A brief comment on Samyang 85mm T1.5

[vlaiv]

Not sure if this is right place to put this topic since it does not involve any astro images.

Just wanted to share quick test on newly arrived Samyang 85mm T1.5. This is same optics as Samyang 85mm F/1.4 but aperture ring does not have hard stops - lens is designed with cinematic application in mind (click stops on aperture ring can be heard when making a video and smooth aperture is advantage there).

Since most of these fast lenses are usually used stopped down for AP, I figured it is better to have smoothly varying aperture instead of preset stops - that way I can have lens at F/2.2 if F/2 is not sharp enough - no need to stop it down all the way to F/2.8 - where next stop might be.

It is chunky piece of glass - feels rather solid in hand. I was surprised by smoothness of both focusing and aperture ring - expected a bit more resistance. This way I worry slightly that it could shift on its own.

There is of course question of sharpness. This is by no means very scientific test, but it does show level of sharpness lens has in the center of the field. I'll be using it with 9mm diagonal sensor (ASI178) to begin with, so not overly worried about corner sharpness at this point.

ASI178mmc has rather small pixels at 2.4um - should be kept in mind when evaluating lens performance.

Methodology - well I used international standard measure for such things:

we'll be examining fine print contrast:

This is screen capture of 200% zoom preview in SharpCap (preview with debayer turned on).

First row is lens stopped to F/4 for reference. Not much difference at F/2.8.

At F/2.0 we start to see some chromatic aberration starting to creep in. This increases with F/ around 1.7 (no way to tell exact f number - I just turned aperture ring someway between F/2 and F/1.4) and letters start to be a little soft. At F/1.4 softness is even more apparent and there is noticeable contrast loss.

I actually believe this lens to be very sharp for AP use - at least the way I intend to use it.

I'll either use it as 85mm with 4.8um pixel sensor and resolution 1500 x 1000 (original is 3096x 2080 with bayer matrix) or as 40mm lens with same sensor. This can be of course achieved with 2x2 mosaic and data further being binned 2x2.

To put this into perspective, here are above letters scaled back to proper size in either of these two scenarios:

85mm super pixel debayer:

or if we see it as ~40mm lens (actually 42.5 but I subtracted a bit because of mosaic overlap):

I think I'll have no issues using this lens at F/1.4 when doing FOV that is similar to 40mm lens, but of course, true test will come under stars (once these clouds go away, but you know, new gear - clouds ... ).

Here is FOV of this lens used in 40mm "mode" (2x2 mosaic) with ASI178:

And here it is as standard 85mm with ASI178mmc:

 

 

[happy-kat]

As comparison I have the canon 85mm f1.8 and wide open stars are fluffy and chromatic aberration is strongest here. The lens gives much tighter stars and less chromatic aberration when used are F4, same for the canon 40mm pancake. Yet a vintage 55mm lens is ok at f2.8. I've found each lens used has to be taken in isolation what works best for that specific example.

 

 

Edited November 30, 2020 by happy-kat