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Saturday, March 25, 2006

Which Focal Length?

Pursuant to a few of the comments raised about H. C.-B.'s picture below, I wonder if it would be possible to figure out mathematically, or by trial and error, whether it was taken with a 50mm or a 35mm. It seems to me that there's a fairly grown kid (based on the proportion of his head to his body) standing right next to the wall (fourth figure from the right). From this, it would seem to be possible to get a ballpark reading on how high the wall is. Then, if we were to replicate a head-sized object in the frame in the same place as the nearest boy, would we then be able to calculate whether front-to-back d.o.f. is possible with a 50mm?

I dunno. It just feels like a 35mm or even a 28mm to me. (Cartier-Bresson carried a 35mm, but not a 28mm, however, so I assume if it was a wide-angle it would have to be a 35mm.)

Posted by: MIKE JOHNSTON

15 Comments:

Blogger Scott Kirkpatrick said...

I don't think it's enough to pin down the heights of the subjects in the picture; you also have to know how far away each was from the camera. The best way to figure this out is to know the height of the wall and the size of the plaza in which the picture was taken. That determines the maximum distance that there could have been between HCB and all those faces that he managed to resolve in one frame. Is there a published contact sheet in which this picture appears? I didn't find it in "HCB, The man, the image, and the world," but a contact sheet might also help to guess the lens.

1:28 AM  
Anonymous Anonymous said...

Can't you just check the exif?
;)

3:29 AM  
Anonymous Wilhelm said...

The only clues to which lens was used for both the "Wall" picture and "Picnic" are the considerable decrease in size of the figures in the background from those in the foreground.
We've all learned that the relative size of picture elements is determined by he location of the camera, not by the lens used, and in both of these images the foreground people are VERY close to the camera.
I have always assumed that "Picnic" was a wide-angle shot, but on inspection there is no distortion of the shapes of those figures at the frame edges, as there would have been with a WA lens (which would have been a 35mm Elmar, incidentally).
What a great master HC-B was.

6:31 AM  
Anonymous dwig said...

It wouldn't likely be a 28mm as Leica didn't market one until 1935, two years after the picture was taken.

To my eye, judging from the shape and appearant view angle of the faces at the extreme edges of the picture, I would guess that it was shot with a 35mm lens rather than a 50mm.

9:10 AM  
Anonymous Joe Decker said...

I suspect your intuition is right, but I don't know how to approach it mathematically.

If the building still existed, and you had copies of the two lenses, I think it's likely you'd find that only one of them could put the windows in the same position and size in the frame. Both 35mm and 50mm are wide enough that there's going to be some mapping of rectangular buildings into our spherical point of view.

10:31 AM  
Blogger Mike Johnston said...

Anonymous,
Unfortunately the hard drive with the exif data on it died a few years ago.

--Mike

11:11 AM  
Anonymous Flaneur said...

It's a wide-angle to me.

1:47 PM  
Anonymous Richard Howe (NYC) said...

If the kid is about 5' tall, then since he's about 1/6 of the frame height, the wall must be about 30' high. Since the wall fills most of the frame (vertically), it had to have been a 35mm lens (assuming no crop). This via direct comparison with shots I made across an intersection here at 28mm, 35mm, and 50mm this morning (not for this purpose though), where distances & heights (including people) are known. Of course, one COULD figure it out mathematically too ....

8:19 PM  
Blogger Scott Kirkpatrick said...

I think the wall of the building looks like there are 5 stories inside, maybe only 4. That means that it is 35-40 feet high. The rest of the math isn't that hard. To create an image 24mm high behind a 50mm lens, a 36' high wall needs to be about 65' to 70' away (you save a few feet because the wall appears to tilt away from a camera placed very low and pointed up). The same wall with a 35mm lens needs only to be 55-60' from the camera. Given the range of apparent sizes of the people, I suspect the larger distance and a big gap between the kids surrounding the photographer and the fat man and others by the wall is reasonable.

8:17 AM  
Anonymous richard howe (nyc) said...

Well, Scott Kirkpatrick's math sure beats my eyeballing it. He is wholly persuasive and is supported by the more or less 1:10 image height ratio between the boy at the wall and the boy on the far right (assuming more or less equal head sizes). This puts the boy on the far right at a distance of 6.5 to 7 feet from the camera. This depth of field (for a 0.03mm CoC) is just outside the range of a 50mm lens at f16 and there may have been enough light on that plaza to permit f16 at a shutter speed that freezes most of the subject motion (2nd boy from left seems to have moved his head maybe 1/2 inch while the shutter was open). -- Otherwise, the depth of field suggests 35mm at f9.6 or thereabouts, which in turn implies a cropped image. Or have I gotten myself totally confused? (This sure beats doing my taxes, which is what I should be doing instead.)

10:42 AM  
Blogger Motto! said...

Does it really matter?

4:54 PM  
Anonymous richard howe (nyc) said...

"Does it really matter?" In the grand scheme of things, of course not. As an amusement, or a way to procrastinate (I should have been doing my taxes), yes, but not very much. As exercise in thinking through the interaction of a number of parameters (focal length, aperture, depth of field, shutter speed, relationship between object sizes and image sizes, perspective, etc.), only if such exercises amuse or help. As a roundabout way into further appreciating "what a great master HC-B was", well, it did that for me, anyway.

8:34 AM  
Blogger Motto! said...

Sweet, sweet procrastination. Which reminds me, I really should be working...

3:31 PM  
Blogger Sean Reid said...

This comment has been removed by a blog administrator.

5:55 AM  
Blogger Sean Reid said...

My gut sense is 35 mm. The figures close to the focal plane seem to be drawn by a 35 and the compression of space feels like a 35.

Does it matter? No, this is just for fun.

Cheers,

Sean

5:58 AM  

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