Sharpness/lens/stopping down quick question

Mountainsandh

Hi all, I might find info about this somewhere else but I'm a bit lazy and in a hurry .

In short : on an average kit lens, would sharpness increase if stopping down a bit more ? don't understand that sharpness/aperture malarchy.

I'm thinking mostly of my 75-300 mm kit lens that came with my Pentax K-x. I like the lens but now that my eye is a bit more critical I'm not super happy with the sharpness of it (obviously especially when zoomed in any bit), and actually I'll throw that in too, I get a lot of "something complicated" light/colour aberration (like a purple tinge around the edges of things).

So, if I stop down a bit and compensate in shutter speed, or Iso, am I likely to get better results ?

I must say I have often been using it pretty wide open (5.6 ) just because I shake a fair bit and I have this notion my pics will be better with a reasonably fast shutter speed (/250).

I'm off to Paris tomorrow and expecting a lot of high contrasty buildings and little rectangular windows sort of thing in my shots .

Thanks !

Prenderb

Yeah lenses might improve if you stop down a little.... try it at f8 or so?

Bear in mind the k-x has in-body shake reduction (yay!) which helps.

Shutter speed, in general just keep it at 1/focal length if that makes sense, should be sufficient to stop motion.

Ghost Train

Not using extreme end of the zoom will normally give noticably better results, at least on cheaper lenses. All about finding best balance of settings for the conditions. Want shutter speed above managable setting for steady shot, iso below what level you consider too noisy, aperture not so small that diffraction makes images soft and not too large that lens give soft result. On a bright sunny day, it might be easy enough to get the results you want on a dull day you might need to sacrifice some (non)noise and sharpness

Is this the lens http://www.pentaxforums.com/lensreviews/SMC-Pentax-FAJ-75-30mm-F4.5-5.8-Zoom-Lens.html , few comments on what other find with it

Mountainsandh

Fantastic link Ghost Train, thanks ! And Prenderb too, yes, I will try F8/11, that's what they advise on the link too, and that's what I had found when I googled quickly, even on Canon lenses apparently.

The Pentax is good with 400 iso, but I think rather than do that, I might just try and be more adventurous and try lower shutter speeds... I'm conscious of always shooting wide open, and I have been trying to take care to be more steady and use lower shutter speed, so there you go, now I'll just have to and that's that.


Better go to bed now, thanks for the quick replies.

Mountainsandh

Thanks again, as it happened, the bloody lens seized at the foot of the Eiffel Tower, so I was 55-300mm-less (don't know why I had the 75 in mind, it's 55) for the 3 days.

So I used my Porst lens, which is all manual, it's an old (very cheap and a bit wonky but I love it) one for film, and this is the kind of shots i was talking about that I would probably have had to stop down on.


I stopped down a fair bit on that too, but it was a very murky day so did have to brighten in PP, didn't want to compromise too much on ISO.

5uspect

Every lens is different and has it's own sweet spot. It may not be a few stops down.

Going back to squareballoon's 80's video games:
http://www.dpreview.com/lensreviews/widget/Fullscreen.ashx?reviews=3&fullscreen=true&av=3.667&fl=18&vis=VisualiserSharpnessMTF&stack=horizontal&lock=&config=/lensreviews/widget/LensReviewConfiguration.xml%3F4


This lets you plot the sharpness of the Pentax 18-55 at different f-numbers and focal lengths. The higher the MTF the sharper the lens.
The left half of the plot shows a "heat map" of the sharpness over half of your image. It's sharper in the middle and falls off at the corners. The graph shows petty much the same thing.

The lower graph shows how good the lens is at not separating colours.