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Comparing Samyang 135mm at f/2 and f/2.8

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Here are a few images taken using the Samyang 135mm at f/2 and f/2.8

The sensor used is a ASI224MC-cool with a UV/IR filter. I've tried to keep as many of the parameters the same as possible, including the image processing. One difference to note is the f/2 images were taken on 10/02/21 on a moonless night and the f/2.8 images were taken on 18/02/21 with 38% moon.

Going forwards I think I will stick with f/2.8 going forwards, but I'd be really interested to hear what you think. Eitherway, I think its a fabulous lens and give amazing results!

The first target is Orion Nebula M42. In each case the subs were 10s long and about 90 mins of data was collected. Based on this image I think the stars are less bloated with the aperture at f/2.8 - especially the brighter stars. There is a little SNR gained by using f/2 over f/2.8

F/2 Image


Now f/2.8 image


Next target Andromeda galaxy M31. In both cases we have about 2 hours of data. Comparing these two images - they look very similar. Very similar SNR and very similar bloating on stars and star shapes.

F/2 image


Now f/2.8. The difference in the core of M31 in this image is that I purposely did not tame the core in this image as much as in the f/2, as it did not look natural.


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Interesting comparisons, thanks for posting. To me, the f2.8 stop-down is certainly worthwhile based on your tests. It would also be interesting to see the results of using an f2.8 aperture mask versus stopping the lens down under aperture control.

I really ought to get myself one of these lenses!!

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Many thanks for this comparison.  Would you say you did the same processing on the F2 images and the F2.8 images?

I have one of these lenses and have been using F2.8 throughout as advised by Uranium for a beginner, but have never attempted to go down to F2.  If the difference genuinely bloats the stars more at F2, then I think I will stick with F2.8

I have been extremely pleased with this lens and it has kept me going this last year when I have been unable to get to a dark location due to Lockdowns. 


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Thanks @carastro.

To the best of my ability yes these images were processed the same (using Startools).

The results at f/2 (and f/2.8) are pretty amazing.  As I was using a colour CMOS I was not using any filters to begin with, and I did notice bloating of the brighter stars but thanks to advice from @vlaiv to use a UV/IR filter the results were massively improved. I was also advised to try f/2.8 hence the comparison here.

I don't think the difference is massive at f/2.8 compared to f/2 but I think it can be detected in the images above.

Also I'd agree with the comment about how pleasing this lens is - this is a wonderful lens that I am also very pleased with. I've had much more imaging time since I've started using it since its on a lightweight set up that is really easy to carry and set up.


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Just now, Alien 13 said:

Might just be me but there is far more detail in the F2 images to my eyes..


Yes I'd agree with this...I think there is more signal to noise in the f/2 image (especially on M42).

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