View Full Version : First post: LF to 35mm focal lenght conversion

soren k jensen
7-Jan-2007, 12:09
Dear users, I hope someone may be able to help with my first post here:

I do video filming in High Definition of children in daycare, schools etc., for educational purposes. I experiment a lot with the storytelling of the films, and this includes mounting all sorts of 35mm lenses, loupes, lensbabies and old lenses on my video camera via an adapter. So maybe you may help me with this question: I am about to find a few LF lenses to use, and I need a large image circle, as extreme tilting etc. is the purpose. Is there a formula that I can use for calculating the equivalent focal length of a LF lens, onto a 35mm film adapter? (My adapter projects 35mm Nikkors on a moving ground glass, that the video camera then films via its normal lens. Cumbersome but fun).

Thanks ,
Søren Kjær Jensen
Development Counsellor,
Learning Lab Denmark
The Danish University of Education

7-Jan-2007, 14:27
If I understood the set up correctly, the image size is equivalent to a 35mm frame (24x36mm)? If so, no conversion is necessary. Say, a 150mm LF lens works the same as a 150mm Nikkor perspective-wise. The conversion ratio comes into play when you expand the viewable area of the projected image outside the 35mm frame at a given focal length.

If it's different from a 35mm frame, the conversion ratio depends on the image size (angle of view). There are more than one answers if the aspect ratio is different (compare diagonal, long side, short side, etc.). An example of conversion chart is here: http://www.viewcamera.com/images/focalchart.gif
Read the "Formats" at the top as the image size on your ground glass.

7-Jan-2007, 15:20
Multiply the LF focal length by 0.3 to get the same horizontal coverage as a 35mm lens. Multiply it by 0.25 (or divide by 4) to get the same vertical coverage. Using these numbers, you will actually get everything on the LF negative that would be on the 35mm negative, plus a little extra due to the different shape of the negatives.

7-Jan-2007, 15:24
Hi Soren,

Here is another table that may be of assistance to you in comparing focal length of different formats with 35mm:



David A. Goldfarb
7-Jan-2007, 15:46
If I understand you correctly, you want to use LF lenses on a 35mm camera or a video camera via a simple mechanical adaptor that has no additional optics. If this is the case, there is no conversion factor.

The focal length of a LF 100mm lens is the same as the focal length of a 100mm medium format lens is the same as the focal length of a 100mm lens for 35mm or super-8 or any other format. The difference between them is mainly the size of the image circle (and of course there may be other differences in optical quality, etc.).

C. D. Keth
7-Jan-2007, 15:50
I'm a filmmaker, I know exactly the kind of adapter you mean. There is no conversion. A 150mm large format lens will be a 150mm lens on your redrock, mini35, or whatever. I would steer away from the idea, though. With a large format lens on the adapter, the whole lens will have to move in relation to the adapter groundglass in order to focus.

Doug Kerr
7-Jan-2007, 17:42
Hi, Soren,

The fact that the lens is an "LF" lens - that is, what format size camera it may have been intended for use on - has no effect on the sigificance of its focal length, which is stated in "format-independent" terms. Any 150 mm lens will give the same field of view when used in connection with a certain format size (such as 36 mm x 24 mm), and all the other inmplications of focal length (such as on depth of field) will also be the same.

If a 150 mm lens of whataver ancestry is used on a camera with a format size of 36 mm x 24 mm (as you describe), the "full-frame 35-mm equivalent focal length" (used to describe the vield of view it yields on this camera) will be 150 mm).

Best regards,


soren k jensen
8-Jan-2007, 00:28
Thanks to all of you! Nice to find such a friendly and knowledgeable forum.

Just to let you know why I attempt such a silly thing: I already now focus a Lensbaby by hand, while filming little children's point of view. I want to be able to do extreme shifts and tilts, with a better lens and one with a large image circle as well. The idea is to mimick or represent the way they see the world. And to search for different looks. So even thouh it is crazy to focus a great LF lens like that, there is actually a point to it... somewhere ;-)
I have built my own bellows with different loupes and bad old lenses, to achieve different forms of expression. Even though it is cumbersome to hold and focus these thingys by hand, while the camera is recording, it actually looks great... sometimes.

Kind regards, Soren