Does a UV Filter Help Sharpness?

wxwax

I was minding my own business, reading a review of the Tamron 200-550 zoom, when at the very end of the review, someone popped-up with this little gem.

Performance Enhancement Tip For All Lenses: To get the best performance from any lens, the owner should purchase a protective skylight 1A filter which also includes UV blocking. If your lens is already slightly "warm", get just a UV blocking filter.

Spherical aberration (which causes softness) is strongest in the UV with any lens. Both film and most CCDs are sensitive in UV unless the digital camera manufacturer has incorporated UV blocking over the CCD. Anyway, doubling up on the UV blocking never hurts and can only improve sharpness and contrast.

-- Michael Marcus, March 7, 2005

Well, blow me down. I've never heard of this before. Anyone else heard of, or even better, experienced a UV filter helping with sharpness?

StevenV

Sid, did you ever find out more about this?

DoctorIt

Hmmm. That's interesting, Sid. I can't find anything immediately wrong with the physics there, but let me think about it.

DoctorIt

I didn't notice this thread was dredged from the depths...

But anyway, I've thought about it. So spherical aberration is not something that is much talked about when dealing with light that has any sort of spectrum (as in, photography). Typically, SA is a function of imperfections in the lens and is a concern with monochromatic light because incoming light rays that hit the lens at different points are focused onto slightly different points at the back. Think several coincidence points. This is common in old lenses, but these days, this primary (or Seidel) aberration is mostly inconsequential [Hecht Optics 2002].

SA is most common in UV. I'm having a hard time confirming this one, although, I admit, I'm no expert. I'm just getting confused, because when you start talking about UV rather than other wavelengths being the cause of softness, you're talking about chromatic and not spherical aberration. Well, you are, and you aren't.

Come on light nerds...

wxwax

StevenV wrote:

Sid, did you ever find out more about this?

No, and I haven't seen it mentioned anywhere else. I do remember Shay Stephens saying that digital camera sensors aren't sensitive to UV the way film is. Which would sorta seem to contradict the article.

MrBook2

I think that we are really talking about chromatic aberration here (see:http://www.vanwalree.com/optics/chromatic.html ) as opposed to spherical aberration (see: http://www.vanwalree.com/optics/spherical.html ). In any case the real question is are CCDs (or CMOS) sensitive enough to UV that a UV filter is going to help prevent softness. To answer that, have a look at the plot here as an example: http://www.maxmax.com/camera_technical.htm

As you can see, the sensitivity for a typical CCD drops off sharply on the UV end (below 400 nm). There is a nice long tail out in the IR (the right hand side of that plot.) This is why most digital cameras have a piece of IR blocking glass in the camera. This is why you can get a camera modded to become an IR sensitive camera; they remove the IR blocking glass and replace it with one that passes nearly all of the light to the CCD.

So, I am going to say that you need not worry about the UV light making your digital images "soft". But that is just my gut feeling.

--Aaron

DoctorIt

MrBook2 wrote:

I think that we are really talking about chromatic aberration here as opposed to spherical aberration.

Ah, so you're having the same thoughts I am (see above)! To really tease the brain, think about how bad an image could be if you had both CA and SA - this it totally possible with chromatic light, NOT with monochromatic.

In any case the real question is are CCDs (or CMOS) sensitive enough to UV that a UV filter is going to help prevent softness. To answer that, have a look at the plot here as an example: http://www.maxmax.com/camera_technical.htm

Yup, that's the answer I was looking for, so that should settle the question at hand, whether an UV filter will sharpen things. But my broader question still remains, is there truth in this, is UV light more prone to cause SA in a well ground lens that with other chromatic light will not cause SA.


know what I'm gettin' at? full nerd-out, we're beyond the photography question nwow.

pathfinder

The downside of any added external filter is two added reflecting/refracting surfaces in the optical system. Not good.

Filters have predictable negative consequences, in addition to any favorable ones - like a real polarizing filter.

I think the biggest advantage of a UV filter is that is keeps sand and salt from getting on the front of your lens. If you are near salt or sand.:):