# Required Distance to Backlight @ 100mm f/8 **URL:** \ **Category:** Guides and How-Tos\ **Created:** [May 4, 2025, 4:17pm UTC](https://forums.negativelabpro.com/t/required-distance-to-backlight-100mm-f-8/9293 "2025-05-04T16:17:14Z")\ **Posts on this page:** 1\ **Showing post:** 3
**Author:** ![Digitizer](https://avatars.discourse-cdn.com/v4/letter/d/57b2e6/32.png) [@Digitizer](https://forums.negativelabpro.com/u/Digitizer)\ **Post date:** [May 5, 2025, 7:00pm UTC](https://forums.negativelabpro.com/t/required-distance-to-backlight-100mm-f-8/9293/3 "2025-05-05T19:00:47Z")
> [@Harry](#): > > a bit marginal in terms of film to panel distance indeed…and some macro lenses aren’t really what their label says. The Canon EF 100mm f/2.8 Macro has an equivalent FL of about 75 mm when set to 1:1. For scanning 645 negatives, the lens is at around 86mm. Read about it here: > [@Equivalent Focal Length And DOF @ Scanning Distances](https://forums.negativelabpro.com/t/equivalent-focal-length-and-dof-scanning-distances/1982): > > All the DOF calculators I looked at were - and probably still are - based on the laws of optics of an ideal, infinitely thin lens. Within this theoretical model and the cases we’re talking about in this part of the forum, the distance between sensor and film should be 400mm when a 100mm lens is used to capture e negative with a scale of 1:1. The usual camera lens is everything but infinitely short, which suggests that calculations based on theoretical models might still apply, but in a complete… --- _[View the full topic](https://forums.negativelabpro.com/t/required-distance-to-backlight-100mm-f-8/9293)._