Zoom vs Prime Dilemma: Sigma 400mm 5.6 APO vs Sigma 135-400mm APO (1 Viewer)

Hello guys. Glad to have found this forum! I am new to birding and am thinking of getting my first lens dedicated to birding and just general wildlife.

I am a Nikon user and everything beyond this range is expensive if considering Nikkor lenses. So was thinking of getting a third party 400mm range.

Currently considering these two lenses:

Sigma 400mm 5.6 APO
Sigma 135-400mm APO non-DG

Wondering which is better? Especially in terms of IQ?

I thought that a prime would generally create better IQ than a prime, but often for primes I have to constantly search for the object because the object is so close up in my viewfinder and a zoom would make it easier to find the object and zoom in?

Any other suggestions would be appreciated.

I have not used either, but given the lack of responses I'll throw you bunch of regurgitated info mixed with my personal opinion. If it has to be between those two, I would unquestionably toss the 135-400 out of the equation. My understanding is that it is slow to focus and soft at all focal lengths. The 400 5.6, specifically the APO Macro HSM, is said to be outstanding, meaning very sharp. I believe there are a few variations of the Sigma 400 5.6, but sticking to the Macro version seems to be the overwhelming opinion. There may be HSM and non-HSM variations here, too. Even early generation HSM was decent and fast focusing. I do believe that most modern DSLR's will not be able to change aperture with the 400mm, thus lmiting use at f5.6. If I understand correctly the Canon version will only work fully with the D30, D60 and 1D. I don't know Nikon compatibility well enough to comment specifically, although I would definitely research this issue before buying. Even the 135-400 may give some difficulties past a certain era of DSLR, for that matter.

Hope some of this helps. Here are some quick reviews that could assist:


Sigma 135-400 MM f/4.5-5.6 ASP AF APO

Sigma 400mm f5.6 APO Tele Macro HSM


Anyway, hopefully someone here with experience using both or either will chime in. Good luck.