| EdZ |
| (Professor EdZ) |
| 07/08/09 02:30 PM |
|
Re: ED glass in binoculars
|
I was asked in another thread (Comparing 22x85, 22x70 and Tak22x60) if I intended to mask the objects of the larger binoculars to compare the slower resulting f# to the smaller and slower Tak. Let's look at that for a moment.
See this note from my original post in this thread
Quote:
Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens.
If I were to mask down the objective of my f/4.7 22x85 to 60mm I would get an f/6.7 60mm, nearly identical to the Tak f/7 22x60. And I'm certain the color correction of the GO22x85 would improve. But I would not be able to isolate how much of the improvement is related to smaller aperture and how much is related to slower f ratio.
We know that given equal design quality and equal f ratio, a smaller objective produces less color. So the color of the 22x85 when masked to 60mm, as a result of smaller aperture should be reduced to 60/85, or to 70% of original.
We also know that slower f ratio in an equal sized equal quality scope would produce less color. The f/6.7 60mm masked objective would produce less color than the full open f/4.7 85mm objective.
The difficulty would be in attributing how much color correction is a result of which changed aspect.
These issues raised here are not specifically relative to ED binoculars. However, you can see from this example, it is quite difficult to assess how much color correction in a binocular is attributable to ED, how much to aperture and how much to f ratio. And although not needed in my example since all are 22x, there would be a difference attributable to power if it were different. If any of those are different in comparison binoculars the assessment becomes rather difficult. And as I stated earlier in this thread, few reviewers are taking the steps to isolate each cause when comparing binoculars.
edz