The 7Artisans 25mm, f/1.8, lens: review. Is it good enough to use for Astrophotography?

A fully manual lens that is available for most APS-C and Micro 4/3rds mirrorless cameras.

Can an all metal prime lens bought for only ~£50 be a worthwhile purchase for use in day to day photography and could it be a suitable first prime lens that is bought to do some astrophotography ?  It turns out that the answer to both these question is yes.  But there are some caveats.  In three respects it is one of the worst lenses that I own − but these problems can be ameliorated in post processing.  [One suspects that anyone who buys this lens will be able to use programs such as Adobe Photoshop.]  However in other, more important, respects it is one of the very best.

This is just one of ~70 articles in the author’s Astronomy Digest: http://www.ianmorison.com

The lens employs a modified Double Gauss design as seen in the images below.

The 7artisans design is above that of a standard Double Gauss design.

Let’s cover the bad points first.

Vignetting

This lens suffers more from this than any other lens that own.  Though it gets significantly better as one stops the lens down, it is still present to a small degree.  In Adobe Photoshop (or other image processing software) one can, when necessary, correct this and this is fine when f stops greater than f/2.8 are used.  [A true rather than indicated f stop – see below re aperture scale.]  In the worst case, f/1.8, if the corners are brought up to the appropriate brightness, the centre of the frame is still overly bright.  But there is a solution; if the central area is selected with an oval selection tool using a large feathering radius of, say, 400 pixels, then the central brightness can be reduced to provide a uniform brightness across the frame.  So the problem can be worked around and, sometimes, vignettes are even added to an image to help concentrate the eye on the central part of the image.    One must, however, remember that increasing the brightness in the corners of the frame will increase the noise in these regions.  

Barrel Distortion

This is present but, again, can be corrected in Photoshop if there are straight features near the edge of the frame.  The effect will be to reduce the area of the frame (a small crop will be needed) and thus slightly increase the effective focal length of the lens.  Whether corrections are needed depends on the scene being imaged.  Again, I do not regard this as a deal breaker.

Edge and Corner Sharpness

If one is honest, the image circle (the region of good sharpness) of this lens, though fine for Micro 4/3 cameras, is not really large enough for use with APS-C sensors such as in the Fugi X-A10.   At f/1.8 neither the corners nor the edges are sharp.  As the lens is stopped down, the edges are greatly improved but the extreme corners never get fully sharp − even at f/8 or greater.   If one were using a wide aperture to isolate a subject from the background so that the background was blurred this would not be a real problem (and, as discussed below, the character of the out of focus part of the image is excellent) but when taking a landscape image it would be.  As in this case one might well be using a stop of f/8 or f/11, it would only be the extreme corners that would constitute a problem and, if the upper part of the image were sky, only the lower left and right corners would appear slightly blurred.  The only solution is to crop the image down so effectively increasing the lens’ focal length.   When photographing landscapes I will usually take three overlapping vertical frames which can be cropped and then combined in Microsoft ICE so removing the problem in post processing (and to also give a higher megapixel result).

One point; the sharpness does not gradually fall off from the centre towards the corners, but (far better) the image remains sharp until it very rapidly falls off just before they are reached.

The Aperture Scale

When I made a set of images to test the image sharpness at increasing focal ratios, I was surprised that, initially, the required exposure (which should have doubled for each full stop reduction in aperture) hardly changed.  I discovered that until f2.8 was reached the aperture barely altered.  Doing comparative tests against other lenses, I then found that the aperture markings were about one stop out, so that f/4 on the scale was really f/2.8, f/5.6 was f/4 and f/8 was f/5.6 etc.   Once understood, this is no real problem but I suspect that the minimum aperture is really f/11 rather that f/16.  This may be a quirk of my lens, but I have found two other reviewers who have noted it

Handling

Because it is a very small lens (a great attribute) the focussing and aperture scales are very close and, when adjusting one, it is easy to inadvertently affect the other.  Care is needed.  When using it for ‘out and about’ pictures in reasonable light, I have actually set the aperture to (a true) f/8 and the focus to just less than infinity and ‘locked’ them in place with a strip of micropore tape.  Problem solved.

Now for the Good Points

Size and Weight

Though not a ‘pancake’ lens it is both small and light.  It sits on my Fuji X-A10 camera beautifully making it an almost ‘pocketable’ combination.

Sharpness

Where the lens is sharp it is very sharp and the central regions of the frame are sharp even at full aperture, though this improves slightly until f/5.6 and beyond.   As the f stop is increased to f/4 sharpness across the frame (except the extremes as indicated above) becomes uniformly excellent.

Contrast and Micro Contrast

The contrast and micro-contrast are excellent − better than that of the Fuji 18-70 kit lens.

Chromatic Aberration

I cannot really see any − which is amazing.  Seriously, it produces one of the ‘cleanest’ images of any lens that I own.

Colour

The colour is very nicely saturated and appears identical to that produced by the Fuji 18-70 kit lens.

Bokeh

Bokeh has been defined as “the way the lens renders out-of-focus points of light”.   Differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting.   The 25mm, f/1.8, lens has a very smooth and pleasing bokeh at full aperture and, due to the fact that there are 12 iris blades so keeping a near circular aperture as the lens is stopped down, this is also true at smaller apertures where there are out of focus elements in the image.  Very good.

Close up ability

The lens can focus down to ~3 inches from the front of the lens – really good for a ‘non-macro’ lens.

An astrophotography example

On a not very good night, I did carry out a test of its abilities imaging Orion under light polluted skies.   Again, with one caveat, the result was excellent.   I imaged at a true f/5.6 so that vignetting would not be a real issue and the only problem was that the very extreme corners suffered from coma (in the common form of small ‘seagulls’) so the field would need to be cropped down a little or a ‘healing brush’ used to repair the stars in these regions.

The image below shows a single 20 second exposure which has been stretched to show the light pollution – not ideal for serious astroimaging, but this was just a test.

A total of 45, 15 second, frames were taken at an ISO of 400 with the camera mounted on my ‘nanotracker’ (see the article ‘Three tracking mounts …’ in my Astronomy Digest) aligned onto the North Celestial Pole.  As can be seen in the image above, there was no star trailing whatsoever in a single exposure. Both Jpeg and raw files were captured and the raw files aligned and stacked in Sequator (see article ‘Sequator: a stacking program….’).  Long exposure noise reduction was disabled; this would remove hot pixels (but Sequator will do this in post processing) and will add a little noise into the image.  More importantly, it would halve the time collecting photons from the stars.

The light pollution was removed using my standard technique described in the article ‘Removing Light Pollution…’ and stretched using several applications of the levels command with the central slider moved to the left to the position 1.2.  Noise was then removed by, in levels, moving the left slider a little to the right to increase the ‘black point’. 

The resulting image below is a crop of the top ¾ of the full frame.  I have enhanced the brightest stars a little to make the constellation pattern stand out better.  The orange star close to the top right of the image is Aldebaran – which still has a ‘clean’ image.  It is only at the extreme top corners of this image that coma rears it head.

The digest article ‘Repairing coma in star images’ shows how to make a ‘coma healing brush’ which can be used to remove the ‘wings’ of seagull like coma.

In all I think that this is really good result and if one had a kit zoom lens this could be a very nice prime lens with which to carry out some astrophotography.  As the focal length is quite short, one could take a set of ~10 second exposure images without using a tracking mount and then align and stack them to give the result of a longer exposure.

There are many examples in the Astronomy Digest that can give guidance such as ‘What ISO..’ and ‘What exposure…’ to use.

Return to the digest home page: http://www.ianmorison.com