[mapguy]

I understand that an older Pentax film lens is cropped or gets narrowed on my K3-II and on the K-1 the same lens is normal. I also get that some of the new DA lenses made for APS-C can't be used on film or the FF cameras like the K-1.

So say my old 40mm smc-m pancake lens if more like a 60mm, and my old awesome 24mm is really not a wide angle anymore. Hence the reason why the old wide angles are popular again with the K-1, and the the DA only LTD lenses aren't useful anymore.

What I still don't understand is how you convert or multiply a film lens focal length to determine what focal length the K3-II will see?

I'd like to know because I was always was curious about the old pentax 28mm shift lenses, but because of crop sensors, they don't seem as useful in a APS-C camera. It would be good to know if my old SMC-A 50mm f1.4 will work as a fast 85mm on a APC-C or the SMC-A 135mm will step in as a fast 200mm.

[Unregistered User 8]

Hi mapguy, this is the info you seek.


The Crop Factor Explained: An Animation - Tutorial Videos | PentaxForums.com

[Adam]

The focal length never changes; APS-C just sees less of the frame. So as you've correctly observed, wide lens will seem narrower, and tele lenses will seem longer.

There are angle of view tables here that you might find helpful:

The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

[Not a Number]

The crop factor for Pentax APS-C DSLRs in 1.5. So just multiple the focal length by 1.5 or if you're like me and bad with multiplication divide by 2 and add that to the focal length. Examples 100 + 50 = 150, 40 + 20 = 60, 24 + 12 = 36, 50 + 25 = 75mm.

Note the focal length is not changing but the effective field or angle of view is affected. So the AOV, from the same distance, a 50mm lens on APS-C is about the same as a 75mm lens on FF

[Sandy Hancock]

So say my old 40mm smc-m pancake lens if more like a 60mm

Correct. Long "normal" instead of short "normal"

my old awesome 24mm is really not a wide angle anymore.

24mm on crop gives an angle of view roughly equivalent to 35mm on full frame. That's a classic wide angle focal length.

What I still don't understand is how you convert or multiply a film lens focal length to determine what focal length the K3-II will see?

Multiply by 1.5

I'd like to know because I was always curious about the old pentax 28mm shift lenses, but because of crop sensors, they don't seem as useful in a APS-C camera.

Agreed. Given one of their main uses is architecture, the angle of view is a bit narrow on a crop camera. Roughly the same as the FA43 on full frame.

It would be good to know if my old SMC-A 50mm f1.4 will work as a fast 85mm on a APS-C

Pretty much: more like 75mm, so close enough perhaps. There's a reason why Pentax's specialist APS-C portrait lens is 55mm.

or the SMC-A 135mm will step in as a fast 200mm.

Yep

It seems to me you actually have it worked out quite well

[mapguy]

So if I put the 1.4 TC on it, I multiple the new focal length I figure out by another 1.4?

So for say any of pentax's 100mm macro lens made for film, in the K3 it would be about 150mm, then with the 1.4 TC it becomes a 210mm?

The 200mm macro becomes a 300mm macro lens in a APS-C body?

Lastly would the DA 100mm macro lens, also be 150mm on k3? or do the DA lenses stay the same focal length they are listed as?

I am getting the sense that one could keep a APS-C body around for telephoto lens applications, and say a K-1 body more for landscapes and general shooting.

[mikesbike]

It is actually not complicated at all. The field of view changes. To answer your question, which essentially indicates you have a grasp of the situation, is just multiply by 1.5 x, or to be more exacting, I read it is 1.53 x for Pentax. A 135mm lens normally used on a 35mm camera body, for example, if mounted on an APS-C DSLR will produce a picture resembling that of what you'd get with a 202.5mm lens on the 35mm film body- or more exacting for Pentax, a 206.6mm lens.

There have been many lenses specifically created for APS-C use with this in mind. For example, the premium and very fine DA* 50-135mm f/2.8 which was designed to replicate results previously obtained with lenses like the FA* 80-200mm f/2.8 on 35mm film bodies, but at a much reduced weight and size factor.

Your 24mm lens would still have a wide angle view, just not as wide- it would produce images similar to a 35mm lens on a 35mm film body. Still a very useful lens on APS-C.

---------- Post added 03-02-18 at 06:06 PM ----------

So if I put the 1.4 TC on it, I multiple the new focal length I figure out by another 1.4?

So for say any of pentax's 100mm macro lens made for film, in the K3 it would be about 150mm, then with the 1.4 TC it becomes a 210mm?

The 200mm macro becomes a 300mm macro lens in a APS-C body?

Lastly would the DA 100mm macro lens, also be 150mm on k3? or do the DA lenses stay the same focal length they are listed as?

I am getting the sense that one could keep a APS-C body around for telephoto lens applications, and say a K-1 body more for landscapes and general shooting.

You've got the idea quite well. This is not to say, however, that good WA work is not attainable with APS-C. The DA 12-24mm f/4 lens, for example is capable of vey fine results, as is the DA 15mm f/4 Limited.

