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12-09-2012, 03:53 AM   #1
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Newly acquired 18-250mm Pentax DA lens issues compared to 55-300mm

I just purchased a used (like new) pentax DA 18-250mm lens. So far, I am not impressed with this lens. After taking few test shots it seemed to me that it wasn't a true telephoto 250mm. At 250mm focal length, it is same as my Pentax DAL 55-300mm at 150mm. I haven't compared the wide end to my other wide angle lenses. Here are the comparisons taken with my Pentax K-30.





Then, there is another issue. The lens depth of field doesn't change properly. Please see the bottom photos that I took at 100mm and 200mm. I have cropped the 100mm. Both were taken at f8. The top left and bottom right edges are more in focus that the top or bottom. I have marked them. I don't know if it is normal for this lens or if I should consider sending it back. I wanted to have an all around lens to use in certain events without needing to change lenses but it might be better just using my DAL 18-50mm and 55-300mm. Any feedback would be appreciated.






Last edited by AH11; 12-09-2012 at 04:03 AM.
12-09-2012, 05:00 AM   #2
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The 18-250 is an IF (internal focus) lens. Thus it is only 250mm at infinity. If you focus closer than that, the focal length reduces (and quite significantly as you have found). The 55-300 is not IF, hence it will remain 250mm what ever you focus at. There are several posts on the pros and cons of IF, with this one being the biggest bugbear.

Can't comment on the 2nd issue....
12-09-2012, 07:14 AM   #3
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The first question or "issue" is a normal effect with all IF lenses. This also occurs with the 18-135 , DA*60-250 especially and even some prime lenses appear shorter FL when focusing at closer distances.


The other shots you've taken appear to be at a pretty close distance also , I'm guessing about 1metre.
If you work out the DOF with simple calcuator at 110mm and 200mm at f8 at this close distance the DOF is only pretty thin anyway about 20mm @100FL and much less at 200mm (or whatever the apparent FL is close focus with this IF lens).

In the 200mm shot only top part of the 4th battery looks to be in focus. That's kind of what you'd expect.
The 100mm shot only the top part of the same battery looks to be in focus and its sort of in focus down the 3rd battery looking at the writing on the side.
The 18-250 is not the sharpest tool anyway, its not a macro lens and you've cropped 3/4 of the pixels out of the 100mm shot to get the same FOV and blown it up so I'm not surprised it looks a touch softer.


Unless the event that you plan to shoot with it is a Flea Circus, I wouldn't do these close pseudo macro shots to test it, go out and shoot what you intend to to use it for. ie people at normal distances where the DOF you would expect would be possibly a metre or more, bounce flash or whatever.

Enjoy the 18-250 its a very versatile and reasonably capable lens but as in any "superzoom" the compromise is ultimate sharpness and of course speed to get that versatility. To do much better you'll need 2 bodies and something like a DA*16-50 and DA*50-135 combination for significantly more coin.


BTW If the event actually is a Flea Circus then I suggest a nice macro like a DFA100WR, Tamron 90 instead of the 18-250


edit: One other thing I noticed is odd about your photos. You have 5 !! of those cheap a$$ Costco batteries lined up and not one of them is leaking yet !!! Amazing!!

Last edited by steve1307; 12-09-2012 at 07:24 AM.
12-09-2012, 08:02 AM   #4
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QuoteOriginally posted by steve1307 Quote
The first question or "issue" is a normal effect with all IF lenses. This also occurs with the 18-135 , DA*60-250 especially and even some prime lenses appear shorter FL when focusing at closer distances.


The other shots you've taken appear to be at a pretty close distance also , I'm guessing about 1metre.
If you work out the DOF with simple calcuator at 110mm and 200mm at f8 at this close distance the DOF is only pretty thin anyway about 20mm @100FL and much less at 200mm (or whatever the apparent FL is close focus with this IF lens).

In the 200mm shot only top part of the 4th battery looks to be in focus. That's kind of what you'd expect.
The 100mm shot only the top part of the same battery looks to be in focus and its sort of in focus down the 3rd battery looking at the writing on the side.
The 18-250 is not the sharpest tool anyway, its not a macro lens and you've cropped 3/4 of the pixels out of the 100mm shot to get the same FOV and blown it up so I'm not surprised it looks a touch softer.


Unless the event that you plan to shoot with it is a Flea Circus, I wouldn't do these close pseudo macro shots to test it, go out and shoot what you intend to to use it for. ie people at normal distances where the DOF you would expect would be possibly a metre or more, bounce flash or whatever.

Enjoy the 18-250 its a very versatile and reasonably capable lens but as in any "superzoom" the compromise is ultimate sharpness and of course speed to get that versatility. To do much better you'll need 2 bodies and something like a DA*16-50 and DA*50-135 combination for significantly more coin.


BTW If the event actually is a Flea Circus then I suggest a nice macro like a DFA100WR, Tamron 90 instead of the 18-250


edit: One other thing I noticed is odd about your photos. You have 5 !! of those cheap a$$ Costco batteries lined up and not one of them is leaking yet !!! Amazing!!
I appreciate the explanations provided. Yes. It makes sense. It is an IF lens. That is why when I was taking the photos about 50-75cm away it wasn't focusing to infinity.

As far as the other explanation of me taking the shots too close, you may be right but the lens doesn't show consistency. If you look at my last picture, where I marked #1 is more focused than #3, but #6 is more focused than #4. #3 and #6 were closer to the camera lens than #1 and #4.

Anyways, as you mentioned, as this lens covers a wide range, I shouldn't expect much. Though I am not doing Flea Circus shots, I will resort to my prime lenses when the occasion asks for it.

I only use the cheap Costco batteries as a backup or for my flash units. I haven't had any issues so far.

12-09-2012, 08:28 AM   #5
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QuoteOriginally posted by AH11 Quote
If you look at my last picture, where I marked #1 is more focused than #3, but #6 is more focused than #4. #3 and #6 were closer to the camera lens than #1 and #4.
Not sure about that, it looks like you shot the picture in an angle slightly from above. So there is not the same distance from the sensor on the top of a battery as on the bottom. Point #1 and #6 probably has the same distance to the sensor and is in focus, #2 & #3 are closer, and #4 & #5 further away from sensor. You probably focused on the middle part of the middle battery, as it is also in focus.
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