[popo]

The other thread on the rear positioning of an external aperture reminded me of an idea I wanted to try but never got around to.



That is, would it be possible to convert a lens to give a Sony STF type bokeh by adding a similar front filter? For those who might not be familiar with the STF, it has an additional different "aperture" which isn't hard, but like an gradual ND filter getting stronger as you go outwards. This makes out of focus points appear as a smoother blur rather than a disk.



I've played about with shaped bokeh before. That is adding a physical mask in front of the lens. The bokeh then takes the shape of this mask with the in focus image looking normal. Would this also work to create a STF like response? For example, if I try printing an approximation of a suitable ND filter on transparency and cutting it to fit the front of a lens. Note I'm looking at proof of concept here and don't expect perfection. I just need the bits to try it out...

[PuxaVida]

[quote name='popo' timestamp='1291969945' post='4920']

The other thread on the rear positioning of an external aperture reminded me of an idea I wanted to try but never got around to.



That is, would it be possible to convert a lens to give a Sony STF type bokeh by adding a similar front filter? For those who might not be familiar with the STF, it has an additional different "aperture" which isn't hard, but like an gradual ND filter getting stronger as you go outwards. This makes out of focus points appear as a smoother blur rather than a disk.



I've played about with shaped bokeh before. That is adding a physical mask in front of the lens. The bokeh then takes the shape of this mask with the in focus image looking normal. Would this also work to create a STF like response? For example, if I try printing an approximation of a suitable ND filter on transparency and cutting it to fit the front of a lens. Note I'm looking at proof of concept here and don't expect perfection. I just need the bits to try it out...

[/quote]



Well, first of all thank you for letting me see such kind of a one-of-a-kind bokeh oriented design. But it seems that the design of this lens (Sony STF 135mm) includes a special glass element which produces the ND filter effect gradually. And both the stepless and normal aperture sit infront of it. I mean, your design would be a real challenge...



Regards,



Serkan

[Brightcolours]

What you propose seems to be not possible at all.



The Sony does not have that element in front of the camera!



What happens when you make the image circle (or rather: opening) smaller inside the lens (with the aperture)?

You get a bigger DOF.



What happens when you make the image circle in front of the lens smaller (think for instance too thick filters)?

You get heavy vignetting.



This should make clear to you that the placement is everything..

[popo]

I have no doubt it is possible, at least in big aperture lens cases, the only question being how effective will it be and if there are any excessive side effects.



Have you ever tried shaped bokeh? Placing a mask on the front of the lens does not result in hard vignetting like too thick filters. Extension in front is different from blocking parallel to the front element. It does by definition reduce the aperture so you get the effects associated with that like light loss. What I'm proposing is instead of a hard mask, to use a gradual one. Just like ND grad filters, but instead of a linear pattern it will be radial.



I do not expect, for example, to convert a random cheap lens into a STF direct replacement, but if this method can assist with more pleasing bokeh without major side effects I think it is worth a try.



And to try I have a relatively simple idea to implement it. Simply create the pattern and print it onto transparency. Then cut that into the right size and see what happens! The cost is insignificant compared to most camera stuff. I just need to order some...

[genotypewriter]

[quote name='popo' timestamp='1291969945' post='4920']

That is, would it be possible to convert a lens to give a Sony STF type bokeh by adding a similar front filter?[/quote]

There's a very easy, widely known way to do this:



http://forums.dpreview.com/forums/read.asp?forum=1039&message=30246830



GTW

[popo]

[quote name='genotypewriter' timestamp='1292199059' post='4953']

There's a very easy, widely known way to do this:



http://forums.dpreview.com/forums/read.asp?forum=1039&message=30246830



GTW

[/quote]

Haven't gone through the whole thread in that link, but aren't they talking about softening?



Edit: have gone though it now, they are talking about general softening effects, which is not what I'm after, and probably would be easier in software these days? I want to keep a sharp focal plane, but have smoother bokeh. Without software trickery.

[Guest]

[quote name='popo' timestamp='1292226339' post='4959']

Haven't gone through the whole thread in that link, but aren't they talking about softening?



Edit: have gone though it now, they are talking about general softening effects, which is not what I'm after, and probably would be easier in software these days? I want to keep a sharp focal plane, but have smoother bokeh. Without software trickery.

[/quote]



I seem to remember people using a match to char the edges of a UV filter to get a similar effect. Can't remember where I read it, but it may give you a lead.

[popo]

[quote name='carpents' timestamp='1292269485' post='4968']

I seem to remember people using a match to char the edges of a UV filter to get a similar effect. Can't remember where I read it, but it may give you a lead.

[/quote]

Ooh, nice trick! Hmm... if I have an old filter I might have to try that, if I don't heat stress the glass to breaking point.



The transparency has been ordered so I can continues on my original plan later this week regardless.

[genotypewriter]

[quote name='popo' timestamp='1292226339' post='4959']

Haven't gone through the whole thread in that link, but aren't they talking about softening?



Edit: have gone though it now, they are talking about general softening effects, which is not what I'm after, and probably would be easier in software these days? I want to keep a sharp focal plane, but have smoother bokeh. Without software trickery.

[/quote]

Sorry about that... my bad entirely.



The darkening of a filter corners seem like a great idea though.



GTW

[popo]

I got the transparency sheets today, so I quickly knocked out some patterns at work. Hmm... even with a greyscale image, the printer decided it wanted to do a green tint, so not a good start. Also the printed areas aren't ND-like as I hoped, but they look semi-opaque and diffuse. Not totally unexpected of course, since the toner is blocking the light, but I was hoping for more like a halftone effect remaining transparent. Maybe I need to play about some more with the dithering options...



As is, the ones I have would probably work as a soft focus effect filter so not what I wanted and I need to try again with the implementation.