Sigma versus Nikon

[sirhc55]

I have tried some very quick and rudimentary comparative tests with the Nikon 18-70DX, Sigma 55-200 DC and the Sigma 105 DG Macro with regards to sharpness.

I had originally thought that I had a problem with back focus on the D70 but after trying Tim Jacksons focus test chart I came to the conclusion that the D70 was OK.

It is actually the Nikon lens that is at fault. In the sharpness tests the 55-200 is superb and also the 105 Macro and the Nikon comes a low third. The Nikon lens is basically “soft”.

If anyone else has experienced this please let me know.

Cheers

Chris

[Glen]

Interesting thought Chris. I have a few other older Nikon lenses if you wish to test versus kit (we are in the same suburb) or I might try myself. Maybe the kit lens is a cheap special?

[Onyx]

Surprising results - maybe you do have a bad sample, as most reports and my experience suggests the 18-70DX being one of the sharpest lenses of modern day Nikkor.

What apertures and focal lengths did you conduct the tests at?

[sirhc55]

Surprising results - maybe you do have a bad sample, as most reports and my experience suggests the 18-70DX being one of the sharpest lenses of modern day Nikkor.

What apertures and focal lengths did you conduct the tests at?

I used f22 with the SB800 on A mode - being as it was comparative.

[MHD]

Hmmm... Well I love the 18-70DX... BUt I also love the 70-300G...
So mabie I am just eaisly pleased!

[Onyx]

That may explain it - f/22 on the kit lens will definitely be noticeably soft due to diffraction. Since you're comparing a wide angle zoom (18-70) with tele lenses (105 and 55-200) at f/22 - the absolute aperture diameter is much smaller and closer to the physical limits of diffraction impacting on image sharpness at shorter focal lengths than it would be at tele focal lengths.
eg. at 50mm focal length, f/22 yields 2.27mm diameter aperture.
At 150mm focal length, f/22 yields 6.81mm diameter aperture.

For various reasons possibly best explained by Lens Sheperd on Nikonians.org, the ideal optimum aperture for many consumer grade lenses on a dSLR is f/6.3. If you were to test the lenses at that aperture, I'm sure the comparative rankings will change.

[Glen]

Onyx, thanks for that interesting snippet (you can see all my photos will now come out at f6.3!). Is there anywhere you suggest to go (I will try Nikonions) to find out optimum aperture for different lenses? Being not good enough or keen to test lenses myself, I have usually just read what someone smarter than myself has suggested as the lenses sweet spot

[Onyx]

For film, the theoretical "best aperture" was f/8. For digital, it's 2/3 stops less than f/8, ie. f/6.3. Len Sheperd on Nikonians wrote up the scientific/optical justification for that, and I tried searching for thread that I'm recalling, but no luck. However, inspite of it all I came across this, also written by him:

"...a theoretically less than perfect "sweet spot" aperture shot with the right dof field for the subject is far better than a "sweet spot" aperture shot with the wrong dof."

[gstark]

For film, the theoretical "best aperture" was f/8. For digital, it's 2/3 stops less than f/8, ie. f/6.3. Len Sheperd on Nikonians wrote up the scientific/optical justification for that, and I tried searching for thread that I'm recalling, but no luck.

I'd be interested to see this; I'm curious as to why it might be different.

However, inspite of it all I came across this, also written by him:

"...a theoretically less than perfect "sweet spot" aperture shot with the right dof field for the subject is far better than a "sweet spot" aperture shot with the wrong dof."

Huh ???

Wouldn't this depend upon the relative quality of the "sweet spot" as compared with the general quality of the lens's DoF?

[Glen]

Thanks for looking Onyx, will keep those thoughts in mind

[Matt. K]

Your information is pretty spot on. However when it comes to lenses sweetspot...every lens is different and the variables are the f/stop and the focus distance and the individual optical elements. No 2 lenses will have the sweetspot at the same settings and finding it is trial and error and sometimes just plain luck. Many lenses may not even have a sweet spot?
Just my opinion

[sirhc55]

Thanks to everyone for their replies. It just shows you are never too old to learn something - thanks again

Chris

[sirhc55]

I just had a thought that in my 10 years of using digital cameras (Kodak/Nikon, Nikon D1 and now the D70) I have never experienced the problem I now have with the “kit” lens. I have used many types of lens - Nikon - Tamron - Tokina - Sigma but this is the first time I have experienced this phenomena.

Basically, the way I see it, if I want to use f22 for a larger DOF then I have to put up with the resulting picture looking out of focus - which seems to me to defeat the purpose of small f numbers. My previous lens (among others) that I used with the D1 was a 28-105 (if I remember correctly) and it was as sharp as a tack.

[Matt. K]

It's a well known fact that f/stops smaller than f16 begin to lose a little sharpness. When doing macro photography you have to sacrifice this sharpness in order to get the depth of field you need. However...it's not a lot of sharpness...just a little...and maybe the softness you are seeing is due to camera movement....the smaller the f/stop then the slower the shutterspeed?

Just an idea.

[sirhc55]

It's a well known fact that f/stops smaller than f16 begin to lose a little sharpness. When doing macro photography you have to sacrifice this sharpness in order to get the depth of field you need. However...it's not a lot of sharpness...just a little...and maybe the softness you are seeing is due to camera movement....the smaller the f/stop then the slower the shutterspeed?

Just an idea.

Hi Matt

It’s not the macro that’s the problem - it’s the Nikon and it is not macro!!