Image size expansion with format size, not with DX to FX?

By gosh, I realized something today that I've been mistaken about all this time. I thought that a given focal length lens produced the same image size of, let's say, a cat, whether it was attached to a full-frame (FX, 35mmx24mm) or a medium format (MF, 6cmx7cm for instance) system. This I had worked out by placing a medium format lens in front of my DSLR, and comparing images of the same cat taken with a native FX lens of same focal length (these types of fool experiments were not possible before digital!).

 

I was mistaken. Today, playing around with medium format (MF) and full-frame digital (FX) cameras with their respective lenses, I realized that the MF lenses I use (6x8 cm) are much farther out from the plane of the film, and project an image of almost double the size. But if you try to use the MF lens on the FX body (just hold it in front, with a tube if necessary to cut out side light) you'll have to bring the back of the lens closer in, thereby shrinking the image to just the same as a native FX lens would throw.

 

Therefore, a larger format does really give a bigger image, whether it be bird, beast, or relative. This doesn't work between FX and DX (24mmx18mm) on the same camera make though, as sensor-to-back-of lens distance remains the same; there is no magnification for a given focal length just by changing from a DX lens to an FX lens.

 

People ought not to talk of the close-in advantage of DX compared with FX, no? They could however justify talking of the image-expanding advantage of larger format, provided that means also longer flange-to sensor distance.

 

Is this sounding utterly nonsensical?!

What is this silliness?

 

The distance between a lens' rear node and the film plane when the lens is focused at infinity is the focal length. This is true regardless is the size of the sensitized surface behind the lens. The distance between a lens' rear node and the film plane when the lens is focused at a closer distance is also invariant with respect to the size of the sensitized surface.

By gosh, [...]

Is this sounding utterly nonsensical?!

Yes. Utterly, completely, absolutely. Nonsense.

• The focal length of a lens is a physical property of the lens, which does not depend on the size of the sensor.
  
• A smaller sensor crops the image of that lens, resulting in a smaller field of view. Sometimes the lens is described with an "equivalent" focal length for convenience in describing the field of view with respect to a FF camera.
  
• There is no "effective" f/stop - the f/stop is also a physical property of the lens and is not affected.
  
• There are inexpensive adapters which fit a MF lens to an FX mirrorless camera, which hold the lens at the appropriate distance from the sensor and exclude ambient light.
  
• A "normal" 6x8 cm lens is 110 mm, which will have the same FOV on an FX camera as any other 110 mm lens.
  

This doesn't explain all you ask, but will hopefully induce you to reflect more carefully on the effect of sensor/format size.

Is this sounding utterly nonsensical?!

Yes.

An 80mm lens is always an 80mm lens, whether it's designed for a 35mm camera, a medium format camera or a large format one.

 

And at any given distance from a cat, will project the same sized image of a cat.

 

If you see any difference, it's probably because lens focal lengths are purely nominal (i.e. in name only) and can vary from what's marked on the barrel by a few millimetres.

 

There are also differences with design that cause focus 'breathing'. A unit-focussing lens will effectively grow in focal length as it's focussed closer. While an infernal internal-focussing lens will almost always shrink in effective focal length when close focussed. This effect is especially noticeable with zooms, whose focal-length markings should be taken with a big pinch of salt.

Edited March 12, 2021 by rodeo_joe|1

o_O Amazing. More agreement among the critics brigade than I thought I would ever, ever see, Thanks for the fun, pjdilip. ;)

There is effective f/stop when lenses aren't focused at infinity.

That is, when they are used for close-ups where the difference can be fairly large.

 

And as noted above, it does get interesting when not all the lens element move together.

There are internal focusing lenses, and front element only focusing lenses.

I suspect effective f/stop can be even more interesting for them when not at infinity.

The image size relative to the subject size depends on the focal length of the lens. The longer the focal length, the larger the relative size of the image.

 

The absolute magnification does not depend on the format size, only how much of the image is visible.

 

Many lenses change the focal length during focusing through the use of internal moving lens groups. In general, the focal length is reduced the closer the subject, in order to reduce the distance the lens must be extended. This is particularly true for macro lenses, long focal length lenses, and AF lenses in general. The position of the focusing ring will affect the magnification if you focus by physically moving the entire lens, as with an extension tube with a focusing helix between the lens and camera body.

It's actually a relief that larger format doesn't magnify the image on the film compared to a smaller format. But my 6x8 (cm) body sure fooled me!

That may be because it gives a magnified image in its viewfinder? (Obviously I can't actually develop a film).

Further experimentation using medium format lenses on a digital sensor (FX) body confirms that the magnification remains the same, compared with an FX lens of same focal length. Of course, the larger format lens will project a bigger circle of coverage or field of view, provided the sensor or film is big enough to register it. To accommodate the same field of view on the smaller sensor, one would need a shorter focal length. Hope this is a correct statement!

It is correct.

 

This, by the way, is something that does not need experimental proof. Just try to imagine a reason or way that the image projected by a lens could or would change depending on whatever you put behind the lens.

For Extra

Here is what the marvelous Life Library of Photography had to say about perspective in one of their volumes (The Camera):

....about perspective.....

The takeaway from the above is:

If you want less background for a given subject size, step back and use a longer lens.

 

Conversely; including more background needs a closer viewpoint with a shorter focal-length.

 

Example - that makeshift bedsheet background isn't quite big enough without the edges showing at a wideangle setting. Move further back and zoom in.