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06-23-2008, 07:49 PM   #1
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85mm vs. 50mm which is which (same subject, same aperture, same magnification)

The two pictures below were taken within minutes of each other, both at f/5.6. One was taken with an 85mm lens and the other with a 50mm lens.

The question is...which picture was taken with which lens and what was your clue? There has been much discussion regarding focal length vs. image size vs. magnification vs. crop-factor in relation to perspective and depth of field. Maybe this little test will create some active discussion and perhaps a few similar posts. (If you cheat and view the Exif, don't spoil the game for others...)

P.S. Please excuse any issues with focus or composition. This was a test exercise only!





06-23-2008, 08:05 PM   #2
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the bottom is 85mm the top is 50mm

the clue is that the backgrouns (althoug out of focus) is smaller in the top picture, Assuming you have the same magnification ratio for the part in focus, you are closer with the 50mm lens to the main subject, but when you consider the magnification ratio of the background, the background has a higher magnification ratio with the long lens and is therefore "bigger" in appearance
06-23-2008, 08:38 PM   #3
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But let's make it clear that the DoF is exactly the same for both of them.
06-23-2008, 08:54 PM   #4
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QuoteOriginally posted by Miserere Quote
But let's make it clear that the DoF is exactly the same for both of them.
I probably should have done test photos that included objects for near DoF as well as far DoF...perhaps chess men or figurines...

06-23-2008, 09:10 PM   #5
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QuoteOriginally posted by stevebrot Quote
I probably should have done test photos that included objects for near DoF as well as far DoF...perhaps chess men or figurines...
Chess men on a chessboard would be great to dispel the myth that DoF depends on focal length.

The only problem with your shots above is that you didn't use a tripod. In this case, a tripod makes a lot of difference so you can frame both of them exactly right. But thanks for doing it, I think everyone should learn a lot from this.
06-23-2008, 09:16 PM   #6
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QuoteOriginally posted by Miserere Quote
Chess men on a chessboard would be great to dispel the myth that DoF depends on focal length.

The only problem with your shots above is that you didn't use a tripod. In this case, a tripod makes a lot of difference so you can frame both of them exactly right. But thanks for doing it, I think everyone should learn a lot from this.
Actually, both were tripod shots with a 2s shutter delay. Getting the framing just right outside of a tabletop setting is trickier than it sounds. I may do this over with a more controlled setup.
06-23-2008, 09:57 PM   #7
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Quote from Wheatfield

QuoteOriginally posted by Wheatfield:
...Focal length plays no part in it, other that deciding camera to subject difference...
To get the same shot as the 85 with the 50, one needs to stand a lot closer to the subject. If that subject is a person, working distance and comfort zone come into play. With a fast 85 you can get a close, fast 50 type shot with a shallow depth of field and lots of bokeh from several feet away, but this is perhaps beside the point of your test. The 50 could get a similar shot. Why only similar?

I'm sure someone can explain the following in exact terms (I'd like to hear them actually), but also by recording images at a distance, perception of depth is altered. Perhaps try another test with a wider gap in focal ranges and a fence or other linear subject to note the differences more dramatically.
06-23-2008, 10:18 PM   #8
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QuoteOriginally posted by thePiRaTE!! Quote
Quote from Wheatfield



To get the same shot as the 85 with the 50, one needs to stand a lot closer to the subject. If that subject is a person, working distance and comfort zone come into play. With a fast 85 you can get a close, fast 50 type shot with a shallow depth of field and lots of bokeh from several feet away, but this is perhaps beside the point of your test. The 50 could get a similar shot. Why only similar?

It would have been VERY helpful if I had taken my other camera and photographed both setups. The difference in working distance was significant and as Lowell pointed out, the background perspective changes quite a bit as a result.

The point of the test was to test a fundamental claim...same magnification at a given aperture yields same DoF.

