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07-06-2012, 10:03 AM - 1 Like   #16

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QuoteOriginally posted by voyager13 Quote
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.
Two primes of the same focal length can and often are different as well, so don't think it is just a zoom thing...

07-06-2012, 10:04 AM - 1 Like   #17
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QuoteOriginally posted by vonBaloney Quote
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.
if you really need to get closer than that a small tube works wonders
07-06-2012, 10:12 AM - 2 Likes   #18
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What you see is probably a result of lens design details; When we say "distance from the lens" what do we mean? from the center of mass? from the glass of the front element, rear element, what?

A camera lens has two positions that are important, the front principal plane - where the lens appears to be when looked at from the front and the rear principal plane which is where the lens appears to be when looked at from the back. These positions are chosen so the thin lens equations work.

Here's an example of a real zoom lens and how its principal plane locations change with focal length.

It would not be surprising if the front principal plane locations for your 28mm prime and 28mm zoom to be very different and help explain the focusing distance differences you observe.


Last edited by newarts; 07-06-2012 at 10:29 AM.
07-06-2012, 10:29 AM - 1 Like   #19
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QuoteOriginally posted by voyager13 Quote
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.
I understand that desire completely. I believe this has been answered very well already, as you have acknowledged, but I wanted to add one important distinction to clear up your initial assumption:

You don't have two 28mm lenses. You have a 28mm and a 28-80 zoom. They are completely different machines. It's like comparing a 2-door purpose-build sports car with a mass-market sedan. Both have 4 wheels and might even have similar range displayed on the speedometer, but you can't expect them to both behave the same way.

07-06-2012, 10:48 AM   #20
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Thanks again guys, especially Newarts for taking time to represent this graphically, and intjonmiller for the interesting comparision, that helps in a big way.
07-06-2012, 09:40 PM   #21
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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)?
The earlier mentioned lens database is one place. And for any current lens, you can find the minimum focusing distance in the specifications. so if you're interested in e.g. a new-to-the-market Sigma that's not in the database yet, go to the Sigma website and check the specification of the lens.
07-07-2012, 07:48 AM   #22
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Definitely not a prime versus zoom thing. In fact, many zooms focus closer than many primes. Different lenses just have different minimum focus distances. The MFD is always one of the published specs for a lens, along with the magnification ratio that results from shooting at that distance. As mentioned, extension tunes or closeup filters can reduce the MFD for a lens.
07-07-2012, 10:27 AM   #23
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Estimating lens thickness

QuoteOriginally posted by voyager13 Quote
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?
I misread this question by assuming op meant distances were different for the same magnification so responded with an illustration of how different the effective location in space a camera lens might be from its actual location (see my above post).

Others talked about MFD variation between lenses. The MFD coupled with the camera's max magnification can be used to locate the effective lens in space (ie. by telling you the distance between the rear principal plane and the front principal plane.)

I assume the MFD reported by the mfg is the actual (minimum) distance from the object to the sensor at maximum magnification.

The theoretical MFD is the sum of the image-lens and lens-object distances, = Focal.length(1+m)^2/m.

The lens thickness (distance between front and rear principal planes) is the difference between actual and measured MFDs:

Lens optical thickness = MFD.reported - f(1+m)^2/m

The M 28:2.8 has a manufacturer reported MFD of 300mm at m=0.12x,

Therefore, M28:2.8 Lens optical thickness = 300 - 28(1.12)^2/0.12 = 7.3mm.

When focused at infinity, the rear principal plane is 28mm from the sensor and the front principal plane is 28+7.3=35.3mm from the sensor.

The distance from sensor to lens mount for Pentax is 45.46mm so when focused at infinity the effective front of the lens is buried about 10mm inside the camera!

I hope this example is helpful in explaining why the object to physical lens distance seems strange sometimes as well as providing a simple mathematical tool to estimate lens thickness.

Dave in Iowa

PS Here's an illustration of the estimated rear and front principal plane locations for the Pentax M28:2.8 lens. Note that both front and rear principal planes are outside and behind the physical lens.

Last edited by newarts; 07-07-2012 at 12:56 PM.

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