Irix Blackstone 15mm f2.4

I heard a few rumors about the new Irix lenses before I went to Iceland back in March, but could I find one to even look at let alone test – no, they seemed as elusive as hens teeth.

But last month I was invited by the Irix marketing and distribution team to give one a beating!

The one lens in the range that piqued my interest the most was the Irix Blackstone 15mm f2.4 – and here’s why:

1. F2.4
2. Supposed lack of CA
3. Supposed lack of barreling
4. 15mm PRIME
5. Focus locking ring
6. Hard focus stops
7. Infinity click/detent
8. Good old-fashioned engraved focus scale with DoF, IR and hyperfocal markings
9. Solid all-metal construction
10. Removable lens hood – the importance of this will be pointed out later.
11. Price point – for the money this lens COULD represent epic value for money – but only testing & evaluation will confirm that.

All-in-all the lens appeared to be a dream for wide-field astro photography and sweeping vista landscapes – just such a shame I couldn’t get hold of one for my Iceland trip.

So a couple of weeks ago I met up with Charles Woods of Charles Talks fame – a real nice guy who moves in far higher circles than I do being the go-to contact for Leaf, Cambo and many other gorgeous lines of photographic loveliness.  If Charles takes on a brand then it’s worthy of some very serious considerations.

The Lens:

The Irix Blackstone 15mm f2.4 packaging is like a babushka – it’s a lens in a semi-rigid case, in a tin, in a box! (excuse the slightly dog-eared box but it’s been around a bit!).

Inside the tin you will also find a world-wide warranty card, a multi-lingual introduction booklet – AND, a SPARE rear lens cap; a nice touch Irix.

Open the zipped case and there is your lens with the lens hood reversed:

Remove the lens, turn the lens hood around, line up the registration lines on the hood and lens, then twist until it locks into position:

A couple of interesting features on the lens you MAY find useful:

On the underside of the lens hood (if it’s on the top you’ve got the hood upside down) you will see a sliding door.  If you push this forward it will reveal a gap  – fit a 95mm screw-in polarizer and you can rotate it as necessary then slide the door back until it ‘clicks’ shut.

And on the rear of the lens there is a built-in gelatin filter holder – Irix have put a lot of thought into the ‘bells & whistles’ on these lenses.

My Thoughts – personally I have to say that these two features may well be very useful for studio/indoor photography BUT – try fitting tiny gelatins in a force eight gale when shooting landscapes!  And as for polarizing filters – they never really work well on lenses with such a wide angle of view because their effects are virtually non-existent on all but the center of the image.

It’s vitally important that the filter access door is ‘clicked’ closed, otherwise it can cause a small amount of flaring – I’ve taped it on the outside so I can’t open it by accident, and I’ll be using 150×150 filters and a 95mm adapter ring for the holder.

Being a bit ‘old skool’ I’m never a big fan of light weight plastic lenses if truth be told,especially when it comes to wide angles. So holding the Irix Blackstone 15mm f2.4 is a real pleasure for me with its all metal construction, and honestly, it’s built like a tank.

The lens itself is manual focus only and available in Nikon F, Pentax K & Canon EF mount options, and features a 9 curved-blade aperture together with 15 elements in 11 groups.

The focus ring is silky smooth and has more resistance than your typical Zeiss Distagon, which in my opinion is a good thing.  The other attribute of the focus ring is the long throw, which is a lot longer than the 15mm Distagon and comparing it to the short throw, fast focus ring on the venerable Nikon 14-24, this Irix lens should be a dream to use for both daytime and night landscape photography.

I have to add a note here about the focus ring on the Nikon 14-24.  It is far too fast/short for precise manual focus without a heck of a lot of practice.

Focus Scale & Focus Locking Ring:

Just ahead of the main focus ring is the focus locking ring – one of the main features of the lens I found piquing my interest in the first place, especially for wide-field astro work where focus is super critical, and accidental movement of the focus ring easily happens.  In the image above the focus is not quite fully unlocked – the unlock indicator should be just about in line with main focus indicator line on the focus scale/vertical white line after the ’15’ on the lens bell for normal focus operation.  Turn the locking ring to the left until it stops – don’t over tighten/force it – and your focus is locked.

The focus scale itself is excellent, and packed with all the information you could ever need if truth be told.

There is a definite ‘click’ as you focus at infinity, and there is even an infra-red focus mark.  You have DoF indicators for f8, 11 & 16 together with hyperfocal distance markings for the same apertures.

