Strange Performance of Sigma 24-70 DG DN II on SL3 - 3 Copies

[JTLeica]

Advertisement (gone after registration)

Hi guys,

I am about to give up on this lens for good, this is my 3rd copy, and every one of them produces a similar result. At and around the 35mm focal length, there is an extremely strange thing happening, I am focussing to infinity, the houses in the distance are 150-200m away, the lens is sharp centrally but always soft on the edges at infinity, however, in front of the subject it seems to be sharp, almost like an extremely exaggerated U-Shaped plane of focus. Its really odd.

I bought a 28-70 Sigma and that is just fine. This is the 3rd copy of this 24-70 Mark II from sigma and I am not sure what else to try. Strange to say the least.

Any thoughts or tests you can try if you have the same combo?

EDIT - just tried on the Sigma FP-L, same performance there too... so I am starting to wonder if at 35mm (as all three copies show the same) the lens just sucks in the corners at wide open.

Thanks again

Edited November 9, 2024 by JTLeica

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[Ivar B]

Very strange indeed. I don`t know, but I would be very surprised if there is a design flaw or something which would produce this poor result. I am sure Sigma would not have launched such a lens design, and VII is supposedly a little better that V1.

So can we have a very poor quality control so three lenses which should have been stopped were indeed sold? Sounds very strange also, but more probable than the first explanation.

You have also ruled out camera error.

I know also that a very talented photographer here in Norway bought the new Sigma 5.6/500 and it was simply not sharp and had to be returned and repaired.

So it may happen that quality control is not as stringent as it should be. 

Edited November 11, 2024 by Ivar B

[Babylonia]

Seems a typical example of "field curvature".
Zoom lenses of all kind of brands can have such flaws in general, for one or the other focal length.

E.g. I have an old manual Voigtländer SL lens  58mm - F 1.4
That have a kind of "parabolic" shape of the field in focus.

[IP54]

I know the 24-70DGDNII very well and i can confirm it has a strong field curvature, the corners are sharp closer than the center.

The 24-70DGDN first version had an inverted field curvature, the borders were sharp behind the center.

Anyway, this V2 is very sharp where you make the focus, just not a good choice for brick walls

[JTLeica]

31 minutes ago, IP54 said:

I know the 24-70DGDNII very well and i can confirm it has a strong field curvature, the corners are sharp closer than the center.

The 24-70DGDN first version had an inverted field curvature, the borders were sharp behind the center.

Anyway, this V2 is very sharp where you make the focus, just not a good choice for brick walls

Gods sake brick walls are my passion!

Man thank you so much. So I have now tried this extensively, the latest one I bought seems much better than the others.

Can anyone here answer this... In regard to my testing method.

When I tested originally, I focussed in the centre, then recomposed and placed the central subject in each corner (4 total shots, one each corner) so I could compare the same subject in each corner. This resulted in horrendous images, well, poor anyway, with the corners being blurry.

However, I then took it to my top bedroom and shot the same scene over the houses in the distance, all at infinity, at 35mm F2.8 and it was fine. Not APO sharp of course, but totally fine for wide open. Why would recomposing from a central focal point change the result here?

[IP54]

What i can tell you is that, with the 24-70DGDNII, you'll get the better results when focussing where you want i sharp, without recomposing, due to it's field curvature.

[JTLeica]

Advertisement (gone after registration)

Thanks IP54.

What I cannot get my small brain to understand, is if I focus on a house (200m away) in the centre and recompose to the corner, its blurred.

If I focus on something else at infinity (A house the same 200m away), that same house is in focus...

Cannot work it out.

[BernardC]

39 minutes ago, JTLeica said:

What I cannot get my small brain to understand, is if I focus on a house (200m away) in the centre and recompose to the corner, its blurred.

If I focus on something else at infinity (A house the same 200m away), that same house is in focus...

That's because the lens is designed to have a flat field at infinity (brick wall syndrome!). So the centre of the image is at 200m, but the corners might be at 210m. When you recompose, you have the corners focused at 210m, but your subject is at 200m so it looks blurry.

Lenses with so-called field curvature have a focus plane that is equidistant from the camera. Think of a circle drawn on the ground, with you in the middle.

