Hi!

I have been trying to understand my responses to pictures from the D3S, D3X and D800E. Not numbers but appearances. A kitchen experiment in bad weather. It is all trial and error so by posting my efforts I may get some really useful replies. I am hoping!

1 I created a trial target in Word of a table of 65 columns and 32 rows with 2.25 point lines and put in Arial bold 12 point letters and numbers. Laser prined on copy paper. Finer lines and letters were tried but though interesting, the images could not be reduced to show anything for posting. The whole sheet is shown in the attachment 1.

2 This was photographed at an angle of roughly 25 degrees to the horizontal. Mixed lighting in kitchen. Shown in attachment 2. (24 - 70 mm @ 24 mm so some distortion - the tripod was upright!) The distance was adjusted so that the width of the A4 target page exactly filled the image area halfway up the image. The sloping target reveals more about focus, resolution, depth of focus, focus shifts and field curvature over the whole image in one shot than a multitude of flat shots.

3 In these shots I used a Zeiss 100 mm f2 Macro lens. Focused at f2 in LV where available. Focus was on the upper half of the letters E in a central row. Exposure by IR remote, Mirror up. EV = +1.7. Auto WB. ISO 100, or 200 on D3S. Manual adjustment of aperture ring, as noted. RAW files.

4 Processed in PS6. ACR 7.1. Sliders mainly set to 0 with minimal sharpening. Saving the images for posting has lost a lot of detail, so I suggest if you are interested you make your own images to pixel peep. Beware this can be addictive!

5 Comparison images at f4, size reflecting pixel number. Attachment 3 - D3S, Attachment 4 - D3X, Attachment 5 - D800E. I could not composite them and save as a jpg. Brain failure of some kind. Attachment 6 is the three scaled to the same image magnification and composited.

THE DIFFERENCES BETWEEN THE IMAGES ARE MUCH LESS THAN I EXPECTED.

6 The next attempt was an aperture series with the D800E. The lens was stopped down between exposures of f2 to f22. No other changes made. Central slices of the images are composited in Attachment 7. f4 is the blackest image of the E, but the in focus plane is painfully shallow. The increasing diffraction softening from f5.6 and increasing depth of 'focus' to f22 is clear. Note this lens has no focus shift. Some other lenses tried had focus shift that wandered back or forwards up to 4 rows of letters by f8, leaving the primary row of Es out of focus even with the increasing depth of focus.

7 Not illustrated, because I am not posting lens comparisons, I looked at the aperture series of a number of prime lenses from f2 (or f2.8) to f8 right across the image. The following were revealed: that flat field lenses have the row of Es in focus across the sheet even at f2. That some lenses never focus at the edges. That any field curvature shows up as the line of focus curving up or down towards the edges. Most illuminating on some very expensive lenses! That focus shift was frequently found, varied with aperture, and was sometimes severe. It will cripple performance of affected lenses if they are not focussed stopped down in LV, or AF fine tune is not adjusted at the aperture that will be used, so it is important to know which lenses have it.

I hope this is not considered too long, but if it is the moderators should remove it.

Roderick

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Cool

I'd like to see a sample with field curvature in it - so I will know what to look for.

Thanks

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Hi James,

Here are two crops showing slight forward curvature. The top one is from the centre of the frame with the line below E in best focus. The second is at the extreme right and focus has shifted down the slope towards the camera to the H row. Taken on a D800E with Zeiss 21 mm f2.8 lens at f 4. It is enough curvature to make me wary of distant trees in the top corners of the frame, they may be soft if the main focus is fairly close. The foreground however gains in sharpness. This much curvature I do not find a problem and the 21 mm is my most frequently used lens.

The same pictures from the Zeiss f2 50 mm & 100 mm Makro lenses, and the Nikkor 85 mm f2.8 Micro PC lens, in comparison are boring. Fully open the same row of letters is in focus right across the field. These lenses are therefore ideal for copying documents and pictures!

Roderick

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>The sloping target reveals more about
>focus, resolution, depth of focus, focus shifts and field
>curvature over the whole image in one shot than a multitude of
>flat shots.

This particular target is good for illustrating depth of focus around the focus plane and field curvature, but because it's made up of regular fine detail and is at an angle to the lens axis, you can't be sure exactly which row of letters the camera chose to focus upon, particularly when the AF sensors are larger that the viewfinder AF markings.

Brian
Welsh Nikonian
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Thanks Brian,

I agree about the doubt there would be in AF. All these tests are done with manual focus with the lens fully open. Focus is not adjusted during a series.

I have made similar tests with Autofocus, but then I use a high contrast target set up parallel to the plane of the sensor. Then for each aperture I do ten repeat shots setting the lens alternately focussed beyond and before the target. The variability in the focus chosen by the camera, even using the central focus point only, is such that I normally use manual focus, with manual focus lenses, in LV for all critical pictures.


Roderick

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