In comparing eyepieces, several factors should be considered including magnification, actual field of view, exit pupil, and optical quality. A 34mm eyepiece with a 72° apparent field of view (AFOV) versus a 38mm eyepiece with a 70° AFOV offers slight differences that can affect observational experiences.
Magnification: The 34mm eyepiece offers a slightly higher magnification due to its shorter focal length compared to the 38mm. This means it will show a slightly smaller portion of the sky but will make objects appear larger, which can be advantageous when viewing smaller details.
Field of View: The 34mm with a 72° AFOV may present a slightly wider true field of view than the 38mm with a 70° AFOV, since the increase in AFOV can offset the slightly higher magnification. This means you could potentially see more sky area with the 34mm depending on the telescope’s focal length.
Exit Pupil: Assuming both eyepieces have a “perfect” exit pupil size for your telescope and observing conditions, this value will determine the brightness and the ease of viewing. It is typically regarded that a range between 2mm to 5mm is ideal for most deep-sky observations, but specific conditions and personal preference can shift this.
Optical Quality: “Perfect” is subjective and can indicate well-corrected optical design with minimal aberrations. The performance is also highly dependent on eyepiece design and build quality. If both eyepieces are from reputable manufacturers, they might each perform very well in their respective designs.
Ultimately, the choice between the two depends on specific observational requirements and conditions. For slightly higher magnification with potentially marginally wider views, the 34mm 72° could be preferable. For lower power, potentially brighter image with slightly less edge native optical correction demands, consider the 38mm 70°. Personal comfort with eye relief, particularly if using glasses, is also a consideration as these can vary between models.