lsaguy Loc: Udall, KS, USA

Okay, I get that a FF lens has a shorter field of view and I multiply by 1.5 to get equivilent FOV for my APS-C sensor. So my 28 - 200 mm is actually a 42 - 300.
Does it go the other way? If I put a 24mm from a Pentax 645 on my K20D does the FOV expand? The 645's sensor is almost 4 times as big as Pentax APS-C. Would the lens become a 6 mm effective FOV?

Rick

speters Loc: Grangeville/Idaho

If you had a 24mm 645 lens it would be equivalent to about a 16mm on full frame, so yes, in a way it goes the other way too, but you can not mount a 24mm APS lens to a 645 mount, all you would get is a little point of light in a dark circle (super strong vignetting). I do not know in my head the exact multiply factor for the 645, but I think it is 0.72 (compared to FF)!

TriX Loc: Raleigh, NC

Maybe this will assist:

User ID

That is verrry much backwards, math-wise.

Look past the numbers and just see reality.
Lens from bigger camera is put onto small
camera. You know the drill !

.

speters Loc: Grangeville/Idaho

That's just what I said, lens from bigger camera is put onto small camera and not the other way around!

User ID

Read what you wrote.

"Does it go the other way?"

What you describe does NOT "go the
other way".

In both cases you swap a lens from a
bigger camera onto a smaller camera.
IOW it goes in the SAME direction.

BTW, forget calculating FOV, as your
concept of FOV is backwards. FWIW,
FOV occurs at the subject which is to
say it's within the subject scene. The
FOV shrinks as the FL get longer [or
also as the format gets smaller]. A
half length portrait has a larger FOV
than a head shot.

FOV does not get longer or shorter.
FOV gets wider/larger or narrower/
smaller.


.

BebuLamar

If you put the 24mm lens from Pentax 645 on your K20D it would still be 24mm but the field of view is equivalent to 36mm lens on a 35mm FF camera.

jerryc41 Loc: Catskill Mts of NY

Play with a lens simulator.
https://www.sigma-imaging-uk.com/focal-length-simulator/
http://imaging.nikon.com/lineup/lens/simulator/
https://hisgallery.com/blog/collection-of-online-camera-simulators-lens-simulators
http://canonfeatures.ca/comparison-tool
https://www.diyphotography.net/use-nikons-lens-simulator-to-pick-your-next-lens-non-nikon-shooters-too/

Jerry G Loc: Waterford, Michigan and Florida

No. If you could put a 24mm from a Pentax 645 on a K20D it would be a 24mm lens. The lens is not what is changing, it is the sensor size that is changing. A lens does not change when mounted on different cameras. It might help if you think of it as changing cameras not changing lenses. The lens will always project the same size image, different size sensors record different amounts of that image. Another way to look at it is that smaller sensors are cropping the image.

CatMarley Loc: North Carolina

No, You just get a vignetted image. The crop lens is made so that the circle of light thrown on the sensor is smaller than that of an FF lens because a small sensor does not need a big circle of light.

amfoto1 Loc: San Jose, Calif. USA

Lens focal lengths don't "change" when used on different image format cameras, as your post suggests.

A 24mm lens is 24mm regardless. A 28-200mm zoom is ALWAYS a 28-200mm zoom.... it never becomes "a 42-300mm".

Lens focal lengths merely "act differently" on different formats.

24mm is an ultrawide lens on a medium format camera like the Pentax 645... or it's a wide angle lens on a "so-called" full frame digital or film camera such as a Pentax Spotmatic.... or a slightly wide "normal" lens on an APS-C camera like your Pentax K20D. The same is true of a 28-200mm zoom.... it's a very wide to short telephoto zoom on a 645, moderate wide to moderate telephoto on full frame, or a very slightly wide to fairly strong telephoto on APS-C.

Lens "factors" (which you're calling "equivalents") merely compare a lens on any other format with how a fictional lens focal length might be expected to behave on a 35mm film camera (or a "full frame" digital, which use the same image size as most 35mm film cameras).

The only reasons to "do the math" and make these comparisons are:

1. You shot with 35mm film cameras for a long time and are accustomed to how lens focal lengths performed on that, but are now trying to select lenses for use on another format.

2. You are changing formats or using multiple formats for different purposes.

A so-called "full frame" lens factor is 1.0X. This is based on the 36x24mm image area that was most commonly used on 35mm (width) film.

