Depth of field and sensor size (and other questions)

[austrokiwi1]

This is probably a very silly question and I suspect I have my answer already. So this post is just seeking confirmation

I had been told that depth of field will be deeper for smaller sensors. Initially I didn't question this accepting it at face value, wondered whether a MFT sensor might be better for some applications, That is when I started thinking hard and realized the statement might be misleading. I found this page on Cambridge color:

http://www.cambridgeincolour.com/tutori ... r-size.htm


I suspect the person who told me DOF field increases with smaller sensor size may have misinterpreted what was written in that Cambridge color tutorial. specifically this is what is said on that page:

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors.

As I understand the quoted section; it is not actually the sensor size that is influencing the DOF but the magnification.

I now use full frame sensor as:
1. when photographing silver dollar sized coins (aprox: 40mm diametre) coins to fill the sensor with an image of a coin requires less image size reduction(from 1-1).
2. For smaller coins ( some just a few mm in diameter) I can maximize the magnification.


From my understanding it is in the second case where the Cambridge color DOF sensor size statement comes into play( the most). On an APSc sensor I would require less magnification to fill the sensor area with the image, so with the lower magnification the DOF would be deeper than if I tried to fill a full frame sensor( where a higher magnification would be required.

IS my understanding correct? ( I know it seems basic but I just want to make sure I have the concept correct)

would this mean that in some closeup photography situations a MFT sensor might have an advantage over APSc?
I am thinking of the case of using a Melles Griot PN 386080 telecentric adapter ( attaches to a SK 50mm F2.8 enlarger lens). The image circle of that adapter almost fills an APSc frame( but with some nasty degradation on the image edges). I am guessing that on a MFT sensor I might see much better performance from that adapter as it was designed for C mount. I assume with MFT I would require even less magnification so I would see a greater DOF.


This leads to a third question: Might MFT be a better option for coin photography? In my limited experience most coin photographers stick religiously to APSc DSLrs ( being a well developed mature technology that approach isn't wrong)

[rjlittlefield]

It seems like discussions & explanations of DOF versus sensor size are always a mess. That's because either a) people don't specify what they're holding constant versus what they're changing, so it's completely unclear what they're saying, or b) they do specify those things, and then the discussion gets so complicated that as a matter of practice you can't tell what they're saying. In addition there's the issue of whether what they're saying is even correct, which cannot be taken for granted especially in older works. Sigh...

Since you're a macro guy, and therefore supremely interested in DOF and diffraction, perhaps I can simplify the explanation.

Assuming that in the end we're photographing the same size field and making the same size final image, then
a) DOF and diffraction track each other in exactly the same way independent of sensor size, and
b) both DOF and diffraction are determined by the angle of the entrance cone.

It turns out that a smaller sensor naturally goes along with a shorter lens, so you need to set a smaller f-number to get that same angle of the entrance cone. If for some reason you persist in setting the same f-number, then the larger sensor will produce an image that has less DOF and also less diffraction blur. It's a different image.

If you carefully slog through the calculations involving magnification, COC, Airy disk diameter, etc., you can confirm the above simplification. But that's the hard way to understand the issue.

--Rik

[austrokiwi1]

Thanks that covers all my questions well