Astrodon Sloan 1.25" U' filters - $100 closeout

[lost cat]

Does the Sloan + U-340(UG11) 1mm stack add up to OD5 for 400nm and up range?

 

Towards answering this pressing question I purchased a 1mm thick U-340 to stack on my Gen1 Astrodon Sloan U. Here is a picture I took of a locally sourced dandelion in sunlight.

 

Camera: Unmodified (undamaged) Nikon D40

Lens: El-Nikkor 80mm/5/6 @ f/5/6

ISO 200

Shutter 1 1/2s (corrected)

UV index 2 (low)

Whitebalanced in Picassa on virgin PFTE

 

 

Now without the filter stack:

(Conditions same as above but exposure changed to 1/800 and no post processing other than conversion to JPEG).

 

[nfoto]

Files straight off a Nikon D40 cannot be balanced for UV unless you process them in say PhotoNinja. Setting w/b against PFTE still leaves a strong reddish hue to the outcome and that is very evident here. You might send me the NEF and I'll process it for you.

[Andrea B.]

Jim, give it another second (or 2) of exposure. The D40 does not have wide dynamic range, so you might have to blow the white PTFE out in order to get a bit more UV light onto your Dandelion, but that is ok. You did capture the dark central bulls-eye very well and that will show up in the longer exposure.

 

Anyway, it looks like your stack will work.

 

Not all converters can handle white balance of raw UV shots. Try the free View NX2 from Nikon for white balancing. I know it will work.

 

Looking forward to some more shots from you!

[lost cat]

Jim, give it another second (or 2) of exposure. The D40 does not have wide dynamic range, so you might have to blow the white PTFE out in order to get a bit more UV light onto your Dandelion, but that is ok. You did capture the dark central bulls-eye very well and that will show up in the longer exposure.

 

Anyway, it looks like your stack will work.

 

Not all converters can handle white balance of raw UV shots. Try the free View NX2 from Nikon for white balancing. I know it will work.

 

Looking forward to some more shots from you!

 

Thanks Andrea,

 

I do have the free View NX2 but I couldn't figure out where the white balance menu was; however, I've since discovered its location. This is a 2s exposure I took this afternoon along with the others; unfortunately it was the longest exposure I took so a redo will have to wait until tomorrow.

 

I took a 5x5 gray point sampling from the PTFE but its still quite red:

 

 

Same image but the gray point is the center of the dandelion:

 

 

I don't think that's how it supposed to work though.

[Cadmium]

Hi Lost Cat, I have a D90 and D7000, and I use CNX2 for white balance (I marquee the whole frame usually). I don't know if you have that program, but I think there are trial versions available for download,

it might be something to try anyway.

Please don't get me wrong, I think you idea is ingenious, but I just question if the stack actually suppresses IR given the graph you posted and the 1mm thick U glass.

Looking at the combined graphs would lead me to expect that there is some IR, some of which might white balance to be white, given the IR range, which might mostly effect the darkness or lightness of the dark UV pattern areas.

The proof is in the pudding, they say. Nice to see your tests.

 

[lost cat]

Hi Lost Cat, I have a D90 and D7000, and I use CNX2 for white balance (I marquee the whole frame usually). I don't know if you have that program, but I think there are trial versions available for download,

it might be something to try anyway.

Please don't get me wrong, I think you idea is ingenious, but I just question if the stack actually suppresses IR given the graph you posted and the 1mm thick U glass.

Looking at the combined graphs would lead me to expect that there is some IR, some of which might white balance to be white, given the IR range, which might mostly effect the darkness or lightness of the dark UV pattern areas.

The proof is in the pudding, they say. Nice to see your tests.

 

 

Yes its possible there is some IR bleeding through. My thought was the known weakness of the U-340 are likely the strengths of the Sloan and vice versa while both filters transmit lots of near UV. But then again there are those pesky missing data points...

 

One possible test woudl be to stack yet another filter, this one a heavy UV blocker and see if ANYTHING makes it through.

 

Any suggestions? Thick bit of polycarbonate perhaps?

[Cadmium]

Yes its possible there is some IR bleeding through. My thought was the known weakness of the U-340 are likely the strengths of the Sloan and vice versa while both filters transmit lots of near UV. But then again there are those pesky missing data points...

 

One possible test woudl be to stack yet another filter, this one a heavy UV blocker and see if ANYTHING makes it through.

