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Rented a medium format camera to see "what is this medium format look" everyone talks about! and evaluate if it is awesome enough to start saving money for it or not.

So my own camera is a Nikon D-810 and the lens I used is Nikon's 85mm, f/1.8 and the medium format I rented is Pentax 645z and the lens is their 90mm macro f/2.8

I shot the Pentax at f/2.8 and the Nikon at f/1.8.

Was my comparison wrong? (using f/2.8 vs f1.8 ) and does aperture really change? I don't think aperture changes but wanted to see what is the best way I can test these two for my own comparisons ? and if I had a f/1.4 on 35mm would it then be more similar to DOF look of medium format?

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    Your basic mistake for the comparison was not selecting a lens for the Pentax 645 that yielded the same FoV as the lens selected for your FF D810. To get the same diagonal FoV as the 85mm lens on the D810 you should have used a 110mm lens on the Pentax. – Michael C Mar 24 '16 at 21:17
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Aperture has an obvious effect on two things: depth of field and exposure level. That means that the answer comes in two parts.

First, exposure. Here, the important thing is the amount of light hitting any given point on the sensor, not total area. See more at Do the same camera settings lead to the same exposure across different sensor sizes?, but the basic point is that for setting exposure (and see What is the relationship between ISO, aperture, and shutter speed? for more on that), f/numbers are equivalent across camera formats and sensor sizes. (This is one of the great things about the system.) ¹

Second — and I assume your main concern since you tagged the question "depth of field" and mention that directly — depth of field. Here, the rough answer is that with a number of assumptions, including printing at the same size, depth of field will be greater with the smaller sensor, by an amount roughly equivalent to that of the lens on the larger sensor stopped down by a factor the same as the ratio of sensor sizes. Whoo, that's a mouthful — basically, multiply the f/number by the crop factor between the two to get an equivalence. (Or divide, going the other way.) More detail (and more caveats) at Can a smaller sensor's "crop factor" be used to calculate the exact increase in depth of field?.

For the full-frame Nikon and Pentax medium-format, that factor is about 1.4× (if you crop the wider 3:2 Nikon frame to get an equivalent image). That means that you should have roughly equivalent depth of field at f/2.8 on the Pentax and around f/2 on the Nikon. Of course, at those apertures, for exposure to be the same, you'll need to adjust shutter speed or ISO by about a stop to match. ²


Notes:

  1. As a tangential dive: the first effect, exposure, is really only constant because when we enlarge an image from the sensor's postage-stamp actual size to view (or from film to print), we "expand" the brightness of each point, rather than dividing it across the enlarged image. Of course, more light overall hits the larger medium. More light means more signal, which means less apparent noise. Of course, differences in sensor technology also make a difference, so it's not always an easy win in the real world, but when it comes right down to it, you can't beat physics.

  2. As Michael Clark notes in a comment, your lenses do not give an equivalent field of view. That means to get the same framing, you'll have to be a lot closer with the Pentax, which would also change depth of field (and the perspective would be different). So this whole exercise would probably be better if using a 60mm Nikkor lens alongside the 90mm Pentax.

  • thanks Matt, so if my images are not getting printed for a wall! and just on phone and laptops, with the Nikon D-810 sensor size that I alreay have, I won't get much difference with that Pentax? so no reason to have it ? right? – Brandon Mar 24 '16 at 14:32
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    I wouldn't say that there's "no reason" in the grand sense — there are plenty of great things about the Pentax 645 digital series, any of which could be a reason to have it. But if you're looking for a dramatic difference in depth of field, right, that's not one of the reasons. – mattdm Mar 24 '16 at 15:06
  • >>Of course, for exposure to be the same, you'll need to adjust shutter speed or ISO by about ⅔ stop to match. --- Is just plain wrong. f/2.8 is f2.8, same exposure on any camera. That is the whole point of f/stop. Fancy lightmeters do not have a menu for camera type, their reading applies to any camera, with any size sensor. – WayneF Mar 24 '16 at 15:23
  • @WayneF Errrr, you're misreading. f/2.8 is f/2.8, but f/2.3 is not. – mattdm Mar 24 '16 at 15:26
  • Maybe so. I see the top part now, which is good. I was just skimming, and your sentence jarred me hard. We do see the same thing in places, a pet peeve here. Maybe you could edit that sentence to word it better, more clearly, more specifically, for readers like me. :) – WayneF Mar 24 '16 at 15:41
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To compare DOF you need two things:

  • FoV (field of view)
  • entrant pupil (the physical place from camera gathers the light)

Given this two measurements you may confidently say which camera from two with same FoV will give smaler DoF (the one with bigger entrant pupil).

You might also want to note that 645Z is only medium format slightly larger than 35mm film (44x33 vs 36x24). The medium format goes all way up to 100x80 mm.

Also, it is not just small DoF which is regarded, it is also specific optical blur because of different optical schemes and, in case of film, greater dynamic range (one stop for each 2x sensor size increase - thus, 3 stops wider dynamic range for 10x8 vs 35mm film of same kind).

  • no clue what are does two things :( can you explain more? – Brandon Mar 24 '16 at 14:07
  • "645Z is only medium format slightly smaller than 35mm film " ? Smaller or larger? – Brandon Mar 24 '16 at 14:09
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    @blake: edited. – Euri Pinhollow Mar 24 '16 at 14:09
  • I would say that a sensor with roughly 5/3 the area and 4/3 the diagonal of the other is more than slightly larger. – Michael C Mar 24 '16 at 21:05
  • @MichaelClark the diagonal looks closer to 5/4 than 4/3 to me. It seems insignificant, but it's still a 2/3 or 3/4 stop improvement. Not quite the improvement you usually get from jumping from one format to the next, so I could see the argument going either way. I say this as a big fan of Pentax. – Mark Ransom Mar 24 '16 at 22:40

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