[mito]

Hi guys,
While trying my Pentax 18-250 lens, I found out that the advertised focal length does not correspond with the actual focal length. The lens is something like a 18-180 (which is still quite amazing, but not what I paid for...).
Here are some quick and dirty shots that prove my claim. All pictures are taken with a tripod, f/8, about 5m far from the subject. The height of the crackers box is about 30cm.
The first one is taken with a Quantaray 100-300mm (which is a rebadged Sigma lens) with a focal length of 250mm.
The second shot is taken with the Pentax 18-250 at the same focal length, while the third is again from the Quantaray but at 200mm. Note that the size of the subject in the third picture is still bigger than the size of the subject taken with the Pentax lens
At 100mm the differences between the two lenses are not so evident.

Of course the advertised focal length of the Quantaray lens may be wrong, that's why I'd like to hear from other 18-250 owners if they can reproduce my measurements.

I did not alter the Exif data in the pictures and I can provide more examples if anybody asks.

Attached Images

View Picture EXIF

File Date

 

View Picture EXIF

File Date

 

View Picture EXIF

File Date

 

[jsherman999]

.

I have noticed the same thing. Somewhere, I forget where, I read something that gave a bit of an explanation - basically, at say 200 meters, you get the full 250mm, but closer in, you get less. I forget why, or where I read it right now....



.

[jake123]

I believe the focal lengths are advertised as when focus is at infinity.

At least on photozone, I think, they mentioned that the focal length gets shorter when focusing closer distances.

[mito]

Both pictures were taken from the same distance, so it should not make much difference. I will post the picture with focus at infinity as soon as I come back home

[Snowcat]

The optical scheme of THIS lens (18-250) acts like this at close focus distances. On macro shots it's focal length is like 150mm only. The further the object gets from the lens, the closer to 250mm it's real focal length gets.
Not all tele lenses act this way, but this one does.
You can check it by shooting a really distant object with these two lenses.

[charliezap]

i just got the pentax 18-250. it falls short as advertised. divide 250 by 18 and you get a factor of 13.888. i took a picture at 18mm and at 250mm and printed them out and measured an object in the shots with a machinist scale and eye loupe.the factor in the shots came out as a 10.185 differential which with my math puts it at 183.25mm--not 250mm.to be fair i will do the same comparison at a greater distance but i still think that this lens will come up short.my quantaray lens set at around 178mm gives me the same size image as the pentax set at 250mm. i think mito is spot on.

[jamesk8752]

It's well established that zoom lenses which have internal focusing may show significant reduction of their focal length when focused on targets shorter than infinity. Many if not all IF zooms show this behavior; it may be more pronounced in the 18-250 because of the extreme zoom factor of this lens. I've noticed it in many of my Pentax zooms.

In other words, this is expected behavior for this lens, and you got what you paid for - you just didn't know it!

[arbutusq]

I believe most zoom lenses behave like this. My old Nikon 18-70 was only around 55 mm at close distances and 70 mm at long distances and infinity. I haven't tested any of my Pentax zooms though, can't be bothered.

[charliezap]

i live just a 1/4 mile from rte 1.i usually check out new cameras and lens by shooting from the overpass which gives me over a one mile look in either direction. the highway has many signs and i like to use the bridge to check resolution on my equipment. i will take some shots at infinity with the pentax 18-250 and do the math to see what the figures tell me.i know that the lens isn't big bucks but the quality isn't too bad. i liked the quantary 70-300 which i have since sold when i sold my pentax k10d. i may repurchase the quantaray.it got me picture of the week in parade magazine 2 years ago when i took a picture of an airliner flying by the moon.you can actually see the craters on the moon with the airliner with contrails to its left.it was a luck shot taken during the day with a deep blue sky.

[frank]

James is right on this one. If you shoot something far away, like a building several hundred meters away, then you compare both, you'll see the 'actual' focal length of this 18250 lens :0 When you focus at close target, the focal length becomes shorter.

[PentaxPoke]

mmmmm crackers...

[Vylen]

out of curiosity, and if anyone knows, is it the same with the 55-300mm?

since i dont have a 300mm lens other than that one, i cant try it out for myself

[frank]

out of curiosity, and if anyone knows, is it the same with the 55-300mm?

since i dont have a 300mm lens other than that one, i cant try it out for myself

55-300 isn't an IF lens, and it's not a mag zoom lens either. Normally this is more obvious to IF and mag zoom lenses

I've done a quick comparison between the DA18250 and DA50200, the subjects taken by DA50200 in close range look quite a bit bigger than those by the DA18250 at the same focal length.

Here is one set of the comparison results, DA18-250 @ 200mm F8 VS DA50-200 @ 200mm F8 (center crop, camera on tripod):

[Vylen]

55-300 isn't an IF lens, and it's not a mag zoom lens either. Normally this is more obvious to IF and mag zoom lenses

I've done a quick comparison between the DA18250 and DA50200, the subjects taken by DA50200 in close range look quite a bit bigger than those by the DA18250 at the same focal length.

Here is one set of the comparison results, DA18-250 @ 200mm F8 VS DA50-200 @ 200mm F8 (center crop, camera on tripod):

hm yea, i figured but i just wanted to make sure

[Alfisti]

All the more reason why those muppets saying the 18-250 covers the 55-300 aree just plain wrong. It barely covers the 50-200.