[filmamigo]

Wondering if anyone can offer their experience with this lens, or an otherwise informed opinion.

I was doing some tests of this lens (looking to choose between the 20mm M and A versions.) The test shots include south-facing sky in or near the frame, so they are a merciless test of any lenses coatings. But this lens showed quite a bit of veiling glare. So much so that I worried about it's general usability. So -- I checked the lens. It is free of fungus, but did have a bit of a fingerprint smudge on the rear element. I cleaned that off, and took some more test shots at different angles from the sun - with no real difference. While some shots are fine (great even), quite a few show localized blooming or veiling glare. In some cases, it seems that minor changes in angle or position will make the glare worse or better.

This isn't something mentioned in any of the reviews of this lens. There are many many sample images posted that look great (but so does my one sample shot almost 180 degrees from the sun.)

Here are more details of my tests:

• glare is apparent at every aperture
• tests were shot with a Pentax K1 (so sensor reflections should be expected to be worse than film)
• the lens was carefully shaded for all tests to prevent light from striking across the surface of the lens

I have attached samples below.

Any thoughts or experience with this lens would be appreciated.

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[Wheatfield]

I have the same lens, though it's not one that I get a lot of use from. Veiling flare is, unfortunately, pretty unavoidable with any ultra wide angle lens. SMC coatings do a good job of limiting the visible artifacts that we normally associate with flare, but the type of flare you are showing isn't something lens coatings do a great job of limiting. For myself, I would say what you are seeing is fairly normal for this lens, or any ultra wide angle, especially one that was designed 3 decades ago.
Pretty much any lens can be made to create flare of some sort.
You've done a good job of identifying the conditions your lens is going to have trouble with. Unfortunately, the only sure cure of it is to limit those conditions which can be difficult with a lens as wide as a 20mm.
The only way I have found to eliminate veiling flare in an ultra wide is to ensure the sun is at least slightly behind the lens.

[Fogel70]

Was the test performed with or without a lens hood? I have no experience of this particular lens, but UWA lenses can be extra prone to these type of problems, making lens hood extra important.

[gofour3]

As mentioned in the above post, are you using the OEM lens hood?

Phil.

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[Wheatfield]

As mentioned in the above post, are you using the OEM lens hood?

Phil.

He did mention the lens was shaded from extraneous light.

[Madaboutpix]

Don't own the smc Pentax-A 20mm myself, but considering that it's a 1985 release, and comparing to my smc DA 15mm and 21mm Limiteds, I wouldn't be all too concerned by what I see in the samples you've posted. After all, veiling flare and highlight blooming can be provoked in all three lenses, although the Limiteds show that they were designed for digital and that their smc coatings are likely not the same as those applied to the much earlier film lens.

As long as your copy of the A 20mm doesn't show other major issues (e.g. decentering, fungus, haze, balsam separation) and the outer optical surfaces are clean with the coatings intact, I would just assume that this is what the A 20mm does in difficult backlight. Legacy film glass is different from modern digital glass, and even the latter benefits from using a lens hood. Still, this wouldn't stop me from exploring how far I could go with backlight, given that it can produce dramatic and highly atmospheric images. Use the A 20mm with a little care, and otherwise embrace and enjoy its character, I'd say.

[Abbazz]

I think it could be balsam separation. I have an FA 20mm f/2.8, which shares the same optical formula, which started showing excessive flare a few years ago. It is now completely hazy and totally unusable. I sincerely hope this is not the case with your lens.

Cheers!

Abbazz

[Wheatfield]

I think it could be balsam separation. I have an FA 20mm f/2.8, which shares the same optical formula, which started showing excessive flare a few years ago. It is now completely hazy and totally unusable. I sincerely hope this is not the case with your lens.

Cheers!

Abbazz

Good call. Could you expand on that. Are there any visible tell tales that might be visible in an external examination? The formula shows two cemented elements near the back of the lens.

[filmamigo]

Thank you all for good feedback. I can compare this lens to the M 20mm and the DA 15mm Limited. Of course you can make any lens flare by abusing it, but I have found the DA 15 remarkably free of flare. And my M 20 is definitely doing better than this A 20.

Appreciate your time and thoughts.

[UncleVanya]

The da15 and m20 are f4 lenses which tend to be less flare prone than f2.8 wide angles. Also the DA15 is atypically strong in its flare handling in my opinion.

[Abbazz]

Good call. Could you expand on that. Are there any visible tell tales that might be visible in an external examination? The formula shows two cemented elements near the back of the lens.

On my sample of this lens, there is visible cloudiness in the cemented doublet which can be seen when I look from the back of the lens, especially while shining a flashlight through the lens. A few years ago, it was only a faint cloudiness, which resulted in some flare like on the OP's pictures. Right now, the cloudiness is really obvious even without a flashlight and the images taken with the lens have a milky appearence, even when the lens is stopped down to f/11.

Cheers!

Abbazz

[filmamigo]

On my sample of this lens, there is visible cloudiness in the cemented doublet which can be seen when I look from the back of the lens, especially while shining a flashlight through the lens. A few years ago, it was only a faint cloudiness, which resulted in some flare like on the OP's pictures. Right now, the cloudiness is really obvious even without a flashlight and the images taken with the lens have a milky appearence, even when the lens is stopped down to f/11.

Cheers!

Abbazz

Thanks Abbazz, I will investigate further. My lens inspection so far consists of examining the elements from both sides of the lens, using strong window light. I didn't see anything, but will take a second look with a magnifier and flashlight.

[Abbazz]

Thanks Abbazz, I will investigate further. My lens inspection so far consists of examining the elements from both sides of the lens, using strong window light. I didn't see anything, but will take a second look with a magnifier and flashlight.

You're welcome. There are a few reports of balsam separation concerning this lens or the FA 20mm on the forums (like here) but I guess that maybe this flare could rather originate in the decay of the so-called "hybrid elements."

Many consumer lenses from that time used hybrid aspherical elements, which were made of a plastic aspherical element sandwiched around a core glass element. The plastic around the glass core could start to show signs of decay after some time, especially if the lens has been submitted to rather extreme temperatures.

But I have found no positive evidence that Pentax used a hybrid element to build its 20mm f/2.8 lenses.

Cheers!

Abbazz