Plossl Eyepiece vs 2x Barlow Lens for More Magnification

You've got your first telescope, and you want to push it harder for planets and the Moon. Two paths lead to more magnification: buying a shorter eyepiece (lower mm number = higher power) or adding a 2× Barlow lens to the eyepieces you already own. The arithmetic arrives at similar magnification numbers. But the practical experience — image brightness, eye relief, apparent field of view, and what your money actually buys — is different in ways beginners often don't hear about before they order.

This guide is based on published manufacturer specifications, optical design principles, and aggregated expert and user reviews. We did not physically test any eyepieces or Barlow lenses. Scope Atlas earns commissions as an Amazon affiliate when you purchase through our links — this does not change our spec-based verdicts.

The Magnification Formula (For Both)

Magnification = telescope focal length ÷ eyepiece focal length (in mm).

For a telescope with a 900mm focal length (a common long-tube beginner refractor spec range):

At 90×, the 10mm eyepiece alone and the 20mm + 2× Barlow are mathematically identical in magnification. The difference is in everything else.

Side-by-Side Comparison: Short Eyepiece vs Barlow

The Eye Relief Problem With Very Short Eyepieces

The most underappreciated downside of high-power short eyepieces is rapidly shrinking eye relief. Eye relief is the distance your eye can be from the eyepiece and still see the full field. Published manufacturer specs for a 4mm Plossl, for example, typically show eye relief of only 3–5mm — meaning your eye must be nearly touching the eyepiece. Glasses wearers need at least 15mm of eye relief to see the full field. This makes very short eyepieces uncomfortable or impractical for many observers.

A 2× Barlow lens inserted before the eyepiece allows the 10mm eyepiece (with its more comfortable ~6–8mm eye relief) to deliver the same magnification as a 5mm eyepiece — with noticeably more comfortable viewing geometry. This is one reason planetary observers favor the Barlow-plus-moderate-eyepiece combination over buying a very short eyepiece.

Cost Efficiency: Barlow Wins for Multi-Eyepiece Collections

Here is the core economic argument for the Barlow:

If you own three eyepieces (say 20mm, 15mm, and 10mm), one 2× Barlow effectively gives you six magnification options:

• 20mm alone and 20mm + Barlow
• 15mm alone and 15mm + Barlow
• 10mm alone and 10mm + Barlow

A 2× Barlow typically costs less than a single quality eyepiece. The leverage ratio makes it the highest-value accessory most beginners haven't yet added to their kit.

Browse 2× Barlow lenses at /go/amazon-barlow-lens. Browse eyepiece kits (which often include a Barlow) at /go/amazon-eyepiece-kit.

When to Buy a Dedicated Short Eyepiece Instead

The Barlow is not always the right answer. Buy a dedicated short eyepiece when:

1. You want to reach the top of your scope's magnification ceiling — a Barlow on a 6mm eyepiece pushes past what most scopes can usefully deliver. A well-corrected 4–6mm eyepiece with better eye relief than a budget Plossl (wide-angle designs, Redline style) is the right tool at the upper limit.

2. You have only one included eyepiece — if your scope came with only a 20mm, one quality mid-range eyepiece (such as a 9–12mm) plus the Barlow gives you more coverage than just the Barlow alone.

3. Image quality at the edge of field matters — at fast focal ratios (f/5 and shorter), a well-corrected wide-angle eyepiece will show a flatter, sharper field than a basic Plossl + Barlow combination.

Image Brightness: The Shared Reality

Both paths darken the image as magnification increases — this is physics, not a product flaw. Exit pupil (the diameter of the light cone reaching your eye) = eyepiece focal length ÷ telescope focal ratio. As magnification rises, exit pupil shrinks and the image dims. A planet at 150× looks fainter than at 75×, no matter how you got to 150×.

The Moon is bright enough that this never becomes a limiting factor. For deep-sky objects, it means that doubling magnification to see a galaxy more closely actually makes it harder to see — aperture and dark skies matter more than power for diffuse objects.

Practical First-Step Recommendation

For a beginner who received a scope with a 20mm and 10mm eyepiece:

1. First: Add a 2× Barlow. This gives you 45×, 90×, 90× (redundant at this point), and 180× with your existing eyepieces. Budget: modest.
2. Second: Add a 9–12mm quality eyepiece (Plossl or wide-angle). This fills the gap between low power and Barlow-boosted 10mm.
3. Third: Only then consider a 6mm or 5mm if you want top-of-ceiling planetary detail — and look for a design with at least 12mm of eye relief.

This sequence maximizes magnification coverage while spending the minimum before you know what targets matter most to you.

How Field of View Changes With Each Approach

One aspect the magnification formula alone doesn't capture: apparent field of view (AFOV) and how it determines the actual sky patch you see.

A Plossl eyepiece has a published AFOV of typically 52°. A more expensive wide-angle eyepiece (Explore Scientific 82°, for example) provides a larger apparent field. But here's the critical point: a 2× Barlow does NOT change the eyepiece's apparent field of view. It halves the true field of view (the slice of sky visible) because it doubles magnification, but the AFOV stays the same.

What this means practically:

The wide-angle eyepiece at the same magnification shows more sky — which matters for targets like star clusters that span larger apparent sizes. But for planets, which are tiny discs, the wider field provides only visual comfort, not additional detail.

Exit Pupil and Image Brightness: The Numbers

Exit pupil = telescope aperture (mm) ÷ magnification. This determines how much light reaches your eye and how bright the image appears.

Exit pupils below ~0.7mm produce dim images and begin to show optical aberrations more prominently. For deep-sky objects (nebulae, galaxies), keeping exit pupil above 2–4mm is preferable — meaning low power (30–50×). For planets, the Moon, and double stars, a 0.5–1mm exit pupil is fine because these objects are bright enough to survive the dimming.

Summary: The Decision Framework

Buy a 2× Barlow first when:

• You already have 2+ eyepieces and want more magnification options from them
• Eye comfort matters (you wear glasses or find very short eyepieces uncomfortable)
• Your budget is tight — a Barlow typically costs less than a quality standalone eyepiece

Buy a new shorter eyepiece first when:

• You have only one included eyepiece and the Barlow alone leaves a coverage gap
• You want to reach near the top of your scope's magnification ceiling with better eye relief than a very short Plossl allows
• Optical quality is a priority and you want a design (wide-angle, Redline) beyond the basic Plossl

For most beginners, the sequence is: 2× Barlow first (see current options at /go/amazon-barlow-lens), then an eyepiece kit that fills remaining gaps (see /go/amazon-eyepiece-kit).