[Matchete]

Hi,

I have a K-1 and a Samyang 14mm f/2.8 lens, which I mainly use for astrophotography with the astrotracer.

I'm considering the Irix 15mm f/2.4 or the Samyang 20mm f/1.8 as a replacement for the 14mm.

They wouldn't have a lot of advantages over the 14mm for astro use as the wider apertures wouldn't be needed thanks to the astrotracer, but quite a few for daytime landscape use : narrower field of view so easier to frame with, less distorsion, better sunstars...

I would like to hear fellow forum members opinions on this matter

Thanks,

Mat

[UncleVanya]

Do you think you would use the lens for anything else like traditional landscapes? If so you might try looking at the 20mm club for full frame shots at 20mm to give you a better idea of the FOV. I personally think 15 on FF is really a bit too wide for most uses but not unreasonable for landscapes.

[dave2k]

It seems that you are looking for a larger maximum aperture and a narrow scene width. The Samyang gives you both of those while the Irix doesn't give you much of either. Personally, 20 mm is not wide enough for some of my landscape shots. I have the Samyang 14, the D FA 15-30 and the FA 20. I very rarely use the FA 20 while the other two get quite a bit of use with the deciding factor between the two being how big of a lens I want to take.

[interested_observer]

Just for the astro aspects of this, the longer focal length glass will be better, given all the other aspects (aperture) remains the same. In that the aperture becomes larger with the longer lenses, then just following that logic the 20/f1.8 will be the best selection.

So, here are some numbers to back up the statement. I'm also assuming that the astro tracking will be for 60 seconds (however the duration really does not matter here, but I'll toss it in anyway).

• 14mm/2.8=5 mm diameter, area = (pi/4)52 = 28.9 sq mm. Exposure time of 60 seconds. Light collection = 1177.5 sq mm seconds.

• 15mm/2.4=6.25 mm diameter, area = (pi/4)6.252= 30.66 sq mm. Exposure time of 60 seconds. Light collection = 1839.8 sq mm seconds.

• 20mm/1.8=11.11 mm diameter, area = (pi/4)11.112= 96.91 sq mm. Exposure time of 60 seconds. Light collection = 5814 sq mm seconds.

Then to compare the lenses...

• 14mm to 15mm = 1839.8/1177.5 = 1.6 - the 15mm collects 1.6 times more light than the 14mm

• 14mm to 20mm = 5814/1177.6 = 4.9 - the 20mm collects 4.9 times more light than the 14mm

The 20mm lens collects 4.9 times more light from objects in the frame. Stars are brighter, star clouds and nebulae will be brighter.

The key in all of this is the actual physical aperture diameter (for the 14mm / f2.8 lens - that is the 14 / 2.8 = 5). Essentially, to simplify the process, just compare the physical aperture diameter of each lens, and you get the same results by

• [(physical aperture long lens) / (physical aperture short lens)]2 i.e., (6.25/5)2 = 1.6x more light for the longer lens.

_________________________

Personally, I went with the 15-30/f2.8 for several reasons. First it was more general purpose for landscapes and I have found myself shooting astro at both ends - 15mm and 30mm. The 30mm end gets substantially more light, while the 15mm gets more of the overall landscape and sky combination. I've found so far, that at 15mm I get plenty of star light for a good image.

---

Last edited by interested_observer; 08-11-2018 at 06:25 PM.

[nocturnal]

Just for the astro aspects of this, the longer focal length glass will be better, given all the other aspects (aperture) remains the same. In that the aperture becomes larger with the longer lenses, then just following that logic the 20/f1.8 will be the best selection.

So, here are some numbers to back up the statement. I'm also assuming that the astro tracking will be for 60 seconds (however the duration really does not matter here, but I'll toss it in anyway).

• 14mm/2.8=5 mm diameter, area = (pi/4)52 = 28.9 sq mm. Exposure time of 60 seconds. Light collection = 1177.5 sq mm seconds.

• 15mm/2.4=6.25 mm diameter, area = (pi/4)6.252= 30.66 sq mm. Exposure time of 60 seconds. Light collection = 1839.8 sq mm seconds.

