[magkelly]

If I am reading some of the posts on here correctly my 50MM film lens is more like a 75MM on my *ist DSLR. That's about 1/3 more distance than it has on an SLR. I'm thinking that means my 75-300MM Vivitar is actually more like 100MM-400MM on the *ist then? It kind of makes sense to me that it would be but I'm admittedly a bit new to this using film lenses on a DSLR thing and I'm not quite sure I am estimating right. I'm also wondering if that means my 105MM Takumar is more like a 135MM and my 135MM Sears lens is actually more like 170MM? (Is there a cheat sheet for this somewhere? I could sure use one! )

I know I am definitely getting almost twice the scope with the Vivitar though. I've photographed birds with the 200MM digital lens and with the Vivitar at 300MM and it's been a whole other ballgame with the Vivitar. I'm getting way closer with the Vivitar than my 200MM AF digital Tamron.

But I'm just not sure if it's a standard 1/3 across the board? Is that a safe bet or is there some other formula for figuring it all out?

Thanks!

[Jewelltrail]

The lens view on Pentax crop sensor is cropped. To get the focal length equivalent for your crop sensor Pentax, compared with Full Frame, you simply multiply by 1.5.

Your example, of 75-300 would become:
112.5mm-450mm

However, the 75-300 lens is always a 75-300mm lens--that doesn't change--the area of the lens you see with your crop sensor changes and crops the view, hence giving the appearance of incresed focal length.

[SpecialK]

I've photographed birds with the 200MM digital lens and with the Vivitar at 300MM and it's been a whole other ballgame with the Vivitar. I'm getting way closer with the Vivitar than my 200MM AF digital Tamron.

That's because 300 is 50% longer than 200. It's not a mystery.

But I'm just not sure if it's a standard 1/3 across the board? Is that a safe bet or is there some other formula for figuring it all out?
Thanks!

Yes, it's 1/3 more (or 1.5x) from film to Pentax digital.

As many other will point out, the focal length does not change, just the angle of view.

[RioRico]

A 50mm lens is a 50mm lens, no matter what size camera it is on. Try this: Cut out a picture from a magazine. Draw a 60x45mm rectangle on it. Inside that, draw a 36x24mm rectangle. Inside that, draw a 24x18mm rectangle. Those are respectively the frame (film or sensor) sizes of 645, FF, and APS-C camera frames. THE PICTURE REMAINS THE SAME but the smaller frames see less of it -- they 'crop' the projected image, that's all.

If you are an experienced 35mm film photographer, then mentally translating focal lengths to various formats is useful. Otherwise, FORGET YOU EVER HEARD OF CRAP FACTOR. Put a lens on your camera; look through it; what you see is what you get.

Back in the day, I worked simultaneously with 135/HF (half-frame, same size as APS-C), 135/FF (full-frame), 6x6 and 6x9 cm MF (medium format), and 9x12 cm LF (large format). I never thought about crop factors -- I just learned what any given focal length would do on any given format. The term "crop factor" wasn't even in the language then, even though some lenses could easily be moved between formats, just as 135/FF lenses are used on APS-C now.

My sister just sent me our dad's ancient 8x14 cm LF view camera. I can't even tell its lens' exact focal length (FL) -- probably 162 mm, which is diagonal of that frame. A lens whose focal length is the diagonal of its frame will give a 'normal' FOV (field of view) no matter the format. A lens 2/3 that 'normal' FL will be wide. A lens 4/3 that FL will be long. That's all that matters.

[jolepp]

The lens specs have stayed the same: the register distance and the focal length of the lenses are still the same. The result is the same as cutting the edges out of a 135 negative (24x36mm) so that the center 16x24mm (APS-C sensor size) remains. Since the dimensions of the film frame are 1.5x larger, the field of view on APS-C is the same as what you'd get with a 1.5x FL (the perspective and DOF are different though so this is strictly speaking not a zoom factor, but a crop factor, as it is really about taking a hardwired center crop).

[magkelly]

So what you all are saying basically in layman's terms is that you're not actually getting more distance you're just getting a tighter cropped shot?

[RioRico]

So what you all are saying basically in layman's terms is that you're not actually getting more distance you're just getting a tighter cropped shot?

BY GEORGE, S/HE'S GOT IT!! (*) Yes, that is exactly right.

For instance, back in the day, I had a Spiratone 400 long lens in T-mount. I could use it on one adapter for a full-frame Nikon F (landscape aspect), and another adapter for a half-frame Olympus Pen-FT (portrait aspect). From the same location, shooting the same lens and subject, I could snap a shot with each. The half-frame image would be EXACTLY the same as if I had cropped 1/4 from each side of the full-frame image. NOTE: the APS-C sensor in you dSLR is just about the same size as half-frame 35mm.

So comparing a lens on full-frame and APS-C, think of the APS-C sensor as performing an in-camera crop.

Where the math get tricky is that field-of-view is computed using the DIAGONAL of a frame, while sensor sizes define the AREA of a frame. The diagonal of FF is about 44mm; the diagonal of APS-C is about 29mm; so FF is about 1.5x the APS-C dimension. But an FF frame has TWICE the area of an APS-C frame. Blame Pythagoras.

(* Please excuse me for not recalling your gender affiliation.)

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Last edited by RioRico; 12-26-2010 at 07:27 PM.

[jolepp]

So what you all are saying basically in layman's terms is that you're not actually getting more distance you're just getting a tighter cropped shot?

Indeed. The only thing that has changed is that instead of a film frame you now have the smaller APS-C sensor at the center of the image circle (focal plane): you get a crop.

[jolepp]

BY GEORGE, S/HE'S GOT IT!! (*) Yes, that is exactly right.
...

This must be the first time our combined efforts in explaining this would seem to have produced a result other than more confusion :-)

[magkelly]

The technical babble doesn't scare me. I may have to read it 3X to sort it out but I do usually get it eventually. LOL

We have hawks out in full force today, and it's really making me want to go out to play with the Vivitar but it's so freakin cold that I can't quite motivate myself. It's not so much the actual temperature. It's so windy that a hard gust just chills you. I just about froze just going from the car into Target yesterday and today is more of the same.

I'm amazed that the birds are so into flying. The wind is up so high the past few days. I would think they'd all be sitting on their nests, but no, they're more active than usual. It's like they are up there having a ball hang gliding or something.

[Ira]

A 50mm lens is a 50mm lens, no matter what size camera it is on. Try this: Cut out a picture from a magazine. Draw a 60x45mm rectangle on it. Inside that, draw a 36x24mm rectangle. Inside that, draw a 24x18mm rectangle. Those are respectively the frame (film or sensor) sizes of 645, FF, and APS-C camera frames. THE PICTURE REMAINS THE SAME but the smaller frames see less of it -- they 'crop' the projected image, that's all.

You've typed this explanation so many times, I think it's time for you to create a visual aid, a JPEG, and just link to it.

I'm not saying I'll give you any reputations points, but you should do it anyway.

[RioRico]

You've typed this explanation so many times, I think it's time for you to create a visual aid, a JPEG, and just link to it.

There ARE a couple images of that here, way down in the thread stack. But it's faster to type the explanation, than to search for the link. Oughta be a sticky...

I'm not saying I'll give you any reputations points, but you should do it anyway.

Hey, hit my rep anyway, eh?

A fiend to all, and a bother to every other scout.