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12-03-2020, 05:27 AM - 20 Likes   #1
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Focal length doesn't affect subject compression - demonstrated

In another thread of mine (related to the benefits of APS-C vs full frame), the subject of "lens compression" came up, suggesting that if a 56mm lens is used on an APS-C camera versus an 85mm lens on full frame, the 85mm full frame setup might produce more subject compression (a desirable flattening of features in portaiture, for example). Some of us know this to be incorrect... we know that "lens compression" is a fallacy; but it's a common misunderstanding that I myself held when I was less experienced, until I was educated by more knowledgeable members here.

For the benefit of anyone still confused by this issue, I've carried out a quick test to prove that focal length does not affect subject compression ("perspective distortion")...

This image was shot handheld with my K-3 and Tamron 28-75mm f/2.8 lens, using a focal length of 75mm:



This next image was shot from the same distance, using the same camera and lens, at a focal length of 28mm:



Here's a crop of the 28mm image:



Note that the subject compression is the same. However, since both the 75mm and 28mm images were shot at the same f/3.2 aperture, the depth-of-field is much shallower on the 75mm shot (as we would expect).

Here's a side-by-side comparison of the 75mm shot (left) versus the cropped 28mm shot (right), as it's easier to view this way:



Conclusion - focal length does not affect subject compression. It's the act of moving closer to, or further away from, the subject that reduces or increases compression respectively. No matter what format of sensor and focal length of lens is used, if you shoot your subject from the same distance, the perspective distortion will be the same.

Shooting a portrait from 5 metres using either a full frame camera and 85mm lens, or an APS-C (1.5x crop) camera and a 56mm lens (or thereabouts), will give you the same subject compression and a very similar field of view... but, depending on the maximum apertures of the lenses, the 85mm on full frame will likely be able to achieve shallower depth of field if required.


Last edited by BigMackCam; 12-03-2020 at 05:09 PM.
12-03-2020, 05:35 AM - 1 Like   #2
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Thanks for that demonstration.
12-03-2020, 05:49 AM - 4 Likes   #3
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Nice demonstration Mike

I had this explained to me once in these terms...... If you have a subject and a background item (the receiver and red thing in your example), the distance between them can be thought of as a ratio to the distance between the subject and the camera. The further you move away from the subject the less that ratio becomes, and both objects appear to be closer to each other in a picture.

The reason why telephotos appear to show more compression than shorter FL lenses is because we are usually standing a lot further away from our subject
12-03-2020, 07:36 AM - 3 Likes   #4
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Nice demo. Shows why the advice sometimes given to "zoom with your feet" when shooting with a fixed focal length is misleading; doing so changes the relationships and relative sizes of objects in the frame.

12-03-2020, 08:46 AM - 1 Like   #5
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QuoteOriginally posted by baro-nite Quote
Shows why the advice sometimes given to "zoom with your feet" when shooting with a fixed focal length is misleading; doing so changes the relationships and relative sizes of objects in the frame.
Certainly. "Zooming with your feet" is not the same thing as "changing the focal length of your lens". Both are useful for different reasons, and whilst it's true that you can often substitute one for the other when the need arises, the resulting shots from each approach will be quite different...

Last edited by BigMackCam; 12-03-2020 at 10:00 AM.
12-03-2020, 09:15 AM - 1 Like   #6
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Mike, you could do a follow up demo showing that a longer focal length lens does not give you shallower depth of field when the subject size in the frame stays constant (ie you step further back with the longer lens)
12-03-2020, 09:54 AM - 2 Likes   #7
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QuoteOriginally posted by BigMackCam Quote
Conclusion - focal length does not affect subject compression.
Thanks for this post. I did a similar thread with a set of comparison shots back in my early days on this site as well as another thread exploring the focal length vs. DOF fallacy. Both were an exercise in personal revelation as I realized how much magical thinking dominated my approach to photography.

QuoteOriginally posted by baro-nite Quote
Shows why the advice sometimes given to "zoom with your feet" when shooting with a fixed focal length is misleading; doing so changes the relationships and relative sizes of objects in the frame.
Just as moving closer changes a human viewer's perspective (both from the perceptive and composition sense of the term), the camera's perspective (composition sense) on the subject changes as well.


Steve

12-03-2020, 09:57 AM   #8
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QuoteOriginally posted by stevebrot Quote
Thanks for this post. I did a similar thread with a set of comparison shots back in my early days on this site as well as another thread exploring the focal length vs. DOF fallacy. Both were an exercise in personal revelation as I realized how much magical thinking dominated my approach to photography.
It's I who should thank you, Steve, as I'm pretty certain you were one of the members who corrected my misunderstanding some years ago
12-03-2020, 10:14 AM - 3 Likes   #9
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QuoteOriginally posted by BigMackCam Quote
It's I who should thank you, Steve, as I'm pretty certain you were one of the members who corrected my misunderstanding some years ago


Those were the "good old days" on PF when were all learning at an amazing rate.


Steve
12-03-2020, 10:29 AM - 2 Likes   #10
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I'm another one who learned from @stevebrot that telephoto compression isn't.

Thanks @BigMackCam for the demonstration.
12-03-2020, 12:25 PM - 1 Like   #11
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Nice demo. Nice shack. 73.
12-03-2020, 01:32 PM - 1 Like   #12
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QuoteOriginally posted by ProfessorBuzz Quote
Nice shack. 73.
Ditto

W8MIF
12-03-2020, 01:46 PM   #13
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QuoteOriginally posted by ProfessorBuzz Quote
Nice demo. Nice shack. 73.
QuoteOriginally posted by AstroDave Quote
Ditto

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Thank you, gentlemen 73 DE M0CQG in the group
12-03-2020, 02:00 PM - 1 Like   #14
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I did a similar thing years and years ago with 24, 35, 50, 105 and 200mm lenses (Pentax, M42) and shot the same view on Kodachrome from the same position for each lens – a picturesque harbour scene with a big red bollard in centre frame. The results "zoomed in" from 24mm on, but if you cropped each image to show the same area seen through the 200mm lens, then the result was the same image exactly with the same perspective.


When these images were shown at the camera club everyone nodded in understanding, but what most didn't get was the second set where the big red bollard was framed to the same size for each image (to fit exactly in the microprism circle in viewfinder). To do that meant moving position for every other lens, so the perspective changed hugely from 24 to 200mm.
12-03-2020, 02:27 PM   #15
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For me these two images (75 mm uncropped and 28 mm cropped) are different enough, I could not agree with your conclusion.
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