• 20mm/1.8=11.11 mm diameter, area = (pi/4)11.112= 96.91 sq mm. Exposure time of 60 seconds. Light collection = 5814 sq mm seconds.

Then to compare the lenses...

• 14mm to 15mm = 1839.8/1177.5 = 1.6 - the 15mm collects 1.6 times more light than the 14mm

• 14mm to 20mm = 5814/1177.6 = 4.9 - the 20mm collects 4.9 times more light than the 14mm

The 20mm lens collects 4.9 times more light from objects in the frame. Stars are brighter, star clouds and nebulae will be brighter.

The key in all of this is the actual physical aperture diameter (for the 14mm / f2.8 lens - that is the 14 / 2.8 = 5). Essentially, to simplify the process, just compare the physical aperture diameter of each lens, and you get the same results by

• [(physical aperture long lens) / (physical aperture short lens)]2 i.e., (6.25/5)2 = 1.6x more light for the longer lens.

_________________________

Personally, I went with the 15-30/f2.8 for several reasons. First it was more general purpose for landscapes and I have found myself shooting astro at both ends - 15mm and 30mm. The 30mm end gets substantially more light, while the 15mm gets more of the overall landscape and sky combination. I've found so far, that at 15mm I get plenty of star light for a good image.

Yes all the maths works but isn't this only relevant for point sources. The bigger clear aperture sees more stars (collects light from fainter stars). Some people say clear aperture isn't relevant for the Milky Way, some say it is. There is a lot of confusion around this area, however I would say a bigger physical/clear aperture is relevant to the Milky Way (my Samyang 14mm doesn't seem to collect much light from the MW as it only has a 5mm diameter wide open at f2.8).

Is the 20mm sharp enough, coma and astigmatism free at f1.8 though? That is the question. 😎

[UncleVanya]

Yes all the maths works but isn't this only relevant for point sources. The bigger clear aperture sees more stars (collects light from fainter stars). Some people say clear aperture isn't relevant for the Milky Way, some say it is. There is a lot of confusion around this area, however I would say a bigger physical/clear aperture is relevant to the Milky Way (my Samyang 14mm doesn't seem to collect much light from the MW as it only has a 5mm diameter wide open at f2.8).

Is the 20mm sharp enough, coma and astigmatism free at f1.8 though? That is the question. 😎

Rokinon 20mm f/1.8 Astrophotography Review: Joshua Tree National Park – Lonely Speck

[UncleVanya]

Samyang 20mm f/1.8 – Review – Aperture Attic

[DeadJohn]

The 15/2.4 is too similar to your current 14/2.8. Get the 20/1.8 from your 2 choices.

[Matchete]

Do you think you would use the lens for anything else like traditional landscapes? If so you might try looking at the 20mm club for full frame shots at 20mm to give you a better idea of the FOV. I personally think 15 on FF is really a bit too wide for most uses but not unreasonable for landscapes.

Yes I would, and I am used to shooting landscapes with 12mm and 14mm lenses on APS-C cameras so I should be quite comfortable with a 20mm lens on FF.

It seems that you are looking for a larger maximum aperture and a narrow scene width. The Samyang gives you both of those while the Irix doesn't give you much of either. Personally, 20 mm is not wide enough for some of my landscape shots. I have the Samyang 14, the D FA 15-30 and the FA 20. I very rarely use the FA 20 while the other two get quite a bit of use with the deciding factor between the two being how big of a lens I want to take.

Actually the larger maximum aperture wouldn't be the deciding factor for astrophotography as I'd mainly use medium apertures such as f/4 with astrotracer. I also like to do panoramas so the 20mm seems to be more suited to this use.

Just for the astro aspects of this, the longer focal length glass will be better, given all the other aspects (aperture) remains the same. In that the aperture becomes larger with the longer lenses, then just following that logic the 20/f1.8 will be the best selection.

So, here are some numbers to back up the statement. I'm also assuming that the astro tracking will be for 60 seconds (however the duration really does not matter here, but I'll toss it in anyway).