 

Yes, this is the genius of your stack idea, that these two filters would cancel each others weaknesses out, but they both transmit some IR in the same range, so that may still be a weak point.

I don't suspect your stack to have a leak in the visual range, however it is a slight possibility knowing what I do about U-340 1mm having an upper visual leak (however mild it may be), which doesn't show up in Hoya data.

It seems hard to say if there is IR or not, or how much, from this white balance.

I think white balance is the first step, then examine your blacks and foliage. Foliage should usually be black/gray, not browns or such.

For IR isolation, You could start with something simple, perhaps a common red filter, 25A, 590nm, or try a 720nm filter, or 830nm.

If you think there is a noticeable leak lower in the visible range, then you can try other filters like 420nm, or above.

However, something will usually "make it through", depending on the exposure time. Even a Baader U will show IR if the UV range is blocked and the exposure time is long enough.

You almost need to compare an isolated stack to an isolated reference filter that has a known good IR suppression.

For example, if a Baader U stacked with a 610nm (or other point) filter shows an empty black frame at whatever exposure time, then your stack should also show an empty black frame at the same exposure time

if you are trying to have the same IR suppression.

Back to white balance, I encourage you to find a trial copy of Capture NX2 and test it out. Take a look at this LifePixel tutorial video, video #5 on this page:

http://www.lifepixel.com/tutorials/infrared-photoshop-videos#

"Infrared RAW file white balance issues & solutions"

 

This is a tutorial for IR, but the exact same method works for UV.

Andrea knows a lot about the other programs, but I only have experience with this, and I know how it works for me, and since you are using a Nikon, then I think CNX2 may be useful to try it.

[lost cat]

One potential issue is I may not have the sloan facing the right way. There have been a few mentions of the need to reverse the Baader else problems occur. I have not reversed my sloan and currently have it mounted with the threaded side on the lens. Is this correct or do I need to reverse it?

[Andrea B.]

The Baader is usually reversed to help prevent flare from the shiny pink side. But there is no difference in the photos which are made with it facing either way.

 

You are white balancing on the Raw file, yes? Not on the jpg?

 

Your white balanced file shows the white PTFE has "blown" hence the cyan cast. So you are caught between a rock & a hard place. Exposure sufficient to reveal the flower blows your PTFE and so it becomes useless for white balancing. Alternately, exposure to control the white area does not reveal the flower. PTFE blows easily because it is not diffuse. But even with my D600 and using diffuse white standards I have to sometimes be careful not to overexpose the white areas.

 

Try white balancing on some magenta area of the raw file. That usually works.

 

I'm beginning think this combo probably has some red leak, so wb efforts may be futile. Do you have a red longpass to test with? Or a 690?

(oh I see that Cadmium has already mentioned this kind of thing...!!)

[lost cat]

Yes, I am white balancing on the RAW. I don't think I have a choice there.

 

I was not aware PTFE was so tempermental, that is good to know. When you say diffuse do you mean this in reference to the reflectivity of the material?

 

Could this overall problem also simply be a symptom of low UV illumination? The UV index was rather low today.

 

 

[Cadmium]

I don't think reversing the Sloan is going to make any difference here. I have never seen any difference in reversing my Baader U, even though I use it in the advised direction.

I would like to see more foliage, which is always what I look to see Red/IR lighting up.

I find PTFE cyan blowout is problematic that way. I use it for tests sometimes, but I usually just Marquee the image, or some small part of the image if I want to play with the white balance, rather than use PTFE.

This may not be relevant, but with my D7000, it lets me set the camera to use a manual lens, so I shoot in Aperture Priority, auto exposure. My D90 is limited to manual exposure with manual lenses.

This is a very interesting test you are doing.

 

I think you have plenty of UV illumination.

I wish I could find a dandelion this time of year here. :-(

[nfoto]

Jim kindly sent me the NEF and I ran it quickly through PhotoNinja with standard settings. The capture is underexposed about one whole stop, but not more than this can be recovered in PN. The PTFE is, as stated before, a little "temperamental" in that it is not providing a true diffuse, Lambertian-type reflection, so it has to be positioned carefully and I tend to make a separate shot with it placed near the middle of the frame. In that position you get a more even reflection and using the PN tools it's easy to get a w/b setting that can be applied to additional images made under the same conditions.