I did a similar example a month or so ago to test the case of whether the same distance from the subject yields the same perspective (it does). For that case, a subject was shot at the same distance with 35mm and 55mm lenses and the results cropped to provide identical composition. Since both photos for that test were taken at the same aperture setting, an unintentional outcome was the demonstration that a shorter focal length will provide greater depth of field than a longer lens at the same working distance and aperture (the inverse of the current test). It was obvious which picture was taken with the 35mm lens because of the greater DoF.

Here are the pictures from that test. Notice that the perspective is identical for both focal lengths:






Last edited by stevebrot; 06-24-2008 at 10:27 AM.
06-23-2008, 10:21 PM   #9
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QuoteOriginally posted by thePiRaTE!! Quote
With a fast 85 you can get a close, fast 50 type shot with a shallow depth of field and lots of bokeh from several feet away...
So, which one is which? There are a few clues in the background since many of the same objects are present in both shots.
06-23-2008, 11:05 PM   #10
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Top is 85, background is larger (less far away than it would appear with the wider 50). I'm just agreeing you can get very similar shots, though the working distance is quite different.

With regards to the perception of depth at various focal ranges (and why wide angles aren't the best for portraits) below is a sample with linear subject matter.

My goal was to keep - as much as possible - the same framing of the subject. I used the top corner of the board, the top of the last rook, the square in front of the bottom pawn and the base of the closest rook as my framing guide to set the appropriate distance and height. EDIT - forgot to mention, focus was on the King.

90mm f8.0


58mm f8.0


and 31mm for good measure f8.0


Note the proximity of the subject to the lens needed to capture the same shot forced the subject to be longer to the wider lens, also causing the depth of field to appear less.

Also note the top left corner, some empty cans. They appear larger in the 90mm shot. The relationship between the subject and background is dramatically different.

Last edited by thePiRaTE!!; 06-23-2008 at 11:56 PM.
06-23-2008, 11:43 PM   #11
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Good job Pirate with the chess pieces. The telling point is the relative size and orientation of the row of opposing white pieces as well as the size of the squares and the converging parallel lines (most obvious in the 31mm example). While you were able to stage essentially similar presentations for the brown pieces, the setting changed dramatically with the change of focal length.

As for which picture for which lens...You are correct. The top one was taken with my Jupiter-9 85/2 and the bottom with my Pentax-A 50/1.7. A little post processing was done to the 50mm version to reconcile the the curve to that of the 85 (shifted a little to the right and reduced the contrast) . The higher contrast of the 50 gave a false impression of greater DoF in the un-doctored image.
06-24-2008, 08:30 AM   #12
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QuoteOriginally posted by stevebrot Quote
Since both photos for that test were taken at the same aperture setting, an unintentional outcome was the demonstration that a shorter focal length will provide greater depth of field than a longer lens at the same working distance (the inverse of the current test).
I had to read your post twice to see that you had cropped the images. For a moment there I was wondering why the DoF was different.
06-24-2008, 08:33 AM   #13
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Pirate, very nice test. I'll be sure to bookmark this thread so I can send people here when this discussion pops up again...and we all know it will, huh?

So is the PentaxForums consensus now that working distance, not focal length, is what changes perspective in a scene? Can we all hold hands and sing Kumbaya at last...?
06-24-2008, 10:23 AM   #14
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QuoteOriginally posted by Miserere Quote
Pirate, very nice test. I'll be sure to bookmark this thread so I can send people here when this discussion pops up again...and we all know it will, huh?

So is the PentaxForums consensus now that working distance, not focal length, is what changes perspective in a scene? Can we all hold hands and sing Kumbaya at last...?
We can also conclude that magnification not focal length determines DoF (the original intent...)

I think that it is also fairly safe to say that crop factor does not change a 50mm into a 75mm despite what the guy in the camera shop says.

I suggest embarrassing confessions followed by a group hug...
06-24-2008, 10:31 AM   #15
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QuoteOriginally posted by Miserere Quote
I had to read your post twice to see that you had cropped the images. For a moment there I was wondering why the DoF was different.
I just edited the post and bolded the important parts for clarity!

Steve
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