FE 24-70mm F4 OSS Here Today .. Gone Today! Locked

Started Mar 4, 2014 | Discussions thread
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blue_skies Forum Pro • Posts: 11,122
Re: Henry says that F4 on FF is F2.8 on APS-C and F2.0 on m43

Miki Nemeth wrote:

blue_skies wrote:

Remember that F4 on FF is F2.8 on APS-C and F2.0 on m43?

Hi Henry,

I have learned so much from You, here on the pages of this excellent forum. This point I do not understand completely. I had the impression that F-number is the same on any camera systems, just like focal-length. Do you mean that the DoF of F4 on a FF camera is the same as the DoF of F2 on a m43? Does this mean that a FF camera with F4 can auto-focus at the same speed as a m4/3 camera with F2?

Another related question: If I change the A7 to APS-C mode the effects/usability of F4 changes down to F5.6?

Thank You for the help, Miki

Hi Miki,

thanks for your comment.

If you expose cameras side by side, they will all measure the same light, regardless of the sensor size.

So if you shoot at ISO 100, then all formats will e.g. choose 1/200th and f/4. This is same normal exposure.

With this exposure, the larger sensor will produce shallower DOF, because, in FF equivalent terms, it uses the widest aperture. If the ISO is not at base 100, but e.g. at ISO 1,000, then the larger the format, then the lower the noise levels.

But for equivalent exposure, the sensor size must be taken into account. E.g if you compare this ISO 100, 1/200th and f/4, then equivalent exposures are:

  1. ISO 100, 1/200th, f/4
  2. ISO 200, 1/200th, f/5.6 (+1 stop)
  3. ISO 400, 1/200th, f/8 (+2 stops)

Or, roughly, the table shows m43 versus aps-c versus FF. The larger sensors can handle a higher ISO level noise free, but require a higher f-number to keep the equivalent level of DOF.

So, the lens speed is a factor, but without knowing the sensor size, it is all relative. Saying that an f/4 lens on m43 is faster than an f/8 lens on FF is incorrect. Technically the lens at f/4 is faster than a lens at f/8, but as you see from the table above, in equivalent terms, they will perform just the same.

Focusing speed may be sensitive to the normal exposure, and not the equivalent exposure, as it needs to see the contrast points. Both PDAF and CDAF need a minimum level of contrast. I find that under normal illumination, both PPDAF and CDAF work fine. If it gets too dark, then the contrast may not be picked up, or not be where you want it (faces are not very contrasty at night, lamps are).

If you stop down too far, your AF point does not matter a whole lot, since you increase your DOF. But if you go wide open, focus becomes more critical, so you need to properly focus. AF works fine with fast (f/1.8) lenses under low light, but you have to give the (contrast detect) system time to find the AF point (it does) - at f/1.8 there are many more steps for the lens to be considered. The A6000 seems to not require the contrast detect fine tune step any longer, so I am curious to find out how it handles faster (than the kit) lenses.

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