Can two different lenses demonstrate a different plane of focus at same focal length?

GMac11 · 3 Jul 2020 at 13:19

Sorry for the long title but I thought it was necessary. This is more of a general wondering (and possible discussion piece) rather than an issue.
I was doing some test shots at home to between my Canon 24-105 3.5-5.6 STM and my 16-35 f4 L IS.
I observed what I initially thought was edge softness in my 16-35 @ 24 and 35mm. However across the rest of the frame the the 16-35 was noticeably sharper than the 24-105.

**Disclaimer: I wasn't using a flat surface. I was shooting in my living room that has many different surface types and decorations which is usually a good test for my lenses.**

I then re-tested and discovered that where the the 24-105 @ 24mm f4 was sharp across the entire frame the 16-35 @ 24mm f4 demonstrated softness in the areas that were ever so slightly closer to the camera - around maybe 12 inches closer.
When I took the shot with 16-35 again (@24mm f4) but this time focussed on the closer subjects they were tack sharp but the other details were slightly out of focus.
At f8 the entire frame was tack sharp.

Essentially I am trying to dig deeper and to find out why at the same FL, same aperture and with the same manufacturer of lens does the plane of focus appear to be shallower on one lens than the other?
Is it just part of the small nuances that come into play in lens technology or is it down to standard of glass/aperture blades etc?

Everything else was constant i.e. mounted on tripod which didn't move, IS off and 2 second timer on the camera.

I tried to google this but I found it difficult to come up with the phrasing to show me the search results that were relevant.

Energize · 3 Jul 2020 at 15:03

Were you using AF? Sounds like the lens is probably slightly front focusing, this is a calibration issue.

GMac11 · 3 Jul 2020 at 15:23

Energize said:
Were you using AF? Sounds like the lens is probably slightly front focusing, this is a calibration issue.

I did initially. The first shots were using centre point AF in live view. However the second set we're done with MF to try and prove/disprove my original findings.

juno_first · 3 Jul 2020 at 17:21

Yes.
The Sigma lens I got for a Nikon needed calibrating manually & setting as a pre-set to get the AF working better & even afterwards the infinity marker is not at infinity on the lens barrel.
Though its only a 35mm lens so just needed some trial & error to remember the position of the focus ring for the hyperfocal distances.

GMac11 · 3 Jul 2020 at 17:29

juno_first said:
Yes.
The Sigma lens I got for a Nikon needed calibrating manually & setting as a pre-set to get the AF working better & even afterwards the infinity marker is not at infinity on the lens barrel.
Though its only a 35mm lens so just needed some trial & error to remember the position of the focus ring for the hyperfocal distances.

If it was a focussing issue would the entire frame not be marginally out of focus? As I said the area where the focus point is, be it centre or on the edge, is always sharp.

juno_first · 3 Jul 2020 at 17:38

GMac11 said:
If it was a focussing issue would the entire frame not be marginally out of focus? As I said the area where the focus point is, be it centre or on the edge, is always sharp.

I'd test by shooting wide open with a flat surface so it should all be in focus, but thats a strange condition if the corners or centre can be in focus.
With a poor quality lens the far edges will be more blurred than the centre esp at extreme apertures.

lsg1r · 3 Jul 2020 at 20:10

If I was to guess (no images) it's probably lens aberration/distortion and/or related to the difference in the number of glass elements and the non constant aperture (it can make a difference) on one of the the lenses.

The 24-105 has a larger range and larger focal length at max zoom, the 16-35 is focusing more on wide angle and has a considerably smaller range, meaning they'll have a different lens arrangement to suit their intended purposes.

You're also comparing them at completely differnet positions across the overal focal range relative to one another. While you picked 'equal' focus lengths you comparing them at different extremes, 24mm on the 24-105 is at it's minimum versus the 16-35 in the middle of it's range for example and in general most lenses show the least distortion/aberration etc in the middle of the overal focal range.

GMac11 · 3 Jul 2020 at 22:45

I finally got round to doing a test on a flat surface - which was more difficult than you'd think. There is no exposed brickwork at my house so I had to improvise with a wallpapered wall with a geometric pattern.
So with a flat surface the 24-105 and 16-35 exhibit fairly equal sharpness at the edges - I need a 200-300% zoom to be able to tell any significant difference. This is both at 24 and 35mm.
As you would expect the 16-35 is sharper in the centre but being an L lens I should think so!

So adding this into the mix it definitely does seem that the 24-105 exhibits a slightly deeper DOF at f4 than the 16-35. It's a puzzler!

juno_first · 4 Jul 2020 at 08:38

GMac11 said:
So adding this into the mix it definitely does seem that the 24-105 exhibits a slightly deeper DOF at f4 than the 16-35. It's a puzzler!

At the same mm & aperture the DOF should be the same.
Personally, I stay away from zooms due to their lower quality per buck, especially the wider the zoom range. There are better zooms but they cost a lot.

lsg1r · 4 Jul 2020 at 12:54

GMac11 said:
So adding this into the mix it definitely does seem that the 24-105 exhibits a slightly deeper DOF at f4 than the 16-35. It's a puzzler!

Then its likely the variable aperture and long focal length at play.

While it might say f4 there's probably a tolerence in play (more expensive lenses will have tighter tolerences) to cope with the longer focal range, essentially while it might say f4 it could in real terms be say +/- 0.5 at different focal lengths.

GMac11 · 4 Jul 2020 at 14:30

Yes, one factor I think it could be is the difference distance of the focussing elements to the subject, that's all I can come up with at this time.
It's just odd and one thing to consider when possibly doing a group shot or a shot where all the elements are not on the same plane. What is in focus using the 24-105 won't be in focus using the 16-35.
That being said (as long as lighting allows) I would very rarely shoot such images wide open anyway but just something to bear in mind.