Objects Appear Actual/Life Size - At What Focal Length?

[Joshua]

I've noticed a property of optics that I don't really know what to call, and not sure how it would be useful, so I hope the community can help me out.

At a certain focal length, somewhere between 50 and 80mm (probably about 70, though it's hard to tell), objects through the lens appear the same size through the lens as they do in real life. They are not magnified, nor are they reduced in size; they are 1:1 I suppose. This does not change if I get closer to the objects or farther away; they remain approximately the same size through the lens of the camera as they do when I look directly at them.

Let me further explain that I tried this with two cameras at approximately the same focal length: a Nikon D5000 with a crop factor of 1.5 and a 35mm format Nikon film camera. Through the 35mm Nikon showed more surrounding objects, as it should, however the relative size of the objects remained the same.

I just want to know what this is called, or if it has a name. I thought a 1:1 reproduction ratio was when something was equally reproduced relative to the film plane, thus an object 24mm tall would vertically fill a 35mm film frame completely. Thank you for any information.

[Dave Humphries]

Hi Joshua, and welcome to the CiC forums.

You are correct that the "1:1" term is more commonly applied to the macro situation where, as you say, for a full frame camera, an object 24mm tall fills the landscape frame.

The other concept you are discussing is the so called "standard" focal length - it is actually more to do with the angle of view (of eye and lens). Traditionally, when I grew up with film, it was 50mm for a film SLR.

In theory, on a 1.5 crop factor camera, it should be 35mm to get the same angle of view.
However, many things can affect our perception, for example; whether the observer wears glasses (or contact lenses, I guess), also viewfinders have a magnification factor. However, you did see a difference (more items around the edges on 35mm FF camera)

The relative sizes of objects is a perspective thing - and that won't alter with focal length, only if you move your viewpoint - typically to compensate for the 'more things around the edges'.

Well, that's my thoughts on it, anyone else got any ideas?

Cheers,

[Peter Ryan]

Hi Joshua and welcome to CiC. Can you please ask an easier question to start with?

I will have a go at this and I am sure others will contribute and/or correct any misinformation I may give.

The human eye has a ‘field of view’ around 43 degrees and the 50mm lens has been coined as being equivalent to the perspective a human eye observes. On the smaller APS-C sensor with 1.5 crop this equates to around a 35m focal length lens. You will not notice any difference in the size of the subject for 35 m film or the cropped sensor, as you say you lose some peripheral data with the tighter crop factor but the magnification is still the same.

When I look through the viewfinder I see the image with the same perspective and scaling as I do when I take the camera away from eye, which will be around 1:1 for that focal length. But this is in the viewing of the object not the printing.

When printed it is scaled proportionality to fit a 35 m format. This is more easily seen when you look through the viewfinder at a bird and it appears life size but when printed it is scaled down.

The true definition of a macro lens (as opposed to one that just provides for close focusing) is one that reproduces the subject life size (1:1) in print.

So there are two different effects here one being viewing the subject through the viewfinder and the other being something totally different – the output. I think you are trying to join them together somehow.