• 14mm/2.8=5 mm diameter, area = (pi/4)52 = 28.9 sq mm. Exposure time of 60 seconds. Light collection = 1177.5 sq mm seconds.

• 15mm/2.4=6.25 mm diameter, area = (pi/4)6.252= 30.66 sq mm. Exposure time of 60 seconds. Light collection = 1839.8 sq mm seconds.

• 20mm/1.8=11.11 mm diameter, area = (pi/4)11.112= 96.91 sq mm. Exposure time of 60 seconds. Light collection = 5814 sq mm seconds.

Then to compare the lenses...

• 14mm to 15mm = 1839.8/1177.5 = 1.6 - the 15mm collects 1.6 times more light than the 14mm

• 14mm to 20mm = 5814/1177.6 = 4.9 - the 20mm collects 4.9 times more light than the 14mm

The 20mm lens collects 4.9 times more light from objects in the frame. Stars are brighter, star clouds and nebulae will be brighter.

The key in all of this is the actual physical aperture diameter (for the 14mm / f2.8 lens - that is the 14 / 2.8 = 5). Essentially, to simplify the process, just compare the physical aperture diameter of each lens, and you get the same results by

• [(physical aperture long lens) / (physical aperture short lens)]2 i.e., (6.25/5)2 = 1.6x more light for the longer lens.

_________________________

Personally, I went with the 15-30/f2.8 for several reasons. First it was more general purpose for landscapes and I have found myself shooting astro at both ends - 15mm and 30mm. The 30mm end gets substantially more light, while the 15mm gets more of the overall landscape and sky combination. I've found so far, that at 15mm I get plenty of star light for a good image.

I was aware of this concept but din't bother to compute everything, thanks for doing it .

Btw I also own the Zeiss 28mm f/2, so don't need a zoom lens.

Rokinon 20mm f/1.8 Astrophotography Review: Joshua Tree National Park – Lonely Speck

The 15/2.4 is too similar to your current 14/2.8. Get the 20/1.8 from your 2 choices.

I already came across both reviews, but thanks for pointing them out to me.

[torashi]

The only reason to get the IRIX is if you need the weather sealing and just a bit more light. Otherwise, 20mm is a great FL. Have you considered the 24/1.4, too?

[northcoastgreg]

Perhaps this isn't true for astrophotography, but I do believe that 20mm is a more useful focal length than 14 or 15mm. On APS-C, I use my DA 15 far more than I use my Rokinon 10mm. However, if I had to choose between the two Samyang lenses and the Irix, I would take the Irix just because I think it's the better lens (better color and contrast). But then again, I already have two lenses covering 20mm. If you don't think there's any appreciable difference in quality between the Irix 15 and Samyang 20, maybe the 20's the better choice.

[Matchete]

The only reason to get the IRIX is if you need the weather sealing and just a bit more light. Otherwise, 20mm is a great FL. Have you considered the 24/1.4, too?

I never reallty warmed up to the 24mm focal length, IMHO it's got the drawbacks of an UW focal length without the actual field of view.

Perhaps this isn't true for astrophotography, but I do believe that 20mm is a more useful focal length than 14 or 15mm. On APS-C, I use my DA 15 far more than I use my Rokinon 10mm. However, if I had to choose between the two Samyang lenses and the Irix, I would take the Irix just because I think it's the better lens (better color and contrast). But then again, I already have two lenses covering 20mm. If you don't think there's any appreciable difference in quality between the Irix 15 and Samyang 20, maybe the 20's the better choice.

I'm leaning towards the 20mm. But I'm surprised that there aren't more user reviews of it !

[northcoastgreg]

I'm leaning towards the 20mm. But I'm surprised that there aren't more user reviews of it !

Check flickr. It's better to see images from the lens then read reviews. Reviews can be misleading (too much emphasis on measured sharpness).

[torashi]

I forgot to mention filters. The IRIX can take a LEE push on filter holder for the 100mm system.

[wanderer2]

I forgot to mention filters. The IRIX can take a LEE push on filter holder for the 100mm system.

Somehow I got the impression that the Irix is threaded for screw on filters - is this incorrect?