I got my 2nd copy of Sigma 24-70mm HSM lens

Qarik

If you haven't been following my travails with this lens...a very short recap. This is a new lens from sigma and it is a total redesign of the older 24-70mm with added HSM focus motor. It is 2x the price of the older lens at $900. I considered this lens in place of the Nikkor 24-70mm which is reputed as the sharpest zoom lens ever created by Nikon which sells for $1700.

I 1st ordered the lens from adorama and they sent me the canon mount by accident. I returned the lens and ordered it from B&H. The 1st copy I got had very bad back focus issue.

(all shots are 100% crop, tripodded, and remote triggered)

1st copy of the sigma lens at 70mm and 2.8


vs

nikkor 24-70mm I borrowed from coworker at 70mm and 2.8



So I sent back the lens to B&H and ordered another copy. This one is sharp. Real sharp. I can't say the nikkor is better or not.



The crop is from center (top button):


The interesting thing is edge sharpness vs sigma and nikkor.

This is 24mm sigma at 2.8..look at the framed cat embroidery in the right upper hand corner


compared to the nikkor version.


Okay granted these were taken at different times but even the 1st copy of the sigma showed this difference. I initially attributed this to the back focus issue on the 1st copy but this 2nd copy is showing the same result. Note the wall texture is more true in the sigma as well. It almost appears that the sigma lens has a larger DOF the the nikkor lens..at the SAME DISTANCE, ZOOM, and APERTURE. Does that even make sense that a lens has more DOF? I suppose the cat sharpness may also be a function of center to edge sharpness characteristics as well.

Note the cat embroidery is about a foot behind the focal plane of padington bear. The camera sensor plane is 92 inches from the paddington and the depth of focus is calculated about 2.5 inches in front and behind for a total DOF of 5 in. I do realize that the cat and bear are not in the same focal plane.

My results show that the sigma lens is as sharp wide open at center at 70mm and 24mm and at least sharper at the edge at 24mm. This is crop sensor as well so this might be magnified on FF. Finally the AF worked well in dim lighting (and good lighting as well). The build quality is very good though not quite as elegant as the nikkor which is exceptional. It has a nice heft to it, it is about 8 ounces less then nikkor and over an inch shorter when not extended. The nikkor is quite heavy and long. One issue is that the sigma filter size is 82mm.

Truth be told, I was looking for an excuse to send it back and order the nikkor. If it wasn't perfect I was going to...now i can't bring myself to. The only nit from these shots on the sigma..if you look real close at the cropped shot around the edge the wood clasp/button border..you can see some banding or pixelation along with CA? Not sure how to describe it. The nikkor shows the CA but not so much the banding/pixelation. :scratch :huh

Moogle Pepper

Good to hear that you got an even better one!

Qarik

The apparent larger depth of field for the sigma lens is vexing me. (again look at the wall texture and cat embroidery behind the chair in the larger photos sigma vs nikkor). How the heck can 2 lenses at the same distance, zoom, and aperture have different depth of field?

I understand that there will be some variation in zoom and aperture for those lens..for instance the supposed 70mm end of each lens may not be exactly 70mm or the 2.8 aperture may bot be exactly 2.8. One might be 69.5mm whiel the other is 70.mm for example. I get that.

But the wall behind the chair is good 12 -24 inches away from the focal plane of paddington bear. Fractional differences in zoom and aperture would only account for fraction of inches inches in DOF at a distances of 92 inches (I remeasured that as well).

I admit "some" of the difference in apparent sharpness may be due to contrasts of walls but it is clear to me that factor is minimal when you study the photos closely.

DOF is determined by lens phyisics and is not a characteristic of the lens itself with exception to the acceptable circle of confusion of the lens. This factor is 2nd order effect though.

Anyway...I can't explain it ..but I will take it. If I can get more DOF from the sigma lens..then I have more flexibility to shoot at wide open. For example I will have more margin shooting a headshot in low light with the sigma lens over the nikkor lens.

ziggy53

Qarik wrote:

The apparent larger depth of field for the sigma lens is vexing me. (again look at the wall texture and cat embroidery behind the chair in the larger photos sigma vs nikkor). How the heck can 2 lenses at the same distance, zoom, and aperture have different depth of field?

I understand that there will be some variation in zoom and aperture for those lens..for instance the supposed 70mm end of each lens may not be exactly 70mm or the 2.8 aperture may bot be exactly 2.8. One might be 69.5mm whiel the other is 70.mm for example. I get that.

But the wall behind the chair is good 12 -24 inches away from the focal plane of paddington bear. Fractional differences in zoom and aperture would only account for fraction of inches inches in DOF at a distances of 92 inches (I remeasured that as well).

I admit "some" of the difference in apparent sharpness may be due to contrasts of walls but it is clear to me that factor is minimal when you study the photos closely.

DOF is determined by lens phyisics and is not a characteristic of the lens itself with exception to the acceptable circle of confusion of the lens. This factor is 2nd order effect though.

Anyway...I can't explain it ..but I will take it. If I can get more DOF from the sigma lens..then I have more flexibility to shoot at wide open. For example I will have more margin shooting a headshot in low light with the sigma lens over the nikkor lens.

I don't see that much difference between the 2 images at the posted resolution but what you may be seeing is either:

1) Field Curvature/Astigmatism, where the image field is not flat but, in this case, the field may be curving rearward, which could mean that a scene which is flat may not be sharp edge to edge. More explanation here:

http://toothwalker.org/optics/astigmatism.html

2) Effective Aperture design differences, applicable to most zooms and especially "constant aperture" zoom lenses, the apparent aperture is constant at the expense of physical aperture, meaning that similar lenses may indeed have the same "effective" aperture while having a different "physical" aperture, and this can impact the DOF (marginally).

See this post for a bit more insight:

http://www.dgrin.com/showpost.php?p=523730&postcount=2

I am not saying that the Sigma is displaying either of these properties, especially as compared to the Nikon lens. Only suggesting possible explanations.

Qarik

interesting ziggy..the curvature issue might help to explain some of the edge difference.