Comparing camera lens when they have different size sensors

[ericmark]

I wanted to compare pictures and found my compact does not give the dimensions of the sensor in the normal mm x mm format but in square inches. My D-SLR's both have different sensor sizes which made comparing rather a problem. So I sat down and wrote a program to convert into 35mm film format.
Camera Lens Calculator
However when I put the info for my Tamron lens into the calculator I found it was a few minutes out. Tamron 18-270 mm used on a Nikon says angle of View = 75º 33' - 5º 51' with a 15.5 x 23.2 mm sensor.
So is it:-
1) I got calculation wrong.
2) The lens is a 18 - 273 mm.
3) Something else.
The Casio EX-Z600 instruction manual said it had a 1/2.5 inch sensor which however I tried seemed to give daft results. However when I tried 1/25 of inch everything dropped into place.

OK I know no real point I was just playing. I would guess the angle of view is the more accurate as the focal length depends on the sensor size.

I have found the Fugi says “1/2.3 – in., square-pixel CCD with primary color filter” and again I can't work out what it refers to? I have modified calculator so I can go round in a circular fashion and all the D-SLR details seem to tally with results I am getting but there seems no way I can relate the readings to the 1/2.3 and 1/2.5 given in the Fugi and Casio compact cameras.

[ericmark]

I have been given three sites
Glossary: Sensor Sizes: Digital Photography Review
Image sensor format - Wikipedia, the free encyclopedia
Optical format - Wikipedia, the free encyclopedia
It would seem it's something from history as yet not updated web page.

[Ron Cardinale]

For a calculated result with an 18-270 lens on a 15.5 X 23.2 sensor, I get 5°55’ and 75°33’ which matches what your program computes. You seem to have typed the telephoto result wrong in your posting as 5°51’, though.

There’s a small error in the size you’ve stated for a 135 film image on your calculator page and in the code. It is not 24mm X 35mm. It’s 24mm X 36mm.

I can’t find an area for the sensor size noted in the Casio manual. I did find “1/2.5-inch square pixel” sensor which means that the pixels are square (which is normal for most cameras) not that the 1/2.5-inch value refers to the sensor’s area.
Unfortunately, the 1/2.5-inch notation isn’t even remotely associated with reality. It’s an arcane and absurd way of describing a digital image sensor’s size. It goes back to vidicon tubes referring to the size of the glass tube. In my view-- and I’ve got lots of company-- using that to describe a digital camera sensor doesn’t make sense but manufacturers sometimes use it. But I digress.
The size of a 1/2.5-inch digital camera sensor is actually 5.76mm X 4.29mm. So, the diagonal is 7.18mm and its area is 24.71mm^2. This area is almost 25mm^2 which explains why using 1/25 for the area in square inches gives you a result that makes some sense since an inch is 25.4mm-- but it’s just a coincidence that it almost works in this case.

Focal length does not depend on the sensor size. It’s the angle of view that depends on the sensor size. Focal length is independent of sensor. Focal length is where an object at infinity forms an image. The size of what’s “seeing” that image doesn’t change where the image is formed but only how much (angle of view) of it is seen.

[ericmark]

Thank you. I have modified program and removed at least for the moment any reference to "Optical format" for calculation. I have looked at the Wikipedia reference on optical format and it does not make it plain what is in inches and what is in mm and if a 1/x or an x measurement. It would seem I could calculate the pixel size but question has to be would this really help? So as it stands it would seem any sensor size given in "Optical format" is similar to chocolate fire guard and next to useless.

Fortunately manufactures also give 35 mm equivalent which would seem to be easier to use but the length of film used in a 35 mm camera was down to camera manufacturer and even the width varied where some still allowed space for the optical sound track. I have however changed to 36 x 24 which seems to be more universal my thanks for pointing that out.

I am still 4 minutes out compared with Tamron data.

[mbrager]

What is equivalence and why should I care?: Digital Photography Review
An interesting article that pertains to what you are discussing.

[ericmark]

That is interesting not quite what I was looking at but very similar. It does go some way to explaining something which was bugging me. I have a Pentax D-SLR with unlikely many other makes allows me to use lenses off the old Ricoh both being Pentax K mount. I have found the old 95 - 210 mm very handy and also the 400mm fixed but the old 28mm f2 lens had seemed little better than my new 18mm F3.5. Now I need to re-read it all and get my head around it all.