[Serkevan]

I'd take a look at something like the D FA 28-105. At ~50-70mm it has quite even sharpness; I certainly haven't seen "completely unsuitable" edges. I can try to get some samples later.

[bdery]

It is not something I have ever seen measured in a lens review test but I now think should be; what appears to be edge softness on a flat test chart is in fact more likely to be this effect. It is a real issue.

As a reviewer having done many lens reviews for Pentaxforums, and as an optical designer by trade, I can tell you that in the context of a lens review, actually measuring field curvature is extremely complex.

Observe it? Sure, I do that all the time and report on it.

Give a feel for it? Yep, by noting how much I have to change the focus position between center and edges.

Measure it? I'd need different tools.

When I was testing the 10-17mm fisheye, I have to come up with a completely different way to measure the lens's sharpness, because when doing that I report the best the lens can offer, not the worst.

[stevebrot]

Thanks for the heads up. I lost touch with Rodger after he left lens rental.

He is still at Lens Rentals (he owns the place) and still posting there as well...

Lens Rentals | Blog


Steve

[stevebrot]

I am an experienced and qualified photographer and I know understand what to concentrate on. I have taken landscapes with the 50 1.4 at f8 on full frame (k1) and whilst diffraction is already taking over, it so front focused at the edges as to be completely unsuitable for this task; that's a fact.

I shoot primarily landscapes and have yet to encounter what you are describing, despite decades of shooting with lenses having readily detectable field curvature at moderate to close distances. That being said, I accept that it is a fact that you have found a way of detecting such under field conditions. An example photo showing the severity of the problem might be helpful. BTW...you aren't doing panos are you?

What would be helpful would be suggestions for an alternative for this task.

As noted above, using a flat-field lens might help. Most quality macro lenses should fit the bill along with higher-end wide-angles. Addendum: At 50mm, perhaps a quality Tessar?


Steve

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Last edited by stevebrot; 04-30-2021 at 12:08 PM.

[Serkevan]

I shoot primarily landscapes and have yet to encounter what you are describing, despite decades of shooting with lenses having readily detectable field curvature at moderate to close distances. That being said, I accept that it is a fact that you have found a way of detecting such under field conditions. An example photo showing the severity of the problem might be helpful. BTW...you aren't doing panos are you?



As noted above, using a flat-field lens might help. Most quality macro lenses should fit the bill along with higher-end wide-angles. Addendum: At 50mm, perhaps a quality Tessar?


Steve

The 50/2.8 Tessars should work, yup... if we're talking Zeiss, maybe the Planar 50/2? More expensive, for sure, but...

[malcolmdavies]

I'd take a look at something like the D FA 28-105. At ~50-70mm it has quite even sharpness; I certainly haven't seen "completely unsuitable" edges. I can try to get some samples later.

Thank you Serkevan. However, I am looking for a suitable 50mm prime for landscape. I note that users of both the A 28mm f2.8 and D FA 50mm F2.8 Macro who wanted a good 'copy lens' say these both have 'flat fields' which suggest that they should also be good landscape lenses, but they are naturally more concerned discussing its macro attributes. I would like to know if anyone has used these particular lenses on full frame K1 for landscape and can comment on field curvature and edge sharpness at distance. I note that these two lenses are completely different optics so am interested in their relative merits - for landscape. Thank you.

[steephill]

I am a little surprised that you claim that diffraction is taking over at f8 on the K-1. The diffraction limit for the 36Mp K-1 sensor is about f9.5 so you shouldn't be seeing significant diffraction effects at f8. Can you show examples which show your diffraction limited images?

[malcolmdavies]

I shoot primarily landscapes and have yet to encounter what you are describing, despite decades of shooting with lenses having readily detectable field curvature at moderate to close distances. That being said, I accept that it is a fact that you have found a way of detecting such under field conditions. An example photo showing the severity of the problem might be helpful. BTW...you aren't doing panos are you?



As noted above, using a flat-field lens might help. Most quality macro lenses should fit the bill along with higher-end wide-angles. Addendum: At 50mm, perhaps a quality Tessar?


Steve

I am posting evidence of field curvature (Petzval-curvature) on the M 50 1.4 as requested. I have fined-tuned the infinity focus end stop a fraction, which has not obviously effected far distance in the centre which is optimum, but seems to have improved front focusing at the edges a little, although as you can see it is still very much there. The first image is f5.6 and the second f8 (optimum for diffraction) both focused at infinity (if I hyperfocus then front focusing will obviously only be worse). Both are 100% crops of the leftmost 20% of the frame with an inset to show sharpness at the centre of the frame.
Firstly, you can see from the centre insets that the images are correctly focused at 300m. The white building and trees at the left edge of the frame are also 300m away but are out of focus, whist the rugby posts in front of these buildings and only 100m away are in focus, clearly showing field curvature increasing at the edge of the frame.
What I really would like to know is if anyone has used the A 50mm f2.8 macro or D FA 50mm F2.8 Macro or even the 50 1.7 on full frame K1 or Sony full frame for landscape and can comment on field curvature and edge sharpness at distance. I note that these two macro lenses are completely different optics so am interested in their relative merits - for landscape. I do not wish to seem ungrateful, however only experience of the K1 or Sony full frame bodies is of value to me regarding this and whilst I welcome all comments I would be grateful if you could kind state the camera being used.

