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 p.2 #14 · Loving the Acratech GXP Ballhead! | |
DaveTCC wrote:
I totally agree, it is best to look at the physical dimensions of a ball head when attempting to judge its stability. The ball diameter is important, but so is the thickness and length of the stem that connects the ball to the clamp. Ball diameter makes the biggest difference in smoothness and locking force, but the stem is usually the limiting factor in the stiffness of the head. The stiffness of the head often seems to be ignored since it is very difficult to notice when handling just the head itself. With a tripod, you can twist it and feel it flex. Not so much with a ball head though since it is very hard to get the right leverage on it to do so. Once you load it up though, this flex (lack of stiffness) can become very noticeable as a long lens provides the leverage you need to feel the weakness of the head. This is why I see stiffness as the most important quality of the head. Admittedly though, this is somewhat self serving as mine is the only site to provide actual measurements for head stiffness.
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And totally appreciate the work you put in measuring this stiffness metric. I agree that it is difficult to assess on a ballhead due to the shortness of the stem. With my 100-400 though, I see more of a flex with my (Wimberly and RRS) lens foot than with the ballhead stem. I say "more" implying some sort of quantitative assessment, but this is misleading. All I mean is that I can actually see the flex on the foot when pushing things around, whereas I cannot with the ballhead (GXP, Markins, or otherwise). That does not mean that it is not there nor that it does contribute (negatively) to the setup, which is why your analyses are so useful!
DaveTCC wrote:
Given that the geometry of the GXP looks basically the same as the older GP heads, I would guess that performance wise, the heads are the same. I may be somewhat cynical, but I think Acratech just increased the load rating to make the head look better against the competition. I'm happy to change my opinion here, though I would need to see a precise definition on how they come up with the weight rating and measure it to do so. In the video above, they put ~1000 lbs of weight on the GP head and it holds up just fine. So clearly this isn't their method.
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I think the "25 lb" figure on the GP, for what it is worth, comes from what they show at 0:08 in the video:
The ball size on both the GP and GXP is the same, 38mm (and so is the stem). However, the overall size of the GXP is larger. I suspect that this size increase is due to the larger housing of the ball, and in particular, the height/width of the "arms" that hold the ball (NOTE: I have not measured this myself, not do I have both the GP and GXP on hand to compare; this is just a speculation based on the different dimensions of the ballhead). If so, then the greater advertised capacity presumably follows, at least in the abstract, from the greater contact area to hold the ball locked, whether or not this increased capacity has been empirically tested through a test like the above or any other.
DaveTCC wrote:
Be careful about using weight ratings to determine the class of a head as well. There doesn't seem to be any consistency here across the industry. For example, which of the following heads has the highest weight rating?
Good point! Naturally, I would guess the Feisol on the right as having the highest rating, because of it's size, and I know it has the highest stiffness, because I follow your site! But because you have set up the question, I'm going to guess the marketing doctors have rated the ballhead on the left with the highest number?
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