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07-06-2012, 09:06 AM   #1
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2 Lenses with different focusing distance

Hi Guys,

I have a SMC Pentax M 28/2.8 and a Pentax A 28-80 f3.5-4.5.
Why is it that when I am focusing on a nearby object, eg. a flower, I can be just a few centimeters away(almost touching the flower) with the 28/2.8 M and it focuses perfectly at that distance, whereas with the 28-80 set at 28mm I have to be a few inches away from the same object, at least, much greater than the 28/2.8 M lens, although I am using the same focal length?

07-06-2012, 09:16 AM   #2
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there is something wrong in here ...technically you can not take pictures from the distance as you described with your M 28mm...

Answering your question:
Min. focus for M 28mm is 30 cm, while for 28-70 it is 80 cm...you can not be closer to an object...
07-06-2012, 09:18 AM   #3
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Every lens design involves tradeoffs. Canon has a great 85mm f/1.2 lens, for instance. It's very sharp with gorgeous bokeh in part because of the razor-thin depth-of-field at f/1.2. But it is massive and heavy (relative to other 85mm lenses).

Focusing distance is one of those tradeoffs. I wondered this myself, particularly about longer lenses (especially zooms). Then I had a chance to talk to one of the upper guys at Tamron when he visited the camera store where I worked. He explained that with long zooms the image quality falls off when they focus too closely. In order to compensate for that they would have needed to make the entire unit bigger, and it's not a strong enough feature for a long lens that will almost always be used for relatively distant focus anyway. This also explains why the "macro zoom" lenses (usually 70 or 100 to 300) only have a "macro" setting at the longest focal length, and still only do 1:2 magnification at best. They made a tradeoff to allow that at one point without designing the optics to handle close focusing at all focal lengths. And considering the mediocre-to-moderate image quality of those zooms the cost of that tradeoff may be quite evident.

Hope this helps. It's not an exact answer for your exact lens selection, but it should help put it in perspective. Especially when you consider how long ago that 28-80 was made (before modern computer-aided lens design developments).
07-06-2012, 09:20 AM   #4
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Hi Vasyl, perhaps I may be wrong, it may be 30 cms for the M. The 80 cms for the A sounds right. If those two distances are right, and I think they are, the question still remains why is there a difference in the focusing distance using the same focal length of 28mm?

07-06-2012, 09:24 AM   #5
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QuoteOriginally posted by Vasyl Quote
Min. focus for M 28mm is 30 cm, while for 28-70 it is 80 cm...you can not be closer to an object.
Agree, something does not sound right unless your definition of "just a few" is larger than mine.

Minimum focus range for any particular lens is part of the physical properties of how the lens is designed. Focal length is not really a consideration. For example the Pentax-A 100mm f/2.8 has a minimum focus distance of 100cm but the Pentax-A 100mm f/2.8 macro has a minimum distance of 31cm. Same focal length but quite different focus distance.

This gets even more complicated with zooms as the minimum focus can change as the lens zooms.
07-06-2012, 09:26 AM   #6
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Intjonmiller, just saw your post after writing to Vasyl. I am a little confused reading your information as its too technical for me, though I understand the lenses are made differently and hence their price varies. I still am looking to know in simple terms why does that difference in focusing distance happen. In my world, focusing distance for all 28mm focal lengths should be exactly the same, otherwise it gets confusing.
07-06-2012, 09:28 AM   #7
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Thanks Jatrax, is there a place here on the forum (or elsewhere) where one can find information on focusing distances for all Pentax (and perhaps non-Pentax lenses)?
07-06-2012, 09:31 AM   #8
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QuoteOriginally posted by voyager13 Quote
Hi Vasyl, perhaps I may be wrong, it may be 30 cms for the M. The 80 cms for the A sounds right. If those two distances are right, and I think they are, the question still remains why is there a difference in the focusing distance using the same focal length of 28mm?
because one is a zoom and it is a compromise made for zoom design. the question was answered i thought above.
the A28 prime does focus to 30CM (no surprise it's almost the same lens)
in fact aside from the K28 F 2 which focuses to 25 cm all K mount 28 primes (k/M/A/F/FA) focus to 30CM. the tak 28 3.5 is 40cm

