Panasonic 35-100 f/2.8 with Sony DH1758 1.7x Teleconverter

I have to admit when I read your post I was a bit skeptical about your claim that the 35-100 was not really 100mm at 100mm. Part of this was simply the logic that it was equally possible that the 100mm of the 100-300 was not 100mm at 100mm and was in fact 170mm at 108mm.

So I did a quick and dirty test of comparing the 35-100mm at 100mm with my A7r and 70-200mm at 200mm (the A7r is FF). And it appears you are right. By my very rough calculations the 35-100 is only about 80mm at the long end. I should qualify that by saying that I am comparing the FOV of a 4:3 sensor with a 3:2 sensor which complicates matters somewhat but the discrepancy is still there.

It would be interesting if someone compared the 35-100 @ 100mm with the 75 1.8 (which I am sure we can be pretty certain is a genuine 75).

 

The focal length of any lens is calculated at infinity. Many zoom lenses have a shorter actual focal length when focused on close subjects, if you're curious about the focal length of your lens, redo the comparison with a distant subject and you may get different results.

 

What EarthQuake said. This is called "focus breathing" and testing against small/close targets like in this example will make it very obvious with most zoom lenses.

The 35-100 is in fact almost exactly 100mm FoV when focused at infinity. It has a wider FoV when focused closer. But that is to be expected, nearly every zoom and even many primes behave exactly that way. Nearly all lenses with internal focusing do this to some degree and of course essentially all mirrorless designs are internal focus.

One of the expensive Nikon zooms had a very strong focus breathing effect and the uninformed all freaked out about "lying" and "misrepresentation" until they actually tested it at infinity and discovered - oh wait, it is exactly the focal length they said it was.

Those most concerned about focus breathing are video folks who may want to rack focus from close to far. The change in FoV when doing this can be very distracting for video. For everyone else it is essentially irrelevant. It can throw off someone trying to compare macro capabilities though - you can't just use the stated focal length and closest focus distance to get magnification ratio because of this effect.

 

I understand the point about 'focus breathing' and I had heard the same point made about one of the 70-200mm Nikon's (while Canon's 70-200 doesnt really suffer from this). I slightly disagree about how relevant it is - apart from the fact it is what it is, so who cares?

You see the 35-100mm is something of a portrait lens. In fact theoretically the 35-100mm @ 100mm 2.8 gives roughly the same DOF as the 75 1.8. But in reality it doesnt. I thought this was a 'DOF isnt bokeh' kind of thing. In fact it turns out it is a 'focus breathing' kind of thing - that when I take a portrait with a 35-100 at 100, I am really only taking it at around 80mm 2.8.

You can see the difference in these 2 photos....

First with the 35-100 @ 100mm f2.8

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And then with the 75 @ f1.8

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Certainly if the 40-150 2.8 doesnt suffer from focus breathing it will be a big plus in my eyes.

 

@robbie36 - Well yes and no...

Yes, the DoF is affected by the lower focal length at closer focusing.

No, that rarely makes a difference in real photography. The problem is your premiss that 100/2.8 and 75/1.8 have the same DoF is technically correct but not actually practically useful. The flaw in the logic is that most people don't care about DoF for a given focal length and subject distance as if they are arbitrary unrelated parameters. The reality in shooting most portraits is you have a desired reproduction size or magnification ratio. For example head, head and shoulders, waist up or full body. So if your 100mm lens focus breathes to 80mm in the end you will step closer to the subject to get your desired composition. By moving closer your reduce the DoF. And in fact you reduce it by exactly the amount to compensate for the lower than expected focal length.

In general the more useful way to consider DoF is that for a given format size (e.g. m43) that the DoF depends on aperture number (e.g. F/2.8) and magnification (e.g. head and shoulders shot). If you think about it this way you can quickly see (and test with a calculator) that focal length is not relevant.

With that in mind we can also clearly see why your two shots above differ so dramatically. First the magnification ratio is different, second shot is slightly higher magnification which will lead to shallower DoF. Second of course the later shot is at 1.8 vs 2.8 which for same magnification would give shallower DoF regardless of focal length.

Basically you've got a double whammy of apples to oranges comparison there. The second shot should have much shallower DoF even if your first was shot with a 100/2.8 with the same magnification.

 

'So if your 100mm lens focus breathes to 80mm in the end you will step closer to the subject to get your desired composition.'

Yes there is a zoom with your feet argument but then again one of the reasons you buy a zoom is so you dont have to.

 

Certainly. And I think this can be an issue with some zoom lenses at "useful" focus distances - like that one Nikon. So I'm not at all trying to dismiss the issue entirely, it is a thing of degrees.

The main issue is in the testing of it - if you test at minimum focus distance that may make the problem with a given lens seem more extreme that it really is. Unless of course you bought the lens to use at minimum focus distance! So really the most useful way to test the impact of focus breathing is to shoot at the distance you plan to use the lens at. That will tell you better how much you've been "short changed" by the particular lens design.

As an aside, I think I recall (but I could be entirely wrong too) that a front mounted teleconverter can affect the degree of focus breathing. So it is possible that the OP's measurements of reduced focal length with the teleconverter on won't be the same as when used with it off. Again, I'm not certain of this - I know rear mounted converters do not affect focus breathing but for some reason I thought I read about front mounted ones potentially affecting it.

 

I've definitely learned more today about focus breathing than I knew before.

The wife is becoming a bit overwhelmed with the number of lenses in the bag (i.e., I'm not sure how well a 100-300mm purchase would go over), so I would love to see some more shots with the 30-100 w/ TC. Any chance you could give us some longer distance shots (especially if you could try and frame them up with the same shot from the 100-300)?

Thanks for being the lab-rat for the rest of us!

 

Just curious, would that mean you see the framing change if you focus from a near object to a far one just behind it? I've not noticed that with my 35-100, but probably will now if I look for it!

 

Yes, that's exactly what would happen. And this is why for people who shoot video "focus breathing" can be a real problem. It is a pretty common cinematographic technique to rack focus from a near object to a far object or the other way around. Again, in most cases to notice the effect you have to change focus from something pretty close to something pretty far - but every lens design is different.

Here's a video someone posted showing focus breathing on the 35-100/2.8:

http://vimeo.com/54754329