Sony Internal debate 135mm f 1.8 vs 70-200ish f 2.8, help, please?


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Somerset, UK
Hello. I have been having an internal debate for the last few days. I've been trying to decide between the Samyang AF 185mm f 1.8 FE and Tamron 70-180mm f 2.8 Di III VXD.

Case usage: very fast and erratic subjects, basically dog from small to large sizes, running and playing out in the open or forests, could also include mild bad weather like rain or dust as well as dogs shaking the water and the mud off themselves so weather sealing might be an important factor.

Intention: Portraits with fast action but possibly, also, portraits still with good subject isolation.

Cost: Samyang AF 135mm f 1.8 FE used: ~640 £
Tamron 70-180mm f 2.8 Di III VXD used: ~815 £

I never owned these kinds of lenses or focal range/aperture so drawing from "past" experience I can not. I have no qualms about image quality, I think it's a penny's throw of which one is better and personally, I don't care much about it (unless it has a significant amount of chromatic aberrations because those are annoying to correct for especially backlit situations, larger than 4 pixels wide or very intense colours).
My main priority is autofocus speed because I want to put emphasis on my value for the customer as action pet photography, no video interests though. I don't know if Tamron's VXD can keep up with erratic subjects and Sony's Real-Time Eye AF with dogs (some reviewers claim that subjects coming towards the photographer the Tamron can struggle after a few frames) or if Samyang's stepping motor can keep up though it has the advantage of AF Focus limiter (1.5 meters to infinity) which would be incredibly helpful.
As for the focal range, this is a strange mix-up that I am not sure I will truly be able to decide until I have months of experience using either lens because 135mm f 1.8 has greater subject isolation and low light capabilities in darker areas like woods (which is a must, it adds a lot of pop and magic to images) while the 70-180mm f 2.8 has greater versatility with movement and reaction to subject and composition and 180mm f 2.8 can pretty much balance in term of subject isolation compared to prime but it will have to be at a High ISO cost, especially for freezing motion.

Now with my (used copy of) Sony a7 IV in hand, I will start offering pet photography and portrait service this spring (instead of summer) and depending on my anxiety level I will expand slowly or focus on what I can handle better first. On the "wider" side I have the Tamron 28-75mm f 2.8 G2 and Samyang AF 35mm f 1.8 FE and (I am trading in my Tokina atx-m 85mm f 1.8 FE for) Samyang AF 75mm f 1.8 (because it's a bit wider and easier to handle indoors, has faster AF motor also smaller and lighter to focus quicker). I just need to cover my telephoto side for outdoor usage.
The 135 is limited to a narrow working range for Portrait work.
The F1.8 speed is fine for low-light, but will not be required for outdoor work.

Look at the Zooms. The F2.8 is more than fast enough for outdoor work. A Zoom with F4.5 would be fine on a digital camera, you can boost the ISO and get fine results.
I recently took a 28~105 F4.5 AF-Nikkor to an indoor event, used my Nikon Df. Boosted the ISO to 3200 and 5000, was fine.
Some random thoughts from someone who shoots often with 135mm primes and 70-200mm eqv zooms.

  • A 135mm prime is a wonderful thing.
  • but for sports and fast paced portrait shootouts where you don't have the required minute to find a suitable perspective for 135mm, I don't think it's a good idea as a first fast tele.
  • Hardly nobody has ever faulted a 70-200 f/2.8 from having too much DOF.
  • If you're worried about high ISO, use flash more
I've used the Tamron 70-180's VXD and Real Time AF with the R4 and 4 and felt the AF speed and accuracy was very good. I didn't use it on fast, erratic objects however. I think that the convenience of the zoom wins in that scenario. And there's the no penalty of having the greater dof to mask AF misses.
That said I prefer having primes like the 75 and 135 and would likely go that route myself despite having to switch lenses and a lack in AF speed(if there is one).

You might check out Dustin Abbott's reviews of both.