[stevebrot]

I am posting evidence of field curvature (Petzval-curvature) on the M 50 1.4 as requested. I have fined-tuned the infinity focus end stop a fraction, which has not obviously effected far distance in the centre which is optimum, but seems to have improved front focusing at the edges a little, although as you can see it is still very much there. The first image is f5.6 and the second f8 (optimum for diffraction) both focused at infinity (if I hyperfocus then front focusing will obviously only be worse). Both are 100% crops of the leftmost 20% of the frame with an inset to show sharpness at the centre of the frame.

Ok...Am I correct in reading that the photo may have been taken with the focus ring at its infinity stop?

Firstly, you can see from the centre insets that the images are correctly focused at 300m. The white building and trees at the left edge of the frame are also 300m away but are out of focus, whist the rugby posts in front of these buildings and only 100m away are in focus, clearly showing field curvature increasing at the edge of the frame.

Are both the "center" inset and the "left edge" reproduced with the same magnification?

I am assuming that the intended point of focus is somewhere in the "center" inset. The "center" inset on neither of the examples is particular sharp, with the rugby post being the object in focus in both examples. Were you up against the infinity stop for these photos? If not, how were you determining focus?


Steve

[malcolmdavies]

Ok...Am I correct in reading that the photo may have been taken with the focus ring at its infinity stop?



Are both the "center" inset and the "left edge" reproduced with the same magnification?

I am assuming that the intended point of focus is somewhere in the "center" inset. The "center" inset on neither of the examples is particular sharp, with the rugby post being the object in focus in both examples. Were you up against the infinity stop for these photos? If not, how were you determining focus?


Steve

Here is a further example at f8. As explained, all crops are 100% (therefore are the same 'magnification') and focused at infinity. Whichever way you look at these, field curvature is clearly evident and what I wish to know is how differently the other lenses I have asked about perform on full frame.

---------- Post added 05-01-21 at 03:34 PM ----------

I have noticed that Pentaxforums has expanded my posted images above 100% and makes them appear softer than at actual 100%. However, It doesn't alter the evidence of field curvature.

[malcolmdavies]

Here is a further example at f8. As explained, all crops are 100% (therefore are the same 'magnification') and focused at infinity. Whichever way you look at these, field curvature is clearly evident and what I wish to know is how differently the other lenses I have asked about perform on full frame.

---------- Post added 05-01-21 at 03:34 PM ----------

I have noticed that Pentaxforums has expanded my posted images above 100% and makes them appear softer than at actual 100%. However, It doesn't alter the evidence of field curvature.

Here is the full image.

[stevebrot]

Here is a further example at f8. As explained, all crops are 100% (therefore are the same 'magnification') and focused at infinity. Whichever way you look at these, field curvature is clearly evident and what I wish to know is how differently the other lenses I have asked about perform on full frame.

Alternatively, your example may show a lens focused much closer than 300m.* The grass at image center might be in agreement.
The comparison is best done using two objects at the same distance and placing focus on one or the other.

I was hoping you would provide an example of a landscape image where the edges were compromised due to field curvature (your description of the problem). Given the nature of DOF,* a subject at less than 300m might be more likely to show the problem such case rather than one where detail is near the limits of sensor resolution. Given sufficient distance, nothing is sharp.

Regardless, several options in the 50mm range are suggested in this thread. With any luck one will meet your needs.


Steve

* Even careful adjustment of the infinity stop using a lens collimator involves setting an arbitrary value for what constitutes infinity.

[mikesbike]

I have both the FA 50mm f/1.4 and the F 50mm f/1.7 both bought new way back. My 50mm f/1.7 is better overall edge-to-edge, especially at wider apertures. It seems the DFA 50mm f/2.8 macro should deliver a relatively flat field.

[malcolmdavies]

Thank you Mikesbike, that is helpful information.

If anyone has experience of the DFA 50 2.8 on full frame for landscape, I would be grateful for your observations of edge sharpness. Reviews of this lens tend to concentrate on it macro function.

[malcolmdavies]

Problem solved. I acquired an F 50 1.7 today. Edge to edge, corner to corner: 5.6 acceptably sharp; f8 very sharp. No Infinity end stop adjustment required!