I'm new to medium format and have a question. Can someone tell me what is the equivalent or close 35mm leses are for a 645 55mm, 145mm, and 210mm? I know that the 80mm is the normal for the 645.

Given a 50mm is 'normal' for 35mm: 50 / 80 = 0.625 Thus: 55 x 0.625 = 35 145 x 0.626 - 90 210 x 0.625 = 131

This is going to get messy Actually, 645 format, 72mm is the image diagonal and computed normal lens, 75mm is the closest standard lens. For 6x6 format, 79mm is the diagonal and computed normal lens, 80mm is the standard normal lens. For 35mm (24x36), 43mm is the diagonal and computed normal lens, but 50mm is the generally accepted normal lens. FYI: For 35mm film cameras (24x36 format), the "computed" normal lens is 43mm. A^2 + B^2 = C^2, 24^2 + 36 ^2 = 1872, C = SQRT of 1872 = 43 50mm is/was the generally accepted normal lens for 35mm. What the manufacturer puts on the camera as "normal" lens can be different than the computed normal lens. The normal lens was different for different manufacturers, for the same format camera. Olympus used a 50mm lens, and Minolta used a 55mm lens. The normal lens can be different for the SAME manufacturer. Example Nikon had 45, 50 and 58mm "normal" lenses. So using the film diagonal as the basis for equivalent lens conversions. 43/72 = 0.60 The equivalents I come up with is close to what sparky's has. With the closest production lens being essentially the same. 55 = 33 > 35mm 145 = 87 > 85 210 = 126 > 135

Cliff Notes version: A 'normal' lens is usually the dimension of opposite corners of the film size / sensor plane. To get even MORE technical, you'll need to consider the aspect ratio of both formats. 35mm is 2:3, while 4x5 is, well, 4:5. So let's use an 8x10 print as a target. It's easy to figure out that with a 4x5 sheet of film, you won't have any cropping needed to make an 8x10. But a 35mm frame isn't the same aspect ratio. To make an 8x10 from a 35mm frame, you MUST crop some of the image out. This means you'll need to lop off 1/6 of the 36mm length of the image, or 6mm. This makes the usable real estate of 35mm film 24mm x 30mm. (1/6 = 0.1667 ....... 35mm x 0.1667 = 6mm ...... 365mm - 6mm = 30mm). So punching 24x30 into ac12's formula above, you get: SQRT(24² + 30²) = 38. The upshot of this is if you're wanting to compare the actual equivalent focal lengths of a print made from both formats, you'll need to take into account the aspect ratio of the prints desired. Thus, a 38mm lens on a 35mm film camera should render the same 8x10 image as a 'standard' 75mm lens on a 645 and cropping the 35mm image to 8x10.

What sparky said about the aspect ratio difference is correct. Which is why I don't really bother with "equivalent" lens when dealing with different film formats. It gets too confusing. What do you match to, the horizontal, vertical or diagonal? I just accept the aspect ratio of that format as the basis to work from. I just use the "normal" lens as a standard reference, and use the magnification of the lens from the "normal" lens. Example a 6x6 has a ratio of 1:1, which is as far from the 35mm ratio of 24:36 or 2:3, as you can get. I just change my thinking to be a 1:1 square, and don't bother thinking aspect ratio difference from 35mm. My 50mm lens is 0.6x of the 80mm normal lens, and my 150mm lens is 1.9x.