[tim mcdonough]

I'm reading a description of a Pentax 35mm DA L F2.4 AL Lens (for DSLR) for sale and the description says "...equivalent to 52.5mm in 35mm format." What does this mean? Thanks in advance!

Tim

[TomB_tx]

Pentax dSLRs use a sensor smaller than the size of a 35mm film frame, so it only captures what would have been the center area of a 35mm film. The result is the angle of view is different when a lens is used on dSLR vs film. So a 35mm lens covers about the same angle of view on dSLR as a 50mm lens did on a 35mm film camera.

[tim mcdonough]

Thanks for the reply TomB. I already use an old non-digital 50mm lens on my K-x. Would the digital 35mm lens have the exact same focal length as the non-digital 50mm lens?

[eddie1960]

nope, this is an instance of the crop factor confusing things. a 35 mm is a 35 mm a 50mm is a 50 mm, they don't change. the effective Field of view changes based on sensor size. because when film slrs transitioned to digital there was no Full Frame (AKA 35mm) sensor that users were used to the idea of a crop factor came up to help with the transition. So your trusty standard 50mm became in essence your trusty short portrait lens (FOV of a 75 mm on 35mm) and your moderately wide 35mm became your trusty standard focal length.
If you never shot film it really means nothing. the lens is what it is. Take a shot of the same scene using a tripod working your way through the various focal lengths you own and it will give you an idea of how the lenses perform on your apsc sensor which in reality is all that matters.
lenses are not digital BTW they may be designed for an apsc sensor (like the DAL35 2.4) or for a 35mm frame (FA50 1.4) but on apsc they will still be a 35 mm and a 50 mm presenting a different FOV

[VisualDarkness]

No, the focal length is always the same no matter the sensor size. Though the field of view (how wide you see) differs when putting the same focal length on different sensors.

[tim mcdonough]

Thanks for all the input, I think I'm starting to understand. I just want to make sure if I shoot with the brand new 35mm lens will the photo more or less look the same as if I used the older 50mm lens?

[JinDesu]

Thanks for all the input, I think I'm starting to understand. I just want to make sure if I shoot with the brand new 35mm lens will the photo more or less look the same as if I used the older 50mm lens?

Yes. It will look similar (if just a tiny bit longer) than the 50mm on 35mm film.

[eddie1960]

Thanks for all the input, I think I'm starting to understand. I just want to make sure if I shoot with the brand new 35mm lens will the photo more or less look the same as if I used the older 50mm lens?

It will look similar to the old 50mm lens shot on film. it will not look similar to the 50 on your digital camera. this is why I said ignore any and all crop factor arguments. they are not relevant if all you shoot is an apsc camera. lens sizes don't change on formats, field of view or angle of view of the lens differs on different formats. As an example I shoot 50 mm on apsc, 35mm film and 645 medium format film
If i am trying to compare how they look and use a 35mm camera as my base then angle of view acts as follows
on 645 my 50 mm performs like a 30mm wide angle on 35mm film (or FF digital)
on 35mm film my 50 mm performs as a standard 50 mm lens
on apsc my 50mm performs as a short tele (or like a 75mm on 35mm film)

in all 3 cases i am using a 50 mm lens but they all provide different FOV based on the size of the sensor (or film frame)

Normal lenses have always varied by format. mmost people think of a 50mm as normal on a 35mm camera, actually normal for 35mm is actually 43mm, but 50's were cheaper and easier to build and not different enough to be an issue

on apsc (at least the 1.5 crop of sony/pentax/nikon not the 1.6 canon crop) a 28 mm is pretty close to a perfect normal, but anywhere from 28-35 is accepted as a normal lens. mostly you will se 35 since they are common and translate [pretty closely to the defacto standard of 50mm on FF(or 35mm)

645 a normal lens was a 75 which is pretty much a 45mm lens in terms of a FF camera.

confused now i bet.
so back to my original statement ignore any and all crap factor arguments you don't shoot on ff or film so it's irrelebvant. use the lenses and you'll get a good idea of what they are best at.
on apsc
Ultrawide is 8-16mm for the most part
Wide is 16-24mm
normal is 28-35
short tele/portrait is 50-85
tele is 100+

[tim mcdonough]

Got it, thanks everyone!

[RioRico]

As mentioned, moving a lens between cameras doesn't stretch or shrink its focal length. A lens projects an image; different-size frames (film or digital) see different amounts of the image. My old example: Cut a picture from a magazine. Draw a 60x45mm rectangle on it. Inside that, draw a 36x24mm rectangle. Inside that, draw a 24x18mm rectangle. Those rectangles are about the frame sizes of 645/MF, 135/FF, and APS-C frames. The picture remains the same, but each format sees a different amount of it.

A 50mm lens on APS-C has about the same FOV/AOV (field/angle of view) as a 75mm lens on 135/FF. If you use a 50mm lens on both a 135/FF camera and an APS-C camera to shoot some subject, you'll move back a bit with the APS-C camera in order to fill the frame, so the perspectives of shots will differ between formats. Moving back further also thickens the DOF. There's about 1 f-stop difference in DOF; when I put a manual lens on my K20D, I may set the aperture to f/11 but I'll read the DOF scale at f/8.

So when you see a lens described as 'equivalent' to some focal length on another camera, remember that only the FOV is equivalent. Everything else changes as you use it. There's a closer equivalence if aperture and DOF are considered too. A 50/2 on 135/FF is FOV- and DOF-equivalent to 75/3 on APS-C. Or going in the other direction, to get the same view as a 50/1.4 on 135/FF, you'd need about a 35/0.95 lens for an APS-C camera! Rotsa ruck...