[Athanassios]

I recently compared two shots taken @300mm with both Sigma 18-300 and Pentax 55-300 PLM and, apart from the quality, I was puzzled that Pentax brought the subject some 10-20% closer.

Other reviewers have also stated that Pentax 150-450 shot @450mm actually brings the subject closer that Bigma 50-500 OS @ 500mm

Reason I ask is that I have a gap between my 300mm zoom and ....900mm (catadioptric) gear and prise wise I feel distressed that in current market, eg ebay, the SIGMA 50-500mm F/4.5-6.3 DG OS HSM is sold 50% CHEAPER on other camera brand mounts.

PS: My 18-300 does work with Pentax HD 1.4x TC (HSM works fine), even if aperture or zoom are shown wrongly, even on the camera eg no 1stop drop, hence I have a SOFT 25-420 in APS-C with psycho exposure @ 350 euro.

I'd be a fool to compare the IQ of the Pentax 150-450 with any Sigma but the price hurts.

[StiffLegged]

eBay is a market, so if more people are chasing Pentax-fit lenses than there are Pentax-fit lenses for sale the price tends to rise. And as we all know (yawn...) lots of people are offloading their other-brand DSLRs and compatible lenses, so other-brand compatible lenses are at lower prices. Nothing new there, it's called supply and demand.

[allanmh]

Another aspect is that the stated focal length is only correct when the lens is focused at infinity. At shorter focus distances the nominal and actual focal lengths will differ. I believe it's called 'focus breathing'. Different lenses will suffer from this in different amounts. That's probably why your Pentax and Sigma lenses show a difference at 300mm.

[Des]

I recently compared two shots taken @300mm with both Sigma 18-300 and Pentax 55-300 PLM and, apart from the quality, I was puzzled that Pentax brought the subject some 10-20% closer.

This is probably due to focus breathing rather than "cheating".

A 300mm lens on an APS-C camera has a field of view (FOV) of 5.4°. But a lens is at its narrowest FOV for a particular focal length only when focused at infinity. When focused at a closer point, the FOV will be at least slightly wider. (There might be exceptions amongst non-internal focus lenses, but put that aside for the moment.) The difference is called focus breathing. Lenses with internal focus (IF) - that is, where the front element does not extend during focusing - have more focus-breathing that non-IF lenses. With some lenses the difference is marginal, with others it is great.

The 55-300mm PLM, which has IF, has more focus-breathing than the earlier screw-driven non-IF 55-300. (I did a comparison here: 55-200, 55-200 WR, 55-300, 55-300 PLM, etc, which telezoom I should get? - PentaxForums.com.) But my conclusion was that it didn't make much difference in practice. For close subjects, the shorter minimum focus distance of the PLM largely offset the focus breathing, which was why the two lenses have about the same maximum magnification. By field-relevant focus distances (about 6m in my test), the difference was negligible.

Wide-ranging IF zooms - and superzooms in particular - are notorious for high levels of focus breathing. The Tamron 18-250 and Pentax DA 18-135 are examples amongst lenses I have owned.

Judging by its specs (Sigma DC Macro HSM (Contemporary) 18-300mm F3.5-6.3 Lens Reviews - Sigma Lenses - Pentax Lens Review Database), I would expect the Sigma 18-300 to have less focus breathing than these lenses. It doesn't have internal focusing and has a maximum magnification of 0.33x (which Sigma dubiously calls "macro") at its MFD. But given its MFD is only 390mm (compared to 900mm for the 55-300PLM), there must be quite a lot of focus breathing close in. At 390mm from the subject, the DFA 100mm macro would have comparable or greater magnification with only one-third the focal length.

If you want to pursue the issue, take a series of images with the two lenses at 300mm at different distances to subject and compare the FOV. Start by comparing subjects at the MFD for each lens (the Sigma should have more magnification), then the MFD for the PLM, then at a few field-relevant distances, then close to infinity. By the time you get close to infinity there should be hardly any difference.

[Edit: @allanmh posted while I was writing this, making the same point.]

prise wise I feel distressed that in current market, eg ebay, the SIGMA 50-500mm F/4.5-6.3 DG OS HSM is sold 50% CHEAPER on other camera brand mounts.

@StiffLegged has answered this. It's just supply and demand.

It might be worse now that more people are selling off DLSR lenses and shifting to ones designed for mirrorless mounts, but the complaint is not new. For as long as I have been a member here (8 years) people have complained about the dearth of affordable AF lenses of 400mm+ in K-mount. Pentaxians who want lenses that are cheaper or longer than the DFA 150-450 (or DA*300 + TC) pay more because they are chasing fewer lenses.

Tamron and Sigma never released K-mount versions of their xx-600mm zooms, and probably never will, because they calculate that they could not make sufficient profit from them. The only way we are ever likely to get such a lens would be a collaboration, probably with Tamron, like the collaborations that produced the DFA 15-30 and DFA 24-70. But Ricoh might be concerned that such a lens would cannibalise sales of the DFA 150-450.

