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03-14-2012, 05:26 AM   #1
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Pentax DA* 60-250 issues

Seems like focal length in my new DA*60-250 is just 60-200 and not 60-250. Although the zoom ring rotates from 200 to 250, I can not see any visible differnce between 200 and 250. It looks like the zooming stops at 200. Any idea? Does it have anything to do with the distance of the shooting objects?

Last edited by sumanm17; 03-14-2012 at 05:49 AM.
03-14-2012, 05:35 AM   #2
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Wow ! thats Odd ! I think the internal lens stop has been set too short.
03-14-2012, 06:39 AM - 1 Like   #3
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Are you shooting at infinity or somewhat closer? The design of the 60-250 is such that at very close distances it doesn't reach it's full focal length.
03-14-2012, 07:33 AM   #4
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I also noticed this on my *60-250. There is apparently very little difference between 200 and 250 mm, but images differ anyhow.

03-14-2012, 07:43 AM - 1 Like   #5
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My understanding of focal lengths vs perspective is that as you go longer, the differences are smaller. 250-200 isn't too different, especially when the lens, under different focal conditions, may not actually be 200-250, but instead 180-210.

Try shooting a very far object at 200 and 250mm, then shoot a very close object at 200 and 250mm. Focus distance does affect the lens focal length a bit.
03-14-2012, 08:11 AM - 1 Like   #6
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You should see my old thread where I complain about the difference between internal focus and external focus lenses. I was convinced it was a rip off, but folks on here set me straight.
03-14-2012, 11:25 AM - 2 Likes   #7
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QuoteOriginally posted by Wheatfield Quote
Are you shooting at infinity or somewhat closer? The design of the 60-250 is such that at very close distances it doesn't reach it's full focal length.
The 60-250 is an internal focus (IF) lens. This is a fairly common trait of many IF lens designs, as the element move around "significantly" relative to one another.

I have heard it being referred to a "lens breathing" If you have a lens focused to infinity and you have a blurry object in the foreground change the focus distance while keeping the zoom constant and you will see the blurry object "move" changing the composition as it comes into focus.
This is because the lens is only at its rated length at infinity or close to it. At shorter distances the focal length is effectively shorter usually.

It's not really an issue unless you are shooting video and bringing a close object into focus and you don't want the compostion of the close objects to change.

Looking at the lens review page specs says: maximum Magnification = 0.15x and minimum focus distance = 1100mm

If these figures can be believed I suppose you can work out the effective focal length at the minimum focus distance with some calculations just by knowing these 2 pieces of info.

from the wikipedia page on "focal length" the focal length formula is : 1/f = 1/S1 + 1/S2

f = focal length
S1 = distance from the rear nodal point to the object
S2 = distance from the rear nodal point to the sensor

Magnification is the ratio of the image size on the sensor to the object size and this is pretty much the same as the ratio of the distances S2 to S1

MAG = S2 / S1

The distance to the object to the sensor is S1 + S2 of course. When calculating the focal length at the minimum distance (MFD) is the distance we're interested in.

MFD = S1 + S2 when at the closest point it can focus

So for a 60-250 (IF) and the MFD = 1100mm and the Max MAG = 0.15 according to the spec

0.15 = S2/S1
S2 = 0.15 x S1

1100 = S1 + (0.15 x S1)
1100 = 1.15 x S1
S1 = 956.5
S2 = 1100 - 956.5 = 143.5

1/ f = 1/ 956.5 + 1/ 143.5
1/f = 0.008014

f = 125 mm (holy cr@p!! that can't be right, can it?)

Big difference assuming that maximum magnification sweetspot is actually at the 250mm setting.
I guess there is probably a bit of rounding in the accuracy of the of the MFD and Magnification figures quoted.

If we try it out with a DA*300 (IF) min focus 1400mm max magnification 0.24x calculated f = 219mm using the same method

DA*200 (IF) min focus 1200mm, max mag 0.2x and calcualted f = 167mm

DA 70 min focus 700mm, max mag 0.12x and calculated f = 67mm pretty close since there isnt much movement in the lens elements in a short lens barrel anyway.

FA43 min focus 450mm, max mag 0.12x and then calculated f = 43mm hooray, its the same !!

So it appears that "prime" lenses can actually change their focal length when focussing at closer distances.

An older style "single focal length" lens that moves the entire lens element assembly to focus I am guessing would not.

Camera lens - Wikipedia, the free encyclopedia

Now I'm confused.

Sorry if I've made it worse

Last edited by steve1307; 03-14-2012 at 11:36 AM.
03-14-2012, 02:41 PM   #8
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THANK YOU GUYS for your constructive feedback!
I did some more testing today and it seems that the difference between 200mm and 250 mm is clearly visible while shooting distant objects. There isnīt much difference while shooting objects in close range.

07-24-2012, 07:31 AM   #9
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A real lens has a rear principal plane and a front principal plane - these are where the lens "appears to be" when viewed from the back or front. The thin lens equations work when measurements are made from the appropriate plane. The physical thickness of the lens is ignored in applying the thin lens equations.

The image distance is measured from the image to the rear principal plane, etc.

1/focal.length = 1/image.distance + 1/object.distance
magnification = image.distance/object.distance

MFD reported by the manufacturer includes the lens thickness.

Lens.thickness = MFD - (minimum. image.distance + minimum. object.distance)

Here's an example showing the principal plane locations for a long zoom lens focused at infinity:

Dave in Iowa

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