Sensor vs Lens constraint

[pgk]

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I find this very interesting. Could you elaborate a bit more on the particular lens constraints you've found?

 

Cheers

Philip

Where to start! Ok as an example. The work that I was doing required macro photography at up to lifesize on sensor but generally a bit less than that. Using a 100mm macro lens it was obvious that at beyond f/16 diffraction was starting to reduce the information (the images did not have the absolute fine detail that they did at wider apertures. Unfortunately opening up the aperture meant greater difficulty in retaining sufficient depth of field (real depth of field - that is to say area of the image which was sufficiently well defined to provide relevant detail) AND ensuring focus accuracy. Opening up wider than f/11 resulted in fewer viable images. So basically the aperture was set at f/11-16 as being optimal. Increased pixel density won't overcome these problems.

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[jaapv]

Which illustrates that lens diffraction is something completely different from sensor diffraction.

[giordano]

Please can someone remind me:

 

When considering diffraction in macrophotography, is the relevant f/number the one set on the lens, or the effective aperture? E.g. with magnification 1:1 and the diaphragm set to f/16, the effective aperture of a traditional prime lens would be f/32.

[01af]

When considering diffraction in macrophotography, is the relevant f-number the one set on the lens, or the effective aperture?

The effective aperture, of course. It's always the effective aperture that's relevant for exposure, depth-of-field, and diffraction ... not only in the realm of macro photography. The effective aperture always is the diameter of the entrance pupil, divided by the lens-to-image distance.

[k-hawinkler]

Which illustrates that lens diffraction is something completely different from sensor diffraction.

 

 

I think I understand lens diffraction.

But what is the definition of sensor diffraction?

Thanks.

[jaapv]

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

[01af]

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Yeah, I saw this half-educated nonsense before.

[lars_bergquist]

The effective aperture, of course. It's always the effective aperture that's relevant for exposure, depth-of-field, and diffraction ... not only in the realm of macro photography. The effective aperture always is the diameter of the entrance pupil, divided by the lens-to-image distance.

 

Would you please enlighten me as to what the 'lens-to-image distance' is? Do you measure it from the entrance pupil, the first principal plane, the second principal plane, the exit pupil, or what?

 

The slightly confused old man

[01af]

Would you please enlighten me as to what the 'lens-to-image distance' is? Do you measure it from the entrance pupil, the first principal plane, the second principal plane, the exit pupil, or what?

From the rear (2nd) principal plane, of course. Note that the rear principal plane might be located before the front (1st) one—in that case the distance between the principal planes would be negative.

[k-hawinkler]

Yeah, I saw this half-educated nonsense before.

 

 

Would you please care to elaborate and point us in the right direction in terms of a proper concept and a reference?

Thanks.

[01af]

Would you please care to elaborate and point us in the right direction in terms of a proper concept and a reference?

I elaborated so many times before, and also several references have been given. Please use the search function.

[pico]

The effective aperture, of course. It's always the effective aperture that's relevant for exposure, depth-of-field, and diffraction ... not only in the realm of macro photography. The effective aperture always is the diameter of the entrance pupil, divided by the lens-to-image distance.

 

The issue regarding effective aperture as it applies to diffraction has puzzled me for some time, but I finally think, but I am not sure that I understand. When working with large format, for example 8x10" with a 14 3/4" (~375mm) lens, I never worried about diffraction until the absolute aperture size was less than 6mm, which was about F/64. However, the 8x10 negative was never enlarged more than 4X (huge, and problematic), and often just 1:1, while 35mm sized formats were enlarged as much as 10X. So, in the end it's all about the acceptable CoC and viewing distance which makes effective aperture relevant. Did I get that right?

[01af]

The issue regarding effective aperture as it applies to diffraction has puzzled me for some time, but I finally think, but I am not sure that I understand. When working with large format, for example 8 × 10" with a 14 3/4" (~375 mm) lens, I never worried about diffraction until the absolute aperture size was less than 6 mm, which was about f/64. However, the 8 × 10" negative was never enlarged more than 4× (huge, and problematic), and often just 1:1, while 35-mm-sized formats were enlarged as much as 10×. So, in the end it's all about the acceptable CoC and viewing distance which makes effective aperture relevant. Did I get that right?

