Top myths in photography

Started Feb 24, 2013 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: Little Known and Shocking Truths in Photography

dsjtecserv wrote:

John1940 wrote:

Detail Man wrote:

dsjtecserv wrote:

Detail Man wrote:

Remove constant framing (FOV) conditions, and DOF varies in direct proportion to sensor size.

And he doesn't mean inversely varies! If focal length, f-number, focus distance, and presumed viewing conditions are held constant, then the only thing that changes with sensor size is the amount of enlargement needed to produce a print of the agreed-upon standard size. The greater enlargement of the image from the small senor enlarges the blur present in the image more than that from the larger sensor. The net effect is that from a given viewing distance, the image will be more blurred, and a narrower range of things will appear to be in acceptable focus. Thus the depth of field given by the smaller sensor is less than for the larger sensor.

In practice, we tend to use a different focal length or subject distance to produce the same framing, and the effect of those factors counteracts the effect of greater enlargement of the smaller sensor, so the net effect is greater depth of field being associated with smaller sensors. But it's well to keep in mind that, left to its own devices and with only senor size being varied, DoF is less for smaller sensors.

An impressive reply. I am shocked that somebody agrees with me on that. People are (understandably) so used to the constant-framing case that they usually "have a cow" over that one - it striking them as a such a radical departure from the commonly believed myth that sensor size (in a constant-framing case) has anything to do with DOF per se ...

... when it is in fact the bottom range of F-Number adjustment values in lens-systems practical to manufacture and which are thus made available for different sensor formats that tells the tale.



DM, what you (amd dsjtecserv) are saying is that it is not logical to think that 100,000 lemmings (or photogs) can't be wrong. I can't fault your logic so you are probably right.

It's also possible that I need more Colombian coffee instead of the decaff I just drank this morning. If you don't hear from me later, then assume I agree with you.


John, I tried to explain the sense in which the conventional wisdom is correct. It is just a matter of being careful about what you assign cause and effect to, and about what variables change and which ones stay the same. So it is correct to say that smaller sensors are associated with greater depth of field, because we normally assume that FoV is held constant, but it isn't correct to conclude that small sensors "cause" or "give" greater depth of field. The sensor itself is quite agnostic on that!


Sensor size is the reason why holding FoV constant results in greater DoF. If it isn't, what is?

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