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UltravioletPhotography

Question for you about comparing lenses


Andrea B.

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I want to compare two lenses' UV-capabilities by checking the exposure speed of UV photographs made with them (at fixed apertures and fixed ISOs), but the two lenses are of different focal length. How do I best do this?

 

Should I make a photo of the scene at equal distances - which will produce different fields-of-view?

Or should I shoot at equal framing - which will force different distances from the subject?

 

Either way, will it be a fair test? It seems to me that the light gathering for each lens could differ in either setup even though both lenses are at the same aperture? But then maybe this wouldn't happen if I was out in the middle of a big yard in full sunshine?

 

Maybe it is the T values I want to use here?

 

Thanks for any input.

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You need to be more specific about what parameter you are actually trying to measure. Is it total UV transmittance wide-open? Is it spectral balance (relative proportion of shorter wavelengths transmitted?) Is it angular resolution? Chromatic aberration? MTF smudging?
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With world enough and time - no, wait -

with time enough and money a-plenty, I would indulge myself in proper spectral equipment and other such tools

in order to measure all those lovely parameters.

 

However, it isn't measurements I'm wanting.

What is wanted here is a practical photographer's point-of-view:

 

Lens A will shoot the scene under a BaaderU at f/11 and ISO-400 on Camera XYZ using {half the exposure time, the exposure time S, the same exposure time} that Lens B will shoot the same scene with the same filter/camera/aperture/iso.

 

That kind of thing.

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enricosavazzi

I think you must add also the same white balance (because a different WB will alter the relative levels of UV false-color). You should also convert to BW in a consistent way, because different spectral transmission curves will result in different overall false color, all else being the same. Of course you will then be unable to detect differences in spectral transmission - for instance you will not be able to detect a flattish -2 stops attenuation across the UVA from a -1 attenuation from 400 to 360 nm that drops to almost no transmission below 360 nm.

 

Other than this, unless I forgot something else you should be able to compare overall UV transmission within a 1-stop precision, or perhaps 1/2 of a stop.

 

Re: same focusing distance vs same field of view, the latter makes more sense. You may have quite different UV reflectance across the field of view, and if the latter changes, all bets are off. Even better would be shooting a completely featureless (or out of focus) test surface.

 

It is not practical to use T because it is an empiric measurement (it is based on actual transmission measurements, not on simple geometric properties like the f/ratio). It would make sense to measure T in the UV with a spectrophotometer in both lenses, but this is exactly what you set out to avoid. Comparing T in the UV with T in the visible range would be possible, but same problem here. Just base your comparison on the nominal f/ settings of the lenses.

 

As long as you don't focus closer than x0.1, there is no significant increase of effective aperture relative to nominal aperture with subject magnification, so don't test-shoot in the macro range and you will have no problem. Or shoot at the same absolute subject magnification to compare two lenses. Virtually none of our UV lenses has internal focus, which would complicate things.

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Re: same focusing distance vs same field of view, the latter makes more sense. You may have quite different UV reflectance across the field of view, and if the latter changes, all bets are off. Even better would be shooting a completely featureless (or out of focus) test surface.

 

OK, that makes sense. I was getting stuck on the fixed aperture gathering the same amount of light for any scene and forgetting that is only a potential - there might be light less light to gather if framing changes. Duh!!! Why do we get brain-stuck sometimes??

 

I have a couple of 135s to test and will try comparing them to the 105 UV-Nikkor using same field of view. Then I can review the experiment and refine it where needed. Or start over if need be. :)

 

***

 

Yes, I use a fixed white balance while shooting in addition to the fixed camera/filter/aperture/iso. Then I shoot the standards and refine white balance as needed when converting.

 

***

 

Here is an experiment for everyone: are your UV exposures shorter when using an in-camera UV white balance versus using other WB settings for UV like Daylight or Incandescent or whatever? I've noticed this to be true, but have never really set up a test. I should do that. :D

Added: Camera for which I've noticed this is Nikon D600. And not just in UV, so I crossed out UV.

 

(I should do a lot of things. "Should do" versus having "time to do". Oh well. Right now I should go prune the Clethra bushes so they no longer cover the garage windows.)

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Here is an experiment for everyone: are your UV exposures shorter when using an in-camera UV white balance versus using other WB settings for UV like Daylight or Incandescent or whatever?

 

Question: What is the best way to get equivalent exposures for such comparisons?

 

I understand fixed filter, aperture & ISO. That then implies shooting a series of shutter speeds, and presumably measuring RGB values on appropriate targets in the image.

 

So, what is your recommendation for selecting which images have equivalent exposure?

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I noticed that when shooting in Aperture Mode using Matrix Metering, the speed changed a little when I changed white balance. But this speed change has seemed mysterious to me because the underlying raw would be the same regardless of not be affected by which white balance was set. So why does it happen? Is it some quirk of the Nikon D600? Remember that Nikons will not white balance well in UV unlike Lumix or Pentax cameras.

 

Edit later: I crossed out the sentence about Nikon white balance in UV because it is irrelevant to the issue I'm describing. The same small speed changes happens in Visible light also. Or unfiltered or under an IR-pass.

 

Added: "a little" = about 1/3 - 1/2 stop.

 

Added: Just now I played with this.

UV-Nikkor at f/4.5, no filter.

D600: Matrix Metering, Active D-Lighting = Off, Noise Reduction = Off, Aperture Mode.

In Live View I focused on the subject.

Then as I rotated through the white balance settings,

the speed was 1" for some white balances and 1.3" for others.

Weird, yes?

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I'm thinking it must have something to do with Matrix Metering? Maybe the D600's scene recognition is permitted to take white balance into account?

 

If anyone does the experiment and sees minor speed alterations when white balance is changed, please let me know.

 

*****

 

Well, I have strayed from my original question which Enrico thoughtfully answered for me.

Thank you, Enrico. :)

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Yes, indeed about 1/2 stop as the channels are equaled out. But since I always have a preset WB, since years I don't shoot w/o that anymore.
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