Yashicamat 124G Optical Performance

by Evan Ludeman ludeman@gate.net (revised 9/16/97)


I recently acquired a second hand Yashicamat 124G and noticed immediately that my photos varied in sharpness depending on the aperture setting. Photos shot at f3.5 - 4 were a little soft, while photos shot at f11 just about leaped off the page. Being of an experimental bent and since it was raining like hell one weekend, I decided to burn a roll of film in pursuit of some quantitative numbers.

My 124G is in LN- condition and has no faults in the optical system. The taking lens is a four element Yashinon 80mm / f 3.5. This lens is generally thought to be a copy of the Zeiss Tessar, although I have no way to confirm this.
To accomplish my test, I set up several USAF 1951 style resolution targets (thanks go to David Jacobson for his terrific post script file, available at http://photo.net/photo/optics/USAF1951.ps) on a vertical wall with the camera supported by a tripod 3.2m away, measured to the front of the lens. A 500 watt halogen shop lamp was used to illuminate the target and the camera was fired via the self timer. Focussing was done in the normal fashion with the view finder and built in magnifier. Kodak T400CN (C-41 process) B&W was used as the test film -- something like tech pan or Tmax 100 would be a better choice, but that's what I had available. Films were read under a stereo microscope with diffuse transmitted light.

Magnification factor was determined directly by measuring the length of the image of one of the photo targets on the negative. This magnification factor was then applied to the line pattern that was resolvable as three lines separated by two spaces.

Tests were run at all apertures at D = 3200mm. The view finder field of view was also marked out on the target wall in black tape, so that view finder coverage could be determined.

Results:

Resolution, Distance to target = 3.2 meters,
magnification factor 1/39.6 Resolution (lines / mm)

f     center  corner

3.5     35      25

4.0     44      29

5.6     63      32

8       63      44

11      79      50

16      71      50

22      56      44

32      35      35


Exposed negative size: 56.1mm tall by 56.9mm wide, width of exposed negative outside viewfinder field of view at 3.2m: 4.5mm top, 4.1mm left side (photographer's left, facing subject), 4.1mm right side, 2.1mm bottom. This is about 86% coverage left to right, 88% coverage top to bottom, 76% of total area. The view finder's field of view at 3.2m is 203cm tall x 198cm wide.

Comments:

There is a 12% "step size" between successive resolution targets, which is pretty coarse. This should tend to yield errors that are conservative rather than strictly random. There is a clearly visible difference, for example, in the films from f5.6 and f8, although the best resolution is reported in both cases as 63 l/mm at center. At f8 I would guess it's probably closer to 67 or 68 l/mm, but not quite good enough to split the next target (71 l/mm).

Measured length of the image of a 11" (27.9cm) sheet of paper (the center resolution target) on the negative was 0.706cm, which yields an apparent magnification of about 1/39.6.

My exposures were a little on the low side throughout -- I used the built in light meter and the subject was a white wall with a few photo targets. Doing it over again, I'd increase exposure a stop or two. Increased exposure would make the film a bit easier to read and might have made have tipped one or two of the measurements above into the next higher bracket. Exposures were dead even across the field at all apertures.

Conclusions:

Overall, I would use caution in the comparison of these results with those that have been recorded and published elsewhere. Test methods and individual cameras, interpretation of film, etc. will all vary. My specific purpose here was to develop a quantitative feel for how resolution varied with aperture setting, not to make any particular claims about the quality of the optical system in this make of camera. As others have pointed out, there is also a hell of a lot more to a good camera that the ability to split resolution test patterns.

The results back up my observation that the photos I shot at f8 - f22 were noticeably sharper than at 3.5 - 4.0. Others have commented on this characteristic of Tessar - formula lenses as well. Clearly, where maximum sharpness and detail is required, shooting at f11 and f16 will be beneficial. Where a slightly softer focus is preferred (e.g. portraits), one can work toward either end of the aperture range, depending on depth of field and shutter speed requirements. What was surprising to me was both the very high maximum resolution and the very wide range of variation with aperture. Given that the usual driving force for MF over 35mm is better resolution for bigger enlargements, this is very useful information to have.


Comments from a reader regarding lens tests:

Thanks for posting your results.

I read your description of the tests. I'm not certain that it said anywhere in it that you varied the focusing.

I recently did a similar test (two weeks ago) for an old 1930s Baldax 645 folder with a Meyer-Gorlitz F4.5 Trioplan scale-focusing lens.

I carefully measured the distance to resolution chart (in this case 4 meters) and exposed a roll of FP4+ using a cable release at speeds of 1/25 and 1/50 at F4.5 and F5.6 with the camera mounted on a tripod. In exposing the roll, I bracketed the scale focusing just slightly. If I had gone only by the negatives scale focused at 4 meters, I would have gotten rather mediocre readings of about 28 lines per millimeter. But negatives shot with the focus set at 3-3/4 meters (even though the tape-measured distance to the chart was 4 meters) produced readings of 48 lines per millimenter wide open!!

I learned two things: that the lens was sharper than I would have thought if I had relied only on results with the focus set at 4 meters; and, more importantly, I learned exactly where in practice the scale focusing actually was.

With a TLR there is the temptation to assume that sharp focus on the ground glass equals sharp focus on the film plane. Not necessarily so. Slight variances between the two, in film plate pressure, etc. can make them not quite the same.

Finally, I believe in my own case that I would have seen an even higher resolution (possibly) with a finer grain film like Agfa25, but since I was conducting the test more for the purpose of determining exact focus (I had had the lens apart for cleaning and had remounted it) numbers weren't that important.

The point of my long-winded story is that it is just possible that the softness you note at F3.5 - F4.5 has more to do with an imprecision in focusing which is masked by greater depth of field at smaller aperatures than it does with the lens underperforming wide open.



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