Focal length comparison: 645 format vs 35mm format
We all know know that when you use a full-frame lens on a crop-sensor camera, that we can consider there to be a new “effective focal length” of the lens on the crop sensor because the field of view changes. When we now change our composition with the crop-sensor camera to match that of the 35mm camera, we change our own position, we then effectively get thane 1.5x or 1.6x focal length increase. This has been explored in the article: Full-frame vs Crop-sensor comparison : Depth-of-field & perspective. But now what happens when you put a medium-format lens on a 35mm camera? How does the effective focal length change? Specifically, let’s look at 645 vs 35mm focal length lenses. What happens when we put a lens designed for a 645 format camera, on a 35mm camera body.
Of course, when we use a full-frame lens on a crop sensor camera, the actual focal length doesn’t change. However, in the way we have to change position between the FF and crop camera (with the same lens) to get the same framing of our subject (perhaps a portrait), our focal length effectively changes. It is easier to think about it this way during an actual shoot, since it directly affects our composition.
I was curious what would happen with a 645 format lens on a 35mm camera, or perhaps a 6×7 format lens on a 35mm camera. With how easy it is to use older manual focus lenses (of any lens mount) on a Sony mirrorless camera, I thought I would give it a practical try, and see.
For example, if you use a 200mm lens for a 645 camera, the angle of view is similar to that of a 125mm lens on a 35mm camera. You get the same composition as if you used a 125mm lens. With that, you can see there is a 1.6x factor involved. Or a 0.625 factor, depending on which way you look at it.
You may wonder if a 200mm f/4 lens for a 645 camera would magically turn into a 320mm f/4 lens. (200 x 1.6 = 320mm). Or does the 200mm lens, which is similar to a 125mm lens, magically turn into a 200mm lens? (125 x 1.6 = 200mm.)
Already knowing the logical answer, I wanted to confirm it properly with actual photographs to satisfy my curiosity.
I got hold of an old Pentax 200mm f/4 lens, as well as a Pentax 645 200mm f/4 lens. Each of the with the proper lens mount adapter to use on the Sony. I use my Sony A7 to use vintage lenses. As mentioned, the way that Sony (and Fuji) implement manual focus, makes these bodies ideal for use with the older lenses.
I met up with Anastasiya in New York, and took a sequence of comparative photos to see the actual results.
The first image was shot with the Pentax 200mm f/4, and the second image with the Pentax 645 200mm f/4 lens … and they look virtually identical. The same composition; the same perspective; and from what I can see from other images, the same depth-of-field.
Exactly what should have happened, happened. If you use a medium format lens on a 35mm camera – it remains the same, actual focal length. The only difference is that the image shot with the 645 lens was sharper – it is a more modern lens, and we only take the central, sharper portion of the image circle. (You won’t be able to notice the difference on these web-sized images.)
This is anti-climactic, if not slightly disappointing though – wouldn’t it have been great to effectively get a superb 300mm f/4 lens buy delving through the medium format lenses on ebay? Unfortunately, we can’t side-step the science here.
The 200mm focal length of the 645 lens is effectively the same as a 125mm lens on a 35mm camera, and when we crop the image by using the 200mm 645 lens on a 35mm camera, it effectively becomes exactly that – a 200mm lens.
BTW, the image at the very top was processed with Aurora software for a mild HDR effect. This is why it looks different in tone than the other two images shown here.
- Full-frame vs Crop-sensor comparison : Depth-of-field & perspective
- More articles on using vintage lenses / classic lenses
645 format focal length vs 35mm lenses
When we calculate the comparative focal lengths between medium format lenses and that for 35mm, the diagonal of the frame / negative is measured. This will give us a sense of the angle of view that the lens covers. However, since the aspect ratio is different between all the formats – 2×3 (for 35mm); 6×4.5; 6×6; 6×7 – there is an approximate focal equivalence when we compare it to a full-frame (35mm) camera.
645 format = 0.625 factor
6 x 6 format = 0.55 factor
6 x 7 format = 0.5 factor,
when converting for an equivalent 35mm focal length.
And for comparison, here are the two Pentax lenses to show their relative size. The optic on the right-hand side is quite petite.
The beauty of the Sony E-mount mirrorless cameras is that with the DSLR mirror box gone, there is the space to adapt any legacy lens to the Sony E-mount. The adapters are usually fairly inexpensive as photography gear goes.
The adapter on the left is the Fotodiox lens adapter for Pentax 645 to Sony E-mount (B&H / Amazon), and has the larger throat for the larger 645 lens. The adapter on the right is the Fotodiox adapter for Pentax K to Sony E-mount (affiliate). It is as simple as that to use pretty much any legacy or vintage lens with the Sony mirrorless cameras.