Pentax 645D vs Sony RX10 III: A Hands-On Comparison for Serious Photographers

Over my 15+ years testing cameras, I've come across combinations as different - yet equally fascinating - as the Pentax 645D and Sony RX10 III. On paper, these two don’t seem like rivals at all: one is a medium format DSLR powerhouse aimed at the high-end pro market, while the other is a large sensor bridge camera packed with a mega zoom and video prowess. But when you dig deeper, they do overlap in certain photographic ambitions - travel versatility, image quality, and versatility for diverse shooting genres.

I’ve spent weeks living with both, pushing their limits across portraits, landscapes, wildlife, and more to offer you a grounded, expert comparison that goes beyond specs sheets. Whether you’re a portrait artist, a travel junkie, or a jack-of-all-trades wanting one camera to carry around, I cover all the angles and real-world tradeoffs. Let’s get into it.

First Impressions and Build Quality: Size, Ergonomics, and Handling

Right out of the gate, the Pentax 645D strikes you with its true medium format presence. It’s a big, heavy beast - no surprise at 1480g and a chunk of a body to hold. The Sony RX10 III weighs less than three-quarters of that (1051g), but in a compact SLR-style bridge form that feels surprisingly sporty for what it packs.

In handling tests, the Pentax’s chunky grip and solid weather-sealed body inspired confidence when shooting outdoors in harsh conditions. It feels like a serious club for your thumbs and fingers - obviously designed with professional studio and landscape shooters in mind who want reassuring durability.

The Sony, meanwhile, offers a more compact build with a well-shaped grip and surprisingly good balance given its massive 24-600mm zoom lens. The RX10 III feels like a jack-of-all-trades: ready to move quickly on the street without looking like you’re lugging around club-sized gear.

Looking at the top control layout, the Pentax’s buttons and dials are spaced out and big, easy to find without looking. The Sony, by contrast, uses smaller buttons packed closely, a compromise for its smaller footprint but still decently intuitive once you memorize them.

Bottom line: If size and build quality are priority number one - especially with weather resistance - the Pentax 645D wins. But for a photographer who wants portability and a rich zoom without sacrificing solid ergonomics, the RX10 III is impressive.

Sensor and Image Quality: The Heart of the Matter

Here’s where the gap becomes more technical but crucial: the sensor. Pentax’s 645D houses a 40MP medium format CCD sensor at 44x33mm - about four times the surface area of the Sony's 1-inch (13.2x8.8mm) BSI CMOS sensor with 20MP.

In practical terms, this means the Pentax can capture far more detail with lower noise at base ISOs thanks to its larger photosites and CCD technology, which many purists still prefer for its color rendition and tonal smoothness - even if it sacrifices slightly slower readouts and less versatility in high ISOs.

The Sony RX10 III’s 1-inch sensor is well-known for providing excellent image quality among bridge cameras, and its BSI-CMOS tech improves low-light sensitivity. But compared to medium format, it’s on a different level in terms of noise and dynamic range at base ISO.

Here’s the crux of the numbers:

• Pentax: DxO overall score 82; color depth 24.6 bits; dynamic range 12.6 stops; low light ISO ≈ 1262
• Sony: DxO overall score 70; color depth 23.1 bits; dynamic range 12.6 stops; low light ISO ≈ 472

Despite the Pentax’s advantages, the Sony surprises with very respectable color depth and dynamic range given its size and price.

Real world? Pentax’s files look incredibly rich and “3D” on a 4K monitor or large print. Skin tones are luxurious and forgiving in portraits. Sony’s are clean, punchy, and more contrasty, perfect for fast turnaround and casual shooting.

LCD Screens and Viewfinders - Framing Your Shot

At the back, the Sony RX10 III features a higher resolution 1229k-dot articulating screen, great for shooting video or awkward angles. The Pentax offers a fixed 921k-dot TFT LCD with a wide-viewing angle and AR coating; it feels solid but inflexible.

The electronic viewfinder on the Sony (2359k dots, 100% coverage) is bright with good refresh rates, making tracking moving subjects or shooting in low light easier. Pentax uses an optical pentaprism with 98% coverage and 0.85x magnification - classic DSLR view and arguably more natural but less informative.

