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On my EM-10 Mk2, I'm testing a Panasonic 25mm f1.8 (top image) against the 20mm f1.7 (bottom image).

I tried to make the lamp take up the same space in the frame before focusing, yet the background in the case of the 25mm is much larger and blurrier than the 20mm, which I actually prefer.

Since the aperture is nearly the same size between the lenses, is this simply a function of focal distance? Any explanation about this would be very much appreciated!

enter image description here

enter image description here

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    That’s a textbook example of field of view. – JDługosz Oct 30 '16 at 5:33
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In the case of the second shot (20mm focal length), the camera was positioned closer to the lamp, thus making it appear bigger than the background.

As the camera moves further away from the subject, the distance between the subject and the background becomes smaller relative to the distance between the camera and the background, making the subject appear more and more closer to the background.

  • Thanks. So in other words, it's entirely a function of focal length? It's an interesting consequence I hadn't thought about if so... So shooting further away from the subject with the 20mm will eventually produce the same subject-to-background size ratio as the 25mm closer up... – zakgottlieb Oct 29 '16 at 13:02
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    Teoretically, it's a function of the distance between camera - subject - background. It just so happens that using a longer focal length forces you to distance yourself more in order to fill the frame with the subject. – D. Jurcau Oct 29 '16 at 13:49
  • I thought it's about the "relative closeness" to the subject... keeping the distance between the camera and subject the same, with longer FL you are relatively closer to the subject than with shorter FLs. And this is the reason alone why smaller sensors (with larger crop factors) produce "less bokeh"... because the lenses are usually of shorter FL to balance out the crop factor. – unperson325680 Oct 29 '16 at 14:54
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    @progo If you keep the distance from the camera to the subject the same the perspective will be identical in both photos. The only thing that changes is the FoV. If you crop the wider lens to match the FoV of the longer lens the perspective and FoV will be identical. To change the relationship of the subject size to background size the camera must be moved no matter which lens is being used. The amount of bokeh will change depending on absolute aperture size (the actual diameter of the entrance pupil, not the f-number). For the same f-number the AA is larger for the longer lens. – Michael C Oct 30 '16 at 20:01
  • (cont.) But one could stop down the longer lens to produce the same amount of blur as the wider lens. If both are shot from the same position and one used f/4 with the 20mm lens (AA of 5mm), one would get the same amount of bokeh at f/5 (AA of 5mm) with a 25mm lens. Assuming the shapes of the aperture openings in both lenses are identical the bokeh would also be the same. Note that exposure would change due to the difference in f-number and would need to be compensated using Tv or ISO.. – Michael C Oct 30 '16 at 20:08
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The easiest way to think of this is with an image. When you use a wider lens, you have to be closer to your subject, which emphasizes the distance between the subject and background by making the background smaller.

Perspective Wide angle

In contrast, if the camera is far away from the subject, you'll have to zoom much farther to get the same size of subject relative to the frame. Think of it as narrowing your frame of view. What this does, however, is show a smaller part of the background. That in turn makes the background appear larger (or closer) relative to the subject.

Perspective zoom

The difference technically is explained quite well by the accepted answer to the question, Is there a difference between taking a far shot on a 50mm lens and a close shot on a 35mm lens?

  • Just use the word "angle" :-) . People seem to have this hangup about linear metrics. – Carl Witthoft Oct 31 '16 at 11:23
  • @Carl Where? Feel free to edit this if something isn't clear :) – Cullub Oct 31 '16 at 16:16
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The differences between the two images are caused by what is known as perspective. The basic idea about perspective is that as the camera position is changed the angles and relative distances between various objects in the scene will change as well. Focal length only comes into play because we use a different focal length to make a subject the same size in the photo when we have changed the subject distance or conversely we change the subject distance to accommodate the difference in focal length. If you use a wider lens and just crop the result it has the same effect as using a longer focal length when both lenses are used from the same position.

If you have to move further back with the 25mm lens to make the lamp the same size in the frame as it was with the 20mm lens then you've changed the distance to the subject by increasing it 25%. But you haven't increased the distance to the background by 25%, because it was further away than the lamp to begin with. So the same increase in feet is a lower percentage of the distance to the wall in the background than it was as a percentage of the distance to the lamp.

Let's say for simplicity's sake the lamp was four feet from the camera and the wall was 16 feet behind the lamp and thus 20 feet from the camera when the shot was taken with the 20mm lens. Now with the 25mm lens the distance to the lamp is five feet, an increase of 25% which perfectly offsets the 25% increase in magnification of the 25mm versus the 20mm lens. But the distance from the lamp to the wall has not changed, it remains 16 foot. This means the distance to the wall from the camera has only increased from 20 to 21 feet, an increase of only 5%. Because we used a lens that increased magnification by 25% and only increased the distance to the wall by 5%, the wall will be magnified more in the second image and will thus appear larger. To make the wall the same size in the image using the 25mm lens as in the image shot with the 20mm lens the distance between the lamp and wall would need to be increased from 16 to 20 feet which would thus increase the distance from the camera to the wall from 20 (4+16) to 25 (5+20) feet or 25%.

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The amount of background blur is determined by the absolute aperture size, not the relative f-number.

Your 25mm lens at f/1.8 has an aperture size of 13.9mm.
Your 20mm lens at f/1.7 has an aperture size of 11.8mm.

Hence the first case has a larger physical aperture and thus more background blur.

More detail can be found in my lengthy article: https://www.nayuki.io/page/absolute-and-relative-lens-apertures

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