There is also one more feature of this lens that I have not mentioned yet, and that’s because it isn’t something you should really ‘mess about’ with unless you know what you are doing!  I am talking about this on the underside of the lens bell:

Details of how to use this feature can be found on Page 8 of the downloadable Extended User manual HERE.

My Thoughts – just about every manufacturer of lenses of this focal length will do a ‘factory infinity calibration’ calculated at beyond 50 meters.

As a landscape photographer your eye level horizon at sea level is around 4700m, and the clouds touching the horizon may be well over 100km away.  Focused at infinity and stopped down to f11 or f16 they WILL be sharp due to DoF.

However, shooting the sky at night at f2.4, f2.8 – in other words with NO DoF to help you out – you will invariably find that you need to focus just before, or more rarely just after, the infinity mark in order to get those stars tack sharp.

A close star like Arctaurus is more than 36.5 light years away, and Polaris – the Pole Star here in the northern hemisphere – is 433.8 light years away.  What I’m trying to say here is that there are many ‘degrees of infinity’ and some types of photography require greater accuracy than others.

Experience has taught me that at 14mm and f2.8 sharp focus on street lights at night from around 15km and an elevated position work perfectly for pin-sharp stars.  Some people then tape the lens focus ring, but using the focus locking ring on the Irix is a tidier solution.

But Irix go a step further than that and give you the ability to MOVE the position of the infinity indicator to correspond with a true visual infinity.  PLEASE READ the manual before attempting to do this yourself.

So this Irix Blackstone 15mm f2.4 is perhaps the most well-appointed lens I’ve ever come across – but now it’s time for the rubber to meet the tarmac and actually do some testing.

Comparative testing

It made sense to compare the Irix Blackstone 15mm f2.4 to possibly the best known and more expensive lenses in this super-wide lens class, namely the Nikon 14-24mm f2.8 and the Zeiss Distagon 15mm f2.8 ZF2.  There seemed very little point in comparing the Irix to the likes of the Rokinon/Samyang 14mm prime as I would expect the Irix to out-perform it by a good stretch, and thus not show how far the Irix ‘punches above its own weight’.

I’ve been wanting a prime to replace the Nikon 14-24mm, especially for night sky photography, for quite some time.  To that end I have tried three different examples of the Samyang 14mm f2.8 and found them all terrible with the aperture wide open when used on a 36Mp camera.  The images from it might appear okay shrunk down to 1920 x 1080 and run in a time-lapse video, but for full resolution stills and large display prints I have found all three examples I have tried to be inferior in terms of sharpness and coma.

So having decided on the two comparison lenses my first test was for vignetting.

Vignette Test Results:

The images below were all produced using the standard DSO Flat Frame production technique of imaging a diffused D65 light source with the lens focused at infinity.

(Click the image to open in a new window and click again to view at full size)

Vignette testing the IRIX 15mm Blackstone lens against the Zeiss Distagon 15mm f2.8 ZF2 and the Nikon 14-24mm f2.8 @15mm. (Click the image to open in a new window and click again to view at full size).

Now before anyone gets ‘all hot and bothered’ about the vignetting on the Irix at wider apertures, here’s where the advantage of a removable lens hood comes into play.  Taking the lens hood off after you have produced your image enables you to easily produce a flat calibration frame using an LCP filter – something that the medium and large format photographers have been doing for years.  Combining these frames is easy inside the likes of Lightroom, and the process removes all vignetting, colour shifts and dust spots from your images.

Of course, you can produce flat calibration frames for the likes of the Nikon or Zeiss – I do it all the time – but you have to make them in the same manner as I made the test shots above.  But it is imperative that you shoot them at the same focus distance, ISO, focal length and aperture as your image frames.

But using the Irix makes the process very simple and it takes just a few seconds to produce a calibration frame which is customized for your composition and lighting levels.

So despite what you might think, vignetting is irrelevant – especially on lenses with a removable lens hood.

Sharpness & Diffraction, Resolution, Chromatic Aberration, Native Lens Color Cast & Contrast Testing.

Please make sure you view these images at 100% magnification by clicking on them.

This test was carried out at a focus distance of 12.5 inches and lacking a proper test bed, getting everything parallel and centered was quite time-consuming I can tell you!

All images shot in manual mode at ISO200 using shutter speeds that rendered a +/-0Ev on the cameras internal meter.

There are some subtle exposure variations between shots from the same lens in each of the three tests which is due to the use of cold cathode lighting, its flicker frequency and the shutter speeds used, but the variation is negligible and for our purposes totally irrelevant.

There has been no sharpening, CA correction, luminance or color noise reduction or other process settings applied to any of the images you see below except for white balance.

