OM-5 Birding: 75–300mm II or 100–400mm II?

Choose the M.Zuiko Digital ED 75–300mm F4.8–6.7 II for lightweight OM-5 birding when you mainly photograph perched birds in daylight and share modestly cropped images online. Consider the 100–400mm F5.0–6.3 IS II when distant subjects, weather sealing and coordinated stabilization justify a substantially heavier lens.

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Neither lens deserves a blanket warning against buying it. However, the 100–400mm II gets the weight priority wrong for casual walking birders: at 1,125g without its tripod mount, it adds 702g over the 423g 75–300mm II. Those figures are documented in the manufacturers’ specifications linked below.

The dimensions, apertures and weights come from OM SYSTEM’s 75–300mm II specifications and 100–400mm II specifications. Both are native Micro Four Thirds lenses; neither needs a mount adapter for the OM-5.

For perched birds, the 75–300mm II is the sensible starting point

The smaller lens makes sense when bird photography is part of a walk rather than the reason for carrying a dedicated wildlife kit. Its 600mm-equivalent field of view already provides substantial reach. The limitation is how much of the frame the bird occupies, especially with small species sitting across a pond or high in a canopy.

The 75–300mm II also starts wider. Finding a nearby bird at 75mm before zooming in can be easier than starting at 100mm. That is a practical inference from the focal ranges, rather than a measured autofocus advantage.

There is a useful detail buried in the specifications for close subjects: the advertised 0.9m minimum focusing distance applies only at 75mm. At other focal lengths, the minimum is 1.5m. If you photograph birds through a window beside a nearby feeder, allow that working distance instead of assuming the lens focuses at 0.9m throughout its range. OM SYSTEM’s detailed focusing specifications

The recommendation has an optical qualification. ePHOTOzine’s Gary Wolstenholme measured approximately one stop of corner shading at maximum aperture and used Imatest on a Panasonic Lumix DMC-G3 to assess resolution. At 300mm, the review found good central sharpness at f/6.7, improving to very good at f/8, while the edges remained weaker. This was a lens test on a different camera, not an OM-5 birding test. ePHOTOzine’s 75–300mm II measurements

That supports using the lens for a bird positioned near the centre and leaving some room to compose afterward. It does not support expecting unlimited crops of tiny, distant birds.

Instagram allows cropping, but distance still matters

Moving from 300mm to 400mm makes a distant bird approximately 33% larger in each image dimension, assuming the same shooting position. That is meaningful, but it is not twice the reach.

Here is the useful calculation: matching a 400mm view by cropping a 300mm photograph retains 56.25% of the original pixels, because (300 ÷ 400)² = 0.5625. The OM-5 records full-size images at 5184 × 3888 pixels, so that crop leaves approximately 3888 × 2916 pixels, or 11.3 megapixels. These are calculated dimensions, not measured lens resolution. OM SYSTEM’s OM-5 image-size documentation

That provides ample pixel dimensions for many social-media uses, provided the original photograph contains a sharply focused bird. Cropping cannot recover feather detail lost to motion, missed focus or the lens’s weaker performance at its longest setting.

Choose the 100–400mm II when you regularly need substantial crops even after reaching 300mm. Its longer focal length puts more pixels across the subject before editing. For a bird already reasonably large in the 300mm frame, the smaller lens is easier to justify.

The OM-5 supports the II lens’s Sync IS

The original OM-5 is explicitly listed as compatible with the 100–400mm II’s 5-axis Sync IS. Advice about the original 100–400mm should not be carried over indiscriminately to the II version. OM SYSTEM’s compatibility footnote

There is also an important qualification to the stabilization advertising. OM SYSTEM specifies 7.0 stops at 100mm and 5.5 stops at 400mm, measured with an OM-1 Mark II under its stated CIPA conditions. Those figures are not an independently measured OM-5 result, and seven stops is not the published result at maximum zoom. OM SYSTEM’s stabilization test conditions

The 75–300mm II has no optical stabilizer, but the OM-5 supplies sensor-shift stabilization. Its lack of lens IS therefore does not leave this combination unstabilized. The camera’s stabilization system is documented in the OM-5 specifications.

Stabilization helps with camera movement. It cannot freeze a bird turning its head, hopping along a branch or shaking its feathers. That distinction matters more than a headline stop rating when choosing a shutter speed.

Shade and moving birds change the calculation

For the 75–300mm II, a reasonable starting point for perched birds is 1/500 second, maximum aperture and Auto ISO, with a small autofocus target placed over the bird’s head. Treat this as suggested technique, not a tested guarantee. Increase the shutter speed toward 1/1000 second when the bird is active; flight often calls for 1/1600 second or faster and continuous autofocus.

At 300mm, try f/8 when there is enough light to maintain the shutter speed. The independent resolution test supports that aperture, but losing shutter speed can undo the benefit on a moving subject. Examine the eye and feather edges before deciding whether a soft photograph needs different settings or a different lens.

The larger zoom is not a major aperture upgrade for dim woodland. Comparing their longest settings, f/6.3 admits only about 13% more light than f/6.7, approximately 0.18 stop, calculated from the aperture ratio. Its stronger reasons for selection are reach and stabilization, rather than a dramatic ability to freeze movement in low light.

Choose the larger lens for weather and repeated distance limits

OM SYSTEM lists the 75–300mm II as not weatherproof. The OM-5’s protected body does not make this lens weather-sealed. If wet-weather birding is routine, that is a concrete reason to choose another lens. OM SYSTEM’s 75–300mm II feature specifications

The 100–400mm II has an IPX1 splash-resistance rating and dust-resistant construction, giving it a documented advantage for outdoor conditions. Its tripod mount can also be removed while the lens remains attached to the camera. OM SYSTEM’s weather protection and tripod-mount details

For daylight walks, perched birds and online sharing, buy the 75–300mm II and accept its long-end sharpness and weather limitations. Choose the 100–400mm II when extra reach or protected construction solves a recurring problem you can identify—and carrying the larger lens will not discourage you from taking it out.