Focus through a magnifying glass

Dan McCormick · January 24, 2008

Hi

I'm about to work on a student film that has a couple of shots through a magnifying glass, both times focussing on the object behind it. The object in focus may move, as will the camera on a track or jib.

My main concerns are the following, so if anyone could help or knows of something I haven't thought of any advice would be great: Does anything change when focusing through a magnifying glass for measuring the initial point? If the point of focus needs to move can it still be measured normally? If the magnifying glass moves in relation to the camera will a pull be needed? Can I rely on the operator being able to see through the viewfinder if the image is sharp?

Thanks

Dan McCormick

Jonathan Bowerbank · January 24, 2008

If what's being magnified in the magnifying lens is the point of interest, I do believe your focus mark has to be set to where he magnifying glass is, and specifically at the point of the refracted image...which would be the side of the glass facing camera.

Sounds like an interesting shot. Will you be dealing with an extremely shallow depth of field on this one?

Dominic Case · January 24, 2008

A magnifying glass works by slightly converging light rays, in the same way as a supplementary close-up lens. So the image actually appears to be (and for the purposes of focussing, it is) further away as well as larger. That is how you can look at a small object such as a jewel or a stamp very close to your eye and still see it in focus.

There is a formula (which I can't bring to mind at the moment, but Google would probably find for you) that will tell you how much further away the image is - and yes, if you move the magnifying glass nearer or further from the object, the image will indeed move and you would have to pull focus.

John Sprung · January 24, 2008

A magnifying glass is a simple lens, just like a diopter. Shooting with a magnifying glass in the shot is very much like using a split diopter. Take your magnifying glass outside on a sunny day, and focus the sun on some preferably incombustible object. Measure the distance from the lens to the object in meters. One divided by that distance is the diopter power of the magnifying glass lens.

You can use diopter tables for ballpark estimates, but things get a little more tricky when you move the diopter far from the taking lens. There must be formulas for that kind of thing, but it's probably not going to be practical to try to apply them on the set. You'd have to know a lot of variables, and measure them accurately. In particular, many camera lens focal lengths are actually quite far from the nominal length engraved on them. A 50 mm may really be 47.2 or 52.6 or something like that.

Bottom line, you're better off trusting what you see in the finder. Set your marks by eye with the taking lens wide open, then light and shoot a fairly deep stop to cover your tush. A wide angle helps, too.

-- J.S.

Jonathan Bowerbank · January 24, 2008

I suppose it does vary, very much by how close the glass is to the front of your camera lens. If you're shooting with a zoom lens, I'd zoom in for critical focus then back out to your preferred focal length, just to be safe.

Dominic Case · January 25, 2008

For what it's worth, there IS a formula. It's the old one for any simple lens: 1/u + 1/v = 1/f.

WE can restate that as v=(uf)/(u-f)

SO how does that help?

u = the object distance - that is, how far the magnifying glass is away from the object of interest.

v = the image distance - in this case, it is how far the object appears to be away from the magnifying glass you are looking (or shooting) through. [notince that the distance from the camera lens is independent of what the magnifying glass does - or vice versa: but your focus setting will be the distance of camera lens to magnifying glass, PLUS the value of v).

f = the focal length of the magnifying glass - use John Sprung's method of focussing the sun to find this out.

In normal imaging, the object distance is greater than the focal length, and the image is formed behond the lens. This is called a camera ;) - u, v and f are all positive numbers.

If the lens is a magnifying glass, then the object distance has to be less than the focal length. The formula will give you a negative result for v - that simply means that the image is on the same side of the lens as the object. It's known as a virtual image.

The formula is a bit of a bugger to use on set, and you may be asked to go away and not come back till you are feeling a bit more sensible. But I worked out a couple of rules of thumb in a simple spreadsheet:

Whatever the focal length of the magnifying glass is, take that as your starting point.

If the object is 1/10 that distance behind the lens, it will appear 1/9 that distance.

If the object is 1/4 that distance behind the lens, it will appear 1/3 that distance.

If the object is 1/2 that distance behind the lens, it will appear 1x that distance (i.e. at the focal point).

If the object is 0.6 times that distance behind the lens, it will appear 1.5 x that distance.

If the object is 0.75 times that distance behind the lens, it will appear 3 x that distance.

If the object is 0.9 times that distance behind the lens, it will appear 9 x that distance.

And finally, if the object is exactly at the focal point of the lens, it will appear at infinity - any further and you won't be able to see an image at all.

Dan McCormick · January 25, 2008

Thanks guys, glad I'm not the focus puller on this one though!

John Sprung · January 25, 2008

Another thought -- Looking at the diopter tables and Dominic's calculations, it's probably easier to work with weaker magnifying glasses -- longer focal lengths, lower diopter numbers. So, buy yourself a few to choose from, and especially look for low power ones.

-- J.S.

Dominic Case · January 27, 2008

I said:-

And finally, if the object is exactly at the focal point of the lens, it will appear at infinity - any further and you won't be able to see an image at all.

Actually of course I've realised that is wrong. Looking at a distant object through a magnifying glass is the same as looking at a distant object through reading glasses. You'll see an image, but won't be able to focus it - it will be blurry, as it is actually focussed beyond infinity. In theory there would be a real image in focus somewhere behind your head, and in practice you can see it if you move far enough back from the lens - when you will see it, upside down.