Accurately preview projector throw and placement on your phone with "Magic Universal Viewfinder"

Magic Universal Viewfinder is a smartphone app which lets you shoot with simulated lenses on your phone. It is useful for location scouting and previewing framing for shots.

It is also useful for simulating projector throw; with the right settings in the app, it can simulate a specific projector model, and you can use it decide on projector model and placement to get the coverage you need.

The tricky bit is to convert a projector's throw angle into a full-frame equivalent lens focal length for use in the app. The focal length of a lens and the throw ratio of a projector are conveying the same information; how wide an area in front of the lens will be in the frame. But the focal length and throw ratio are described in completely different ways, so you have to correctly convert between them.

Know your projectors

You can look up projector throw ratio on the web, e.g. projectorcentral.com - the site's calculator is useful for situations when you know the exact measurements of the space you are working in, but the app is more useful when visiting a site to decide what is possible, or exploring unconventional projection.

For instance, my Optoma EH465 has maximum (distance:width) throw ratio of 2.09 - this means that if the projector is 2.09m away from the projection surface, the image will be 1m wide. The zoom lens allows a wide range of throw ratio, but this maximum value is what you get when you "zoom out" to get the smallest possible image, which is appropriate if the projector is going to be placed far away from your projection surface.

Convert throw ratio to focal length

You can find the full-frame equivalent focal length of your projector lens by multiplying the throw ratio by 36mm.

2.09 * 36mm gives me a focal length of 75.2mm

In the app

In the Magic Universal Viewfinder app, click the upper right hand corner, select "camera", and select "Custom sensor 36x24" at the bottom - this is a generic full frame sensor, it is important to match the "full frame" we used in the calculation above otherwise you won't get the right results.

Now I scroll the wheel on the right side until I find 75mm, then long-press to lock the wheel.

Now the orange frame in the viewfinder shows exactly the coverage my projector would have, if it was placed where the phone is.

If I repeat the calculation for the "zoomed in" end of this projector's zoom range, I find that it has a range of 50-75mm. Now I can fully explore all possible uses of this projector in the app.

Confirm it

Set up your projector, place your phone right next to the lens, and verify that the orange frame in the app matches the projected image frame. If it is off, you might need to calibrate the app to work with the camera in your android phone (all iPhone models, and major Android models, should work out of the box).

Testing my Acer H6510BD, the fit was perfect... I actually moved the phone back a few inches to get both top corners into the frame for demonstration purposes.

Use it

Now you can scout sites with just your phone. Calculate and note the full-frame focal length equivalents of your projectors, and you can preview exactly what coverage you get from different placement of different projectors.

For short-throw projectors like this 0.5 throw ratio, your phone's camera lens might not be wide enough to capture the entire projected frame... but the app helpfully shrinks down the live camera preview to give you a sense of how much larger the projected frame will be.

And of course you can use it the other way too... from your desired projector location, use the zoom in the app to set the orange frame to match your desired projection coverage, then take the focal length mm shown onscreen and use the table below to look up what throw ratio you'd need.

Cheat sheet

Look up your projector's throw ratio, then use these numbers in the app to quickly approximate it.

This is simply a table showing throw ratios multiplied by 36mm, so you can do the math yourself - but in the real world you can't expect more than 2 significant digits of precision.

Throw ratio (distance:width) - Full frame equivalent focal length (rounded to nearest mm)

0.5 - 18mm

0.6 - 21mm

0.7 - 25mm

0.8 - 29mm

0.9 - 32mm

1.0 - 36mm

1.1 - 40mm

1.2 - 43mm

1.3 - 47mm

1.4 - 50mm

1.5 - 54mm

1.6 - 58mm

1.7 - 61mm

1.8 - 65mm

1.9 - 68mm

2.0 - 72mm

2.1 - 76mm

2.2 - 79mm

2.3 - 83mm

2.4 - 86mm

2.5 - 90mm