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View Full Version : Is there a way of estimating how bright a lens will look in the viewfinder?



newmoon2night
1-May-2010, 05:56
I'm thinking of buying a Nikon 90mm F/8, and have no way of trying this lens before I buy (and it will be imported, so postage is expensive).
When using a 110mm XL lens, I know I can view the edges perfectly acceptably wide open, and I've tested stopping it down, and viewing is perfectly acceptable at F/16, but getting difficult at F/22.
I also know that when using a 72mm XL, I have great difficulty viewing anything at the edges that isn't well lit, even when wide open at F/5.6.
At the edges the 110mm lens at F/22 seems clearer to me than the 72mm lens at F/5.6.
I'm not sure I understand this, as the centre filter for the 72mm is two stops, so I would have expected it to get equal at F/11. Is it to do with the image circle used by each lens?
I'm using a dark cloth, Maxwell bright fresnel, and 6x lupe, with no lens shift.
Has anyone any idea what F stop I should set the 110mm XL at to get an approximation of how the Nikon 90mm will look at F/8?
Thanks.

ic-racer
1-May-2010, 06:53
f8 ?

Jack Dahlgren
1-May-2010, 07:19
It has something to do with the viewing angle on the GG. The fresnel is a lens too, and it can either focus the image back towards you or not. It is likely designed better for the 110 than the 72. You could get a fresnel which is optimized for wide angles, but it would perform worse for longer lenses.

As for estimating what the 90 would be like, it should be somewhere in between what your 110 shows at f/8 and what your 72 shows at f/8. Remove the center filter for this comparison.

For the wider lenses try using a regular gg or a fresnel designed for wide angles.

newmoon2night
1-May-2010, 07:22
f8 ?
Well .... yes!!
But it doesn't seem to work that way in practice.
Is it just me, or is it generally the case that the wider the lens the harder (darker) it is to view on the screen/fresnel/ground glass?

Brian Ellis
1-May-2010, 07:30
Well .... yes!!
But it doesn't seem to work that way in practice.
Is it just me, or is it generally the case that the wider the lens the harder (darker) it is to view on the screen/fresnel/ground glass?

It's generally the case. The wider the lens angle of view the more light is striking the ground glass at an oblique angle, which means it has to travel farther and hence is weaker than more direct light traveling through the center of the lens (layman's explanation). It's especially true with Fresnels, which have to be viewed straight on for best results, unless the Fresnel is designed specifically for a wide angle lens. The only Fresnel I've ever used, and I've used quite a few, that allowed the edges and corners of the screen to be easily viewed from a straight-on position with my 80mm lens was the Maxwell.