[vonBaloney]

Crop factor between old film lenses and new digital lenses is supposedly the same, right? But it isn't, at least with what I have here. Up until now, I've had the 18-250mm zoom as my only lens (never used the kit lens). But recently bought several old m42 lenses, and have been testing them out. So I have two manual zooms -- one that goes to 205mm, another to 210mm. Also a 200mm prime. The prime and both zooms zoomed all the way in all give me a greater telephoto than my normal zoom at 250mm by a noticeable amount. What gives? (Or is it an m42 adapter thing -- are the screw mount lenses sitting slightly closer the the sensor than they would with a native mount?)

[Aegon]

The DA 18-250mm lens measures 250mm only at the closest focus distance.

Yes, it cheats. But they all cheat.

[vonBaloney]

But even if it is only 200mm, the 200mm prime is still more magnified -- significantly. (The others go to 205mm and 210mm, so basically impossible to set them at exactly 200mm.) I will do a test at 50mm and see what it looks like.

[Adam]

FL's are approximate- this applies to almost all lenses. Look at the manufacturer's FOV spec to be able to judge the effective FL you'll be getting from a lens.

Adam
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[vonBaloney]

Yeah, found an old thread about this lens that explained it -- sounds like it is more of a 150-180mm on closer subjects. Learn something new every day. Back in the old days when I learned (20 years ago), I only had a single prime lens (K-1000 w/ a 50mm like most students) -- never learned that much about the actual optics and lens construction. When I got back into photography a couple years ago, got this lens to be a catch-all to fit the budget and haven't had anything to compare it to until now...

[sterretje]

I'm not any kind of specialist on this subject but as far as I understand it, it depends on the lens internals what happens to the focal length while focusing. It does not have anything to do with crop factor; behaviour will be the same on a SLR film camera.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/135882-what-tr...r-if-lens.html might be worth a read.

[Lowell Goudge]

Crop factor between old film lenses and new digital lenses is supposedly the same, right? But it isn't, at least with what I have here. Up until now, I've had the 18-250mm zoom as my only lens (never used the kit lens). But recently bought several old m42 lenses, and have been testing them out. So I have two manual zooms -- one that goes to 205mm, another to 210mm. Also a 200mm prime. The prime and both zooms zoomed all the way in all give me a greater telephoto than my normal zoom at 250mm by a noticeable amount. What gives? (Or is it an m42 adapter thing -- are the screw mount lenses sitting slightly closer the the sensor than they would with a native mount?)

the focal length is only measured at infinity,

new zooms that internally zoom and focus, reduce the focal length as you focus at shorter than infinity focal length,

there is no trickery as such.

[Stone G.]

I'm not any kind of specialist on this subject but as far as I understand it, it depends on the lens internals what happens to the focal length while focusing. It does not have anything to do with crop factor; behaviour will be the same on a SLR film camera.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/135882-what-tr...r-if-lens.html might be worth a read.

That is a very accurate and true statement. What "cheats" is that lenses - be it zooms or primes - may have internal focus and/or moving rear lens groups.

You can only count upon the nominal focal length at infinity focus. You may read a bit more here:

Focal Length Experiments

[vonBaloney]

But it is mainly a phenomenon with internal focusing lenses? (At least to such a large degree as I am seeing on this 18-250.) Are IF lenses a relatively new innovation or have they been around forever?

[Lowell Goudge]

But it is mainly a phenomenon with internal focusing lenses? (At least to such a large degree as I am seeing on this 18-250.) Are IF lenses a relatively new innovation or have they been around forever?

to my limited knowledge, IF lenses began at around 1976, and with pentax, the SMC 28 F2.0 which had a Fixed Rear Element. At that time, it was noted that there was a slight decrease in the focal length as the lens was focused in from infinity.

The issue is much more apparent in high ration zooms compared to any other type of internal focused lens. Unfortunately, while the focal lengths are measured only at infinity, there is no reporting of the focal length consistancy etc, as a function of focus distance reported.

While it is understood it happens, and has even been covered in photo magazines, it would be nice for the consumer to know the full performance of their lens. In this respect, I am disappointed by the photo press, who seem to concentrate less on some of the physical atributes of a lens relitive to specification.

[RioRico]

An ideal (maniacal) enthusiast's project: Measure the true focal lengths (and max apertures!) of zillions of extant lenses. All the PK's and M42's and M39's anyway. And publish the results. Any volunteers? Can we get a grant?

[vonBaloney]

One big project I *am* planning to do is to make some Lightroom/Photoshop profiles of these old lenses so I can just hit a button and remove CA and distortion, etc. Except with manual lenses you really can't just hit a button because the aperture and focus info (and focal length on a zoom) won't be in the EXIF data, which means keeping a notebook with that stuff in it as I'm shooting so I can type it in later -- which I suppose I really ought to be doing in any case. THEN I can hit that button.

[sterretje]

An ideal (maniacal) enthusiast's project: Measure the true focal lengths (and max apertures!) of zillions of extant lenses. All the PK's and M42's and M39's anyway. And publish the results. Any volunteers? Can we get a grant?

Lowell started thatmore or less in the link that I posted in #6. It ended more in a discussion than in actual facts You can start with your 200+ lenses. I will add my 2 AF primes