An APS-C camera's lens factor is 1.5X (in some cases it's 1.6X or 1.7X). Multiply or divide the focal length by this factor if you are "thinking" in full frame/35mm terms or switching from that format and trying to select what lenses to buy.

A medium format digital Pentax camera's sensor isn't quite as large as those cameras image area on film... The Pentax 645Z uses a sensor that's about 44x33mm, while Pentax 645 film cameras made images 56x42mm. As a result, depending upon which camera you're referring to....

645 "digital" lens factor is 0.78X.

645 "film" lens factor is 0.614X.

For example, say you have a 24mm lens that can be used on all three cameras, but you've been using it on the 35mm film camera. To anticipate how it will perform on the APS-C camera, you multiply 1.5 X 24 to see that 24mm will "act like a 36mm lens would" on your 35mm film camera. Or instead multiply 0.78 X 24 to see that on the 645Z that same lens would "act like a 19mm would" on the 35mm film camera.

Or, say you're looking for a lens that gives you the same angle of as you have enjoyed using a 24mm on your Pentax Spotmatic 35mm film camera, but are now using...

For use on an APS-C K20D.... divide 24 by 1.5 to find out that you'll need a 16mm lens to get similar angle or view. Or...

For use on a medium format digital 645Z... divide 24 by .78 to find out that you'll want to buy a lens around 31mm focal length, for use on that camera.

This is actually nothing new... there have always been different formats of film and, before that, glass plates, daguerreotypes, etc. In fact, with all the different image formats, you can take this to even greater extremes such as...

A larger format such as 4x5" film (approx. 121x97mm image area... 0.29X lens factor).

A tiny format like digital 1/2.3" sensors used in many extreme-range, point-n-shoot cameras (approx. 6.17x4.55mm image area.... 5.64X lens factor).

If such a thing existed, a 28-200mm zoom on the 4x5" film camera would behave approx. like an entirely theoretical 8-58mm lens would on full frame/35mm film (28 x 0.29 and 200 x 0.29).

Or, that same 28-200mm zoom on the digital 1/2.3" sensor camera will "act like" approx. 158-1126mm lens would on full frame/35mm film (28 x 5.64 and 200 x 5.64).

On the other hand, if you were looking for a zoom that gave the same angles of view as that 28-200mm lens does on full frame/35mm film...

For the 4x5" film camera you would need to find and buy approx. 96-690mm zoom (28 / 0.29 and 200 / 0.29).

For the digital with the tiny sensor, you'll need to look for one fitted with approx. 5-35mm lens (28 / 0.29 and 200 / 0.29).

All of this relates other formats back to the image area standard with 35mm film cameras... 36x24mm. Someone who never used that might find it all irrelevant. Or, someone who has used another format all their life and is now contemplating a change to something other than full frame/35mm film format may need to use different lens factors to arrive at equivalents that have any meaning to them. Alternatively, they could convert their old format first to the full frame/35mm film equivalent, and then use the lens factor to convert from that to their new format.

It's with point-n-shoot digitals that you see a lot of "equivalent" focal lengths stated, rather than the actual focal length of the lenses. This is done because those types of cameras use a wide variety of different sizes of image sensors, which in turn makes for many different lens factors. In fact, there are probably close to two dozen different sizes of sensors used... with lens factors ranging from 14.14X to 2.47X. So manufacturers commonly refer to them using full frame/35mm film equivalents.

And among interchangeable lens digital cameras another common format is the so-called micro Four/Thirds sensor format... with a lens factor of 2X.

rcarol

Exactly. Finally, somebody gets it right.

User ID

Here ya go .... read what you wrote:

"If you had a 24mm 645 lens it would be
equivalent to about a 16mm on full frame,
so yes, in a way it goes the other way too,
...... ... ....... .."

As I remarked, your math is hopeless.

.

Bill P

You may be able to make the math work, but that won't make the lens work. A part of lens design includes the intended format. Three of the 50 mm lenses for my OM film cameras would fit inside of the 50mm for my Hasselblad. That's because the 50's for 35 FF cameras won't cover a 6x6 frame.

So this discussion is simply mental masturbation.

speters Loc: Grangeville/Idaho

Maybe you should use your glasses when reading! It is exactly as I said, if you use a 24mm lens on a 645 , it would be equivalent to about a 16mm on a full frame, that's just the way it is, no hopeless math needed!