---

Last edited by mikesbike; 03-02-2018 at 06:11 PM.

[tvdtvdtvd]

The 200mm macro becomes a 300mm macro lens in a APS-C body?

Lastly would the DA 100mm macro lens, also be 150mm on k3? or do the DA lenses stay the same focal length they are listed as?

The focal length doesn't change. A 100mm lens is always a 100mm lens, regardless of the size of the sensor behind it. What does change
is the apparent field of view, (FOV), that that focal length covers. A 100mm lens will cover a wider FOV on a FF sensor than on an APS-C
sensor. The FOV covered by a 150mm lens on FF is equivalent to the FOV of a 100mm lens on APS-C. Thus it has become common place
to refer to the apparent change as a relative focal length 'equivalence', despite the confusion this causes. 100mm on APS-C has the FOV
equivalence of 150mm on FF, but the 100mm lens is still a 100mm lens.

edit: Whoops, got my equivalence wrong the first time. Fixed!

[UncleVanya]

The way I try to explain it is if you hold a picture frame up close to your face and then hold it out farther - the amount in the frame is different but the magnification of the objects didn't change - focal length is like this. The 100mm lens (or 200 or 15) works the same no matter what size sensor is behind it. Some lenses are only good for projecting a smaller image circle and if you put them on a larger sensor they fail to cover it fully or have distorted areas outside the part that would be covered well on a smaller sensor. Some smaller sensors are very high density so that they give higher detail than cropping a larger sensor. So for example shooting a 300mm on an APSC 24MP camera gives more detail than the same lens on a k-1 where the equivalent crop would give about 15MP. (There are other reasons for using one vs. the other). The thing to remember is that an 8x10 camera 300mm lens mounted on an APSC camera will not give different results generally than an APSC 300mm lens. Reversing this and mounting an APSC only lens onto an 8x10 camera will result in a very large amount of black and a tiny round spot a bit larger than the APSC width of the sensor. The part shown inside the illuminated portion of the 8x10 sensor would be the same as if the same scene was captured with the 8x10 version of the 300mm. (Not at all very telephoto)

[normhead]

Do the easy thing, look through the view finder.

[UncleVanya]

Do the easy thing, look through the view finder.

That helps immensely.... until you are planning before buying. LOL. I totally agree. While the back of the envelope helps me sometimes plan when shooting multiple formats it is easy to just look.

[twilhelm]

One way to simplify it is this: If you take the old K200, DA200 and the 645 200mm lens and mount each one on your K3, you’ll see the same magnification or photo. The crop factor applies to the sensor or film plane with 35mm being the standard.

[Sandy Hancock]

So if I put the 1.4 TC on it, I multiple the new focal length I figure out by another 1.4?

So for say any of pentax's 100mm macro lens made for film, in the K3 it would be about 150mm, then with the 1.4 TC it becomes a 210mm?

The 200mm macro becomes a 300mm macro lens in a APS-C body?

Lastly would the DA 100mm macro lens, also be 150mm on k3? or do the DA lenses stay the same focal length they are listed as?

I am getting the sense that one could keep a APS-C body around for telephoto lens applications, and say a K-1 body more for landscapes and general shooting.

You are more or less spot on all counts. Strictly speaking, focal length per se doesn't change between formats. When we talk about equivalence most of us are referring to angle of view.

[acoufap]

You are more or less spot on all counts. Strictly speaking, focal length per se doesn't change between formats. When we talk about equivalence most of us are referring to angle of view.

We should not mix the concepts. This would be my take:

1. (Already said) focal length of a lens doesn't change
2. FF vs. APSC: APSC sensor area makes a smaller crop of the lens's projected image circle. That implies the narrower angle of view of APSC vs. FF
3. Using a TC1.4 makes a new optical formula. That's something different than FF vs. APSC crop. The combination of a 100mm lens plus TC1.4 makes an optical 140mm focal length construction comparable with a real 140mm lens. Because you use 2 parts put together it may be not an optimal "optical lens construction" so image quality can suffer in some way.
4. If you use this 140mm construction you can of course apply info no 2. (FF vs. APSC image circle crop)

[WPRESTO]

So if I put the 1.4 TC on it, I multiple the new focal length I figure out by another 1.4?

So for say any of pentax's 100mm macro lens made for film, in the K3 it would be about 150mm, then with the 1.4 TC it becomes a 210mm?

The 200mm macro becomes a 300mm macro lens in a APS-C body?

Lastly would the DA 100mm macro lens, also be 150mm on k3? or do the DA lenses stay the same focal length they are listed as?

I am getting the sense that one could keep a APS-C body around for telephoto lens applications, and say a K-1 body more for landscapes and general shooting.

Your calculations are correct. When I put a FF on my K3, I do not think of it as getting, let's say a 150mm lens instead of a 100mm lens. I think of it as "pre-cropping" the outermost 1/3 of the picture area .