Cross platform equivalency

Started Apr 25, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 38,500
Re: I would be interested in a response to this

Rick Knepper wrote:

Ian Stuart Forsyth wrote:

One thing I don’t see with pentax is fast glass, the fastest I could find in production is F2.8 so this would be like F2.2 on FF

My question, can a FF 1.4 lens stopped down to F2.2 be as sharp as a 645crop F2.8 shot wide open?

My understanding from elsewhere in the thread is that the FF lens has to be 1.3x sharper given the example given.

At the equivalent f-ratio, yes.

I think a FF 1.4 lens with equivalent settings can be sharper than the MF equivalent, but are they? As Just Another Canon mentioned, the sharpness levels are not equivalent. That's why I asked about comparing MTFs which it looks like GB has confirmed below.

Overall, it would be better to wait for verification from someone with better footing on equivalencies.

It's not a matter of "better footing on equivalencies", but measuring the required variables, such as the sharpness of the lens and the effect of the AA filter.

would this not give an edge to FF ?

The 24-70 II has crazy sharp corners for a zoom even at f2.8. F2.8 is more than fast enough for landscapes as I shoot them. Will the Pentax lens in question, stopping down to f3.5, be as sharp in the corners? I'll refer to the MTF charts if I can find them.

Manufacturer MTF charts for FF lenses measure contrast at 10 lp/mm (on the sensor) and 30 lp/mm (on the sensor) and only wide open and at f/8.

Lens [System] tests measure resolution in the photo (lw/ph) at 50% contrast, but generally do so at one stop intervals from wide open.

If the cropped 645z was a true 645 I could see the advantage of the F2.8, but not having anything faster for the cropped 645 really equals out the playing field in noise and resolution would it not ?

It depends on how sharp the lenses are.  If they the medium format lenses are the same sharpness, then the 645Z will have a marginal advantage over 51 MP FF.  But if they're sharper, the advantage could be more substantial.

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