Periskop vs Achromat?

JPD · Jan 16, 2023

The year is 1920-something and you can't afford a camera with a fine lens, but you absolutely don't want a simple meniscus! You are in the store and trying to decide between two affordable cameras, one with a Periskop lens and the other has an Achromat. You want it for everyday use, photographing groups, sometimes portraits and landscapes. Which lens would you perfer?

Donald Qualls · Jan 16, 2023

Based on my experience using both (Achromat meniscus in one folder and multiple box cameras, Periskop in a couple Speedex Junior) the Periskop is the better lens. Both of these are typically fixed aperture, neither is commonly seen faster than f/11, but in my experience the Periskop type does a better job of eliminating geometric aberrations -- and if made with achromats, should be at least as good on color fringing as well.

JPD · Jan 16, 2023

Donald Qualls said:
Based on my experience using both (Achromat meniscus in one folder and multiple box cameras, Periskop in a couple Speedex Junior) the Periskop is the better lens. Both of these are typically fixed aperture, neither is commonly seen faster than f/11, but in my experience the Periskop type does a better job of eliminating geometric aberrations -- and if made with achromats, should be at least as good on color fringing as well.

I have seen lenses with the confusing name "Periskop-Aplanat" and always wondered if they were two or four element lenses.

Donald Qualls · Jan 16, 2023

Aplanat was a term that meant the lens had a flat focal plane, as opposed to the field curvature seen with a meniscus or achromat (especially if convex toward the subject). This was one of the biggest advantages of Periskop lenses over meniscus, achromatic or not. My Speedex Jr. was capable of allowing brick counting in a chimney a block away (6x6 on 120), and that level of sharpness carried into the corners of the frame. One shutter speed and two apertures, but the best image quality I've ever seen from such a simple camera.

JPD · Jan 16, 2023

Donald Qualls said:
Aplanat was a term that meant the lens had a flat focal plane, as opposed to the field curvature seen with a meniscus or achromat (especially if convex toward the subject). This was one of the biggest advantages of Periskop lenses over meniscus, achromatic or not. My Speedex Jr. was capable of allowing brick counting in a chimney a block away (6x6 on 120), and that level of sharpness carried into the corners of the frame. One shutter speed and two apertures, but the best image quality I've ever seen from such a simple camera.

So less distortion and field curvature with the Periskop. Stopping down an achromat, if possible, should improve the field curvature, or at lest the effects from it, somewhat.

Donald Qualls · Jan 16, 2023

JPD said:
Stopping down an achromat, if possible, should improve the field curvature, or at lest the effects from it, somewhat.

True, it does help -- that's why simple cameras with meniscus lenses (basically the cheap little brother of an achromat) are no faster than f/11, often even f/16. The achromat fixes (most of) chromatic aberration, but doesn't change field curvature or spheric aberration. Periskops generally run f/8 or smaller, though, so there's not a lot of speed difference.

Periskop vs Achromat?

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Donald Qualls

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