Cross platform equivalency

Started Apr 25, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 40,592
Re: My ears were burning. ; -)

Rick Knepper wrote:

Great Bustard wrote:

Rick Knepper wrote:

FF > MF that is.

Yes this will concern a rumored Canon camera, not an actual one (at least for the moment).

I am pretty comfortable in my understanding that more MPs per given area results in more detail.

Let's say this rumored camera, casually referred to on this forum as the High Resolution Monster, has 51 MPs just like the Pentax 645Z.

Those same number of MPs will be laid out on .79x less area on Canon's FF sensor as opposed the Pentax sensor.

I have admitted in the past that I am fuzzy on cross platform equivalency but until now I have not been interested in anything but FF. The Pentax 645Z may change that.

Noise and DR aside, which set of 51 MPs will produce more detail, less detail or the same detail in the respective systems all other things being equal?

GB mentioned large pixel blur the other day so I am interested if that concept applies here in any way since the pixels on the Pentax sensor will be larger.

Anyone not familiar with the Pentax 645Z can catch up here:

The pros of the 645Z, $8499, 51 MPs. on a sensor larger than FF, weather sealed, 25 cross-type AF points, and wait for it.............. articulating LCD.

The cons: 3 fps.

Lenses for the system mostly look reasonably priced although you won't find any pity-fifty pricing. The MF>FF crop factor is .79 according to DPR.

Here we are:

In short, the resolution as a function of sensor size and pixel count can be summed up as follows:

For a given lens sharpness, the difference in resolution (lw/ph) between two systems will be between the ratio of the sensor heights (enlargement ratio) and the ratio of picture height in pixels (linear pixel density). The sharper the lens, the closer the balance will be to the ratio of linear pixel density, the less sharp the lens, the closer the balance will be to the enlargement ratio.

More specifically, the lenses for a 51 MP FF DSLR with the same AA filter as the 645Z will have to be 1.3x sharper than the lenses used on the 645Z to resolve as well.

I'll draw from a statement you made in another thread: even dull lenses will produce more detail when combined with a higher resolution sensor vs. a lower resolution sensor [both sensors having the same area].

Correct. However, the sharper the lens, the greater the benefit. For example, a mediocre lens might show a 15% increase in resolution going from 22 MP to 51 MP whereas a sharp lens might show a 35% increase in resolution (the max possible increase is sqrt 51/22 = 1.52 = 52% increase, which presumes a very sharp lens, indeed, negligible motion blur and negligible diffraction).

You also mentioned (probably off hand) something about large pixel blur.

Pixel size is one form of blur. The more pixels you have, the less blur you have. However, that doesn't mean that larger pixels on a larger sensor are not more blurry than smaller pixels on a smaller sensor.

Sorry to be dense, but assuming all lenses are equivalent (FF = 1.3x sharper than MF), and noise and DR aside, will the two systems capture equal detail?

Yes. Except, we can't cast noise entirely aside as noise does affect resolution. However, in this case, for equally efficient sensors, I don't see that as contributing anything but a minor, if not entirely insignificant, difference.

Conversely, if the lenses from both systems are equal in sharpness (not equivalent), will the larger sensor produce more detail?


To determine the equivalency of FF vs. MF lenses, does one compare MTF charts and do you have to use a multiplier (1.3z) to determine relative sharpness.

Yes. For example, if using the manufacturer MTFs, you would compare 10 lp/mm and 30 lp/mm on FF to 7.7 lp/mm and 23 lp/mm on 0.77x. You don't just multiply or divide measured or calculated MTFs by 1.3, by the way -- you have to measure resolution at the equivalent lp/mm.

Or does anyone know off the top of their head? Are Canon lenses 1.3x sharper?

I don't know anything about the sharpness of medium format lenses. I imagine one would have to take it on a lens by lens basis. Consider, for example, how different the 24-70 / 2.8L and 24-70 / 2.8L II are.

Post (hide subjects) Posted by
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow