Well, how does the saying go: "Lies, damn lies and statistics
The deeper you dig, the more your head hurts...
I was just concentrating on analysing the 120-400mm data, when it came to me as a shock that the base data above (that is for my shake without IS-support) are quite different depending on which lens I take. The above statistics were derived from the sum
of shots from the Sigma 120-400mm at 400mm plus the Sigma 400mm (naturally at 400mm
But when I look at my favourite shutter speed of 1/125
sec there is a hugh difference in blur between both lenses and guess which lens comes out on top? Well yes, the lighter, leaner 400mm fixed focal! (I'll come back to this a little later)
There are 23% shots rated at >=3 stars fom the fixed focal vs only 3% from the zoom
From the worst shots (1+0 stars) the fixed-focal had 53%, the zoom had 69%
The picture changes at 1/60
sec shutter speed. There the blur distribution between both lenses is much closer.
sec shutter speed you have 56% >=4 stars with the zoom and 56% >= 4 stars with the fixed-focal with a clear advantage for the latter lens at the 5 star rating.
What do we get out of these statistics?
1. It seems that the 1/125 shutter speed is the "critical speed" at 400mm where the results are most sensitive to influences. At the shorter speed of 1/250 sec things are getting more equal because shorter speeds should eliminate shake more often. The same is true with 1/60 (and should be at even slower shutter speeds): The odds are aginst getting any sensible IQ out of those shots.
2. If this is so (the 1/125 sec being the discerning speed) and the samples are large enough (in this case around 30 shots each) the results could be statistically significant.
3. The only explanation that comes to my mind are:
3.a The leaner fixed-focal lens is easier to grip
3.b the lower weight of the fixed-focal (1300g vs. 1600g) is easier to hold steady and aim (but there is certainly a limit to this logic: my 600g 500mm mirror lens is much harder to hold steady than the 400/5.6!)
Are there any consequences to m review of the 120-400mm?
Yes, there are. If I compare the 120-400 with OS="on" with the statistic that mixes the results from the fixed-focal and the zoom one could say that this is not fair. Ok so for the sake of an apples-to-apples comparison I present here the results for the 1/125 sec shake statistic purely for the 120-400mm lens:
Relative as % of total shots at a given speed:
1/125sec..|...0%....3%....0%...28%..34%..34%.....100% (only 120-400mm)
((any numbers that don't add up are due to rounding errors!))