35mm-equivalent focal lengths for Kiev 88 lenses

from Shutter, by Lewis Collard

	More or less	6x6 (W)	6x6 (H)	6x6 (avg)	6x6 (diag)	6x4.5 (W)	6x4.5 (H)	6x4.5 (avg)	6x4.5 (diag)
Arsat 30mm f/3.5	18mm	19mm	12mm	15mm	16mm	19mm	17mm	18mm	18mm
Mir-26 45mm f/3.5	28mm	28mm	19mm	23mm	24mm	28mm	25mm	27mm	27mm
MC PCS Arsat 55mm f/4.5	35mm	35mm	23mm	28mm	30mm	35mm	31mm	33mm	34mm
Mir-38 65mm f/3.5	40mm	41mm	27mm	33mm	35mm	41mm	37mm	39mm	40mm
Arsat 80mm f/2.8	50mm	51mm	34mm	41mm	43mm	51mm	45mm	48mm	49mm
Vega 120mm f/2.8	75mm	77mm	51mm	61mm	65mm	77mm	68mm	72mm	74mm
Kaleinar-3 150mm f/2.8	90mm	96mm	64mm	77mm	81mm	96mm	85mm	90mm	92mm
Telear-5 250mm f/5.6 Jupiter-36 250mm f/3.5	150mm	160mm	107mm	128mm	136mm	160mm	142mm	151mm	154mm
Tayir-33 300mm f/4.5-22	180mm	192mm	128mm	154mm	163mm	192mm	171mm	181mm	185mm
MC APO Arsat 500mm f/5.6	300mm	321mm	214mm	257mm	273mm	321mm	285mm	302mm	309mm

What this all means

There's more than one way to work out the 35mm equivalent of lenses on medium format cameras, which usually use a different aspect ratio to 35mm film.

Method 1: divide the width of the 120 frame by the width of the 35mm frame, then divide the medium-format focal length by the resulting number. ("W" in the table above.)
Method 2: do the same as above, but use the height of the two frames. ("H" in the table above.)
Method 3: divide the medium-format focal length by the average of the magic numbers calculated above, or average the two resulting numbers from the above. ("Avg" in the table above.)
Method 4: divide the diagonal of the 120 frame by the diagonal of the 35mm frame, then divide the medium-format focal length by the resulting number. ("Diag" in the table above.)
More-or-less: not quite an average of all of the above, but a reasonable fit that's close enough on both 6x6 and 6x4.5 to a 35mm focal length that is actually used.

None of these methods are objectively correct. Depending on what you're shooting, one could make a case for using a different one for each photograph you take. (The diagonal and average numbers are close enough that you won't worry about the difference.)

Another way would be to calculate the diagonal angle of view, then work out what lens would be required to do this on 35mm format. I didn't.

Any of these methods will give identical results for comparing formats with identical aspect ratios, such as 35mm film vs 24x16mm digital SLR sensors, or 6x9 medium format.

Calculating it for other lenses

I've included all the lenses in the Kiev 88 system that I know of. If you want to work it out for some other medium-format lens (if you're fitting a Pentacon Six mount lens to a modified Kiev 88, for example), then divide the real focal length by one of these numbers:

6x6 (W): 1.56
6x6 (H): 2.33
6x6 (avg): 1.94
6x6 (diag): 1.83
6x4.5 (W): 1.56
6x4.5 (H): 1.75
6x4.5 (avg): 1.65
6x4.5 (diag): 1.62

I generated most of the table using a Python script I knocked up in five minutes. Do anything you like with this.

The calculations assume a 645 frame is exactly 42mm high. It's actually 41.5mm, but 42 is close enough, and is also a much better number.

Alternative method

TRA of the great Pentacon Six website has an even better method: Divide by two and add about 10%.

Acknowledgements

I got a list of Kiev lenses from the excellent Kievaholic site.