0

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?

mattdm
  • 143,140
  • 52
  • 417
  • 741
Brandon
  • 2,460
  • 5
  • 21
  • 39
  • 2
    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

2 Answers2

3

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.

Community
  • 1
mattdm
  • 143,140
  • 52
  • 417
  • 741
  • 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
  • 3
    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
  • @WayneF Yes; actually edited already — is that better? – mattdm Mar 24 '16 at 15:47
  • 1.2x is only in relation to the linear width of the two sensors. The linear height is closer to 1.4x. This is why we normally use the measure of a sensor's diagonal to compute crop factors. The diagonal also measures the diameter of the image circle that is needed for any particular sensor. The Pentax 645 (NOT a true 6 x 4.5 cm format) has a crop factor of 0.78x when measured along the diagonal. Or to look at it the other way, a FF 35mm camera has a crop factor of 1.27x when compared to the Pentax 645. – Michael C Mar 24 '16 at 21:11
  • @MichaelClark Agreed. I was just trying to not get into all that. Which measurement is most useful depends on what you're doing, and using the diagonal is sort of a compromise. It really only makes sense if you're printing both (or scaling both onscreen) so the diagonal is the same, which in the real world seems unlikely, since it's easier to make an edge match. (And crop, or letterbox on screen.) I picked 1.2× because that's what's relevant if you crop the 4:3 image from the 645z to match. But perhaps more importantly, for perceivable effect on DoF, it's all pretty close together anyway. – mattdm Mar 24 '16 at 21:18
  • I think most would actually crop the 3:2 image to match the 4:3 image, rather than the other way around, which means 1.4x would be the more practical comparison. – Michael C Mar 24 '16 at 21:20
  • @MichaelClark I guess it depends on what your subject is. I dunno... you think I should write some of this into the answer? – mattdm Mar 24 '16 at 21:22
  • Especially since you mention printing to the same size rather than viewing at the same size, I'd go with the 1.4x height factor before the 1.2x width. It's not uncommon at all to crop a 35mm negative or 3:2 digital image to make a 5x7 or 8x10 print. It was quite a bit more unusual to crop a 1.33:1 or 1.25:1 MF negative to make a 3.5x5 or 4x6 print. – Michael C Mar 24 '16 at 21:31
  • And in the context of "phones and laptops" I think 4:3 or even 5:4 is more common than 3:2 as a display form factor, especially phones and tablets. – Michael C Mar 24 '16 at 21:33
1

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).

Euri Pinhollow
  • 5,071
  • 17
  • 36
  • 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
  • 1
    @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
  • 1452mm² divided by 864mm² is 1.680555. That's almost exactly 5/3. 55mm divided by 43.3mm is 1.270208 so yeah, it is a little closer to 5/4 than 4/3. But 2/3 stop is significant. That's larger than the difference between µ4/3 and APS-C, less than the difference between APS-C and FF. – Michael C Mar 24 '16 at 23:50
  • @michael-clark: still more remains before it becomes 645 format. – Euri Pinhollow Mar 25 '16 at 05:37
  • So anything less than than 1 3/4 stops difference between 35mm and 645 is not significant at all? That means APS-C is only slightly smaller than FF, after all it is only 1 1/3 stops! – Michael C Mar 25 '16 at 07:58
  • @michael-clark: that's why I use 1,5x crop lol. Seriously, I calculated square difference, it is almost exactly 3/4 stop, not even the difference between "crop" and "ff". Also keep in mind that there are even no F2 objectives for 645Z. – Euri Pinhollow Mar 25 '16 at 13:31
  • 3/4 of a stop is still significant. Sometimes it is the difference between getting the shot you want and not getting the shot you want - whether in terms of shallow DoF or fast enough Tv to prevent blurring or low enough ISO to process the way you desire. 3/4 of a stop is far from slight. – Michael C Mar 25 '16 at 21:08
  • As one who shoots with both APS-C cameras and FF cameras with the same generation of sensor technology from the same manufacturer I can tell just by looking at shots taken in bright sunlight which ones are from the FF camera and which are from the APS-C camera. The difference isn't huge, but it's also a lot more than slight. – Michael C Mar 25 '16 at 21:11
  • @michael-clark: "3/4 of a stop is far from slight." - there is not a single objective faster than F2.8 (or F2.4) for Pentax 645Z. Do you refuse to take it into account? It is also not sensitivity which is discussed in the question but medium format LOOK. – Euri Pinhollow Mar 26 '16 at 06:03
  • LOOK at the sensors! One is 5/3 the size of the other! If one were 16/15 the size of the other than would be slight. 68% larger is more than slight. – Michael C Mar 26 '16 at 08:04