Question on using a FF lens in crop mode on A7RIV

Started 3 months ago | Discussions thread
PWPhotography Forum Pro • Posts: 10,756
Re: Question on using a FF lens in crop mode on A7RIV

Trollmannx wrote:

SQLGuy wrote:

A couple of other ways to think about it:

1. A bit over simplistic, but crop mode is very much like using a 1.5X teleconverter. A 1.4X TC adds 40% to the focal length, and drops one stop (1.4 X the lens' rating).

Do not quite agree as the lens does NOT change and remain unaltered despite whatever is put behind the lens. Any 1.8/85mm lens is still a 1.8/85mm lens.

Putting a 1.4x teleconverter behind the lens will extend the physical focal lenght. The physical aperture will remain, but obviously the f/number will change.

When cropping an FF camera to APS-C format the field of view becomes more narrow, and that is where the added 1.5x tele effect comes in.

The lens does not change. The light density does not change. The plate scale does not change. Only the field of view change when using a larger or smaller image sensor, simply because the area of the image sensor differ.

And obviously the larger FF image sensor can collect light falling outside the APS-C format. That is where the confusing "collects more light" comes into play.

It's not confusing but FF sensor does collect more total light. The light density in per square mm is the same. But FF sensor is much bigger therefore collects more total light. If you compare between F1.8 FF and F1.8 APS-C lenses, you will see F1.8 FF lens opens hole (diameter) bigger on the same aperture and shutter. Believe the same FF f1.8 lens open the hole or diameter less under APS-C mode than under FF mode in the same aperture/shutter.

The equivalence part (which can be sound) is often expressed in confusing ways and surrounded by lots of equivalencenonsense...

The larger FF sensor collects more total light and then projects to the same output size with less pixel magnification (or enlargement). Really no confusing here in equivalency.

Just imagine an old projector.  The larger film (sensor), the less enlarged after lens (glass) projecting to a big curtain (final image), better quality.   Really the sensor format is directly related to pixel enlargement.  Larger sensor -> less pixel enlargement -> better IQ, not that difficult to understand.

2. F stop is the ratio of the lens' effective diameter to its focal length. Cropping, or a teleconverter, increases the focal length, but doesn't make the front element any bigger, so the calculated F stop obviously increases by the crop factor as well.


For instance, a 100mm F2 lens will have an effective diameter of 50mm. E.g. the Canon 100/2 USM takes 58mm filters and probably has about a 56mm front element... there being some losses along the way in getting to 50mm effective diameter. Putting that lens on a 1.5X TC, or using it in 1.5X crop mode, will make it act like a 150mm lens. 100/50 = F2.0. 150/50 = F3.0.


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