Help with arithmetic!

[Bill Caulfeild-Browne]

I'm trying to figure out what the effective focal length is if I crop an IQ4-150 frame to 1/4th of its size.

The frame is 14,204 px by 10,652 px. Assuming I'm using a 240 mm lens, how much do I have to crop to get the equivalents of a 480 mm and 960 mm lenses?

My logic is that I'd get 480 mm equivalent by cropping to 50% laterally and 50% height. That is, 7,102 px by 5,326, or a 37.8 px image.

A 960 mm equivalent would require cropping by 25% each way, giving a 3,551 by 2,663 or a 9.5 mpx image.
Is my logic correct?

 

[dougpeterson]

It might help you to think about the effective size of the sensor when cropped to a given percentage of the original.

For example if you crop down to a 36x24mm worth of the 645 sensor then the lens can be considered in full-frame-35mm terms.

 

[tsjanik]

I'm trying to figure out what the effective focal length is if I crop an IQ4-150 frame to 1/4th of its size.

The frame is 14,204 px by 10,652 px. Assuming I'm using a 240 mm lens, how much do I have to crop to get the equivalents of a 480 mm and 960 mm lenses?

My logic is that I'd get 480 mm equivalent by cropping to 50% laterally and 50% height. That is, 7,102 px by 5,326, or a 37.8 px image.

A 960 mm equivalent would require cropping by 25% each way, giving a 3,551 by 2,663 or a 9.5 mpx image.
Is my logic correct?

Bill, I believe you are correct. You can calculate the diagonals of each crop to verify, but it is perhaps easier to confirm your results using the known properties of existing systems: micro 4/3 has a 2x focal length factor and has a sensor area roughly 1/4 that of full frame, consistent with your results.

Tom

 

[MGrayson]

Bill,

You are correct. Of course, those focal length "equivalents" are with respect to a full size IQ4150 sensor, that is, the FoV of a 960mm lens on the full sensor is the same as the FoV of the 240mm on 1/16 of the sensor (1/4 in each linear dimension).

It's only confusing if you think about it :grin:

Matt

 

[Jack]

Format diagonal = "normal" focal length. While this is true for any aspect ratio, tradition uses a crop of 2:3 or 3:4 or 4:5...

Soooooo... Simply stated, if you cut your diagonal in half by cutting each side by 2 and now using only ¼ of your sensor, you effectively double all of your focal lengths; cut each side by 3 for 1/9th your sensor, and you triple your focal lengths.

 

[Bill Caulfeild-Browne]

Thank you all for your responses - it's nice to know age has not totally diminished my reasoning power.....
And yes Matt, it IS confusing if you think about it!

Bill