Laowa 25mm F2.8 2.5x-5x Ultra-Macro Image Quality Analysis (ver 1.1)

Introduction

The Laowa 25mm F2.8 Ultra-Macro (LW25UM) is my lens of choice when I am unsure about the magnification I need. Although I have a collection of prime lenses for each magnification between 1x to 5x, it is sometimes difficult to figure out the exact magnification I want for subjects such as minerals.

Despite some disadvantages, the lens is incredibly reasonably priced and offers a bridge towards higher magnification macro photography, whilst allowing the photographer to acquire new skills such as focus stacking and DIY diffusion setups.

In my previous long-term review which can be found HERE, I focused on the lenses’ performance, handling, and packaging. I included samples of real world subjects all which a technique called focus stacking was utilised. The article was not overly technical, as I only discussed the image quality briefly.

I did not compare the apertures closely either, which led me to believe that F4.0 is the optimum aperture. This delusion turned out to be factually incorrect. Some suggested to set the aperture at around F3.2, between F2.8 and F4.0. This is possible because the lens features a fully manual aperture ring and larger apertures are adequately spaced apart. This also turned out to be untrue.

Version 1 of this article will only feature images at either 2.5x or 5x. Having all magnifications featured will compound the amount of work and lead to confusion. I will slowly update the article as time goes on.

Method

The following three targets were used to produce the results:

  • Resolution test target, chromium deposited onto quartz glass, goes down to 400lp/mm
  • Alignment mask, features rounded squares
  • Diffraction grating mask

I tried my best to align the targets with the Thorlabs GNL20 goniometers. This was incredibly difficult as my setup wobbles a lot. I do not have a permanent setup for these tests, what I must do is rebuild my current one. The make-shift setup does not offer much precision. I aim to refine the setup in the future.

A Thorlabs MAX3SLH microscopy slide holder was used, which could only adequately hold the 50x50mm resolution target. I need to design a custom holder for larger masks, I am thinking about thick Perspex.

I gave up on alignment after being fed up with the imprecision of the setup, decided to use focus stacking for resolution tests instead. I will build an autocollimator for better alignments if I am willing to do more tests of this kind.

Distortion

Distortion becomes quite visible with the use of an alignment target. The LW25UM suffers from pincushion distortion.

LW25UM at 2.5x, F2.8

-1 via Photoshop’s custom lens correction under “filters” seems to do a decent job.

I have a target coming that allows distortion to be measured precisely and quantified, I will update this section later.

Longitudinal Chromatic Aberration

Longitudinal chromatic aberration (LoCA), colloquially known as bokeh fringing is characterised by chroma fringing on defocus elements of high contrast.

Putting the diffraction grating mask at an incline reveals purple and green fringing.

The LW25UM suffers from moderate longitudinal chromatic aberration.

Lateral Chromatic Aberration

Lateral chromatic aberration occurs when different wavelengths of light is focused on the same plane but the foci is not placed along the optical axis.

Magnified by 400%

Quite a colourful square we have here. Focus stacking easily rectifies this issue.

Shooting at a smaller aperture does not help.

Resolution

On the Nikon D810, the lens can resolve up to 400lp/mm at 5x. The lens will struggle with anything above that.

The lens performs the best at F2.8 yielding the most resolution. Image quality is compromised when stopping down to F3.2 and further degrades at F4.0. However, the difference is barely visible between the 3 apertures unless one zooms in 400% to pixel peep. At F5.6, the image quality turns bad which gets unusable at F8.

Resolution target at 400lp/mm, aligned to be at the centre of the frame, focus stacked, cherrypicked for best region.

At 400%, we can see that F2.8 offers slightly more resolution than F3.2 and a bit more than F4.0.

400% export of the image

Below is an animated gif I made.

gif

Vignetting

There is slight vignetting at F2.8, which clears up at F5.6. This is observed from my lateral chromatic aberration test photos, samples will be included in a future update.

To-do List

  • Quantify distortion
  • Resolution in the corners

Conclusion

As the test illustrates, the Laowa 25mm F2.8 Ultra-Macro lens performs the best wide open at F2.8 in the centre and corners of APS-C. Image quality degrades at F3.2 and F4.0, which becomes worse at F5.6 and unusable by F8.0 due to diffraction.

Unless more depth of field is absolutely needed, the lens should be used at its maximum aperture of F2.8 at all times if only the centre is a concern, and the technique of focus stacking will provide greater depth of field. I have yet to analyse the performance in the corners, F3.2 or F4.0 might bring significantly improved corners. Since the difference in resolution in the centre is only observable at a pixel level, this trade-off might be worth it.

Many extreme macro photographers have techniques and success in hand-held focus stacking up to 5x. It is certainly doable and the results are worth the effort.

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