Difference in DOF/Bokeh - Pentax 67 105mm vs Contax 645 80mm

[santiago_murillo]

<p>One of the biggest reasons why I want to get the Contax 645 is because of the Zeiss 80mm f2. However, the camera is way too expensive right now so I'm looking at other alternatives. One being the Pentax 67 w/ 105mm f2.8. <br>

I was wondering is the Pentax 67 could rival the bokeh and shallow DOF from the Zeiss 80mm f2? </p>

<p>Thanks!</p>

[jon_porter]

<p>Yes, since the 6x7 is a much larger negative. I don't own a Contax but I have a Mamiya M645 and 80mm f/1.9 and for shooting wide open I prefer the Pentax. BTW, the 105mm is an f/2.4.</p>

[ondebanks]

<p>First, calculate the linear aperture of each lens - this determines the wide-open bokeh blur "size" and dof at a given shooting distance:<br>

80 mm / 2 = 40 mm...Contax<br>

80 mm / 1.9 = 42.1 mm...Mamiya a bit bigger<br>

105 mm / 2.4 = 43.75 mm...Pentax just bigger again</p>

<p>Second, calculate the FOV of each lens...or as a quicker proxy, the ratio of the frame diagonal to the lens focal length. If the values are about the same, then the lenses compose scenes about the same, and shooting distances will be about the same:<br>

645: 69.7 mm diagonal / 80 mm lens = 0.87...both Contax and Mamiya<br>

6x7: 89mm diagonal / 105 mm lens = 0.85...Pentax is close enough to the others; a very slightly "longer" lens in terms of framing.</p>

<p>Taken together, these values show that the 3 lenses will behave very similarly in terms of the <em>quantity</em> of bokeh (of course the <em>quality</em> depends on other things, like spherical aberration)...with a slight edge to the Pentax.</p>

[dave_redmann]

<p>You don't need to calculate the linear apertures to compare, at least when the lenses being compared give similar fields of view. Multiply the aperture on one by the ratio of the linear film (or sensor) sizes to find the aperture you'd need on the other to get the same depth of field:</p>

<p>f/2 x (89mm/70mm) = f/2.5 for the Pentax to have the same DoF</p>

<p>which tells you that theoretically the Pentax 105mm f/2.4 can get depth of field just a tiny bit shallower than the Contax 80mm f/2. (As did Ray, I have used the diagonal dimensions of the respective film frames. When comparing formats with different aspect ratios (these two are <em>slightly</em> different), if you want to assume some particular output aspect ratio, you can use the respective effectively-used dimensions for each format.) Among medium format options, these differences are modest; if you want to figure out, say, what aperture to use on your Sony A7r when you're used to using f/16 on your RB-67 Pro SD, this sort of calculation will tell you that, for U.S.-standard larger print sizes with 4:5 aspect ratios, the answer is f/16 x (24mm/55.6mm) = f/6.9.</p>

<p>Agreed that the <em>quality</em> of the bokeh depends on many factors, and is difficult to impossible to determine from any ordinary specifications.</p>

[john tonai]

<p>Bokeh, being a quality, cannot be measured. The depth of field can, but these are two unrelated aspects of the image.</p>

[santiago_murillo]

Thank you for the response. I do agree that broken quality is more subjective. In your opinion which of the two are better at rendering the

out of focus areas? I've always heard great things about Zeiss and it's microcontrast, but not much about the Pentax.

[donbright]

<p>I don't think anything compares to the out of focus characteristic of the SMC 105 2.4. Very distinctive, I can generally pick it in online streams. </p>

[ondebanks]

<blockquote>

<p>which of the two are better at rendering the out of focus areas? I've always heard great things about Zeiss and it's microcontrast</p>

</blockquote>

<p>Microcontrast is only applicable within in-focus areas. Out of focus areas will, by definition, have no contrast over small scales.</p>