Simple Lens Tests

[Andrea B.]

The following are meant to be very informal tests which can give you a better idea of how a lens performs. They require no special gear although I definitely think you should perform these tests on tripod -- especially any Diffraction or Sharpness tests.

 

We aren't concerned here with the definitions and technical aspects of lens problems but only with a simple way to find them.

 

I am hoping hoping hoping to be able to illustrate these simple tests.

But, hey, why not contribute some of your own tests?

 

Comments, corrections and suggestions are always welcomed.

I reiterate this at the bottom of this topic.

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Vignetting

Effect: center brightness and corner darkness

• Photograph the sky or a evenly-lit wall with the lens slightly defocused.
  Don't blow out the center.
  

• Vignetting increases with stop-down, so test at at least 3 apertures - wide, medium and narrow.
  

• Look for corners which are darker than the center.
  

• Make test photos both with and without your UV-pass filter.
  Is there any difference in the amount of vignetting when a filter is added?
  Sometimes filter mounts can cause some vignetting in addition to the lens's light fall-off.
  NOTE: Some converters automatically correct for vignetting. Turn that OFF.
  

• FIX: Look for the vignetting correction in your converter.
  

Comment: I'm pondering whether shooting with a Monochrome setting would be useful for this test,

because we are only interested in light fall off.

 

 

Diffraction

Effect: blur

• Tape a sharply printed page onto a flat wall or cabinet door
  and align the sensor plane parallel to the wall. (Not so easy.)
  

• Photograph the page at all apertures using a monochrome setting.
  

• Look for the aperture where edge blur begins to set in on the center of the photo
  so that you can determine the aperture where details are the sharpest.
  Write it down for future reference.
  

• Make another aperture series under your favorite UV-pass filtration.
  You should find that UV diffraction does not set in until one or two stops
  (maybe more with some lenses) past the visible diffraction limit.
  

• FIX: Shoot at your sharpest aperture. If stopped down beyond that,
  then applying acutance sharpening in your converter can give the appearance
  of less diffraction.
  

Comment: Usually a diffraction series is done without boosted sharpness or contrast settings.

I must remember to go determine whether that makes any difference.

 

 

Lateral Chromatic Aberration

Effect: purple fringing

• Photograph something backlit like tree branches
  or a pile of metal tools/cutlery topped by a crumpled ball of aluminum foil.
  

• Purple fringing decreases with stop-down, so test at at least 3 apertures - wide, medium and narrow.
  

• Test both with and without the UV-pass filter.
  

• Look for purple edges in both the Visible photos and the white-balanced UV photos.
  Did stopping down help mitigate the purple fringing?
  NOTE: Some converters automatically correct for chromatic aberrations. Turn that OFF.
  

• FIX: Look for the chromatic aberration corrections in your converter.
  

Longitudinal Chromatic Aberration

Effect: red/green fringing, color smear

• Photograph a well-lit, high contrast scene using at least 3 apertures - wide, medium, narrow.
  

• Test both with and without the UV-pass filter.
  

• Look for red/green color fringes in Visible OOF areas.
  I do not know how false color behaves in this test.
  Must figure that out.
  NOTE: Some converters automatically correct for chromatic aberrations. Turn that OFF.
  

• FIX: Look for the chromatic aberration corrections in your converter.
  

Comment: Remember that some converters automatically correct for chromatic aberrations

so that correction must be turned OFF.

 

 

Sharpness: Center vs. Corner Sharpness

Determining measureable sharpness of a lens is too technical for the very informal testing I am suggesting in this topic. Although if you want to buy a line resolution chart or discuss Airy disks at certain distances, don't let me stop you.

 

For basic information about resolution, acutance and MTF charts, this article by Roger Ciccala of Lens Rentals is a good place to start: Have You Seen My Acutance?

 

Technical difficulties aside, there is a useful bit of easily acquired sharpness knowledge about a lens -- how does the lens's sharpness vary between center and edges or from side-to-side or top-to-bottom? This test can also help you find a decentered lens.

• Photograph a brick wall after carefully lining up the sensor plane to be parallel with the wall.
  

• Look at the center and the edges to see how sharpness varies across the photograph.
  It is quite common to see photo corners/edges somewhat less sharp than the center.
  

