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08-28-2009, 11:26 AM   #1
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why is this happening?

Two shots 5x5" center @ 100% crop taken with a Tamron 28-75 set at 50mm and a Pentax A-50 f/1.7, same settings for both lenses from a tripod.
They show how good the Tamron is for a zoom and how very good the prime lens is. But here's the puzzle: if they're both at 50mm, how can they be different in image size? Metadata shows the Tamron is at 50mm, does that mean that the prime is really a bit longer, maybe really closer to 53-55mm?
Brian
First, the Tamron


Then, the A-50 prime


08-28-2009, 11:35 AM   #2
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welcome to the wacky world of physics and optics.

first of all, can you advise when setting up the shot if the tripod was in the same place or the front element of th elens was the same distance to the subject.

remember here you are working close, and this little difference could be the cause.

also note that for zoom lenses focal length is only accurate and measured at infinity focus.

actual focal length can vary greatly when close focused.
08-28-2009, 11:51 AM   #3
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In addition to the points above, the sensor in the lens which measued the focal lentgth for the metadata entry does not usually have a continuous measurement range but measures in discrete steps. It is quite possible that although the metadata indicates a 50mm effective focal length, the lens was actually at at a slightly shorter focal length.
08-28-2009, 11:56 AM   #4
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Most zooms are not the focal lenght that you dial in when you are at close focus distance.

Nikon 18-105 becomes a 78mm when focusing at 4 feet for example. Its called varifocal. Varifocal lens - Wikipedia, the free encyclopedia

There are some formula to calculate the true Focal lenght goes something like this

Focal lenght = d / (2 + m +(1 / m))
d = distance to object in mm
m = magnification (1:1 = 1, 1:2 = 0.5 1:5 = 0.2 etc)

Sigma 28-135 macro zoom gives you. Closest focus gives you distance to objekt, 240mm, magnification is 1:2 = 0.5)
Focal lenght = 240 / (2 + 0.5 + (1 / 0.5)) = 53.3mm

So your zoom is most likely true 50mm only at infinity.


Last edited by melander; 08-28-2009 at 12:07 PM.
08-28-2009, 12:08 PM   #5
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QuoteOriginally posted by melander Quote
Most zooms are not the focal lenght that you dial in when you are at close focus distance.

Nikon 18-105 becomes a 78mm when focusing at 4 feet for example. Its called varifocal. Varifocal lens - Wikipedia, the free encyclopedia

There are some formula to calculate the true Focal lenght goes something like this

Focal lenght = d / (2 + m +(1 / m))
d = distance to object in mm
m = magnification (1:1 = 1, 1:2 = 0.5 1:5 = 0.2 etc)

Sigma 28-135 macro zoom gives you. Closest focus gives you distance to objekt, 240mm, magnification is 1:2 = 0.5)
Focal lenght = 240 / (2 + 0.5 + (1 / 0.5)) = 53.3mm

So your zoom is most likely true 50mm only at infinity.
Note varifocal is not the issue we are talking about here Varifocal is the term used to describe a lens that changes its focus as you zoom in, as opposed to a true zoom (forget the term at the moment) that maintains focus as you zoom

the issue here is the change in actual focal length of the lens as you focus in from infinity,
08-28-2009, 02:41 PM   #6
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Happens to be a nice comparison of contrast, color and sharpness too. IMO, with a tiny tweak of contrast (levels?) the A50 wins all catagories.

Incidentally, if the tripod/body didn't reposition there's a 65mm difference between the faces of the two lenses and the subject - significant at that distance.

Years ago it was a common practic to 'round-off' lens FL to the nearest 5mm for marketing purposes. (Even then, some measure-bater would argue that it was REALLY only a 48.82mm lens!)

A look at some of the older lens reviews reveals that the ACTUAL FL was one of the descriptive items often reported -- especially at various zoom settings. (E.g., Modern Photography's reports) Even then, some measurbater couldn't resist pointing out that it was REALLY only a 48.82mm lens!

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08-28-2009, 03:02 PM   #7
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you cant trust that your 50mm prime is actually 50mm exactly. and forget about a zoom set at 50mm actually being 50mm. I don't see a difference big enough in these photos to worry about it. whats the big deal really?
08-28-2009, 03:39 PM   #8
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Ah hah...

QuoteOriginally posted by pacerr Quote
Incidentally, if the tripod/body didn't reposition there's a 65mm difference between the faces of the two lenses and the subject - significant at that distance.

H2
I didn't take that into account; as I have now learned, that difference becomes less important as the focal distance increases. The things you learn when you ask questions... and that's why I asked,
Brian

08-28-2009, 07:43 PM   #9
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Ohh, did I forget. . .

. . . to tell you? That particular A50 was the evil twin. It gets in a snit and acts up if you don't "walk it" twice a day. I kept the well behaved one.

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08-28-2009, 08:13 PM   #10
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And, by the way, no prime lens is truly accurate in focal length to its specified focal length. As far as I know, Leica is the only manufacturer that measures the as-assembled focal length of their lenses and include the results in the paperwork. From what I've read, most lenses differ from their specified focal length by not more than a millimeter so it's not an issue.
08-28-2009, 08:17 PM   #11
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The "evil twin" curse strikes...

QuoteOriginally posted by pacerr Quote
. . . to tell you? That particular A50 was the evil twin. It gets in a snit and acts up if you don't "walk it" twice a day. I kept the well behaved one.

H2
Ah hah, that explains what happened. I tried out the A50 on the K20, and then wanted to see how it worked on the K100. I think it didn't like being downgraded, so when I attempted to put the original Pentax screen back in the K100...

Brian
08-30-2009, 07:01 PM   #12
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QuoteOriginally posted by FHPhotographer Quote
I think it didn't like being downgraded,
Yeah, that one insisted on at LEAST a K100D Super.

Very fashion conscious -- I think that one was female.

H2
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