The first set of images show the low right optical center 770 pixel square from the D800E raw files.  The second set of images show the same 770 pixel square from the bottom left corner of the frame, and the third set that of the top center.  I chose the low right center because we are interested in the rendering of the higher lpm (line pairs per millimeter) resolving power of the three lenses being tested.  The axial center is located in the middle of the concentric circles:

Sharpness & Diffraction, Resolution & CA, Native Lens Contrast & Colorcast test results.
Image Center Low Right
Camera used: Nikon D800E.

Immediately one or two things are apparent.

The Zeiss is the sharpest at apertures wider than f5.6, and both the Nikon and Irix catch up with it by the time we get to f8.

So above (wider than) f8 the Irix would appear to be less sharp than the Zeiss – but things are never quite so cut and dried as they appear.

The Zeiss is not quite as sharp as you think because it’s got a higher degree of transmitted contrast – something a lot of people would be mistaken in thinking is due to the so-called ‘Zeiss pop’**.  If I remove -20 contrast from the Zeiss f5.6 image and equalize the black and white greyscale values in Photoshop, then the difference between it and the Irix at f5.6 appears to be significantly less than we first thought:

Zeiss Distagon @ f5.6 (left0 vs Irix Blackstone @ f5.6 (right) with black point and white point greyscale values in the Zeiss image modified to closely approximate those of the Irix image.

** the so-called ‘Zeiss pop’ is down to sublime MICRO-CONTRAST, and the 15mm Distagon has very little of it.  You need to look at Zeiss lenses longer than 25/30mm before you even begin to see it.

And the Nikon exhibits the same tendency, though not as extreme as the Zeiss.

So all in all, from an image center sharpness point of view the Irix has good level of performance.

The Irix seems to consistently produce a ‘warmer’ image at f2.4 than at f2.8 and beyond, which is a bit odd, but again easily correctable in post so not of any real concern – unless you are crazy and shooting jpeg-only of course.

The next observation we can make is image center CA.  All 3 lenses exhibit good anti CA in their image centers, but if you look carefully you will see that the best performer is the Irix.  The Zeiss is second best, and the most obvious CA can be seen on the Nikon images.

Diffraction:

On the diffraction side of things we can see diffraction begins to show at apertures smaller than f11.  But diffraction is caused by the inter-relationship of lens aperture Airy Disc and sensor CoC (circle of confusion).  As long as the lens aperture Airy Disc is SMALLER than the sensor CoC, diffraction will not be a problem.

When the Airy Disc and sensor CoC are of equal size the combination of lens and camera sensor is said to be at its ‘diffraction limit’.

These images were produced using a 36Mp Nikon D800E, which is diffraction limited to f14 at best, and more usually f13.  Switching out to a 20Mp Nikon D4 I’m diffraction limited to f16 or 18, and on a 12Mp D3 I can get away with f20.

So diffraction has little to do with the lens and everything to do with the camera sensor, and if I had used a Nikon D3 for the above tests then f22 on all 3 lenses would look a lot sharper!

Lens Native Color Cast:

We can also just about see the Zeiss has a vestige of a green cast, and the Nikon a slight blueish one while the Irix stays fairly neutral (other than at f2.4).

The CA/Color & Resolution story takes on a whole different meaning though when we move off-axis and look at the image sides and corners:

Sharpness & Diffraction, Resolution & CA, Native Lens Contrast & Colorcast test results.
Image Lower Left Corner
Camera used: Nikon D800E. (Click the image to open in a new window and click again to view at full size).

Between the Nikon & Zeiss that’s £3500 worth of glass at f2.8 out-performed by a much cheaper lens at f2.4 – gives you something to think about doesn’t it.

Now let’s take a look at the top center:

Sharpness & Diffraction, Resolution & CA, Native Lens Contrast & Colorcast test results.
Image Top Center
Camera used: Nikon D800E. (Click the image to open in a new window and click again to view at full size).

In both of the off-axis comparison tests you can easily tell which lens is the winner in the CA stakes – the Irix, and by a huge margin too.

Yes it does exhibit some CA at the image margins, but far less than the other two lenses.

CA removal can sometimes have a detrimental effect on convoluted edges in your images due to the manner in which it works.  Ordinarily these errors don’t have much of a visual impact in your shots, but the larger your sensors pixel count the more edge-halo problems you can experience if you try the sharpen the image, or try to create or apply masks.  I shall talk more about this later.

The inherent ‘native’ color cast of the Nikon and Zeiss lenses is plainly obvious in the off-axis test shots, as is the neutrality of the Irix.

