William Optics RedCat 51 250mm f/4.9 Gen 1.5 APO Telephoto Lens

Note: The RedCat was upgraded in late 2019 with exciting new features! In a nutshell:

• Now includes a tiltable rear plate for precise squaring of the camera sensor
• Installing a 48mm filter now involves simply un-threading the camera mounting ring--no tools required!
• The mounting ring has its accessory mounting surface at the top, not at an angle as was the case previously
• An optional dovetail/carrying handle for mounting a guidescope, finder, etc., is available
• A more precise focus scale, with vernier, is built in
• A longer dovetail mount is included

The new RedCat 51 is the answer for those seeking a very highly corrected lens in the common telephoto focal length range. Today's capable camera sensors don't quite demand the kinds of fast f/ratios in vogue in the past. This permits to design a highly optimized system possessing of high resolving power, well controlled aberrations, a large, flat focal surface, in a light-weight package, all at an attractive price.

The FPL-53 substrate used in its objective has dispersion characteristics not much different from that of the much vaunted fluorite crystal, and so is termed a synthetic fluorite. This permits the employment of a fairly fast f/4.9 while delivering superb chromatic aberration suppression.

Don't let the seemingly small element count of just four lenses put you off. The Petzval configuration contributes to the flat field over the considerable image circle of 44mm. And the uniformity of spot sizes and hence resolving power from on axis out to the edge of that field is remarkable.

The familiar helical focus scheme common to so many camera lenses is adopted. A focus tension adjustment ring permits to set the focus action to the desired degree of 'tightness.' Filter wheels can be accommodated, or 48mm filters can be installed inside the removable 48mm mounting ring.

The field rotator ring allows to orient the camera to any desired angle for framing of your target. A scale graduated in degrees at the rear of the lens body permits precise setting. The mounting dovetail bar is reversible, depending on whether your mount supports the Vixen or Arca-Swiss system.

To aid in setting precise focus, a patented Bhatinov mask is provided. For quick setting based on the instrument configuration, the lens barrel has a printed, calibrated scale in millimeters, with vernier for a more precise initial setting.

The lens body at the rear has an M48 thread. Instrument configurations requiring longer back focus, such as filter wheels, can be attached directly.

In the Box

• RedCat 51 f/4.9 telephoto lens
• M48 camera mount
• Patented Bhatinov focusing mask
• 210mm long dovetail bar
• Soft carry case

Features and Specifications

• Lens Type: Prime focus telephoto
• Aperture: 51mm (2")
• Focal Length: 250 mm
• Focal Ratio: F/4.9
• Glass Type: Synthetic Flourite FPL 53 APO
• Lens Structure: Petzval (4 elements in 3 groups)
• Fully Multi-Coated
• Image Circle Diameter: 44mm
• Lens Shade: Reversible for compact storage
• Focuser: Manual helical, calibrated in millimeters
• Focuser Tension Adjustment: Yes, via a ring ahead of the focus ring
• Focuser Travel: 33mm
• Field Rotator: Yes, with mounting holes for finder or microphone
• Rotation Angle Scale: Yes, at the rear
• Accepts 48mm filters in the easily removeable camera mounting ring
• Bhatinov Mask Cover for Critical Focus
• Camera format: Up to full-frame 24mmX36mm, or 44mm diagonal
• Mount: Vixen or Arca-Swiss, reversible, 210mm long, with 1/4-20 threaded holes
• Instrument Mount: M48 x 0.75mm
• Interchangeable T-mounts: Canon EF, Nikon F, Sony E, Pentax, micro 4/3
• Dimensions: 225mm x 80mm x 80mm
• Weight: 1.47kg (3.24 lbs)
• Two year warranty