to longer focal length lenses? Are there any ways to minimize it

while still using the filter for protection? Is this peculiar to only

some filters? Thanks for any help you can offer.

Well, as far as I know, a filter should NOT cause a change in Bokeh. SHOULD not, if it's plane and parallel. Are you sure that in an identical shot (i.e. same focus, lighting, subject and exposure) with and without the filter, that the "bokeh" changes? If it does, I'd try another filter (because there's nothing else you can do!).

 

The effect you are seeing isn't that unusual. I've seen it myself in some shots, though I've never really correlated it with a particular lens or zoom setting or focus distance. It probably depends on all three factors as it's related to the aberrations in the out of focus image. It's nothing to do with diffraction. In principle a filter (if defective) could affect the out of focus aberrations, but in doing so it would affect the in-focus aberrations too.

 

If it's a real effect then it's a very interesting observation!

Bob,

 

Thanks for your response. Yes, it is a real effect, and is definitely originating from the filter. I played with the filter/lens combination on a tripod-mounted camera for quite awhile a couple of days ago to convince myself that it was so. The parallel lines that are so apparent in the viewfinder when viewing some out of focus highlights COMPLETELY disappear when the filter is removed and the same scene is viewed again seconds later. I've had this lens for a couple of years, and for the first year and a half didn't use a filter (a la John Shaw's opinions about sharpness). After reading several discussions here about most photographers never reaching the resolution limits of their equipment due to imperfect technique, I figured a filter wouldn't make a noticeable difference, and started using it. I first started noticing these bokeh artifacts in some slides soon thereafter, but really didn't make the connection to the filter until recently (these artifact are only obvious under fairly specific lighting and background conditions). Maybe I just got a bad filter - but I really don't feel like kicking $20 for a replacement if its going to happen with any filter I replace it with.

That's really odd! I can't imagine the optical origins of that kind of bokeh from a filter. Does the effect remain constant if you rotate the filter? Have you tried shading the lens? I use a UV filter on my 400mm all the time, never seen anything like that.

 

The other question was what kind of filters are you buying for $20? The Simga 400mm APO takes a 77mm filter, right? I've tried everything from $45 Hoya SMCs to $80 Nikon filters in the 77mm size with no problems, but never tried anything less.

Peter: Actually, this issue of whether or not to use protective filters on lenses is periodically hotly debated here. Personally, I prefer to protect my EOS lenses with B&W Schneider multicoated UV-haze filters, and I will temporarily remove them only when flare or ghosting is likely to result from them. I routinely use these protective filters because in the event of an accident, it is, for me, less costly and time consuming to replace the filter than to live with a damaged front element or incur the cost and downtime to return the lens to Canon for repair. Also, because I use protective filters, the front elements of my lenses are pristine, which helps greatly if/when I decide to sell one of them. If you decide to use protective filters, buy the absolute best that you can reasonably afford, and for me this means B&W Schneider. In an ideal world, it is indeed true that protecting the front lens element is the job of the lens cap, not filters, and those with enough money and/or those who make their living from their photography can choose to follow the "lens cap" rule, but can you? At this time, I can't, and I find filters like B&W multicoateds to be the best compromise. For those who are new to photography, I believe that recommending an inexpensive protective filter like Tiffin is sound advice, in part because many such folks will eventually decide to clean their front lens element (often in an improper manner), and we read their postings here in which they are asking why their front glass now looks so funky after trying to clean it with Windex and a Handi-Wipe. As they either grow as photographers or decide to sell their gear because they realized they are not interested in photography, the cheap filters can be shed for better ones or none at all, or they can sell their gear in good condition.

 

As for the excellent photo you posted that clearly illustrates the problem you've noticed, I offer the following thoughts. 1) When you tilt this filter and carefully scrutinize it in sunlight, do you see any ripples? I have found Tiffen filters to be absolute junk--they often use cheap, "windowpane" glass, and it's often not a solid piece of glass but rather is laminated, and glue bubbles are commonly visible within the glued laminate; 2) Were you shooting at an aviary or zoo where the background enclosure was a screen or mesh--I have encountered the lines you illustrated in such situations when my aperture was more stopped down. I looked very carefully at your example and noticed I could not see these lines within the bird, but the surrounding areas had the lines; 3) I have observed very similar concentric, curved lines when looking through the viewfinder of an EOS A2, and I finally realized they were due to the fine etchings in the camera's focusing screen; however, they of course did not appear in any slides, because they were not in the actual light path to the film.

Addendum: On further viewing of the image, I can maybe see the lines in the tail feathers...At any rate, I would suggest dumping the Tiffen and try B&W or Heliopan filters instead.

I use the same lens and filter and have noticed the same effect on some photos but just chalked it up as "one of those things". Thinking back on it I think most of them were in brighter light and at smaller apertures. It certainly doesn't do it on macro type work with a uniform colored background. I will certainly try some shots without the filter to see what happens.

Thanks to everyone for your thoughtful answers. The $20 figure for the filter was off the top of my head; the point was that I didn't want to spend more money for a better filter only to experience the same effect. I don't particularly like it. I haven't carefully inspected the filter for the aberrations you suggested, Kurt, but there was nothing obviously weird about it on superficial inspection. The artifacts are most apparent in a brightly lit scene, with lots of detail in the background. Sunlight coming through foliage produces it big time, but only when the the foliage is way out of focus. I have no feeling for the effect of aperture, since my N70 doesn't have DOF preview, and with the 400 I'm usually shooting at or near wide-open. If I have any more profound insight (yeah, right) about this subject I'll pass it on.

can someone define "bokeh". is it the same thing that fine wines have?

>>can someone define "bokeh". is it the same thing that fine wines have? <<

<br><br>

Try this link: <a href="http://www.altavista.com/cgi-bin/query?pg=q&sc=on&q=bokeh&kl=XX&stype=stext"> bokeh</a>

Bokeh (n) : 1. The subjective appearance of the out-of-focus areas

of a photograph, esp. when considered as a property of the lens

that took it. 2. One of those mysterious factors factors that,

despite being nominally a property of a photograph, seems to matter

more when discussing photography on the internet than when actually

practicing it.

I have had the bad Bokeh happen to my photos also. I never thought it was the filter until now. I have also had

the double sun, and I knew that was from the filter. I, personally, am trying to get rid of the unsharpness seen

on bright daylight subjects edges. The glare. I have to run some specific tests, but you can see my other

examples here:

 

http://picasaweb.google.com/Wuud52/BadPhotosFromFilterExamples?authkey=OSBukbjgrbs

 

Hope this helps.

<p>I just ran into this with a Hoya UV on a 100-400mm zoom. I was thinking the lens was going to have to go for service until I found this thread (and the one it was linked from). Phew! And thanks :)<br>

James.</p>