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I think I almost get the concept of equivalence and the fact that a 70mm lens mounted on a full-frame body is not the same as 70mm on aps-c sensor.

Let's work with some concrete examples:

  1. Sony A7 II + 24-70mm F2.8 lens, full frame sensor
  2. Sony A6300 + 55-200mm lens, APS-C sensor (almost)
  3. Sony RX10 III F2.4-4 24-600mm, 1" sensor

Are they shown as 'equivalent' lengths or actual, ie. at 70mm am I comparing like for like?

I am looking for the best possible travel setup and trying to understand just how much better would A7 be at 70mm compared to the other options.

How much more light would I get with the A7 vs A6300 and RX10?

Regards,

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    Please be more precise about what you mean by "how much more light". – user29608 Apr 7 '17 at 9:57
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    Possible duplicate of What is crop factor and how does it relate to focal length? – mattdm Apr 7 '17 at 11:11
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    70mm is the same, regardless of what is behind it. The difference in field of view is due to the sensor size, not the focal length of the lens. – Michael C Apr 7 '17 at 12:52
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    @MichaelClark Except the RX10 III is marketed with the 35mm-e focal length, to compete with point & shoots which do the same. It's actually a 8.8-220mm lens. – mattdm Apr 7 '17 at 14:35
  • @mattdm What it is marketed as doesn't change the actual focal length of the lens, though. It is actually marked 8.8-220 on the front of the lens. – Michael C Apr 7 '17 at 14:38
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I wish the so-called crop factor (magnification factor) never existed. They serve only us gray-hairs that lived and breathed the 35mm film camera. Cameras evolve! My mentors used the 8x10 view camera. My boss used the 5x7 portrait camera. I used the 4x5 and the medium format before switching to 35mm. We never worried about a crop factor as we progressed. Why a crop factor? We gray-hairs discerned the field of view delivered by our arsenal of lenses we used on our 35mm camera. If we converted to a compact digital, we thought a handy prompt would help us identify what lens on our new little camera would delivered a similar angle of view. Hence the crop factor; I think it is of little use to those without a history of 35mm film camera usage.

How the crop factor is computed: We find the diagonal of the larger camera we have grown accustomed to. We use the Pythagorean method to find the diagonal. The 35mm format is 24mm wide by 36mm long the diagonal via square root of (24x24+36x36)= 45 mm. Next we find the diagonal measure of the Sony A6300 a compact digital 16mm wide by 24mm long – diagonal = 30mm. Now we divide to find the crop factor --- 45 ÷ 30 = 1.5.

Another way to look at the 1.5 crop factor! Convert the 1.5 value to a percent. Thus 1/1.5 x 100 = 66%. This tells me that DX (compact digital is smaller. It is 66% of the bigger guy's format.

That’s the crop or magnification factor. What does this tell us? If we know the angle of view delivered by a 70mm mounted on a FX (full frame), we divide by the crop factor -- thus 70 ÷ 1.5 = 45. Therefore a 45mm on a compact digital will deliver about the equivalent angle of view. (Values rounded for convenience).

The Sony RX10III format is 8.8mm wide by 13.2mm long and the diagonal measure is 16mm. The crop factor to compare the venerable 35mm to this format is -- 45 ÷ 16 = 2.8. If a 70mm is mounted on an FX than 70 ÷ 2.8 = 25. This tells me that a 25mm on a Sony RX10III delivers an equivalent angle of view. Another way to look at a crop factor of 2.8 is 1/2.8 x 100 = 35% (the imaging chip is 35% of the size of a FX format.

Nobody said this stuff is easy!

  • given my confusion, I think having 'equivalence' still makes sense, even if the original 35mm is not that widely used anymore - much like not many of us carry 1kg weight on them, it's still useful to have a common scale. Even 1kg/1lb is confusing so I think this is a bit similar. – Lech Rzedzicki Apr 8 '17 at 22:58
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Is it possible that you might have confused the direction in which the crop factor works? Your examples are quite different setups:

1) "real" 24-70mm (Full frame sensor, so crop factor 1)

2) Here, like above, the focal length is the one of the lens only. Crop factor is 1.5, so equivalent focal length is 83-300mm

3) With bridge cameras most of the time the equivalent focal length is given, so 24-600mm is what you need to compare to the two others in terms of only focal length.

So from this you can see that (regarding focal length) these three are not really comparable!

Apart from that you would have to chose whether traveling light or having real good image quality is more important for you. Few DSLR/Lens Combinations can ever top the huge zoom range of some bridge cameras while still being small and lightweight, but the costs of overall image quality and low light capability are high.

  • ok so if I need to compare the image I'd get from A7 at 70mm, what do I need to compare it to on the other cameras? – Lech Rzedzicki Apr 7 '17 at 13:42
  • The closest you can get to that on the 2nd combination is the shortest focal length of 55mm, which will still only show you a little less of the scene than the on shot whith the A7 at 70mm. The third you can set to 70mm without problems and then compare the pictures. – smow Apr 7 '17 at 14:40
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I am not aware of any interchangeable lens camera which does not use actual focal length. That includes The A7 and A6300 you list.

However, many fixed-lens cameras are marketed with the focal length not of the actual lens but with the focal lengths of a lens which would give the same field of view on a 35mm film camera or digital "full frame". That is the case with the RX10 III.

However, even on the RX10 III, the actual focal length range — 8.8-200mm — is written on the lens:

enter image description here

You can basically take two approaches. One is to always convert everything to 35mm terms. Some people do this; I think it's kind of silly, but, it's pretty typical for things like flash head zoom to also use this standard, so, okay. The other is to always think in terms of the actual focal length and keep in mind your camera's format. For this, you need to understand crop factor and how angle of view relates — but it's really not that complicated.

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