Crop Factor, Low Light and Aperture with m4/3 lenses?

Started Jun 15, 2014 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: Very simple:

Just another Canon shooter wrote:

Great Bustard wrote:

oneohone wrote:

I am in the market for a DSLR/mirrorless camera and after initial research and trying various cameras the Olympus M10 has made my final shortlist. Now I have some experience shooting 35mm film and in my experience I often take my 50mm f1.7 lens which I enjoy a great deal and it s a good fit for the types of photography I do, candid street, a little landscape and also portraits.

Now I wonder, I have mostly shot film (not counting snapshots with my cellphone) and then crop-factor never enterted my mind. The M10 has a larger crop factor than the other cameras on my shortlist (1.5 or 1.6) how does that impact the choice of lenses? To get something similar to the 50mm focal length I would need a 24mm lens, but what about aperture? If I understand correctly (and maybe I don't) aperture is also influenced by crop factor, then how is it possible to get a f.2.8 lens or lower for the m10? And how does that further influence low light performance? I often take pictures in low light without flash or tripod and from my experience with the film camera I often use f.2.8 or lower.

I know the M10 is a good camera, and the m4/3 system is also really good, and obviously people are satisfied using this system, but I just need a little help understanding the problem with low light and lens selection for the m4/3 system.

50mm f/2.8 on FF is equivalent to ~30mm f/1.8 on APS-C which is equivalent to 25mm f/1.4 on mFT. By "equivalent to" I mean:

  • The same diagonal angle of view.
  • The same DOF for a given perspective (subject-camera distance) and same diffraction.
  • The same total amount of light falling on the sensor for a given shutter speed.

Any differences in noise will be due to differences in sensor efficiency (as opposed to differences in the ISO setting that is used to get the same brightness for a given shutter speed).

Keep in mind that the lower noise of larger formats (since, for a given exposure, as opposed to same DOF and shutter speed, more light falls on the larger sensor) necessarily results in a more shallow DOF, for better or worse.

The best way to think about this is in equivalence terms. FF does not have a lower noise advantage (for equivalent images) - it has the same noise (but better resolution in general, which can offset some of that).

FF has the same noise for equivalent images only if the sensors are equally efficient. But as a rule they are not. Smaller sensors tend to be more efficient than larger. See here:

http://www.dpreview.com/forums/post/53189207

You can trade in DOF for less noise with any system. What FF does better is that it has more latitude, with the common lenses, to get a shallower DOF, which means more total light and therefore less noise.

Since the OP mentioned APS-C - the m43 has the advantage of having more lenses designed for it, and IBIS. If I know that I will never go FF (well, I already did ), I would rather get an m43 system because of the better suited lens selection, IBIS, weight and size. The recent mirror-less Fuji, etc. are other interesting options.

EDIT: About the post above: "assume the same ISO..." - don't. There is no reason to assume that. Think about auto ISO, or even better, no ISO at all. You chose the aperture and the shutter speed; the ISO is a secondary thing.

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