• BTW, for 5 points extra credit, is your lens curved or flat-field?
  

• FIX: None.
  

Comment: You can also photograph anything in a row at the same distance from the sensor plane:

cereal boxes, leafy shrubs, a row of tulips, the front of an apartment building. You get the idea, I'm sure.

 

 

Focus Shift between Visible and UV

Effect: UV photo is OOF at the same in-focus Visible focal length.

• Using your lens's sharpest aperture (see Diffraction above), make a Visible photo.
  

• Without refocusing, add your UV-pass filter and shoot again at that same aperture
  after, of course, setting an appropriate UV exposure length.
  

• Look for an out-of-focus UV photo.
  

• How much readjustment is needed to bring the UV version into focus?
  It can be useful to make a note of this for future reference.
  

• FIX: Always refocus whenever any filter is added to a lens.
  

Comment: Even well-corrected UV-dedicated lenses can show some tiny bit of focus shift

depending on distance from subject and chosen aperture.

Also, the correction for UV-Vis focus shift might be better in one UV range than another.

I'm thinking UV-Vis focus shift correction, where it exists, might be best between 350 - 400 nm?

 

 

Focus Shift by Aperture

Effect: loss of focus after aperture change

This is an exceedingly annoying characteristic of some lenses.

 

Note: I'm suggesting this test for manual focus lenses only

because I don't want to get into the details of the various ways auto-focus works.

Our UV-dedicated and UV-capable lenses are almost all manual focus.

 

UV light is scarce in the world, so we quite often set UV focus in Live View

with the aperture wide open while shining a UV-LED onto the subject.

Then we stop-down to make the actual UV photo at a longish exposure.

This can induce the aperture focus shift for those lenses prone to it.

The only way around it for such shifty lenses is to focus stopped-down from the start,

so you might need to shine two UV-LEDs onto your subject.

 

Indeed, all the old manual focus, preset aperture lenses, which we have repurposed for UV,

were designed to operate this way -- preset your aperture, rotate to wide-open and focus,

then rotate back to the stop-down aperture for shooting.

• Shoot a stable, reasonably close subject at a wide aperture, for example f/4.0,
  
• Without refocusing shoot again stopped down this time at, for example, f/11.
  

• Look for to see whether the second photo is OOF.
  

• FIX: Always refocus after an aperture change.
  

Comment: You could spend a lot of time comparing all possible pairs of apertures for this kind focus shift.

So just try a couple of combos initially. Use your typical UV aperture setting for the second one.

 

 

Bokeh

Effect: unusual OOF blobs or just plain boring?

This one is just for fun.

• Photograph a well-lit, contrasty close landscape with both near and far elements at a wide aperture.
  Focus on the nearest subject, of course, so that you have lots of OOF background.
  

• Look to see the shapes and features of the roundish out-of-focus blobs in the background.
  

• Test both with and without the UV-pass filter.
  Any differences in the appearance of the Bokeh under UV?
  

• Alternate Bokeh Test: Photograph deliberately out-of-focus a string of Christmas lights or little fairy lights. You will get a nice colorful abstract photo with some bokeh blobs to investigate.
  

 

Comment: The more aperture blades a lens has, the rounder the Bokeh blobs.

There can be some quite unusual Bokeh with some lenses such as rings or swirls,

internal waviness, or dark/light dots.

 

 

Barrel and Pincushion Distortions

Effect: curvature of straight lines

• Shoot a chess board or photograph that metal grid thing on which you cool baked goods.
  Align the board or grid carefully to be parallel to the sensor plane. (Not always so easy.)
  

• Look for, well, distortion.
  Do the chess board or metal squares and/or metal remain square?
  Note: Converters quite often have built-in distortion correction. Turn that OFF.
  

• FIX: Look for distortion correction in your converter.
  

Comment: Doors and doorways are also a good subject for distortion tests.

There are sublabels for distortion such as barrel distortion or pincushion distortion.

Some distortion is "wavy", not a sign of a good lens.

 

Veiling Flare

Effect: hazy flare over the entire photo

• Shoot anything outdoors in good strong light at your lens's widest aperture.
  Veiling is mostly seen at the very widest apertures like f.1.2 or f/1.4.
  

• Look for a slightly hazy appearance over the entire photo with some loss of very small detail.
  

