Nikon 50/1.8 AFD: A Few UV Shots

[Andrea B.]

This lens was being discussed in another topic along with a U340 x 4.0 mm UV-Pass filter. I made a few test shots to see how this lens performs with that U340.

 

Camera: Nikon D610 Full-spectrum Conversion

Lens: Nikkor 50 mm f/1.8 AFD

Illumination: Strong sunlight.

 

It was very windy out there today. I was not able to make any UV photos which did not show motion blur. That's OK though. I was looking for hot spots or other performance issues and motion blur won't interfere with that. And also I was wanting to compare photos from an IR-blocked U340 x 4.0 mm with an unblocked U340 x 4.0 mm.

 

Conclusion up-front #1:

In strong sunlight you really do need to add an IR-blocker to a U340 x 4.0 mm.

Over time I have made several good unblocked U340 x 4.0 mm photos under the light from either a modified SB-14 flash or the UV SB-140 flash. Those always show reflected and absorbed UV areas quite well along with some interesting false color. But the outdoor photos I made today definitely show serious leakage under good old broadband Sunlight.

 

*****

 

Conclusion up-front #2:

The Nikkor 50/1.8 AFD shows some evidence of center discoloration in the false color versions.

This could be caused by simple vignetting or by placement of a UV-pass filter on a wide-angle lens. (We see more of that with dichroic filters, but it can happen with absorptive UV filters too.)

I'm also thinking the lens might be a little bit hot in the center. So at the end I'll show you a couple of sky photos which might indicate that slightly hot center.

 

Nothing showed up in these few simple tests which would indicate that you shouldn't enjoy using it for reflected UV photos. Just stop down and be careful of the angle of illumination from the sun. (The usual advice, eh?) And you can always convert a UV photo to a nice B&W or other monotint colored photo to get rid of circular discolorations. Some of my very best UV photos were converted to monotint or split-tones for just that reason.

 

Note: This conclusion might possibly only be relevant for my particular copy of the 50/1.8 AFD. There is variation amongst these older lenses due to age or wear-and-tear.

 

*****

 

All photos had some detail sharpening added to get the fuzz off.

 

I increased saturation for green, blue and yellow, our typical false colors in UV. I did this to help determine whether there were any wide-angle circular discolorations.

 

The conversion of the UV photo to B&W was very simple. I only increased the Illumination/Exposure slightly and opened up the shadows just a small amount. No work on zones or tones for this simple test.

 

i cropped a bit of boring stuff off these photos so the center of the original photo is a little to the right. I hope that is OK for when I refer to "hot centers" below.

 

The false-color white balance for the UV-pass photos was *not* standardized, that is, white balance was not made against Spectralon or PTFE. Instead, I chose a certain area on the rocks and used that area for each of the three UV-pass photos.

 

 

Visible Reference Photo

Baader UV/IR-Cut + BG38 x 2.0 mm

We have here some basic foliage, sky and rocks.

The foliage will be useful to determine either Visible or IR leak under UV-pass filters.

 

 

Ultraviolet Reference Photo: False Color

363FWHM50 UV-Pass Filter: LaLaU

This is a typical UV-pass photo at a peak wavelength reached by many non-dedicated UV-capable lenses.

 

 

Ultraviolet Reference Photo: Black & White

363FWHM50 UV-Pass Filter: LaLaU

The foliage is UV-dark, the gravel is moderately UV-reflective and the little daisy like-flowers and the sky are both UV-bright. All is as it should be.

 

 

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U340 x 4.0 mm + S8612 x 2.0 UV-Pass Stack: False Color

The exposure time naturally increased in this photo because of the thicker filtration and the lesser amount of 340 nm in the sunlight. From 1/13" to 1/2" is 8/3 stops, almost 3 stops. I don't know how much of a role the 50/1.8 lens played in this exposure increase because I don't know the transmission for that lens from memory. (I'll see what I can find later.)

 

The 50/1.8 is certainly capturing the UV well enough under this stack. However, there is definitely some circular discoloration going on in this 340 photo which I don't see quite as much in the UV Reference photo.

I don't know what is that blue strip on the extreme left side. That wasn't in the UV Reference photo either. I do see Dust Bunnies creeping in. Sigh. Kindly ignore.

 

 

U340 x 4.0 mm + S8612 x 2.0 UV-Pass Stack: Black & White

The preceding photo looks much better in B&W. All the tones are as they should be.

 

 

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U340 x 4.0 mm Unblocked: False Color

In this strong New Mexico sunlight, the U340 x 4.0 mm doesn't stand a chance when used without its IR-blocker. This photo comes fairly close to being a Visible rendition in both false color and in the next B&W rendition. And I note that we lost the expected UV false yellow sky.

I don't think these unblocked U340 nm photos show much IR contamination.

What do you think?

 

 

U340 x 4.0 mm Unblocked: Black & White

Too bright. Does not have that Dark Look of authentic UV.

