Full frame or not ???

Nigel, perhaps this might be something left over from the early days of FF DSLRs? If an APS-C field of view was cropped from the image back then, the few pixels remaining would have limited the scope for making a large print, so it was better to use a longer focal length lens to fill the field of view of the FF sensor with the subject. With the multi-mega-pixel FF sensors around today, cropping and then making a detailed large print is a now a viable option.

Cheers.
Philip

A very interesting observation Philip, and I'm sure that's right. It's focal length and distance that combine to produce the required magnification. The required magnification only has to be 'enough' .... Enough depends on the resolution and intended output size (print or screen).

For most purposes a frame filled aps-c size image at our modern resolutions is 'enough'. If that was taken with a 300mm lens than with modern resolutions there's no reason that it is not ' enough' for a full frame photographer as well. The superfluous pixels will just be cropped away.

Our FF shooter might feel cheated of course ... His bird or whatever looked smaller! Well, he doesn't need to spend a fortune on a FF diameter 400mm lens ...... What he might like is a viewfinder control to increase his viewfinder magnification to, say, 140% ..... Then at least he'll feel happier when shooting .... ! And his back balance will be as well .

No, perspective is only affected by the distance from the camera to the subject. Focal length in this respect makes no difference.

But I could take 2 portraits from the exactly the same distance from the subject ... One at 14mm and one at 135mm .... Would the perspective be the same? The shape rendering on the face would surely be different .. Is that not 'perspective'? (you'd have to really go to 200% or so to see it on the 14mm shot I guess ....)

And to think all the OP wanted to know was if the Sigma 120-400mm HSM would work with their K-r and K-5IIs

Well it would be a boring old forum if we'd just ended it with 'yes' ....

I liked Peter's use of perspective because it would help explain in to a non photography geek. I think talking to non photographers in terms of magnification might actually confuse, because looking on the back LCD and image produced by APS-C you could easily conclude that APS-C has more magnification. Look, the bug is bigger on the screen, must be better magnification!!!

Regarding your test at 14 and 135mm... try it If you have the same effective depth of field you might be surprised at the result

I agree with nass, try it.

How could the perspective be any different, you haven't moved....

It would be very interesting to test it for sure ... I'll try, but may not be able to post the results until back from holiday .... Nothing is simple it seems ... Well, at least for photography geeks

Bottom line... complicate the issue all you like chaps, but put my beloved Pentax 28mm 3.5 on my Dad's old K1000 and I see a lot more of the world than if the same lens is fitted to my K10.

(Have a great holiday, Nigel...)

Thank you all for your comments and from all that I take it, "it is a No for now" and see what the future brings and a "yes for the lens" read all the comments and am slightly more confused than before but will get my head around one way another, Please to see it brought a number of debaited subject's again thank you all for taking the time to reply and enjoy your hols N.Mc

Field of view changes as a lens focal length changes, and is different on different sized sensors.
Perspective though is determined by how far away you are from a given subject physically, regardless of the lens/sensor combo.

Agree the wide angle/fish eye type effect does make things a bit difficult to get your head around, but it can't let you see any further around objects/corners etc from a certain distance away.

Thanks Couls and Andrew ... Weather has been good generally and Swanage is a beautiful area. These are always stimulating discussions ... Some things keep on coming up again and again of course. I'm looking forward to my 14mm and 135mm portraits for perspective comparisons and will post them here in future.

Go and enjoy the Sigma zoom on aps-c for now ... And lets hope you get the chance to experience it's wider field of view (but not different perspective!) on a Pentax FF very soon! You might just have to crop a bit if you want things to fill up the sensor as much.

You often hear the claim that long lenses "compress" perspective. It's an interesting experiment to take the same shot with, say, a 300mm lens and 16mm lens. Enlarge a distant feature in the 16mm shot to the same size as it is in the tele shot and you'll get exactly the same perspective "compression". As John says, camera-subject distance is the sole determinant of perspective. It's just not normally visible in the fulll wide-angle field of view.

Hi couls,

I hope you've got your answer! The guys are of course correct about focal length, perspective etc etc, but it's how the image looks that will confirm for you whether a film-frame-compatible lens is even relevant. For example, I have both DA55-300 and A70-210 zooms (the second a 1980s film-frame) that I can use on my K-5, but the 55-300 is noticeably sharper. Because a modern lens is FFC (film frame compatible), it doesn't necessarily mean that it's automatically 'better'. Purchase of a FFC lens may then possibly be down to whether you'd want to trade in APS-c for a camera with a film-frame sensor when it comes along.

I don't regard film-frame an upgrade for me from APS-c, and won't even be considering it. Why? It's horses for courses. If an APS-c camera/lens combination delivers satisfyingly stellar results on a HD monitor then where's the advantage of film-frame, especially with the not-inconsiderable potential downside of increased size and weight. I don't print my pictures, and since the pixellage of my HD monitor is nowhere near the resolution capability of my standard lens and 16MP APS-c sensor then film-frame holds no draw for me at all. I would not have invested in the marvellous 20-40 Ltd if it did! 😄