[adwb]

I have 3 x f2.8 zoom lenses all of which are soft when looked at on a monitor never mind being zoomed in on even a little, but by the time you get to f4.0 there is discernible improvement and by f5.6 at 100% zoom the images are sharp.

( they might not be sharp where you expect them but that another subject, back/front focus and focus locked on the wrong contrasty object)

The three lenses are a;-

Sigma 24-70_ 1:2.8 EX DGhsm ;

Sigma 24-60 mm 1:2.8 EXDG;

and a Di SP Tamron 70-200mm f2.8 [IF] macro

is it because they are zoom lenses? is it because they are 3rd party lenses? or am I just unlucky?

Would spending the extra money on genuine pentax lenses fix this?

There seems little point in having these lenses if they cannot sensibly be used wide open

I have not bothered to post samples as I don't think looking at them will help us and just uses band width up.

I have both A series and manual series Pentax primes that are sharp at f2.8 as primes should be.

thoughts and comments anyone?

[alamo5000]

I would say to first check your focus. Try manual focus with live view and then see if they are still "soft".

[adwb]

I would say to first check your focus. Try manual focus with live view and then see if they are still "soft".

same result , reviews of the lenses say they are "soft at 2.8" from I can see.

[alamo5000]

What kind of lighting conditions are you shooting under?

[maxfield_photo]

Few zooms are faster than 2.8. All lenses are softer wide open than stopped down a bit, that's the nature of optics. Therefore any zoom is likely to be soft at 2.8, regardless of whether it's OEM or 3rd party. If you need sharpness at 2.8 you may want to use a fast prime.

[stevebrot]

s it because they are zoom lenses? is it because they are 3rd party lenses? or am I just unlucky?

Conventional wisdom is that very few lenses are as sharp at maximum aperture as when stopped down a bit. Some are worse than others. This is true for primes as well as for zooms.


Steve

[dms]

Most lenses are soft (all are softer) wide open. Close down 1/2 stop and things (should) get much better. I think it is a mistake to expect otherwise. The wide open character may be interesting, but wide open generally allows you to focus more critically.

Maybe my 1/2 stop is a bit optimistic--although I have several lenses where I see this--but it it is often more about other lens attributes. Most lenses should get much better in sharpness by about 2-3 stops closed--as you observed. That is normal.

Throw enough money into the mix, make the lens large (so it starts out already [effectively] stopped down), limit the FL range, etc., and things get better--but such lenses are not a viable solution for most of us.

---

Last edited by dms; 08-04-2014 at 10:22 AM.

[Nick Siebers]

The standard focus screen makes it hard to visualize the exact focal point when you are wider open than f/4 or so. I use an S-type screen in one of my cameras and it helps quite a bit. Hard to get perfect focus at f/2.8 or faster. Live view is generally better, especially zoomed in.

[adwb]

What kind of lighting conditions are you shooting under?

makes no difference

[Kerrowdown]

Should you decide to go the fast prime route, many of these would be already stopped down a bit before your even get to F2.8 and hence performing optically better.

[Miguel]

The cream of the crop are very sharp at f2.8 and lend a real three-dimensional rendering to people in action. I'm talking about Canon 70-200, Sigma 120-300mm Sport, and the Nikon 70-200mm f2.8. But these are a couple of thousand dollars each and not lightweight. Unfortunately these lenses (well, the Sigma) are not in K-mount, so your available choices may indeed be optically inferior.

M

[VoiceOfReason]

Well, the new Sigma 18-35 f/1.8 is nice and sharp at f/2.8. It's also decently sharp even wide open.

[kenafein]

I found the results with my Tamron 70-200/2.8 were pretty good at 2.8, but not prime quality. The new version of the Tamron, with VC, is supposedly excellent.

[wtlwdwgn]

Most lenses have a sweet spot where they perform the best. If the max aperture is f/2.8 the sweet spot will likely be f/5.6 and f/8. Mike Johnston has a good article about getting the best out of lenses.

[HenrikDK]

Like most everyone else has mentioned almost all lenses are softer wide open. The bigger question is really "is it good enough". Shooting wide open may produce better results than f8 if you are handholding even if you are within "handheld limits", even with higher ISO. Unless you go tripod I personally err on faster shutter speed. I have plenty of shots that are taken wide open with lenses that supposedly don't perform well wide open, yet the results worked really well.

In short don't expect most lenses to do great wide open, but know the limitations and don't be afraid to use lenses wide open.

The picture below is FA 35mm at f2.8 and 1/40 with a K-01. Tries to balance depth of field, ISO, and shutter speed. I would not have hesitated to use f2 if I wasn't tried to increase the depth of field - which was still insufficient. F8 and a tripod would have worked much better, but this was a snapshot...