No, I'm afraid you didn't. It has nothing to do with formats, circles of confusion, or viewing distances.

 

Instead, it's about the difference between geometrical aperture and effective (actual) aperture. Geometrical aperture is defined as entrance pupil diameter divided by focal length at infinity focus (the latter part is important for lenses which change their focal length with focusing distance). Effective aperture is defined as as entrance pupil diameter divided by lens-to-image distance. At infinity focus, the two definitions are just the same because here, lens-to-image distance is the same as focal length ... in fact, that's how focal length actually is defined—it's the lens-to-image distance at infinity focus.

 

At focusing distances shorter than infinity, the two definitions usually will be different, and geometrical aperture becomes a totally irrelevant, purely abstract concept. At 'normal' shooting distances that are not close-range or macro, the difference between the two apertures typically is negligible. Still, everything that depends on aperture, like exposure, depth-of-field, diffraction, always and naturally depends on effective aperture, never on geometrical. As simple as that.

 

Formulas that compute things like depth-of-field or exposure correction factors usually accept the geometrical aperture as one of their input parameters and compute the effective aperture from either magnification or focal length and focusing distance. Or they neglect effective aperture altogether—which at 'normal' shooting distances is just fine.

 

And oh, by the way—there's another parameter that everyone believes to depend on focal length but actually depends on lens-to-image distance: angle-of-view (and hence, field-of-view). That's why the difference between the field seen in the M camera's framelines and the field actually captured changes with focusing distance.

[pico]

My post may have been muddied by referring to 'effective aperture' which is indeed a mathematical factor, however I was concerned specifically with diffraction caused by the diaphragm (aperture). A 375mm lens which has a 6mm (for example) physical aperture might have an effective aperture of 64, while a 55mm lens at an effective aperture setting of 64 will have a smaller physical aperture so that the 55mm lens will have more degradation due to diffraction.

 

Larger format cameras require less enlargement to reach, for example, 10" on the long side than smaller ones. Less enlargement lessens the observance of diffraction further.

 

Should I use the term interference instead of diffraction?

[jaapv]

If I recall correctly, diffraction is not caused by the aperture as number but by the actual diameter of the aperture in relationship to the wavelength of the light passing through.

[pico]

If I recall correctly, diffraction is not caused by the aperture as number but by the actual diameter of the aperture in relationship to the wavelength of the light passing through.

 

That is what I was trying to express, but not in so few words as to be cast aside easily.

.

[haroldp]

If I recall correctly, diffraction is not caused by the aperture as number but by the actual diameter of the aperture in relationship to the wavelength of the light passing through.

 

That is essentially correct. In oversimplified form, light bending around the actual edges of the iris essentially reduces contrast, it is logical that wavelength would affect this.

 

At wider openings, even though the edges are larger, the relative area for other (focused) light to pass through the lens is gets larger as a square function, so the 'scatter' around the iris edges gets less significant.

 

This explanation would never get past physics 101, but is useful for paractical photographic purposes.

 

When sensor pixel pitch gets below the longest wavelength of visible light (around 0.7 microns), this stuff will get really interesting. Some cellphone sensors are approaching this even now.

 

Regards .... H

[jaapv]

Well, I suppose one (not me!) could carry it into quantum physics, but it would not add anything that is remotely of interest in a photography forum I think.

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[luigi bertolotti]

If I recall correctly, diffraction is not caused by the aperture as number but by the actual diameter of the aperture in relationship to the wavelength of the light passing through.

 

I think it's so... is the real diameter of iris that accounts : I think to have also observed it in some rough home test (magazine's page) : if you close at f22 a 21mm you see easily an appreciable degradation of image definition... not so, for instance, on a 280 at f 22... (but, expecially in long lenses, I seem that the real diameter of iris depends significantly also on where it has been positioned within the elements, by designers)

[mjh]

Diffraction depends on the entry pupil, i.e. focal length / f-number.