Nikon Reflex-Nikkor 500mm f/8 (mirror)

Donut bokeh is the whole reason this lens exists, and you either want it or you don't. Every out-of-focus highlight behind your subject comes back as a bright ring with a hole punched in the middle, the optical fingerprint of the central mirror blocking the middle of the light path. Shoot dappled light through trees, or a chain-link fence with sun on the water beyond it, and the background dissolves into a field of glowing rings. Plenty of 500mm mirror lenses do exactly this and most of them are small and light, the Tamron SP and old Tokinas included. The Nikkor is just the one people trust most for build and coatings.

The folding is the whole trick. A catadioptric design lets light enter the periphery of the front element, hit the annular primary mirror at the rear, reflect forward to a small silvered spot on the back face of that front element, then pass through a correcting group set into the center of the primary. Fold the path that many times and a 500mm focal length ends up about the length of a coffee mug at roughly a kilo. Nikon ran this one from 1968 to 2005, a remarkably long life, and the center stayed sharp across that run. Stopped-down sharpness is not a question you get to ask, because there is nothing to stop down. The aperture is fixed at f/8. No diaphragm, no blades. You control exposure with shutter speed, ISO, and a set of rear 39mm screw-in glass filters (UV (L37C), orange (O56), amber (A2), blue (B2), and ND (ND4)), and one of them must always be in place because each filter's glass thickness is matched to the optics. The factory-installed UV, the L37C, is the one you leave in by default.

Center sharpness wide open, which is the only open, is genuinely good. Crisp enough for the moon, distant birds, or a compressed ridge line. Contrast is the soft spot. Mirror lenses scatter a little light internally, so images come back lower in contrast than a refractive 500mm, blacks sitting gray, and you will be adding it back in the darkroom or in post. Off-center detail falls away faster than a normal prime telephoto too. This is a center-of-frame tool, full stop.

Who reaches for it: birders and wildlife shooters on a budget, plus photographers who go looking for that ring bokeh on purpose as a style. It is no portrait lens, though people do use the donuts as a deliberate background. Manual focus only, and at 500mm on film the depth of field is paper thin, so handholding is a fight. A tripod or at least a monopod is close to mandatory.

The honest weakness, past the contrast, is the locked f/8. You cannot open up for a dim morning hide, and you cannot stop down to tame a blown highlight. Exposure lives entirely in shutter and ISO, which makes metering the discipline that saves the shot. Meter the bird or the moon against the sky in Zone Light Meter and place that key tone exactly, because with a fixed aperture you have one fewer lever to recover a misjudged frame. Cross-shopped today against the Tamron SP 500mm f/8 and old Tokina mirrors, the Nikkor stays cheap because autofocus mirror lenses are rare and the few that exist never came cheap. The 39mm spec is the rear filter slot, not a front thread; front filters do not apply.