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Viewfinder eyepiece field stop


leif_goodwin8

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<p>I recently read that Nikon DSLR viewfinders effectively limit the lens aperture to about F2.8 and my question is why and how.</p>

<p>My guess is that to ensure that the viewfinder eyepiece is compact and affordable, they introduce a field stop that limits the lens aperture to F2.8 and below. The alternative would be to use the full light cone from an F1.4 lens, which would require a complex and large eyepiece design, and an associated increase in bulk and cost of the DSLR. I guess the pentaprism could also play a role in stopping down the lens.</p>

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<p>Find answers yourself by performing simple test. The results will stick to you for much longer than reading something that may not be true. </p>

<p>The test: (if your camera does not have the DOF button... then cannot use this test.)</p>

<p>Open up the 1.4 lens to f=2.8, tha value that troubles you. Use Manual or Aperture Priority mode to make sure the aperture you set is what you test. See some bright and consistently lighted test surface.<br>

Press and hold for a moment the Depth of Field button and observe in the viewfinder what and how it changed. Release the DOF button from holding, and see how picture returned to maximun aperture view.<br>

Repeat the test using all apertures of 1.4, 2.0, 2.8, 4.0, 5.6, etc... and you get your own undisputed knowledge how your Viewfinder - Mirror - Prism work on your camera. There will be differences in the brighness, and in the DOF.<br>

Things may look different on FX cameras and on DX, and also on different grades of the camera.</p>

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<p>Hello Frank. That is a good idea. I did think about photographing the viewfinder image with 50mm F2.8 and 50mm F1.4 lenses mounted and looking for a difference. What you suggest is much simpler, and better. Unfortunately I have no lenses faster than F2.8 so I cannot do this test myself.</p>
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<p>My understanding---someone please correct this if it's wrong--- is that the full light cone of the f/1.4 lens---or any lens--- stops at the screen, and that the f/2.8 "limit" is created in the lenticular design of the actual focus surface of the finder screen and not related to any physical stops or restrictions in the post-screen optical path. </p>

 

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<p>My understanding---someone please correct this if it's wrong--- is that the f/2.8 "limit" is created in the lenticular design of the actual focus surface of the finder screen and not related to any physical stops or restrictions in the post-screen optical path. </p>

 

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<p>"I recently read that Nikon DSLR viewfinders effectively limit the lens aperture to about F2.8" - What you've read is patently wrong, at least in the case of the D700 and D800. With both of those cameras I can see a clear difference in viewfinder brightness <em>and</em> depth-of-field when stopping an f/1.4 lens down from full aperture to f/2.8.</p>

<p>However, the difference in viewfinder brightness between f/1.4 and f/2 is not quite so marked, and neither is the apparent change in DoF. The most likely cause for this is the prism angle of the condenser fresnel used to brighten the "ground glass" image, coupled perhaps with the size of the mirror. In other words the mirror/fresnel doesn't gather light from the very perimeter of the lens's exit pupil at around f/1.8 and above. This is a very small issue that really shouldn't worry anyone in practise.</p>

<p>The DoF as seen through the viewfinder is in any case totally unrepresentative of what appears in the final image, regardless of lens aperture. The reason for this is that the ground glass surface is designed for maximum brightness and not to show DoF accurately. The same is true for most DSLR viewfinder screens on the market today. You can have a nice bright image, or you can see the DoF properly; but you can't have both. Liveview, of course, shows DoF as it will be captured - at wider apertures and in good light at least.</p>

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<p>Thanks for the answers.<br>

Rodeo Joe: It seems that you have confirmed that what I read is not patently wrong, except that the value could be a bit less than F2 rather than F2.8. And as you suggest the mirror may be slightly undersized, and (as also suggested by Keith B.) the focus screen may play a role.<br>

One further point though is that human eye's iris also acts as a stop, so it is possible that you saw less of a brightness change between F1.4 and F2 compared to between F2 and F2.8 because your iris was acting as a stop at the wider aperture. That is only a guess, which I think is why the only accurate way to know is to take photographs of the viewfinder image. Thanks for the useful feedback. </p>

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<p>The slightly undersized mirror doesn't really make much of a difference, if any, the light emerges as a diverging cone from the lens, if the mirror was too small, you would be missing part of the image on the screen. </p>

<p>The screen on the other hand makes a huge difference. Note that some older film cameras that meter thru the prism underexpose with a brighter screen and overexpose with a ground glass screen without a fresnel lens.</p>

<p> </p>

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<p>Leif, the iris diameter of the eye has no bearing on what happens before the image is formed on the GG surface of the viewfinder. If it was an aerial image, then your theory might hold some water, but it's a real image. Just as real as the image on the computer monitor in front of you.</p>

<p>If this trivial issue is really bothering you, then I suggest you study <a href="http://tokyobling.wordpress.com/2008/12/17/nikon-d3-cut-in-half/">this x-section of a D3</a>, where the viewfinder optical path is clearly shown. I believe the image-forming etched surface is on the upper side of the flat fresnel condenser just above the mirror. What's not obviously shown is that there's also an LCD membrane sandwiched in there to display AF points, grid lines, etc.</p>

<p>Nikon's FF DSLRs use one of the largest mirrors of any make; made possible by the deep lens register. So if the mirror really is responsible for any exit pupil occlusion, then no other SLR is going to do any better in this respect. Incidentally, I've just checked and my old F2 film camera shows almost identical viewfinder behaviour, despite the fact that it has a much coarser "grind" to its GG surface. The F2 has been around for over 40 years, and AFAIK in all that time nobody lost a picture because of its viewfinder optics.</p>

<p>Bob, with any positive power lens, light emerges from it as a <em>converging</em> cone.</p>

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<p>Ah! I see what you mean now Bob. I still think you have to consider the path of a bundle of rays coming from the exit pupil as a converging cone though. Otherwise any optical obstructions can't be easily visualised.</p>

<p>After pondering whether the mirror might be involved, I'm beginning to think it probably isn't. Mirror obstruction would tend to "square off" the exit pupil and turn out-of-focus highlights into rectangles. An effect that can't be seen in the viewfinder.</p>

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<p>If you obscure part of the lower edge of the mirror, it is very visible in the eyepiece, easily observed by leaning a piece of opaque plastic film against the lower edge of the mirror so it sticks up a few mm. Just don't have it tilted back enough to rest on and damage the mirror surface. Replacing and realigning the mirror is not for the faint of heart or a semi skilled DIY'er. :(<br>

</p>

 

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