Flare on a Petzval

[Shangheye]

Please excuse the digitally manipulated attachements. They are intended as a way to emphasise the point of this post.

I have an old @ 1860's Petzval lens with no waterhouse stop. There are two unusual things about this lens...

a) a slot on the left hand side of the lens hood has been cut to allow stops to be put in (front of lens style)

b) The lens which is around 55mm wide in the glass, has a chip in the inside of the inner rear element which is around 5mm in diameter, but right at the edge. As installed in the lens right now, this chip is positioned at the bottom right (around 5 O'Clock if viewed from the rear)


So here is the problem...I get unusual flare from this lens. It appears to come from the bottom in a positive image, and is shaped as a crescent (see the highly contrasted images below that show clearly where the flare is). Clearly these images are exagerated, but I wanted people to see the shape and exact location of where the low contrast is occuring. Please also note the white area at the top is the tree line and not flare.

Now given the chip is at the bottom of the rear element, and with all my knowledge of how lenses work and especially Petzvals, I am loathe to think the chip is causeing this, since given it is at the bottom...would the flare not then appear at the top of the image? If everyone agrees with that assesment (I am open to being convinced otherwise), then the only possibility is the cut out in the hood (or maybe how big the hood is?) Since the cut out is located on the side (my left as I look at these images, Why would the flare appear so uniform centrally is the next question?...I would have expected a flare that ran from side to side....not so? The other wierd thing is that the Flare looks like is just fixed there...i.e. as I get in closer, it is just like magnifying it to a larger size, but it's the same shape in the same location??

I am working around it at the moment, and actually quite enjoy using the lens, but would love to understand what I am missing here, and if there is a way to remove it, then I would like that at my disposal! I intend to shoot a few plates this weekend to understand a little more...but that is a very time consuming way to work it out...;-)

For reference the lens is an 8in f3.8 Petzval and these plate sizes are 8x10.....for a real view of the flare effect, you will find the image marked xx in my gallery...


Appreciate any help that can be given.

Rgds, Kal

 

[brian d]

Why not try some black tape or darkcloth, whatever is handy over the slot for a shot or two so you can rule that possibility out?

 

[pgomena]

That, too, is my first guess. Cover the slot. It looks as though light is coming in through the slot and causing your problems. My second guess would be that there might be an area inside the lens that has lost its black paint over the years and is causing light to bounce around inside the lens.

Peter Gomena

 

[Shangheye]

Thanks guys..I am actually going to try those two (will check inside the lens), and I will also stick some dark insulation tape on the chip too...I reckon if it then disappears, I can at least know I can get rid of the issue. If after that it is still there, it could be some deformity in the lens design I have been told by a friend...any recommendations on the best paint if I need to paint the inside of the lens black?...I have some school blackboard paint. Rgds, Kal

 

[brian d]

I have used black magic markers for problems like that. The blackboard paint sounds like something that would be good for the inside of the barrel but its not something I would have ever thought of. If you try it let us know how it works out. Good luck!

 

[pgomena]

I'd use flat black enamel meant for plastic scale models. I've used it inside my Hasselblad's body and it doesn't appear to flake off. I've seen people use the same paint on lens chips. I doubt the little chip is the source of your problem.

Peter Gomena

 

[nick mulder]

My method involves placing my eye in the film plane and just looking into the lens ... Move your eye around the film plane, in and out of the area that is affected, you'll soon spot where the excess light is coming from. If you're focused on something uniform in colour then, the only colour you should see is that colour - any flare will be, well, flare coloured ! Use a darkcloth to assist ...

 

[Shangheye]

thanks Nick...I had tried that (and saw nothing) but without the dark cloth...will try that out tonight. Thx. K

 

[Sirius Glass]

Send the lens back to the manufacturer and demand a warantee repair!

Steve

 

[Shangheye]

Send the lens back to the manufacturer and demand a warantee repair!

Steve

Heh! Heh! I think they have a No return policy

So....I have moved the chip around to a different position, I have sealed the slot and I will put an extended hood to cut back the flare.

Will shoot tomorrow (subject to light) to see what happens. K

 

[Sirius Glass]

That is what happens when you buy from eBay.