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Last edited by Des; 01-16-2023 at 02:21 PM.

[Sandy Hancock]

Another aspect is that the stated focal length is only correct when the lens is focused at infinity. At shorter focus distances the nominal and actual focal lengths will differ. I believe it's called 'focus breathing'. Different lenses will suffer from this in different amounts. That's probably why your Pentax and Sigma lenses show a difference at 300mm.

^This.

To be fair, some Pentax lenses also have quite significant focus breathing. The otherwise superb DA*60-250 is one of the most dramatic examples, but the newer DFA*70-200 and DFA70-210 have also been criticised for it. Personally, I don't consider it to be a particularly big issue, but some people like getting bent up over things.

Mind you, some companies have been accused of being a little optimistic with their on-paper specifications

[mlag]

Mind you, some companies have been accused of being a little optimistic with their on-paper specifications

Which means that if manufacturer's design team achieved 293,4568865 mm lens, the marketing would put it in the market rather as a 300mm rather than a 293mm (or a 295mm) lens.
We all like rounded numbers, especially on the longer tele side, don’t we?
( on wide lenses <40mm it's more mm acccurate, as it translates into noticeable different angles for field of view)

Albeit, I have some exceptions in my lens collection
Like the soligor 95-310mm, probably trying to outsell other 100-300mm lenses...
and a vivitar 75-300mm, probably running short of the more common 70mm.
And even the pentax 43mm, 55mm or the 150-450mm, 560mm are a bit odd numbers in the market...
Most others seem to follow the 50 70 100 135 200 250 300 400 500 600 800 1000 series.

On the tele-side a few mm difference for distant objects hardly matters in my experience , a 290, 300 or 310 mm will feel similar in use.
It is only when one abuses its tele’s as pseudo-macro lenses for objects at close distance that things get really different, but it’s more a mix of minimum focus distance & breathing(lens design) rather than pure focal length ... a 500mm with mfd of 6+ meter is useless for macro style usage and yields less magnification than a 70mm with mfd of <50cm.

[kypfer]

I'd be a fool to compare the IQ of the Pentax 150-450 with any Sigma but the price hurts.

I asked this question on these forums some time back and found someone who actually had owned both the 150-450mm Pentax and a 150-500mm Sigma at the same time, so had been able to make a direct comparison … apparently there was very little difference!
On the basis of this opinion I've continued to be perfectly satisfied with my "bigma"

[Paul the Sunman]

^This.

To be fair, some Pentax lenses also have quite significant focus breathing. The otherwise superb DA*60-250 is one of the most dramatic examples, but the newer DFA*70-200 and DFA70-210 have also been criticised for it. Personally, I don't consider it to be a particularly big issue, but some people like getting bent up over things.

Focus breathing is a matter of basic optics. Internal focus lenses such as the DA* 70-200 are particularly prone to it, but internal focus also brings many benefits (better weather sealing, faster focusing, constant lens length, etc.). It is a trade-off; which do you value most? Buy accordingly.

[Sandy Hancock]

Focus breathing is a matter of basic optics. Internal focus lenses such as the DA* 70-200 are particularly prone to it, but internal focus also brings many benefits (better weather sealing, faster focusing, constant lens length, etc.). It is a trade-off; which do you value most? Buy accordingly.

As I said in my earlier post, I don't think focus breathing is big deal. The DFA* 70-200 is an amazing lens, which I can't imagine parting with.

[Bruce Clark]

I have a Sigma 170-500mm and a recently aquired Pentax 150-450. I have felt the same thing on my first outing but have not as yet got around to doing any serious examination of the issue. Apart from this, the focusing of the Pentax is so far superior in speed and accuracy, that I doubt I will use the Sigma again. I will put it for sale as soon as I can locate a misplaced lens cap and hood.

[Paul the Sunman]

As I said in my earlier post, I don't think focus breathing is big deal. The DFA* 70-200 is an amazing lens, which I can't imagine parting with.

Absolutely! Would you believe it's even great for street photography (from a distance ).

[lotech]

Another aspect is that the stated focal length is only correct when the lens is focused at infinity. At shorter focus distances the nominal and actual focal lengths will differ. I believe it's called 'focus breathing'. Different lenses will suffer from this in different amounts. That's probably why your Pentax and Sigma lenses show a difference at 300mm.

Yes, as far as I noticed lenses used in motion picture does not 'breath' as much, but they are big and expensive.

[UncleVanya]

I think a combination of focus breathing and loose labeling is involved. A 308mm Pentax and a 294mm could both be marketed as a 300mm for example.

[lotech]

I found on some 'fancy' zoom that has digital display, I believe they can't 'cheat' ha !

[SFX]

I have both lenses (P 150-450 & S 50-500). The Sigma is "good" and the Pentax is "better" regarding sharpness. 50mm difference is irrelevant (because of cropping). Often the 80/20 rule applies. Here too, you pay up to 80% more for about 20-30% better quality.