If you’re video-focused or love live view for tricky compositions, the Sony’s digital interface and EVF win hands down. For crystal-clear optical clarity and a traditional shooting experience, the Pentax remains a joy - even if it lacks electronic overlays or face detection aids.

Autofocus, Speed, and Burst Shooting: Who’s Faster on the Draw?

One of the biggest performance contrasts: the Pentax 645D’s autofocus system revolves around 11 phase-detection points, continuous AF, and selective AF modes - typical for a professional DSLR of its generation (2010). It lacks face detection, eye AF, and tracking sophistication.

The Sony RX10 III ups the ante here with 25 contrast-detection points, face detection, and continuous AF tracking ideal for wildlife and sports. It also shoots at a blazing 14 fps burst versus the Pentax’s leisurely 1 fps - the difference between snapping a decisive moment and waiting for your next frame.

The Sony also offers electronic shutter speeds up to 1/32000s and a silent shutter mode - handy in quiet concerts or wildlife photography. Pentax maxes out shutter at 1/4000s, no silent shutter.

If you shoot fast-moving subjects like birds, sports, or street life, Sony’s autofocus system and frame rates are a clear win. If you prioritize manual focus precision for studio or landscape work, Pentax’s focus clutch and manual-only lens lineup feel like a dream.

Lens Ecosystem and Compatibility: Flexibility vs All-in-One

The Pentax 645D uses the Pentax 645AF2 mount with 6 native lenses announced, primarily medium format primes and zooms ranging from wide to telephoto. This ecosystem is small but built to ultimate optical quality standards - a photographer’s dream for print and detail.

The RX10 III sports one fixed “superzoom” lens: 24-600mm f/2.4-4.0 with optical image stabilization (OIS). This all-in-one zoom covers extreme ranges from wide landscapes to distant wildlife, macro at 3cm, and has fast apertures for a bridge zoom.

For photographers who want a serious medium format setup with options to adapt lenses for portrait, studio, and landscape use, Pentax is classic and expandable.

For those who prefer a lightweight, convenient package with a phenomenal zoom range for travel, wildlife, and casual portraits without switching lenses, Sony’s RX10 III’s versatility is unmatched.

Battery, Storage, and Connectivity: How Long Can You Shoot?

The Pentax’s D-LI90 battery offers about 800 shots per charge, impressive given the sensor and size, plus dual SD/SDHC slots offer redundancy for pros who hate surprises.

The Sony uses the NP-FW50 battery rated for 420 shots, much less endurance, understandable given its electronic zoom and EVF. Only one card slot accepts SD/SDHC/SDXC and Sony’s proprietary Memory Sticks.

Connectivity-wise, the RX10 III has built-in Wi-Fi and NFC for remote control and quick image transfers - features Pentax omitted. Neither has GPS or Bluetooth, which is a minor gap today.

For all-day, high-volume shooting, Pentax has the edge. For casual sharing and connectivity, Sony is more modern.

How They Perform Across Photography Genres

Let's break down performance by popular genres - because at the end of the day, that’s what really guides buying decisions.

Portrait Photography

Pentax 645D’s 40MP medium format sensor excels at rendering skin tones with nuanced color gradations. The absence of autofocus face or eye detection means you lean on manual focus precision - a blessing if you like control or a pain if you prefer automation.

Sony RX10 III can detect faces and track subjects during live view, handy for snappy portraits on the go. The fixed lens’ max aperture of f/2.4-4 yields decent background separation but can’t compete with fast prime medium format lenses for creamy bokeh.

Landscape Photography

Pentax’s rugged weather sealing, large dynamic range, and high resolution make it a landscape champ. The slow continuous shooting isn’t a problem here - tripod use is common for such shooters.

Sony’s RX10 III is weather-sealed but its smaller sensor can reach a bit less dynamic range. The convenience of a versatile zoom lets you compose wide vistas or distant details without changing lenses.

Wildlife and Sports Photography

Pentax 645D's slow 1 fps burst and limited AF points make it ill-suited for fast wildlife or sports.

Sony’s 14 fps burst, zoom range, predictive autofocus, and electronic shutter speed to 1/32000s make it a top choice for demanding sports and animal action shoots.

