a6k Loc: Detroit & Sanibel

I used two cameras+lenses:

Sony 𝜶6500 with Minolta 500/8 AF Reflex (EXIF reports 496 mm = 744) The lens is fixed length.
Sony RX10 IV with 220 mm lens (EXIF reports 220 mm=600). The zoom lens is not interchangeable.

Using a tripod, I shot the same target at the same distance with the RX10 at longest setting (Minolta is a prime). The picture, exposure, etc. are not important because the dimensions of the lenses, sensors and distance to target are the only things being compared.

744 / 600 = 1.24 so if the equivalent focal length were really only about the physical size of the sensor, the displayed image from the 𝜶6500 would be 1.24 times the size of the one from the RX10 IV.

Skip the math. The screenshot shown here tells me that in order to make the shot taken with the RX10 IV as large as the one taken with the 𝜶6500 + Minolta 500 it was necessary to increase the smaller image by 49%.

If we care about what the equivalent focal length means then either the 𝜶6500 is 894 or the RX10 is 499. Of course, the truth could be some mixture/hybrid of the two discrepancies.

It's a shame there is no standard to use for pixels per mm on the sensor so all we can do is compare lens and sensor combinations to each other. The discrepancy here appears to be the difference in pixels per mm in the horizontal dimension of the sensor.

The second screenshot that I added as an edit shows the RX10 shot with one using the 𝜶6500 with a Sony 70-400 on which the EXIF says 400=600. Same two cameras, supposed to be the same equivalent length. Twelve % difference.

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Linda From Maine Loc: Yakima, Washington

Can you give me an example of when equivalent focal length needs to be that precise?

jerryc41 Loc: Catskill Mts of NY

This seems to be a very contentious topic. I wish camera makers could have thought of a better way to state the focal length for the various sensors. Someone will post something about the focal length he used, and someone else will say he's incorrect. It's the "equivalent" focal length. Does it really matter? No. You select your lens or your zoom length, and you take a picture.

A similar topic is "depth of field." That's been beaten to death, but it keeps coming back to life. Something is either in focus or it isn't.

Linda From Maine Loc: Yakima, Washington

Oh lordy, Jerry. This is so wrong But it's a hijack to continue discussing. Please start a new topic.

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a6k Loc: Detroit & Sanibel

No. If you don't care then you don't.

a6k Loc: Detroit & Sanibel

👍

a6k Loc: Detroit & Sanibel

Jerry, the equivalent length might matter to someone who needs the longest zoom for birds (example) and is considering what camera plus lens to buy. For me, it's partly that and partly just curiosity.

Longshadow Loc: Audubon, PA, United States

"Standard" focal length on a 35mm was selected as 50mm. It makes something that is 20 feet away in real life look 20 feet away in the picture. Little or no subject magnification (positive or negative).
Less than 50mm pushes the subject back, greater than 50 brings it closer.
"Standard" for different <film/sensor> formats is different, that's why they use a conversion.
Base reference is the 35mm film format.

Linda From Maine Loc: Yakima, Washington

Excellent! Thanks for satisfying my curiosity

imagemeister Loc: mid east Florida

To compare any FL You NEED to be focusing @ INFINITY with ANY zoom because most zooms loose FL close up ! - primes can also loose FL close up - but to a very lesser degree .....It is just that SIMPLE
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imagemeister Loc: mid east Florida

imagemeister Loc: mid east Florida

ALL advertised focal lengths - equivalent or not - are assumed to be @ INFINITY ! !

rcarol

Knowing the focal length of a lens used for a particular situation does absolutely no good for making a decision on what to buy because the viewer has no idea what distance the photo was taken.

jerryc41 Loc: Catskill Mts of NY

I was referring to the fact that there is a lot of needless debate over terms in photography

Bye!

BebuLamar

There are 2 problems.
1. The aspect ratio of the 2 cameras are not the same and thus the equivalent is based on the diagonal so the width and height of the FOV are not the same.
2. You need to do the test with distance subject because for some lenses the focal length shorten as it focuses closer than infinity and the degrees varies from lens to lens.