The DSLR crop factor (again)

[PentaxManiac]

OK, so I can understand that the 1.5 crop factor makes a lens with a focal length of 50mm appear to behave like a lens with a focal length of 75mm when mounted on a DSLR. In real terms its focal length of course remains 50mm.

What I don’t understand is whether this has any effect on the ratio at which an object appears on the sensor of a DSLR as opposed to a frame of film and the resulting negative. I’ll use as an example the lens I’m thinking of buying: the Tamron 90mm F2.8. I can see on a DSLR with a crop factor of 1.5 this will behave like a 135mm F2.8, but will it still reproduce an object at life size (1:1) or larger or smaller?

[AlexMonro]

Life size 1:1 is still 1:1 whatever format you mount the lens on. But the size of the image formed is the size of the sensor / film frame, so an object 36mm wide will fill a 35mm film frame, but only 24mm wide is needed to fill a 1.5 crop DSLR (all at 1:1).

I use my Tamron 90 on both my K20D and ME Super - I get full frame images of smaller things with the K20D, but they're both "life size", just with more magnification required from the DSLR to produce a 6"x4" print.

[PentaxManiac]

Ah yes, I see.

You've actually answered another question with that, namely that the Tamron will work on film, and I presume full-frame should Pentax go that way, as well as digital. I'd presumed that would be the case, but one is never clear with Tamron because they, more than any other manufacturer, give their lenses such long and complicated titles.

Thanks

[sillyconguru]

one is never clear with Tamron because they, more than any other manufacturer, give their lenses such long and complicated titles.

Tamron lenses designed solely for crop-camera use have 'Di-II' in their designation.

[Benchista]

Tamron ..., more than any other manufacturer, give their lenses such long and complicated titles.

Yeah, not a bit like Pentax, with their "smc DA* 16-50mm F2.8 ED AL (IF) SDM"

IMHO, all manufacturers have got silly with the names, and it's certainly potentially confusing trying to work out what coverage a given lens has as each manufacturer uses a different convention.

[AlexMonro]

The other compatibility issue, particularly with manual film cameras, is whether there is an aperture control ring. Sadly, Tamron appear to have no designation in their lens titles to indicate the lack of this, unlike Nikon's "G" or Pentax's "DA" or "FA-J", and although my lens, purchased just under a year ago, does have this, there's no indication that current production still supports this feature - indeed, the product images on the Tamron website appear to show a lens without an aperture ring (though this may only apply to some non-Pentax mounts).

[PentaxManiac]

Yeah, not a bit like Pentax, with their "smc DA* 16-50mm F2.8 ED AL (IF) SDM"

Point taken. But at the risk of "he would say that, wouldn't he" I have that lens - my bank account is still recovering - and the SDM bit is very significant. Those who have tended to rate Pentax below other systems for speed of focus would be surprised to find it so much quicker and smoother and most of all quiter, because in a first for Pentax the motor is in the lens rather than the body.

In fact here's a confession. When I first got the lens, mounted it on a camera and put first pressure on the shutter button to focus I thought nothing was happening and the lens wasn't working, until I looked through the viewfinder

[Benchista]

To be honest, the ultrasonic motor is a lot of the reason why I use Canon. Having tried quite a few different cameras back in the early 90s, most of them made a hell of a racket and weren't very fast. I tried an EOS 100 with a USM lens, and it was magical - focus just appeared as if by magic. I still love that! Of course, now everybody has such motors, and they really are good.

As to the Pentax lens, when I attended the AP Awards ceremony last year, this lens won lens of the year, but at the rehearsal, the comedian co-presenting took the best part of 5 minutes just to read out the title... I was <s>pi</s> laughing quite a lot.

[Roger_Provins]

To be honest, the ultrasonic motor is a lot of the reason why I use Canon. Having tried quite a few different cameras back in the early 90s, most of them made a hell of a racket and weren't very fast......

... if you really want to hear a nice noisy lens my Sony 100mm macro takes some beating. I've a theory that insects are deaf, or I've deafened all the local ones