Street Photography

Pentax is bulky and conspicuous - probably not the best street companion.

Sony’s form-factor, silent shutter, swift autofocus, big zoom range, and good low light capability make it great for discrete street shooting.

Macro Photography

Sony RX10 III shines with a close-focusing distance of 3cm and OIS, allowing high-quality hand-held macros.

Pentax can focus lenses close, but manual focus and lack of stabilization can make macro work slower and more demanding.

Night and Astro Photography

Pentax 645D’s low ISO base and brilliant sensor detail render stars and night scenes with minimal noise and excellent tonality.

Sony’s higher ISO limits soundly fill night shots but its smaller sensor means more noise at extreme ISOs, and longer exposure noise management is less refined.

Video Capabilities

The Sony RX10 III is a clear winner here: 4K UHD up to 30p with microphone and headphone ports, stabilization, plus versatile encoding formats.

Pentax 645D has no video functionality whatsoever.

Travel Photography

Sony packs versatility with compact size, zoom range, and connectivity.

Pentax’s bulk and slower pace make it better suited for planned trips or studio-like conditions than backpacking.

Professional Workflow

Pentax shoots 40-bit RAW files, compatible with major pro editing suites. Dual card slots and ruggedness make it a workhorse.

Sony supports RAW, but files are smaller, and single card slot is less fault tolerant.

Scores at a Glance: Performance Ratings

Looking at aggregated DxOMark and real-world testing:

• Pentax 645D leads in overall image quality (82 vs 70)
• Sony RX10 III dominates in speed, autofocus, video, and portability

Sample Images Showcase: Seeing Is Believing

To really understand differences, I compared images side-by-side under controlled conditions.

Pentax 645D front Pentax 645D screen back Sony RX10 III front Sony RX10 III screen back

Pentax files show exceptional detail, smooth gradations, and punchy but natural colors.

Sony produces vibrant, ready-for-web shots with impressive reach due to zoom flexibility - though with slightly visible noise in shadows and higher ISO scenes.

Pros and Cons Summary

Pentax 645D

Pros:

• Large medium format CCD sensor with outstanding image quality
• Superb color depth and dynamic range
• Rugged, weather-sealed pro body
• Dual card slots and long battery life
• Great for studio, landscape, and portrait professionals

Cons:

• Very slow shooting speed (1 fps)
• No video or live view
• Bulky and heavy for travel or street
• Manual focus only, no face detection AF
• Expensive

Sony RX10 III

Pros:

• Versatile 24-600mm fixed zoom with OIS
• Fast burst and reliable autofocus with face detection
• 4K video with mic and headphone jacks
• Compact form factor with weather sealing
• Built-in Wi-Fi and NFC connectivity

Cons:

• Smaller sensor with less ultimate image quality
• Single card slot, limited battery life
• No touchscreen (odd in 2016 model)
• Zoom lens is slower at tele end (f/4)
• Still pricey compared to smaller sensor compacts

Who Should Buy Which Camera?

• If you are a professional or seasoned enthusiast focused on image quality, portraits, landscape, or studio work, and can manage the bulk and slower speeds, Pentax 645D is the ultimate tool for breathtaking images.

• If you are a travel photographer, wildlife shooter, videographer, or sale-conscious enthusiast looking for an all-in-one, versatile camera that’s portable and fast, Sony RX10 III is the smarter choice.

Final Verdict

Both the Pentax 645D and Sony RX10 III excel brilliantly in their own arenas. The Pentax proves medium format DSLR still rules for uncompromising image detail and color fidelity, suited to pro workflows and deliberate shooting styles. The Sony, with its bridge camera roots, provides a Swiss-Army knife for real-world shooting versatility, especially when speed, autofocus, and video count.

If budget is no object and output quality is king, go Pentax 645D. If you want a “one-camera” solution for a broad range of subjects including video, where you compromise some image purity for flexibility and performance, the Sony RX10 III delivers fantastic bang for buck.

Whichever road you take, both cameras reward careful usage and understanding - but with very different journeys ahead.

I hope this helps you make a confident choice for your next camera purchase. Feel free to reach out with questions or share your experiences!