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# Thread: Lens equivalancy to 35mm

1. ## Lens equivalancy to 35mm

I heard a while ago that if you take the lens and divide by 3, you get the 35mm equivalent. I have used this as my rule of thumb for quite some time. However, I was just looking at the 300mm lens I currently have mounted on my 8x10, and IMHO it doesn't look like a 100mm zoom on a 35mm ...

2. I believe the ratio is more like 8.

3. First of all, the factor of 3 is used for 4x5" to 35mm, NOT for 8x10" to 3500m. Double it for 8x10".

Second, the aspect ratio is different enough that the "equivalency" is of limited useability.

Third, at moderate distances the bellows extension needed to focus a LF camera (beyond infinity focus) throws the whole "equivalence" off.

A 300mm lens on 8x10", focussed at infinity, has a focal length slightly shorter than the film diagonal. So would a 40mm lens on a 35mm camera.

4. The secret is to forget about 35 mm equivalence. Learn to see the world via the lenses for your 8x10. With practice it will become intuitive.

5. Exactly what Walter said.

6. Measure the height of the frame in millimeters. Now use that number as the divisor, and the lens FL as the dividend...e.g. 150mm lens on 4x5 is 150 / 90 = 1.66, so this would be equivalent on 135 format to 24 * 1.66 = 40mm FL

This technique is what Sinar uses.

7. There is a great spreadsheet on the ALPA website with many photographic calculations including 35mm [or other] equivalent lens.

If I could only afford their cameras...

8. As ever – nothing is straight forward in LF

There is no good hard lens equivalence between 35mm & LF

The aspect ration of 35mm is 3:2, with LF its 5:4

Get the vertical equivalence right and the horizontal is off & visa versa

There are several ways to do it:-

Guess - it’s somewhere between 3 & 3.6 at infinity compared to 5x4 (double that for 10x8) but the ratio goes out of the window as you rack out the bellows.

Experiment - find a friend who has a bunch of lenses and try them out for size to find some that suit you

Scale up or down from a Lens you already own in that format (the ratio of 1.5 to 2:1 seems a reasonable spread in LF – you can then go back and fill in the gaps as your needs require)

Good luck

Martin

9. Just use the rough rule of thumb Ole & others mention 3x for 5x4 & 6x for 10x8 formats.

As Brad & Walter say it's not really that relevant it's how you see with that format.

Ian

10. Get yourself a piece of dark colored cardboard and cut a 4" x 5" hole in the center. Glue a piece of string, 12" or more long, to the bottom center of the card. In other words, make yourself a viewing card. For 4x5, the distance from your dominant eye (measured from your cheek is close enough,) to the cardboard is the focal length of the lens which would reproduce the view. Double the measurement for the equivalent 8x10 lens. Now you can walk around and expose "air film" all day, and in the process learn what focal length lenses your vision gravitates toward.

Mike

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