Kit lens soft in when held in portrait format


Pentaxophile

Link Posted 01/08/2009 - 22:25
I was reviewing my shots when I noticed some of the ones taken with the kit lens, were soft at one end. I have noticed this more with shots taken in portrait format for some reason. I wonder if something is wrong with the lens?

The effect is noticeable here: link

And this image shows a crop from a shot with the camera held in portrait format with the grip held from below (left) and from above (right) - which is clearly worse. The crop is from the top of the image.



[link=https://500px.com/will_brealey/[/link]
Last Edited by Pentaxophile on 01/08/2009 - 22:27

George Lazarette

Link Posted 01/08/2009 - 22:43
Most lenses (though not macros, generally) have a curved field of focus - so if you focus on a particular point, every other point on the curve will tend to be in focus. A flat surface like a brick wall will not all be in focus unless you stop down.

So to be sure you have a problem, you need to be certain that the points that are NOT in focus are in fact MEANT to be in focus.

A tree is not the right subject for a test of this nature.

G
Keywords: Charming, polite, and generally agreeable.

Pentaxophile

Link Posted 02/08/2009 - 00:20
Thankyou George, I will find a secluded brick wall and conduct some proper tests...
[link=https://500px.com/will_brealey/[/link]

kerrowdown

Link Posted 02/08/2009 - 11:24
I know it may sound daft, but you say it happens more in portrait mode, have a quick check at the way your holding the camera, might it may be partially down to technique, just a thought.

Just reading my reply, Nah, it sounds daft me even saying it, how could it be down to that, ignore me I'll go away.
Kerrowdown... Aka Romantic Highlander.

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Last Edited by kerrowdown on 02/08/2009 - 11:32

George Lazarette

Link Posted 02/08/2009 - 12:18
When looking up, the subject will be further away at the top than at the bottom. Both can't be in focus, unless you are stopped well down. Portrait mode will emphasise this point.

G
Keywords: Charming, polite, and generally agreeable.

fatspider

Link Posted 02/08/2009 - 12:57
Goerge's explanation sound the most likely cause, however you also need to take into account the focal length of the lens too, do a few test shots at various apertures and also vary the focal length from wide open to full zoom.
If the wide open shots at large apertures give worse results (ie say 18mm and f5.6) than the zoomed in ones stopped down (say 50mm and f16) then there is no problem and you've just discovered the innate characteristics of lenses.

EDIT. sorry I didn't spell GEORGE correctly (and you know what he's like
My Names Alan, and I'm a lensaholic.
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Last Edited by fatspider on 02/08/2009 - 12:59

Pentaxophile

Link Posted 02/08/2009 - 15:01
Thanks all.

Quote:
When looking up, the subject will be further away at the top than at the bottom. Both can't be in focus, unless you are stopped well down. Portrait mode will emphasise this point.

Yes! I thought that was what it might be, especially as the issue came to light with this shot, and similar, which I was aware would need perspective correction (and therefore the top of the building is further away):


But further testing shows the soft effect is only at the top when holding the grip from above (as is my habit), or on the right hand side when shooting landscape (therefore the portrait thing seems to be a red herring, sorry about that)... it is progressively more pronounced as you zoom in.

At 55mm, f5.6, camera held in portrait. Crop from centre-top on the left / centre-bottom on the right:




Same test at f8:




And I took this in landscape format at 55mm, f8... which I would have thought would eliminate any focus issues as the bank is straight and directly opposite my boat:




Crop from left / centre / right:



[link=https://500px.com/will_brealey/[/link]

George Lazarette

Link Posted 02/08/2009 - 15:28
Well, there are three problems here. The first is that most lenses have a curved field of focus (which is why brick walls are a bad test subject, unless you know what you are doing. For this reason, if the centre of the bank is in focus, the edges won't be.

Second, most lenses are noticeably sharper in the middle, so soft edges are not unusual.

Third, the right side does seem noticeably worse than the left. This could conceivably be a sensor alignment problem. I can't see how it could be the lens (unless it has been dropped).

G
Keywords: Charming, polite, and generally agreeable.

George Lazarette

Link Posted 02/08/2009 - 15:29
impotentspider wrote:

EDIT. sorry I didn't spell GEORGE correctly (and you know what he's like

Growl.

G
Keywords: Charming, polite, and generally agreeable.
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