Something else you may or may not have noticed in the above 3 test sequences is that the Nikon 14-24mm @ 15mm images all display an increased ‘image magnification’.  This is due to excessive FOCUS BREATHING of this particular lens.  Focused at such a short distance – 12.5 inches – 15mm on the Nikon carries the same magnification you would associate with perhaps a 16mm or 17mm lens.

Lens Barrelling:

Lens barrelling & angle of view of the Nikon 14-24mm f2.8 @ 15mm
12 feet and f14.

Lens barrelling & angle of view of the Zeiss Distagon 15mm f.8 ZF2.
12 feet and f14.

Lens barrelling & angle of view of the Irix Blackstone 15mm f2.4.
12 feet and f14.

The barrelling test loser is the Nikon – and you can also now see that, even at 12 feet, the focus breathing of the lens is resulting in a wider AoV – check the right edge of the air vent in the brick wall.

So the Nikon – at reasonable working distances – has a wider Angle of View (AoV) than the Zeiss, which in turn is marginally wider than the Irix.  I can’t say that I’m particularly bothered by this as the overall effect is minimal.

The Irix has slightly less barrel distortion than the Zeiss and so has to be the clear winner.

Coma Test

Seeing as the Irix originally piqued my interest as a super-wide prime with astro potential I’m really keen on looking at the coma it produces.

What’s Coma Andy and why does it matter to you?

First things first, here is what Coma looks like:

Top Right corner Canon 16-35 USMII at 16mm & f2.8

The artifacts produced by Coma at wide apertures can make certain lenses redundant for astro photography – every single one of those upside down Cylon Warbirds is a star that’s supposed to look like a pin-prick of light.

Coma, or Comatic Aberration, is simply a distortion of off-axial point light sources.  The direction of the distortion can be sagittal (parallel to the lens diagonal), meridional (perpendicular to the lens diagonal), or a mix of both.

In the Canon lens above (that was over £1000 worth when new)  the coma is strongly meridional which is then pin-cushioned – terrible, and virtually impossible to correct for in Photoshop.  Yes, the coma vanishes at around f7/f8, but you can’t use those small apertures when shooting astro.

Contrast the image above to the one below from the Nikon 14-24:

Top Right corner Coma on the Nikon 14-24mm @ 14mm and f2.8

Okay, the Nikon sets the bench mark – the Coma is a fairly even mix of mild meridional and sagittal distortion which can easily be corrected in Photoshop where necessary.

Let me just say that the images above were shot under perfect dark sky conditions.  The image below by comparison is most definitely NOT – the sky is full of light pollution and we can’t see the stars at their full brightness, so the next shot is a slightly unfair comparative:

Top Right corner Coma on the Irix 15mm @ f2.8

As I said, due to light pollution we can’t see any of the fainter stars, and the brighter ones look smaller.

And here is the Zeiss 15mm Distagon ZF2 under the same conditions as the Irix above:

Top Right corner on the Zeiss 15mm f2.8 Distagon ZF2 @ f2.8

To be honest, the level of coma in both the Irix and the Zeiss are in no way unmanageable in Photoshop, and so are of little concern. Both are mild in their extremes of the image frame corners, the Zeiss being slightly biased towards sagittal and the Irix perhaps a tiny bit more meridional.

In both lenses the distortion ‘coma tails’ are smaller than the stars diameter and so a piece of cake to remove with a coma brush in Photoshop.

And if you want to know what a ‘coma brush’ is then go and buy my My Complete Guide to Photographing and Processing Night Sky Images.

Light Transmission

When I was doing the light polluted night sky shots I mentioned earlier I became very aware of something I hadn’t really noticed before – a T-stop difference between the Irix and the Zeiss:

Click the image to view larger

A 6 second exposure at the same aperture (f-number) and ISO on the Zeiss is roughly as bright as a 3 second exposure using the Irix.

The middle image is 3 seconds using the Zeiss, and is noticeably darker – I would say that the Irix transmits somewhere between +0.6 and +0.75Ev more light than the Zeiss – which let’s not forget is THREE TIMES THE PRICE!

So, what do I think of the Irix Blackstone 15mm f2.4 so far – bearing in mind that I need to test it for flare and focus breathing.  Well, I’m off to Norway for a week and hopefully I’ll get a few seascapes and high country midnight sun landscapes done in between the eagle action – and the Irix is the only superwide I’m packing!

I will conclude my review of this lens on my return from the land of the Vikings, but suffice to say, at the moment I consider possession of this lens a complete NO-BRAINER.

See my UPDATE on this lens HERE.

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