While we naturally desire to maximize light across the full field, the vignetting induced by even a 'skinny' dew shield of I.D. equal to the objective is less than one might surmise. And that includes wider bino TFoVs.
An example. Consider a 50mm bino delivering a 10 degree TFoV. Further, it offers the amazing characteristic of full illumination to the field edge. That is, it suffers no intrinsic vignetting. (Note that many real world binos have edge-of-field illumination in the range of 50-70%.)
Suppose a dew shield of 50mm I.D. is installed. How long must it be in order to reduce edge-of-field light to 50%?
The bino's semi field angle is 5 degrees, and objective semi diameter is 25mm. To a first approximation, half the light is clipped when the shadow of the front lip of the dew shield creeps inward to the objective center. This point is reached when the dew shield length equals
25mm / TAN(5)
= 25mm / 0.0875
That's 11-1/4 inches, or a bit more than 5.5 objective diameters. When the shield is of the (minimalist) same diameter as the objective, no less!
And because the typical bino already suffers mechanical vignetting, the *relative* contribution by our skinny dew shield is even less than it would be for a vignetting-free instrument.
Indeed, such 'excessively' long dew shields offer the signal benefit of more aggressive interception of out-of-field light. Particularly when the shield is not itself a source of scatter/reflection. A coating of fine sawdust, sealed and blackened with flat black paint, serves nicely. Or the shield can be increased in diameter, with a number of ring baffles installed. Why, a sufficiently long shield could intercept much or all of that light contributing to the false pupil segments, or 'fingernails, which bracket the principal exit pupil.
If such a measure were to realize tangible benefits as regards reduced intrusion of unwanted light, the payment exacted in the form of reduced outer field illumination would be worthwhile.
Contrast beats brightness! And besides, the central field brightness is never reduced, no matter how long the dew/light shield. Of course, some care in alignment of very long dew shields is warranted, else the region of full (or maximal) brightness moves off-axis.
Edited by GlennLeDrew, 06 February 2019 - 04:19 AM.