 

 

There are complication in the test image due to the presence of vignetting accompanied by a w/b gradient, as seen over the PTFE sheet. I selected the part of the PTFE close to the flower head for w/b and this provides an acceptable balance for the main subject.

 

A lower UV input level is easily counteracted by a longer exposure so on its own it has little significance. However, if there is ever so slight wind present, flower subject may shift to cause loss of sharpness. For testing, I thus prefer studio flash(es) with uncoated Xenon tube or the SB-140 in the field.

 

On an unrelated note, Jim's camera has a very dirty sensor ....

[Cadmium]

That look good to me. Now I am wondering what a Sloan shot looks like on its own.

Also, just curious what filter was used for the visual pic?

[colinbm]

I don't know if Jim is using a block or sheet PTFE, but perhaps a roughing with some 40 to 60 grit abrasive cloth & a drop of dish washing detergent in water, to also clean the PTFE, may help ?

Col

[lost cat]

I don't know if Jim is using a block or sheet PTFE, but perhaps a roughing with some 40 to 60 grit abrasive cloth & a drop of dish washing detergent in water, to also clean the PTFE, may help ?

Col

 

This was a block of about 2mm thick PFTE. I also have a block of wood wrapped with several layers of PFTE plumbers tape as a backup :)

 

I can try roughing up the plastic to increase its diffusitivity. Also try to keep it cleaner. Who knew junk sticking to Teflon could be such a problem?

 

That look good to me. Now I am wondering what a Sloan shot looks like on its own.

Also, just curious what filter was used for the visual pic?

 

I'll try the sloan alone when I get a chance. No filter on the visual pic, just a bare lens.

 

On an unrelated note, Jim's camera has a very dirty sensor ....

 

Yeah, my many, many lens changes haven't helped that.

 

And this was the undamaged Nikon - you should see the the D40 my nephew smashed onto a tile floor! I'm just happy that one still works at all!

 

Time to break out the cleaning kit. Ugh, I hate to think what my Canon's sensor looks like by now.

 

I did use the El-Nikkor wide open. I can stop it down a bit to see if that helps the vignetting.

[Andrea B.]

Yes that looks better in Bjørn's edit. I would have applied the Black &/or Shadow slider in tiny amounts to pull some detail out of that dark center. As mentioned the D40 lacks dynamic range, so blocked shadows and blown whites must be dealt with. [And we note that you still must deal with this kind of thing even with the best of sensors. It is just easier to handle with the newest Nikon sensors.]

 

PTFE mostly works for white balance settings. Being non-diffuse, it is prone to specular blow-outs. So try angling the illumination. Being a non-standard, it can sometimes produce very very slight colour casts. But for what we do, it works Well Enough. [bTW, having equipment work "Well Enough" is the state of grace we should all strive for.] When lacking PTFE, just white balance on something magenta in the UV shot and you can get very close to Well Enough with a bit of experience in a good converter.

 

I do not think it is a good idea to "rough up" the surface of your PTFE. You cannot attain a diffuse reflection this way. You will simply scratch the surface of the PTFE and make it more prone to stains and dirt.

 

An important point to note is that to attain a "true" False Colour in a UV photo (......and there we go with our amusing UV terminology: True False Colours lol !!......) you must both profile (calibrate) the camera's Visible colour after removal of any internal filters and perform white balance in the given illumination of a scene. Of course, UV shots almost always work Well Enough without colour calibration. I have some post lurking around here somewhere which shows the few differences that the colour calibration step can make. There could be photographic situations where you would want more control over the False Colours by profiling your camera.

 

I think it might be important to point out that we are practicing UV photography here, not UV scientific documentation via photographs. Most of us have come to like the look of a white balanced UV photograph, but there is no particular necessity for performing colour calibration and white balance in a UV photograph. Artistically you can pick any colour schema you want to for your UV photographs.

 

Bjørn and I chose to use a standardized white balanced blue/yellow/grey palette in our UV Botanical sections so that there could be some uniform look to our photographs when making high level comparisons of the different UV signatures. We currently have no way to perform an actual false colour calibration through the UV filter because there has never been (as far as I know) a standard set for what a UV false colour should be. So we are simply calibrating the camera's visual colours and applying that to the UV False Colours. OTOH, when looked at from the point of view that there are surfaces which reflect all of R, G, B and UV (call it UV-white, why not?), then the white balance step makes a lot of sense because UV-white 'should be' mapped to Visible-white.