 

 

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UV340 x 4.0 mm: Sky Photos in Black & White

Aimed at the sky. Note there was more sunlight to my left than to my right. Exposure time was such that the B&W histogram was in the middle.

 

f/1.8 for 1/10" @ ISO-100: As Shot

And so you see this photo is bright in the center. Does it look that way just because of fall-off/vignetting? Or is this lens a little bit hot in the center with the UV filter?

 

 

Same photo with black and white point adjustment.

Here I have brought in the two endpoints of the histogram to see how bad things get. This is an extreme edit. In the next set I will show how things improve when you stop down to f/8.

 

 

f/8 for 1.3" @ ISO-100: As Shot

There in only a small amount of increased center brightness when the lens is stopped down to f/8.

 

 

Same photo with black and white point adjustment.

Again I've used te extreme edit to bring out a couple of interesting things. There is an area of flare in the lower right. We can ignore that. I was shooting at an angle to the sun, not directly at it. But not at such an angle that I couldn't catch a bit of flare, which obviously I did.

 

Now if you look at the circled area in the center, then you can see a faint hot spot. But it took an extreme edit to make it obvious. And....this might just be another area of flare?

 

However I don't think that the center of this lens causes enough trouble to avoid using it for UV photos. This lens appears to be quite nice for typical UV-pass photos around 360 nm. Just be careful of your shooting angles and stop down as much as possible.

[lukaszgryglicki]

Great, thanks for a very detailed review - looks like I was wrong thinking that thick 4mm U-340 alone is enough for UV photos, need to get some IR blocker too.

[Andrea B.]

It seems to depend strongly on the lighting when using U340 x 4.0 mm. To reiterate, up close and with a UV flash, the U340 x 4 shows only a little "contamination". Outdoors an IR blocker is needed for sure.

 

I have a link somewhere showing indoor/flash use of that U340x4 which I will post here: See Next Post

 

Our recommended IR blocker to use with dual bandpass UV/IR glass is the S8612 glass. I do not know what is the less expensive Chinese equivalent for that. There may not be one. Let me go find the next best BG IR-Blocker.

 

From this attached transmittance chart by Cadmium, you can see that S8612 x 2.0 mm is best

followed by BG39 x 2.0 mm. Both are OD4 at 700 nm.

ADDED: Please see the transmittance chart in Post #5 below and the remark in Post #6. A 1.0 mm thickness of S8612 is sufficient to block IR with the 4.0 mm U340. But the 2.0 mm thickness of S8612 might be more useful if you are only going to purchase one IR-blocker.

[Andrea B.]

Raw color example of unblocked U340 x 4.0 mm with SB-140 UV flash.

f/11 for 1/30" @ ISO-200

This UV flash photo was made in a room with ambient Visible skylight.

The dark area of the flower petals and the dark center are probably 1.5 - 2.0 EV brighter than the same areas in next photo made with the BaaderU.

 

I can live with that for the interesting false color the U340 x 4.0 can provide due to a bit of green leakage. Certainly the UV-signature of this flower has been well-captured. If you used a UV flash in a completely dark room, then such a UV flash photo would not show the small leakage.

As always, YMMV on this!

 

 

Raw color example of BaaderU with SB-140 UV flash.

f/11 for 1/30" @ ISO-200

 

 

Example of unblocked U340 x 4.0 mm with SB-140 UV flash.

(Just a quick render to show color possibility, processing not complete.)

I love this green.

 

 

Example of BaaderU with SB-140 UV flash.

(Just a quick render to show color possibility, processing not complete.)

[Cadmium]

It seems to depend strongly on the lighting when using U340 x 4.0 mm. To reiterate, up close and with a UV flash, the U340 x 4 shows only a little "contamination". Outdoors an IR blocker is needed for sure.

 

I have a link somewhere showing indoor/flash use of that U340x4 which I will post here: See Next Post

 

Our recommended IR blocker to use with dual bandpass UV/IR glass is the S8612 glass. I do not know what is the less expensive Chinese equivalent for that. There may not be one. Let me go find the next best BG IR-Blocker.

 

From this attached transmittance chart by Cadmium, you can see that S8612 x 2.0 mm is best

followed by BG39 x 2.0 mm. Both are OD4 at 700 nm.

 

You would only need to use S8612 1mm thick for stacking with U-340 4mm, which will give you OD5, and give you better UV transmission.

The actual OD is a calculation of the U-340 and the S8612. You see how the S8612 plot doesn't show the deeper plot of the real combination of the U-340 4mm + the S8612 ,of course I am using 1mm here,

but the point remains the same.

[Andrea B.]

Cadmium, thanks for the stack chart. I will go back and add a note.

 

I suppose if a person is only going to buy *one* S8612 for general use as an IR-blocker when shooting UV, we would recommend the thicker S8612 x 2.0 mm. It is possible the S8612 x 1.0 mm might not block enough IR when used with some thicknesses of dual bandpass UV/IR to make a UV-pass stack. So economics play the usual role here in acquiring filters.