Lens Focal Length Rip Off.

debmalya · 08-10-2011, 02:47 PM

My test results: I took pictures with my Nikon AF-S 18-200VR and Pentax DA 18-135WR set at 70mm focussed at infinity. The subject measured almost the same in size in both the photographs. I repeated the test with both lenses set at 135mm and noticed the same difference, that is - almost none, with respect to size of the subject recorded in the photographs. I conclude that this proves the magnification of both lenses remain equal at the same focal lengths, when focussed at infinity.

newarts · 08-10-2011, 03:26 PM

Originally posted by debmalya

My test results: I took pictures with my Nikon AF-S 18-200VR and Pentax DA 18-135WR set at 70mm focussed at infinity. The subject measured almost the same in size in both the photographs. I repeated the test with both lenses set at 135mm and noticed the same difference, that is - almost none, with respect to size of the subject recorded in the photographs. I conclude that this proves the magnification of both lenses remain equal at the same focal lengths, when focussed at infinity.

If you can find the time it would be helpful if you would check them at the same focal length, closest focusing distance for which you can have the same distance for both lenses..

Thanks
Dave

debmalya · 08-10-2011, 05:07 PM

Originally posted by newarts

If you can find the time it would be helpful if you would check them at the same focal length, closest focusing distance for which you can have the same distance for both lenses..

Thanks
Dave

Sure. I will post the results of my observation after I try that out. If I understand you correctly, you want me to set both my lenses at same focal length (say 70mm - since this # is marked on both my lenses), focus on a subject placed as close as possible to the lens with the greatest minimum focus distance (say 12"). Then try the same shot with the other lens with the subject at the same distance from that lens (eventhough it is capable of a lesser focussing distance). Measure the size of the subject on the recorded picture and report the results. Correct me if I misunderstood any step.

newarts · 08-10-2011, 05:41 PM

Originally posted by debmalya

Sure. I will post the results of my observation after I try that out. If I understand you correctly, you want me to set both my lenses at same focal length (say 70mm - since this # is marked on both my lenses), focus on a subject placed as close as possible to the lens with the greatest minimum focus distance (say 12"). Then try the same shot with the other lens with the subject at the same distance from that lens (eventhough it is capable of a lesser focussing distance). Measure the size of the subject on the recorded picture and report the results. Correct me if I misunderstood any step.

That's right. If the two lenses are at the same distance from the subject (actually the camera's tripod hole should probably be the same distance from the subject.) Then there will be little to argue about in comparing results to predictions; besides the info might be useful to you, especially if you also take a photo at the shortest possible distance with the shorter distance lens.

A collection of raw data about distance and apparent ruler size (like take a photo of a yardstick) would also be very useful.

Whatever you have time for will help.

Thanks for responding!

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08-10-2011, 02:47 PM	#46
debmalya Senior Member Join Date: Jul 2011 Location: NJ USA Photos: Albums Posts: 209	My test results: I took pictures with my Nikon AF-S 18-200VR and Pentax DA 18-135WR set at 70mm focussed at infinity. The subject measured almost the same in size in both the photographs. I repeated the test with both lenses set at 135mm and noticed the same difference, that is - almost none, with respect to size of the subject recorded in the photographs. I conclude that this proves the magnification of both lenses remain equal at the same focal lengths, when focussed at infinity.

08-10-2011, 03:26 PM	#47
newarts Inactive Account Join Date: Dec 2008 Location: Ames, Iowa, USA Photos: Albums Posts: 2,965	Originally posted by debmalya My test results: I took pictures with my Nikon AF-S 18-200VR and Pentax DA 18-135WR set at 70mm focussed at infinity. The subject measured almost the same in size in both the photographs. I repeated the test with both lenses set at 135mm and noticed the same difference, that is - almost none, with respect to size of the subject recorded in the photographs. I conclude that this proves the magnification of both lenses remain equal at the same focal lengths, when focussed at infinity. If you can find the time it would be helpful if you would check them at the same focal length, closest focusing distance for which you can have the same distance for both lenses.. Thanks Dave

08-10-2011, 05:07 PM	#48
debmalya Senior Member Join Date: Jul 2011 Location: NJ USA Photos: Albums Posts: 209	Originally posted by newarts If you can find the time it would be helpful if you would check them at the same focal length, closest focusing distance for which you can have the same distance for both lenses.. Thanks Dave Sure. I will post the results of my observation after I try that out. If I understand you correctly, you want me to set both my lenses at same focal length (say 70mm - since this # is marked on both my lenses), focus on a subject placed as close as possible to the lens with the greatest minimum focus distance (say 12"). Then try the same shot with the other lens with the subject at the same distance from that lens (eventhough it is capable of a lesser focussing distance). Measure the size of the subject on the recorded picture and report the results. Correct me if I misunderstood any step.

08-10-2011, 05:41 PM	#49
newarts Inactive Account Join Date: Dec 2008 Location: Ames, Iowa, USA Photos: Albums Posts: 2,965	Originally posted by debmalya Sure. I will post the results of my observation after I try that out. If I understand you correctly, you want me to set both my lenses at same focal length (say 70mm - since this # is marked on both my lenses), focus on a subject placed as close as possible to the lens with the greatest minimum focus distance (say 12"). Then try the same shot with the other lens with the subject at the same distance from that lens (eventhough it is capable of a lesser focussing distance). Measure the size of the subject on the recorded picture and report the results. Correct me if I misunderstood any step. That's right. If the two lenses are at the same distance from the subject (actually the camera's tripod hole should probably be the same distance from the subject.) Then there will be little to argue about in comparing results to predictions; besides the info might be useful to you, especially if you also take a photo at the shortest possible distance with the shorter distance lens. A collection of raw data about distance and apparent ruler size (like take a photo of a yardstick) would also be very useful. Whatever you have time for will help. Thanks for responding!

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