I am over-simplifying. No lens has a perfectly flat field, or a perfectly circular field. The shape of the focus plane isn't even consistent from centre to corner.

The moral is to focus on what's important!

[jaapv]

1 hour ago, IP54 said:

What i can tell you is that, with the 24-70DGDNII, you'll get the better results when focussing where you want i sharp, without recomposing, due to it's field curvature.

That is nothing new Gűnther Osterloh had a standard phrase in his lectures for the Leica academy over a decade ago: ,If you want something sharp in your image, focus on it. 

[Richardgb]

On a general point, you might be interested in this article from Lensrentals about 24-70mm zooms. It predates the latest Sigma / Leica zooms, but the principles are worth taking on board.

[IP54]

35 minutes ago, Richardgb said:

On a general point, you might be interested in this article from Lensrentals about 24-70mm zooms. It predates the latest Sigma / Leica zooms, but the principles are worth taking on board.

excellent article, thx

[jaapv]

Like all Lensrentals articles 😉

[JTLeica]

Thank you, very interesting indeed

[Stuart Richardson]

I say this without trying to be shaming and with no Schadenfreude, but this is why I use the APO summicrons. I struggled with this a lot on the Leica S as well. The 70mm lens has pretty noticeable field curvature and I live in a country where there are lots of mountains in the distance, often with little in the foreground. So I found that my very sharp and excellent Leica lens could not take fully sharp pictures of the landscape unless I stopped it down to f6.8. This was harder for me, as I did a lot of night work and photography in the winter, where it is always dark. The 120mm did not have this behavior at all, nor did the 45mm, but the 70 has it noticeably. When I transitioned to the SL, I noticed that the APO Summicrons that I had (35mm and 50mm) do not have this issue. People love zoom lenses because of convenience, but they are, even in 2024, an optical compromise. Even the mighty 90-280mm APO zoom does not equal the performance of a small 90mm APO Summicron M from 1998.  I find field curvature to be one of the most frustrating properties in lenses that have it, as it can be difficult to visualize when and where it will occur. For me at least, having to switch between lenses is a small price to pay for getting rid of it completely. It may not be such a bother for you if you live in a more three dimensional country (Iceland has so much emptiness in the foregrounds, due to the lack of trees and the wide open vistas), and of course a zoom lens is practical in many situations, but if you want to get rid of field curvature, a good prime lens (like the APO Summicron) might be in order. 

[IP54]

same, apo28 and apo50 for 90% of my industrial and packshots work, in any case, anywhere, it's sharp, period.

I recently tested the sigma 50/1.2 , fantastic lens, almost equal sharpness with the apo50, but some field curvature too, so only for portraits, or bokeh style

[hdmesa]

On 11/9/2024 at 5:51 AM, JTLeica said:

Hi guys,

I am about to give up on this lens for good, this is my 3rd copy, and every one of them produces a similar result. At and around the 35mm focal length, there is an extremely strange thing happening, I am focussing to infinity, the houses in the distance are 150-200m away, the lens is sharp centrally but always soft on the edges at infinity, however, in front of the subject it seems to be sharp, almost like an extremely exaggerated U-Shaped plane of focus. Its really odd.

I bought a 28-70 Sigma and that is just fine. This is the 3rd copy of this 24-70 Mark II from sigma and I am not sure what else to try. Strange to say the least.

Any thoughts or tests you can try if you have the same combo?

EDIT - just tried on the Sigma FP-L, same performance there too... so I am starting to wonder if at 35mm (as all three copies show the same) the lens just sucks in the corners at wide open.

Thanks again

You simply can't expect a zoom to be sharp in the corners when focused ~infinity and shot wide open. Even primes struggle with this unless they are very flat field lenses. The only thing that you should be looking for is equal performance in all the corners – that is, corner performance is uniform comparing them all to each other.

Stop down until you find the aperture that gives you the corner sharpness you want. Even the weakest focal length on a 28-70 zoom should sharpen up in the corners to acceptable at f/5.6 and very good by f/8.

[Planetwide]

This will make sense of it all. Once you learn the field curvature of your lens, you can use it to your advantage.

I still have an old minolta 24mm with user variable field curvature!

https://photographylife.com/what-is-field-curvature

Edited December 13, 2024 by Planetwide