• FIX: Try some detail enhancement in your converter which can sometimes mitigate some of the hazy look although it cannot of course restore lost detail.
  

Comment: Some lenses are prized for their veiling flare because of the artistic effect when used for portraits or certain ethereal subjects. I'm thinking here of the old original Noct-Nikkor 58 at f/1.2 or the old manual Nikkor 50/1.2 AIS at f/1.2. Then there was quite a craze a few years ago for using converted TV Rodagons having a fixed aperture of 1.0 to obtain an extremely narrow depth of focus combined with an overall misty, hazy look. I stil lhave one of those somewhere.

 

 

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Coma

Effect:

I haven't decided whether I want to get into this one because I'm thinking we mainly need to know about coma when shooting night scenes. This is not at all common in reflected UV photography although we do have some lovely UV night scene examples here on UVP.

 

Astigmatism

Effect: directional blur

Also not sure we need to get into this one as I'm thinking it is not often seen? You will notice any lens astigmatism when doing the preceding Diffraction test using a sharply printed page.

 

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Additional Reference: https://en.wikipedia...ical_aberration

 

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Comments, corrections and suggestions are always welcomed !!

 

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[Andrea B.]

Completed 15 Jan 2021

[ulf]

Thank you for listing all these optical problems. That is a very good start for expanding the UV Lens testing.

 

The distortion types, both methods and characteristics, might be scrutinised more in detail and possibly be improved upon.

 

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Regarding Chromatic Aberration, I prefer the type indications Axial and Lateral. Those names are more clear to my mind. Especially Axial is quite descriptive for me.

I admit that I have used all four type indicators before, but when seeing them in your Wiki-link things became less confusing and I will try to stick to Axial and Lateral in the future.

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The effects you describe colour-wise might be valid for the VIS wavelength colour sequence blue - green -red, but not for the UV false colour sequence green - yellow - blue.

The purple fringing, is that a case of CA or something else even if it is somewhat related?

 

For me Axial CA is a focus phenomenon and Lateral CA is an image scaling phenomenon.

 

Axial CA is evenly spread over the entire image, for parts out of focus and is decreased by stopping down to increase DOF.

One problem investigating it is that few lenses has a completely flat focus plane.

 

For me Lateral CA is an image scaling phenomenon, not present in the image's center, but increasing towards the corners.

See my Sunex post where Lateral CA can be seen. It is yellow - blue.

For complex lens designs the increase is not linear, by radius change, making corrections, by scaling the different colour channels, more complex.

 

In the real world there is likely a combination of both Axial and Lateral CA.

 

Let us further discuss and improve the methods investigating both Axial and Transversal CA.

[Andy Perrin]

Andrea, not sure why you think coma applies only to night scenes. In VISIBLE light that is true, because it matters a lot to taking photos of stars, but for UV, everything tends to be dark all the time, so the same situation occurs (any small bright reflections will show the coma in a mostly dark scene — think about the conical cells!).

[nfoto]

The coma causes issues for most focus stacking software as well.

[Andrea B.]

Andy, Birna - point taken.

 

So how can we test for coma?

The primary goal here is to provide very simple tests that anyone can perform without special gear.

[Andy Perrin]

I would just take a photo of the clear night sky with visible light? I mean, I don't know how much the coma would change in UV, it may be the same I'm thinking?

[nfoto]

Small pin-point lights towards the corners of the frame will show coma aberrations. Look for any change to the point shape, for example, like spreading wings of a bird.

 

You will get coma in UV as well. Non-specialist lenses often show lateral and/or axial colour aberrations, spherical aberration (loss of sharpness, veiling flare or blurring towards the outer parts of the frame) in addition. For some issues, stopping down helps.

[ulf]

I wonder how much different aberrations vary with focus distance.

Some lenses are optimised for a certain type of distance / magnification, while others work better over a wide range.

Testing a specialised macro lens with stars in a clear night sky might not be a good way to do it, if at all possible.

[nfoto]

True, one should only expect a lens to perform at its peak under its designated conditions of usage. That applies to any lens. Never believe, for example, the myth that added extension is 'harmless because it's only air, no glass'. That being said, sometimes an optic can provide surprises of the positive kind thus there is no substitute for actual testing of the lens.