Zooms will vary depending on the level of compromise and what they want to acheive (just like a 70-200 zoom is really only 200mm at infinity and at closest focus is probably closer to 135mm)

this is one of the reasons for shooting with primes

07-06-2012, 09:40 AM   #9
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QuoteOriginally posted by eddie1960 Quote
the A28 prime does focus to 30CM
You meant M28 focuses to 30cm, because the minimum focusing distance for the A28, as mentioned by Vasyl, is 80mm which I just checked is correct.
But yes it's beginning to make sense why most primes are at a premium compared to zooms.
07-06-2012, 09:42 AM   #10
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QuoteOriginally posted by voyager13 Quote
In my world, focusing distance for all 28mm focal lengths should be exactly the same, otherwise it gets confusing.
Well, that assumption is just wrong, so you'll have to get over it. The focal length doesn't even stay the same as you focus -- the stated focal length is only true focused at infinity.
07-06-2012, 09:53 AM   #11
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QuoteOriginally posted by voyager13 Quote
You meant M28 focuses to 30cm, because the minimum focusing distance for the A28, as mentioned by Vasyl, is 80mm which I just checked is correct.
But yes it's beginning to make sense why most primes are at a premium compared to zooms.
he mentions the A 28-80 zoom (80cm) - the A28-135 is 170cm MFD
the A28 primes are both 30CM

MFD on the zooms is in part determined by the long end of the zoom, and by how small they want it, zooms are all about balancing compromises to give a good general purpose lens
Even the best of zooms have the compromise the FA* 28-70 is 43cm MFD Which is excellent for a zoom of this length (the nikon 24-70 is around 38cm)
07-06-2012, 09:54 AM - 1 Like   #12
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QuoteOriginally posted by voyager13 Quote
Thanks Jatrax, is there a place here on the forum (or elsewhere) where one can find information on focusing distances for all Pentax (and perhaps non-Pentax lenses)?
Yes, here is a link to the lens database it includes all Pentax and Asahi lens made as well as most of the third party lenses made in Pentax mounts.
07-06-2012, 09:56 AM   #13
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QuoteOriginally posted by voyager13 Quote
You meant M28 focuses to 30cm, because the minimum focusing distance for the A28, as mentioned by Vasyl, is 80mm which I just checked is correct.
But yes it's beginning to make sense why most primes are at a premium compared to zooms.
in fact at 28 mm compared to the prime both at MFD the zoom is probably not really as close to a true 28mm
zooms are a series of compromises for convenience. Some of the best ones (read also stupidly expensive and of much more recent vintage than an A zoom) still suffer from compromises. just less of them when you pay big bucks.
07-06-2012, 09:59 AM   #14
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QuoteOriginally posted by vonBaloney Quote
Well, that assumption is just wrong, so you'll have to get over it. The focal length doesn't even stay the same as you focus -- the stated focal length is only true focused at infinity.
Yes I have now got over it after knowing differences in MFD for primes and zooms. A good lesson in lens basics. Thanks guys. Truly appreciate it.
07-06-2012, 10:02 AM   #15
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QuoteOriginally posted by jatrax Quote
Yes, here is a link to the lens database it includes all Pentax and Asahi lens made as well as most of the third party lenses made in Pentax mounts.
Keep in mind focus distances are from the sensor plane, not the end of the lens barrel, so some of that distance is eaten up inside the camera and lens itself. But 30cm still leaves about 20cm from the end of the barrel to the subject, so it doesn't sound like you can get right on top of subject with the M28 as the OP stated. I have noticed that with many lenses you can in-fact get closer than they say, but that's a pretty big difference.
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